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Late Transition Metal Complexes of Mixed
NHC / Phenolate Tripodal Ligands for Small
Molecule Activation
Komplexe später Übergangsmetalle tripodaler, gemischter
NHC/ Phenolat Liganden zur Aktivierung kleiner Moleküle
Der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität
Erlangen-Nürnberg
zur
Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von
Martina Käß
aus Schwabmünchen
Als Dissertation genehmigt
von der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 13.06.2014
Vorsitzender des Promotionsorgans: Prof. Dr. Johannes Barth
Gutachter: Prof. Dr. Karsten Meyer
Prof. Dr. Nicolai Burzlaff
Experience is what you get when you didn’t get what you wanted.
- Randy Pausch, The Last Lecture
IV
Acknowledgements…………….…………………………....………….……..……..........V
Publications…..………………………………………………...……………………......VII
Table of Contents……………………………….……………………………...………VIII
V
Acknowledgements
First and foremost, I would like to thank Prof. Dr. Karsten Meyer for giving me the chance
to work in his team on this exciting project. Thank you for your trust in me to design my
own experiments, your support with my publication and your help and guidance. The past
few years have been an exciting trip, and I have gained plenty of invaluable experience.
I am indebted to the Studienstiftung des Deutschen Volkes for both financial and
non-material support. Particularly, I would like to thank Prof. Dr. Heidrun Stein-Kecks and
the interdisciplinary group of scholars, for some refreshing nights out and insights beyond
the field of chemistry.
My thanks also to the GSMS (Graduate School of Molecular Science) of the FAU
Erlangen-Nürnberg for ideational support, and for two interesting and enjoyable winter
schools.
Many people at the Chair of General and Inorganic Chemistry have contributed to my
research and experiments. I would like to thank Dr. Matthias Moll for collecting a part of
my NMR spectra, and for pulling the strings behind the scenes. Thanks to Dr. Jörg Sutter
for all Mößbauer spectra, help in their interpretation, and assistance with the EPR
instrument - and for all the jokes! I want to thank Dr. Andreas Scheurer for various
discussions, and for sharing your knowledge of organic chemistry which helped get my
ligand synthesis on track. Concerning organic chemistry, I also need to thank Dr. Alexey
Nizovtsev for his assistance. Thanks to Dr. Frank Heinemann for recording and for solving
all the crystal structures. Thank you also, Panagiotis „Panos“ Bakatselos, for your help in
crystallography and the „sweet“ support in some frustrating moments. I want to thank
Christina Wronna for carefully executing the elemental analyses. I am grateful to Dr. Marat
Khusniyarov for his help in making sense of EPR and other spectroscopy, and the
computational study on my nitride. Thank you Dr. Stephan Zündt for countless discussions
both silly and serious, and for speaking German with an adorable Swiss-American accent.
VI
I would like to thank Prof. Dr. Sven Schneider for discussions about chemistry and beyond.
I want to thank Prof. Dr. Susanne Mossin, particularly though not only for her help with
EPR.
I am grateful to Dr. Eckhard Bill and Dr. Maurice van Gastel (MPI Mühleim) for
welcoming me in Mühlheim and helping me with EPR and ENDOR spectroscopy.
Many thanks to my fellow Meyer group graduate students, those who went before me and
those who joined during my time:
Thank you Dr. Carola Vogel for preparing the way in (Erlangen’s) Meyer group. My warm
thanks to Dr. Oahn Phi Lam, for countless distractions, many laughs, the exploration of
Erlangen’s culinary scene, and for being just as coco-nuts as you are!
Special thanks to Eva Zolnhofer, for being the bestest labmate, a great friend, and for
taking up and carrying on my chemistry where I left it. To her and Jana Korzekwa, my
thanks for numerous “girls’ nights” and discussions about pretty much anything (including
chemistry): A good support system is vital for surviving and staying sane in grad school!
Likewise, I want to cordially thank Mario Adelhardt and Johannes Hohenberger. You
complete more than the ligand series! Thank you also for many “Wired” nights.
The rest of the Meyer group, as well as the Streb, Khusniyarov and Schneider groups, I
want to thank for every bit of support, and for the friendly and productive working
atmosphere in Erlangen.
I would like to thank my lab-students Martina Göckel, Dominik Lungerich, Theresa
Mekelburg, and my Bachelor student Sibylle Frieß, for taking a little part of this journey
with me and adding their pieces of findings to the final picture.
I want to express my heartfelt gratitude to my partner Frederik Naraschewski, whose
support in numerous ways I deeply appreciate.
VII
Publications
M. Käß, A. Friedrich, M. Drees, S. Schneider, Rutheniumkomplexe mit kooperativen PNP-
Liganden: bifunktionale Katalysatoren für die Dehydrierung von Amminboran,
Angewandte Chemie 2009, 121, 922-924.
M. Käß, A. Friedrich, M. Drees, S. Schneider, Ruthenium Complexes with
Cooperative PNP Ligands: Bifunctional Catalysts for the Dehydrogenation of
Ammonia–Borane, Angewandte Chemie International Edition 2009, 48, 905-907.
A. Friedrich, M. Drees, M. Käß, E. Herdtweck, S. Schneider, Ruthenium Complexes with
Cooperative PNP-Pincer Amine, Amido, Imine, and Enamido Ligands: Facile
Ligand Backbone Functionalization Processes, Inorganic Chemistry 2010, 49,
5482-5494.
M. Käß, J. Hohenberger, M. Adelhardt, E. M. Zolnhofer, S. Mossin, F. W. Heinemann, J.
Sutter, K. Meyer, Synthesis and Characterization of Divalent Manganese, Iron, and
Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand
Environments, Inorganic Chemistry 2014, 53, 2460-2470.
E. Jang, C. McMullin, M. Käß, K. Meyer, T. Cundari, T. Warren, J. Am. Chem. Soc. 2014,
accepted.
K. Meyer, M. Käß, A. Hätzelt, C. Kropf, Bleichendes Wasch- oder Reinigungsmittel,
Deutsches Patent- und Markenamt, DE102012207949 A1, filing date: 11 May
2012.
* This publication reports results of my doctoral thesis. Reproduced in part with permission
from the American Chemical Society. Copyright 2013, American Chemical Society.
VIII
Table of Contents
1 Introduction.................................................................................................................. 1
1.1 Motivation and Bio-Inorganic Background ....................................................... 1
1.2 Tripodal Ligands................................................................................................... 3
1.2.1 Tripodal Ligands in Literature ................................................................................ 3
1.2.2 Geometry and Steric Bulk....................................................................................... 9
1.2.3 Electronic Structure .............................................................................................. 11
1.3 Carbenes .............................................................................................................. 13
1.4 Phenolate Ligands............................................................................................... 18
1.5 Objectives............................................................................................................. 21
2 Results and Discussion............................................................................................... 23
2.1 Synthesis of Mixed Phenolate-Carbene N-Anchored Ligands........................ 23
2.1.1 Motivation............................................................................................................. 23
2.1.2 The Basic Building Blocks: Imidazoles and Phenols ........................................... 24
2.1.3 The Original Plan: Mannich-Chlorination-Route ................................................. 25
2.1.4 The Tosylation Route............................................................................................ 32
2.1.5 The Bis(imidazolium) Routes ............................................................................... 34
2.1.6 Final Synthetic Route for (BIMPNR,R’,R”)– and (MIMPNR,R’,R”)2–:
Chloromethylation of the Phenol and SN2 Reaction with Bis(imidazolium) Salt
p2-R ...................................................................................................................... 39
2.1.7 Crystal Structures of a (BIMPNR,R’,R”)– Ligand and its Protonated Precursor...... 42
2.1.8 Summary of Ligand Synthesis and Outlook ......................................................... 45
IX
2.2 Iron Complexes of (BIMPNR,R’,R”
)–
................................................................... 47
2.2.1 Iron(II) Complexes................................................................................................ 47
2.2.2 Essays at redox chemistry..................................................................................... 57
2.2.3 Photolysis Experiments with [(BIMPNMes,Ad,Me)Fe(N3)] (2) ............................... 65
2.3 Manganese Complexes of (BIMPNR,R’,R”)–........................................................ 68
2.4 Electrochemistry ................................................................................................. 74
2.5 Diamagnetic Complexes of (BIMPNR,R’,R”
)–
..................................................... 78
2.6 Cobalt Chemistry of (BIMPNR,R’,R”
)–................................................................ 82
2.6.1 Cobalt (II) Complexes........................................................................................... 82
2.6.2 Co(I) Complexes and their Reactivity .................................................................. 94
2.6.3 Reactions of [(BIMPNMes,Ad,Me)CoI] (8) with Organic Azides ............................. 98
2.7 Towards a Cobalt Nitride................................................................................. 106
2.7.1 Introduction......................................................................................................... 106
2.7.2 Photolysis of [(BIMPNR,R’,R”)Co(N3)] (7).......................................................... 106
2.7.3 Photolysis of [(TIMENMes)Co(N3)]+ ................................................................... 123
2.7.4 Outlook and Suggestions for Further Experiments............................................. 131
3 Summary in English and German.......................................................................... 133
3.1 Summary............................................................................................................ 133
3.2 Zusammenfassung............................................................................................. 141
4 Experimental Part.................................................................................................... 149
4.1 Methods, Procedures and Starting Materials ................................................ 149
4.1.1 General ................................................................................................................ 149
X
4.1.2 Starting Materials................................................................................................ 149
4.1.3 Analytical Methods............................................................................................. 149
4.2 Synthetic Details................................................................................................ 151
4.2.1 Ligand Syntheses ................................................................................................ 151
4.2.2 Iron Complexes................................................................................................... 171
4.2.3 Manganese Complexes ....................................................................................... 184
4.2.4 Diamagnetic Complexes ..................................................................................... 188
4.2.5 Cobalt Complexes............................................................................................... 191
4.2.6 TIMENMes Cobalt Chemistry .............................................................................. 211
4.3 Magnetism Data and Simulation Parameters ................................................ 214
4.4 Crystallographic Details................................................................................... 215
4.4.1 X-ray Crystal Structure Determination Details................................................... 215
4.4.2 Crystal Data, Data Collection, and Structure Refinement Details:..................... 219
4.4.3 Geometrical Structure Parameters ...................................................................... 224
4.4.4 Crystal Structure of [(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) .................................... 238
5 Symbols and Abbreviations .................................................................................... 239
6 Numbered Compounds............................................................................................ 242
7 References ................................................................................................................. 244
1
1 Introduction
This thesis concerns itself with tripodal, mixed NHC/phenolate ligands, their coordination
chemistry, and the possible use of their complexes in small molecule activation. This
introductory chapter will first map out some of the biological background for the work,
then go on to discuss the specific properties of tripodal ligands. Subsequently, the two
different classes of binding groups – N-heterocyclic carbenes and phenolates – will be
elaborated on in more detail, to give a summary of the history and use of the respective
ligand classes, and the inherent properties of these binding sites. Finally, this background
on the state of the art is linked to the questions I tried to answer with this thesis.
1.1 Motivation and Bio-Inorganic Background
Certain small molecules such as N2 and CO2 possess extremely high thermodynamic
stability and hence low reactivity. The bond dissociation energy for the N≡N-triple bond,
for instance, is 945 kJ/mol high, and the highest dissociation energy is found in carbon
monoxide C≡O (1077 kJ/mol).[1] It is therefore a challenge to coordinate these molecules to
a metal center and activate them for further reactions. Biological fixation of atmospheric
dinitrogen is the basis for all nitrogen containing molecules in life forms, from DNA to
amino acids. And the exhaust gas CO2, of which an estimated 33 billion tons are emitted
into the atmosphere every year,[2] may become a source for sustainable fuel and chemical
production – if ways are found to bring this molecule, which is at the deep end of the
thermodynamic well, back into the industrial process through reduction or
functionalization.[3-5]
Nature has developed some very effective enzymes for these tasks. Nitrogen fixation is
catalyzed by nitrogenase, a protein with a reactive FeMo cluster at its center[6-7] that is
found in certain bacteria and archaea.[8] Some of the proposed intermediates during the
transformation process are high-valent metal species with metal-nitrogen multiple bonds.[9]
In the corresponding industrial process (Haber Bosch) atmospheric N2 is converted to
136 million tons of ammonia per year.[10] The cleavage of the N2-triple bond also proceeds
via imido and nitrido species that have been observed spectroscopically on the iron
catalyst’s surface.[11-12]
2
Nature’s way of CO2 fixation is photosynthesis, which turns CO2 and water into
carbohydrates. At the other end of that process, dioxygen is formed by a light-driven four-
electron oxidation of water. The dioxygen is released in photosystem II from the oxygen
evolving complex (OEC), an enzyme with a Mn4CaO5 cluster at its core.[13] The
mechanism of O–O bond formation is still under debate. There are mainly two reaction
pathways discussed in literature today: Firstly, an acid/base pathway (AB, or nucleophilic
attack mechanism), in which a water molecule (the nucleophilic oxygen or base) attacks a
terminal oxo-moiety (the electrophilic oxygen or acid); secondly, a radical coupling
mechanism (RC) between two terminal oxo species.[14] While there is experimental
evidence for the AB mechanism in Mn model systems that perform O–O bond
formation,[15-16] more recent computational studies using the latest OEC crystal structure[13]
favor an oxo/oxyl radical coupling mechanism.[17-18]
The development and characterization of well-defined model complexes of species with
metal-to-ligand multiple bonds are vital to aid in the elucidation of these processes and
their mechanism.[19] Functional models or reactivity studies may also promote a deeper
understanding of the underlying principles. With deeper insights into reaction pathways at
reactive metals centers, not only may Nature be better understood, but more efficient
catalysts may be developed for industrial processes. The aforementioned Haber-Bosch
process consumes vast amounts of energy and natural gas, as it requires high pressure and
high temperatures as well as dihydrogen, which is in turn produced in the energy-intensive
water gas shift reaction that also sets free carbon dioxide. Catalysts that generate ammonia
from N2 at milder conditions may help preserve large amounts of energy – Nature’s
nitrogenase is still the best role model for this.
Furthermore, complexes with oxo-, imido-, and nitrido species may be used in catalytic
atom- and group transfer reactions, i.e. for epoxidation and aziridination reactions, or even
direct C–H-bond amination.[20] For all these reasons, the synthesis of species with
metal-ligand multiple bonds is an attractive research goal. For late transition metal
complexes, tripodal ligands have emerged as a most suitable class of ligands to stabilize
such M=E and M≡E multiple bonds.
3
1.2 Tripodal Ligands
Polydentate ligands give more robust complexes than their monomeric counterparts, owing
to the (kinetic and thermodynamic) chelating effect. Thus, tripodal ligands with their three
(or more) binding sites often form stable complexes with a large variety of metal ions. The
steric bulk around the metal center depends on the nature of the coordinating groups, but
can often be tuned conveniently by exchanging their substituents, and in that way bulky
ligands may be easily prepared. The anchoring units of tripodal ligands – usually B, C, or N
– can provide further structural and electronic flexibility, and they can effectively block
undesired side- and decomposition reactions by shielding one side of the reactive metal
center entirely.
As a result of its trigonal nature, a tripodal ligand enforces ‘facial’ (as opposed to
‘T-shaped’ or ‘meridional’) binding. This can be used to mimic a variety of non-heme
metallo-enzymes, many of which contain a triad of amino acid side chains that binds
facially to the metal center.[21-23] In contrast, other 3-coordinate geometries, like the
‘T-shaped’ binding of pincer-type ligands for instance, have little to no significant
biological relevance.
All these beneficial characteristics of tripodal ligands as well as their complexes’ electronic
structure (see below) have promoted the design and development of numerous tripodal
ligand systems.
1.2.1 Tripodal Ligands in Literature
One old and widespread class of tripodal ligands are tris(pyrazolyl)borates (Tp)
(compound A in Scheme 1)[24-25] and their derivatives. They were first published by
Trofimenko in 1966,[26] who named them “scorpionate ligands”,[27] evoking the picture of a
scorpion that grabs its prey (the metal ion) with its two pincers before reaching over the
plane to attack with its stinger (the third ligand arm).1 The steric and electronic properties
1 The Tp ligands are usually referred to as “scorpionate ligands” in literature, as are some other ligand
systems mentioned here (e.g. Tm). The differentiation between scorpionates and tripodal ligands is nebulous,
however, and the question of whether a ligand system is called a “scorpionate” or simply a “tripodal ligand”
in literature seems to be a question of the school or chemical heritage of a publications’ author or ligands’
creator, rather than a logical classification or distinction between ligands.
4
of the pyrazolyl donor can be modified by changing its 3- and 5-substituents (R and R’).[24-
25] Numerous derivatives of the Tp ligand and of the related tris(pyrazolyl)methane (tpm,
B) have been developed, although the synthesis of the latter is more challenging.[28-31]
Also very popular and effective, the tris(amido)amine ligands, based on inexpensive tris(2-
aminoethyl)amine, have been intensively studied.[32] Schrock and coworkers employed a
bulky tris(amido)amine to catalytically reduce dinitrogen to ammonia.[33] They isolated and
BNN
N N
R
H
R
N
N
R
R'R'R' C
NN
N N
R
H
R
N
N
R
R'R'R'
tris(pyrazolyl)borate, Tp
B
P PP
FeIV
N
N
N NMoIIIN
iPr iPr
iPr
iPr
iPriPr
HIPT
HIPT
N
N
N
N NFeIIIN
O
N
O
N
ON H
HH O
2-
-
A
tris(pyrazolyl)methane
B
a tris(amido)amine complex
C
tris(ureayl-amido)amine,
D
B
S SS
NiII
Fe-oxo complex
AdAd
O O
tris(thiomethyl)borate,
E
Ni-superoxo complex
tris(phosphino)borate,
F
Fe-nitrido complex
BNN
N N
R
H
RN
N
R
tris(imidazol-2-thione)borate, Tm
-
H
SS
S
N
O O
N
N
O
tris(oxazoline)amine
G
N
R R'
R R'
R R'
Scheme 1. Several representative tripodal ligands and complexes (R, R’ = various alkyl and aryl
substituents).
5
characterized a number of potential intermediates,[34-37] one of which is shown in Scheme 1
(compound C, HITP = hexa-iso-propyl-terphenyl). Borovik and coworkers prepared
terminal iron(III) oxo complex D using a ureayl-functionalized tris(amido)amine ligand,
which stabilizes the oxo-ligand through hydrogen bonding.[38-39]
In contrast to the “hard” amido donor ligands, tripodal ligands bearing ‘‘soft’’ donor atoms,
such as sulfur[40-41] and phosphorus, are more suitable for stabilizing electron-rich low-
valent and late transition metal centers. Thus, nickel complexes of a sulphur-borane tripod
were synthesized, as was its side-on dioxygen adduct identified as a Ni(II)-superoxo
complex (E).[40] Peters and coworkers demonstrated the high potential of tripodal ligands in
small molecule activation by utilizing a tris(phosphino)borane[42] to synthesize iron nitrido
species (F),[43] as well as terminal Co(III) imido[44] and several Fe(III) imido[45-46]
complexes.
Further tripodal ligand systems included in Scheme 1 are the tris(oxazolines), which have
been developed with either methane-, nitrogen- (G), cyclopropane- or mesityl-anchoring
units,[47] and tris(imidazol-2-thione) ligands (H, TmR: hydro-tris(R-thimazolyl)borate).[48]
Tripodal NHC Ligands
Tripodal NHC ligand systems have so far been synthesized with mesityl-, borane-,
methane- and nitrogen-anchors. The mesityl-anchored ligand I (Scheme 2) was isolated by
Dias and Jin, but no metal complexation was achieved.[49] Hu and Meyer were able to
introduce the exceptionally large Tl(I)-ion into the cavity, but it was concluded that the
ligands’ cavity was too large (and rigid) to allow the complexation of any transition metal
ion.[50]
As analogue to the common Tp ligands, Fehlhammer synthesized a tris(carbene)borate.[51]
However, this ligand formed complexes with two ligand molecules chelating one metal
center, resulting in coordinatively saturated hexakis(carbene) complexes that did not bind
any further ligands (see complex J in Scheme 2), which decisively limits their applicability
in small molecule activation.[51-53] By derivatizing this ligand with larger substituents on the
boron anchor, and more importantly on the NHCs, Smith was not only able to isolate 1:1
complexes,[54-56] but also synthesize iron- and cobalt imido-complexes,[57-58] as well as iron-
nitridos[59-60] that form ammonia in the presence of an H-atom donor[61] (Scheme 3).
6
N
N
NN
N
N
BNN
N N
R
H
RN
N
R
BN N
NN
R
H
RN
N
RMII
M = Co, Rh, Fe
2+
I J R = Me, Et
N
N
NC
N
RC
N
MN
C
N
R
R
C
N
NC
N
RC
N
MN
C
N
R
R
hypothetical
12
3
456
7
8 12
3
45
6
1:1 TIMER metal complexTIMENR metal complex
Scheme 2. Tripodal tris(carbene) ligands and complexes.
BN
N
N
N N
N
Fe
N
NN
NC
NC
N
FeIV NC
N
R
RR
N
+
NN
NC
NC
N
Mn NC
N
N
n+
MnIII (n = 0),
MnIV (n = 1),
MnV (n = 2)
R = H, Me
n+
FeIV (n = 0),
FeV (n = 1)
Scheme 3. Nitrido complexes from the groups of Smith (left) and Meyer (middle and right).
7
The methane-anchored NHC tripodal ligand TIMER (1,1,1-tris(3-R-imidazol-2-ylidene)-
(methyl)ethane, R=me, tBu) by Hu and Meyer formed complexes with group 11 metals, but
never in a 1:1 ligand : metal ratio.[62-64] This may be explained insofar as a hypothetical 1:1
TIMER : metal complex would contain three eight-membered metalla-rings, with much
lower stability than the more commonly observed five- or six-membered rings (see
Scheme 2).[64] Therefore, Hu and Meyer sought to incorporate a coordinating atom at the
anchoring position of the carbene tripod, which would result in more favorable six-
membered rings. The N-anchored tris(carbene) ligand TIMENR (tris[2-(3-R-imidazol-2-
ylidene)ethyl]amine)[64] finally gave the desired 1:1 complexes with, among others, Mn, Fe,
Co, Cu,[65] and Ni.[66]
TIMENR (R = mesityl, 2,6-xylyl) has enabled the stabilization and full characterization of
terminal Fe(IV) nitrido complexes, and these were the first crystallographically studied
mononuclear iron-nitrides.[67] Recently, a series of TIMENR (R = 2,6-xylyl) Mn nitrides in
different oxidation states (III, IV, and V) has also been synthesized.[68] Their reactivity is
currently investigated. In the cobalt peroxo complex of the TIMENXyl ligand, two of the
carbene arms are pushed apart, giving the complex an overall distorted octahedral
symmetry, thus allowing for side access for organic substrates.[69] Consequently, the
complex was employed in O-atom transfer chemistry to organic substrates. In contrast, the
three-fold symmetrical iron nitrido and cobalt imido complexes of TIMENR (R = mesityl,
2,6-xylyl) did not undergo any atom or group transfer chemistry. Instead it could be shown
that the Co(III) imido and the one-electron oxidized Fe(V) nitrido intermediate insert the
RN2– and N3– ligands into the metal carbene bond, forming bis(carbene) imine species
(Scheme 4).[70-71]
Therefore, reduction of the steric pressure of the chelating ligand was desired, to open up
the reactive cavity and allow side access of substrates to the functional entity. The first
strategy to reduce steric pressure was to remove the ortho-CH3-groups of the mesityl-/
xylyl-substituents on the imidazolylidenes. However, the introduction of reactive
hydrogens in the ortho positions leads to unexpected new reactivity, resulting in C–H bond
activation and three-fold metalation of the ligand.[72] Subsequently, side access by
exchanging one or two carbene arms with phenolates was sought (section 1.5 Objectives).
8
NN
NC
NC
N
M NC
N
R'
R'R'
N
R
N
N
NC
NC
N
MNC
N
R'
R'
N
R'
R
R' = H, Me
a) M = Fe, Co,
b) M = Fe,
R = Ar
N = nitride
R' = H, Me
a) M = Fe, Co,
b) M = Fe,
R = Ar
R = H Scheme 4. Insertion reactions of TIMENR imido and nitrido complexes.
Tris(phenolates)
Another N-anchored tripodal ligand system routinely employed in the Meyer group
laboratories[73-75] is the tris(phenolate) ligand ((R,R’ArO)3N)3– (trianion of tris[(3,5-R,R’-2-
hydroxyphenyl)methyl]amine)[76] (Scheme 5). A tris(phenolate) derivative of this ligand
system with methyl substituents in the ortho and para position of the phenol was employed
by Kleij and coworkers in the catalytic cycloaddition of CO2 to epoxides and oxiranes with
iron.[77] The tris(phenolate) ligand derivative ((Ad,MeArO)3N)3–, developed in our lab, has
recently been used to activate CO2 and other heteroallenes at reactive uranium coordination
complexes.[78]
It is worth mentioning that the equivalent tris(thiolate) system has been developed about 2
decades ago,[79] and particularly its chemistry with iron and molybdenum has been explored
in the hope of mimicking as far as possible the FeMo-cofactor of nitrogenase.[80] Very
recently, even a corresponding tris(stibine) (i.e. with three antimony binding groups) has
been prepared and its coordination to Fe and Mn probed.[81]
N
OR
R'3
N
SR
R'3
N
SbR
R'3
Scheme 5. Tris(phenolate) (left), tris(thiophenolate) (middle) and tris(stibine) N-anchored ligands.
9
Enantiomery in Tripods
The TIMENR ligand (and probably other tripods) forms helical enantiomers upon
coordination, which are observed however only in the solid state (i.e. by X-ray single
crystal analysis). In solution, the system can be considered dynamic, with a rapid exchange
between the two enantiomers resulting time-averaged in C3v symmetry.
In search of catalysts for enantioselective catalysis, a number of chiral, enantiomerically
pure tripodal ligands have been synthesized, including derivatives of tris(oxazolines),[47]
tris(pyrazolyls) and tris(phosphanes).[82] While bidentate C2-symmetric ligands reduce the
number of possible diastereomers in catalytic intermediates for square planar complex
geometries, C3-symmetry can do the same for octahedral intermediates.[47, 82]
Mixed Tripodal Ligands
All tripodal ligands mentioned so far are also C3 symmetric, with three equivalent binding
groups, but a number of tripodal ligands with mixed ligand arms are known.
Of the borane-anchored „scorpionate“ ligands, mixtures between pyrazolyl and imidazole-
2-thione ligand arms[48], and for the carbon-anchor, between pyrazolyl and carboxylate
binding groups[22] have been published.
Some ligand arms that are easily mixed on a nitrogen anchoring unit are N-donors like
pyridines, imidazoles, benzimidazoles, or amines, and O-donors like phenolates or
alkoxides, as has been nicely demonstrated by Palaniandavar and coworkers.[83] Most
impressively, a monomeric Mn complex of an N-anchored, mixed ureayl and pyridyl
ligand has been shown by Borovik and coworkers to catalytically reduce dioxygen to
water.[84]
So far, and to the best of my knowledge, no tripodal mixed NHC / phenolate systems have
been reported.
1.2.2 Geometry and Steric Bulk
For a tripodal ligand system, the sterics around the complex’s metal center depend to a
large extent on the nature of the binding groups. Scheme 6 demonstrates that for
tris(amides) and –(phosphanes), the organic substituents R are directly bound to the
coordinating atom, and therefore point away from the metal center. This leaves the reactive
site wide open, encouraging dinuclear decomposition pathways. Accordingly, the iron(IV)
10
nitride of Betley and Peters dimerizes and could not be characterized by X-ray
crystallography: The single crystals contain merely the dinuclear, dinitrogen-bridged
coupling product.[43]
Similarly, Yandulov and Schrock had to introduce three exceedingly bulky hexa-isopropyl
terphenyl substituents at the tris(amido)amine ligand in order to prevent dimerization of
their Mo-nitride intermediate.[33] The synthesis of these extremely bulky ligand derivatives
is often challenging and time-consuming.
In contrast, the sterics of tripodal NHC ligands are controlled by the substituents at the
imidazole N3 position, which can align nearly perpendicular to the metal-carbene
coordination plane. Even for less bulky alkyl or aryl substituents, this results in a narrow,
well-protected cylindrical cavity around the reactive center, which allows axial access for
small molecules and is able to stabilize reactive species like the terminal nitride ligands
shown above in Scheme 3.
M
P PP
B
Ph
R
R
R
R
RR
M
C C
N N N
B
Ph
NMN
N
N
R
R
R
NM
N
N
N
N
C
C
N
C
N
R
R
R
NN C
N
RR
R
Scheme 6. Sterics around the reactive metal center for tripodal ligands with different binding
groups; arrows pointing in the direction of the steric bulk created by the substituents R (adapted and
expanded from Hu et al.[64]).
11
The comparison between Smith’s borane-ligand and Meyer’s TIMENR demonstrates that
the steric bulk can also be strongly influenced by the anchor or the “bridge length” between
anchor and binding groups: Both ligands are tris(carbenes), but in Smith’s case, the NHCs
are directly bound to the boron anchor. Thus, the carbenes are forced to coordinate at a
flatter angle to each other, which causes the substituents R to point somewhat away from
the reactive center.
1.2.3 Electronic Structure
Tripodal ligands provide a powerful platform for small molecule activation chemistry,[32-33,
35, 84-87] as has been highlighted most impressively by Peter’s synthesis of Fe(III) imido,[45-
46] terminal Co(III) imido,[44] and iron nitrido species,[43] plus further examples given in
section 1.2.1 (see Schemes Scheme 1 and 3). The ligand field splitting resulting from their
trigonal coordination environment is suitable for the stabilization of highly unusual metal-
ligand multiple bonds, even for relatively electron rich late transition metals,[87-88] which
will now be illustrated and discussed by means of the d-orbital splitting diagram in
Scheme 7 (see also reviews by Nocera[88] and Peters[87]):
Scheme 7. d-Orbital splitting patterns for complexes with metal-ligand multiple bonds M=E and
M≡E in C4 and C3 symmetric environments.
12
In an octahedral or square pyramidal complex with only σ donor ligands, one finds three
degenerate, non-bonding orbitals (dxy, dxz, and dyz) and two anti-bonding ones (dz2, dx2-y2).
Through interactions with π-donors, some of the non-bonding orbitals may become
antibonding. For strongly π-donating ligands, such as oxos or nitridos, this results in the
1+2+1+1 splitting pattern ( 7a) ) first introduced by Ballhausen and Gray for the vanadyl
ion in VO(H2O)5.[89] This pattern has been used by Gray for trans-dioxo and nitrido
complexes of Tc(V), Re(V),[90-91] and Os(VI)[92], and adapted for many similar complexes.
The more covalent the ligand-to-metal multiple bond becomes, the more the dxz and dyz
involved in the π-bonding are destabilized. For trans-[(cyclam)M(N)(X)] complexes
(M = Cr, Mn; X = µ-N3, CH3CN), a 1+3+1 orbital splitting with almost degenerate dx2-y2,
dyz, and dxz orbitals has been deduced,[93-94] and finally in [Cr(N)Cl4]2–, the π* orbitals are
even higher in energy than the dx2-y2, leading to a 1+1+2+1 splitting ( 7b) ).[95] In the latter
complex, the π-perturbation from the nitride far outweighs the σ contribution of the
chlorides, which puts the dx2-y2 and dxy orbitals far below the anti-bonding π* involved in
the triple bond to N, giving the complex a d-orbital splitting more comparable to a linear
geometry as reference system than to a distorted octahedron.
In either case, 7a) and 7b), only one non-bonding orbital remains which can house only two
d-electrons, rendering it obvious that this geometry is unfavorable for the higher electron
count of low oxidation states or late transition metals as the population of anti-bonding
orbitals would destabilize the metal-to-ligand multiple bond. This is made evident in the
difference in stability of the three nitrides sketched in Scheme 8: While the Cr(V) and
Mn(V) nitrides (d1 and d2) are stable at RT, and therefore characterized
crystallographically,[93, 96] the iron(V) nitride (d3) can only be generated at very low
temperatures and was studied in frozen matrix by low-temperature Mößbauer and EPR
spectroscopy.[97]
N N
N N
MV
N
L
HH
H H
M = Cr, Mn L = NCMe
M = Fe L = N3
Scheme 8. Cyclam metal-nitrido complexes by Meyer et.al.
13
In a trigonal symmetry, in contrast, the orbitals lying in the xy-plane remain non-bonding
(and degenerate). For an M=ER double π-bond as found in imido-complexes, three non-
bonding orbitals are available (which may even be strictly degenerate) for up to six
d-electrons ( 7c) ).[98] With an oxo or nitrido triple bond, wherein the dz2 is strongly
destabilized through σ-bonding, two non-bonding orbitals still remain to accommodate four
d-electrons. DFT calculations verified this 2+1+2 pattern ( 7d) ) nicely for Vogel’s
[(TIMENR)FeIV(N)] (R = Xyl, Mes).[67] It has also been stated that two frontier π–orbitals
that are strictly degenerate are favorable for metal–ligand multi-bond formation.[32]
In conclusion, the electronic environment created by C3 symmetric ligands can greatly aid
in the synthesis of species with metal-ligand multiple bonds, like terminal imido, oxo and
nitrido species, especially in mid- to late first-row transition metal complexes.
1.3 Carbenes
A carbene is defined as a molecule containing a neutral, divalent carbon atom with only six
valence-shell electrons, in which the carbon atom possesses two covalent bonds to adjacent
atoms. The history of their research can be traced back almost 200 years,[99] and carbene
chemistry today still represents an exciting and rapidly developing area of research.
History and Use in Catalysis
Isolable carbenes have been sought after since the first half of the 19th century.[99-102] The
first notable experiments were conducted by Dumas,[103] who reacted methanol with
dehydrating agents, like phosphorous pentoxide or conc. sulphuric acid, in the hope of
freeing the CH2 unit. His efforts and many others[104-107] failed, however, until at last
carbenes were thought of as unstable, fleeting intermediates by most chemists and treated
as such in standard textbooks. At best, it was thought, they could be stabilized in the form
of metal complexes.
Meanwhile, carbenes were introduced by Doering as synthetic intermediates into organic
chemistry in 1954[108] and by Fischer into organometallic chemistry in 1964,[109] and these
intriguing species became involved in many reactions of high synthetic interest.
14
NC
N
Ph
Ph
H
CCl3
∆∆∆∆
- HCCl3
2N
CN
Ph
Ph
N
N
Ph
Ph
CN
CN
Ph
Ph
2
Scheme 9. Synthesis of an ene-tetraamine (“carbene-dimer) by α-elimination from an imidazolidin.
Wanzlick tried to generate a free carbene through α-elimination of chloroform from the
corresponding imidazolidine compound (Scheme 9),[110] but only the dimer could be
isolated. In 1968, Öfele[111] and Wanzlick[112] independently synthesized chromium and
mercury complexes of imidazolydenes (N-heterocyclic carbenes, or NHCs). Finally, after
renewed efforts,[102] Arduengo was able to isolate, characterize, and even
crystallographically analyze a free carbene (Scheme 10).[113]
Arduengo’s discovery was followed by a sheer explosion of new isolable carbenes. Today,
even a number of non-cyclic stable carbenes have been isolated.[114-116] But NHCs in
particular began to play an important role in transition metal catalysis, as several extremely
active catalysts were developed, perhaps most prominently illustrated by the second-
generation olefin metathesis catalysts developed by Grubbs[117] and Nolan,[118-120] or by the
cross-coupling catalysts introduced by Organ and commercialized by Aldrich.[121]
Catalytic applications of NHC complexes have been extensively reviewed in the last
decade.[122-133] The ongoing popularity and explosive development around this fascinating
class of molecules is demonstrated by dozens of review articles published each year on
carbenes in all fields of chemistry.
N
NH
- NaCl, -H2
(THF)
NaH
[DMSO]
NC
N
Cl
Scheme 10. Synthesis of the first isolated, stable carbene by deprotonation of an imidazolium
chloride, sterically encumbered by bulky adamantyl substituents.[113]
15
Properties and Electronic Structure
Depending on their hybridization, carbenes can be either linear or bent. The simplest of all
carbenes, methylene CH2, is a linear carbene with triplet ground state. The linear geometry
implies sp hybridization, which leaves two degenerate p orbitals for the two non-bonding
electrons, resulting in a triplet ground state. This geometry is adopted only when the
substituents are lower in electronegativity than the carbon atom. Most carbenes possess a
bent coordination mode (sp2 hybridization), and the exact angle between substituents
depends on their electronegativity (which influences the degree of hybridization) and, in
part, on their steric demands (for cyclic carbenes, this includes the ring size).
Scheme 11. Possible electronic configurations of a carbene.[101]
Scheme 11 gives an overview of the possible electron configurations for an sp2 hybridized
carbene. The non-bonding orbitals are one of the hybrid orbitals and one p orbital,
conventionally denoted as the σ and pπ orbital, respectively. Four electronic configurations
can be envisioned:[101] A singlet with both electrons located in the hybrid orbital (σ2), a
triplet ground state (σ1pπ1), an energetically higher singlet state with both electrons in the
p orbital (σ0pπ2), and an excited singlet state with one electron in each orbital (σ1pπ
1) but
with antiparallel spin.
The energy gap between σ and pπ necessary to induce a singlet ground state (σ2) vs. a triplet
(σ1pπ1) was determined by Hoffmann to be 2 eV.[134] The energies of the carbenes’ frontier
orbitals depend on the substituents, which can be explained through inductive (Scheme 12)
and mesomeric (Scheme 13) effects:[101] The inductive effect involves the σ orbital, which
is stabilized by electronegative substituents, leading to larger σ–pπ energy gap and singlet
ground state, whereas substituents with +I–effect decrease the energy gap and,
consequently, favor the triplet ground state. The mesomeric effect engages the pπ orbital,
which is destabilized by π-electron-donating substituents and stabilized by π-electron-
withdrawing substituents.
16
carbene substituent
2px,y,z pπ (b1)
2s
b2
σ (a1)
a1
carbene substituent
2px,y,z pπ (b1)
2s b2
σ (a1)
a1
C C
Scheme 12. Perturbation orbital diagram illustrating the inductive effect of σ-donating (left) and
σ-withdrawing substituents (right) on the carbene orbitals.[101]
pπ
pπ (b1)
σ
a2
σ
b1
CX X
CX X
δ−
1/2 δ+ 1/2 δ+
px pπpy
σ
CZ Z CZ Z
δ+
1/2 δ− 1/2 δ−
σ
pxpy
CX Z CX Z
δ+ δ−
pπ
Scheme 13. Perturbation orbital diagram showing the mesomeric effect on the carbene frontier
orbitals; X = π-donating substituent, Z = π-withdrawing substituent.[101]
While the substituents determine the carbene’s electronic structure, the carbene’s ground
state multiplicity strongly influences its reactivity: Since singlet carbenes possess one filled
HOMO and one empty LUMO, they can be considered ambiphilic, whereas triplet carbenes
possess diradical character.[135]
17
NHCs have replaced phosphines in many areas, for they are temperature stable and do not
oxidize like their phosphine counterparts, which greatly facilitates preparation and storage.
The excellent performance of NHC catalysts derives not exclusively from the pronounced
σ-donor properties of NHC ligands, but also from the possibility of π-back-bonding within
the metal-carbene unit,[136-139] which enables them to stabilize both high and low oxidation
states formed in catalysis resulting in higher turnover numbers and longer catalyst
lifetimes.[140]
Furthermore, they provide one of the few series of ligands that support homogeneous
catalysis and for which, at the same time, steric and electronic parameters can be
extensively tuned.[140-142] Serviceably, while phosphines PR3 have only one variable (their
substituent R) which influences both their steric and electronic properties, an NHCs’ steric
and electronic tuning is separable to a greater extent: Their sterics are predominantly
controlled by substituents on the atoms adjacent to the carbene center (usually the N-
atoms);[143] electronic effects are also influenced by substituents on the other ring-atoms,
and by the nature of the azole ring and the ring position to which the metal is attached.[144]
Classification
Classically, metal carbene complexes have been divided into Fischer and Schrock
carbenes.[145-147] Fischer carbenes are electrophilic and are usually found with mid- to late
transition metals in low oxidation states, and also π-acceptor ligands on the metal and
π-donor substituents on the carbene ligand (typically OR or NR2) that give them further
stability. The nucleophilic Schrock carbenes, on the other hand, are found with early
transistion metals in high oxidations states, with π-donor metals, and normally without
heteroatom-substituents on the carbene carbon atom. Consequently, metal-carbene bonding
in Fischer carbenes is usually described by help of the Dewar-Chatt-Duncanson model,[148-
149] with carbene–metal σ–donation and metal–carbene π–backbonding, whereas the
situation in Schrock carbenes is more aptly described as covalent double-bonding between
a triplet carbene and a triplet metal fragment.[145]
Since the introduction of N-heterocyclic carbene (NHC) complexes, however, the
Fischer/Schrock classification needs to be revisited and/or extended because of the very
different electronic character of these ligands. On first glance, their heteroatom-substituents
18
would put them in the group of the Fischer carbenes. Accordingly, the –I and +M-effect of
the heteroatoms deliver ideal stabilization of the singlet ground state. Nevertheless, “One of
the most characteristic features of NHCs is their extraordinary electron richness.”[150]
Finally, in addition to classic N-heterocyclic carbenes, a growing number of carbenes with
unusual coordination mode have been reported in the last decade,[141, 151] as has recently
been excellently reviewed by Crabtree:[144] abnormal NHCs (aNHC), alternatively called
mesoionic carbenes (MIC), and remote NHCs (rNHC) (see Scheme 14). For free aNHCs,
no six electron structure can be drawn, but one is obliged to assign formal charges to the
non-binding NHCs (thus the name MIC). Therefore, aNHCs are not strictly carbenes
according to the terms’ definition. However, upon binding, both types of NHCs, normal
and abnormal, form compounds with similar behavior and properties, and so the NHCs
nomenclature “holds for aNHCs because it emphasizes the many factors that are common
to both classes of ligand. The distinction between mesoionic and non-mesoionic carbenes
in the free state is largely lost on binding.”[144] Lastly, in rNHCs, the carbene carbon is not
situated α to any heteroatom. An rNHC can be a normal NHC or an aNHC.
NC
N R'R
C
N N R'R C N
Scheme 14. Normal NHC (nNHC, left), abnormal NHC (aNHC, middle) and remote NHC (rNHC,
right). The rNHC in this example is also a normal carbene.
1.4 Phenolate Ligands
Aryloxides look back on a long history of coordination to transition metals.[152] A large
variety of lanthanide[153] and actinide[154] phenolate complexes have been characterized as
well, since metals as diverse as vanadium, iron or uranium[155-156] form strong metal-
oxygen bonds.
Electronic Versatility
Owing to the two lone pairs on the oxygen atom, phenols possess the potential to donate
either one, three, or five electrons. They can do so through π-donation to a single metal
19
center or, in a bridging coordination mode, to different metal atoms. Even as a pure σ-
donor, their strong inductive effect provides stabilization for higher metal oxidation states.
Redox Non-Innocence
In the last two decades, phenolates have become well-established as redox non-innocent
ligands.[157-158], 2 Studies on the phenolate ligands’ redox-activity have been spurred by the
discovery that the reactive center of several metalloenzymes contains phenoxyl radicals. A
tyrosyl radical can be found, for instance, in class I ribonucleotide reductase and
prostaglandin endoperoxide synthase.[159-160] A well-studied case is the reactive center of
galactose oxidase (GO), where a phenoxyl radical is coordinated to a single copper
atom.[161-162] The active form of GO had been regarded as a copper(III) species,[163-164] until
Raman spectroscopy revealed the formation of the phenoxyl radical species and the Cu(II)-
phenoxyl radical bond.[161-162, 165-166]
Since the radical coordination in the active center of enzymes was discovered, many metal-
phenoxyl radical complexes have been synthesized and characterized to reach a better
understanding of the enzymes’ mechanisms and the properties of metal complexes with the
coordinated phenoxyl radicals.[167-168] Copper-phenoxyl complexes have been studied in
particular,[169-170] due to their kinship with the GO center, but the first synthetic phenoxyl
radical complex formation was discovered by Wieghardt et al. after chemical and
photochemical oxidations of Fe(III)-tris(phenolate)-tacn complexes (see Scheme 15).[171]
The historic confusion about the GO reactive center demonstrates nicely that a formally
assignable oxidation number does not have to be the correct one. In the presence of
potentially redox non-innocent ligands, one must always keep in mind that a redox-event
may take place at either the ligand – leading to an open shell radical ligand – or the metal
center (Scheme 16). Whether the oxidation of a metal-phenolate complex really leads to a
metal-phenoxyl or a high-valent metal-phenolate, which are isoelectronic to each other, can
only be determined by suitable spectroscopic methods.
2 Catechols, which are oxidized to semiquinones, have been considered redox non-innocent for a longer time,
while simple phenols have been recognized as redox-active later on.
20
chemical oxidation
NFe
N
N
O
tBu
O
tBu
OtBu
NFe
N
N
O
tBu
O
tBu
OtBu
+
photochem. oxidation
irradiation at 254 nm
Scheme 15. Iron(III)-phenoxyl radical formation through chemical or photochemical oxidations of
an iron(III) 1,4,7-tris(3-tert-butyl-2-oxybenzyl)-1,4,7-triazacyclononane complex (from [170],
Scheme adapted from [171]).
To facilitate the study of redox-behavior of the phenolate ligand, complexes with redox-
inactive, usually diamagnetic metal centers can be prepared for comparison of experimental
data, Zn(II) [172] or Ga(III)[74] complexes for instance. A nice “tutorial overview” of the
catalytic potential and mechanisms for redox non-innocent ligands has been written by
Lyaskovskyy and de Bruin.[173]
O MII
+
M(II)-phenolate complex
O M
2+one-electron
oxidation
formal oxidation state: M(III)
O MIII
M(III)-phenolate
O MII
M(II)-phenoxyl radical
possible experimental oxidation numbers
Scheme 16. Two possible outcomes of a one-electron oxidation of a metal-phenolate complex
(adapted from Shimazaki et al.[170]).
21
Tunability through substituents
Like carbenes, the phenolate ligand offers different sites for modification of its steric and
electronic properties. Usually, substituents are introduced ortho and para to the phenol
oxygen, since these positions are synthetically readily accessible.[152, 167] Upon
complexation, the ortho group is in close proximity to the metal, offering a control element
for the steric environment. Adapting the para group, on the other hand, allows modification
of the electronic properties and solubility of the ligand without affecting the steric demand
of the system.[174]
1.5 Objectives
The objective of this dissertation work was to develop new tripodal mixed NHC/ phenolate
ligands and to explore their coordination chemistry with late transition metals. Mixed
NHC/phenolate ligands with other binding geometries have been reported, such as
bidentate NHC/phenolates,[175-176] vaulted, 4-coordinate bis(NHC-phenolates)[177] or pincer-
type ligands with an OCO binding motif (O = phenolate, C = NHC).[178-181] Most recently,
Bercaw and coworkers published tridentate dianionic mixed ligands with one NHC
moiety.[182] However, to the best of my knowledge, (BIMPNR,R’,R”)– and (MIMPNR,R’,R”)2–
are the first tripodal ligands that combine NHC and phenolate chelating arms.
As stated above, TIMENR iron imido and -nitride complexes were too sterically hindered to
promote group- or atom transfer chemistry. Desirably, side access may be improved by
exchanging one or two carbene arms with phenolates. Although the ortho-phenol groups
can possess substantial steric bulk, as in the case of adamantyl functionalization, the
phenolate’s substituents are further away from the metal center, causing less steric pressure
while still providing sufficient protection, e.g., to prevent bimolecular decomposition
reactions. The new ligands (BIMPNR,R’,R”)– (anion of bis[2-(3-R-imidazol-2-ylidene)ethyl]-
[(3,5-R’,R”-2-hydroxyphenyl)methyl]amine) and (MIMPNR,R’,R”)2– (dianion of mono[2-(3-
R-imidazol-2-ylidene)ethyl]-bis[(3,5-R’,R”-2-hydroxyphenyl)methyl]amine) (Chart 1) can
be regarded as hybrid ligands or cross-overs between the N-anchored tris(carbene) and the
tris(phenolate) ligand systems. Together with the TIMENR and ((R,R’ArO)3N)3– ligands,
they provide a complete ligand series, ranging from tris(carbene) to tris(phenolate), from
22
NN
N
3
R
TIMENR
N
O
3
((R,R'ArO)3N)3-(BIMPNR,R',R'')-
NN
N
2
RO
(MIMPNR,R',R'')2-
NN
NR
O
2
R'
R'' R''
R' R
R'
Chart 1. Series of tripodal N-anchored ligands from tris(carbene) (left) to tris(phenolate) (right).
which the ligand with the most suitable steric and electronic properties for the desired
reactivity may be chosen.
A series of metal complexes with mid- to late transition metals was to be synthesized and
thoroughly characterized. The thesis work focused on (BIMPN)– complexes of Mn, Fe, and
Co as central metals: For these, TIMENR chemistry can serve as comparison to evaluate the
differences and new qualities of the mixed ligands. Furthermore, the capacity of the new
ligand’s complexes for small molecule activation is to be explored, starting with – but not
limited to – the generation of nitrides analogously to Vogel’s work.
23
2 Results and Discussion
About the numbering of the compounds.
Numbers are assigned to the different (BIMPNR,R’,R”)– complexes to refer to them
shorthand. Superscripts may designate different substituents on the NHC and phenolate
groups. If no superscripts are given, the number refers to the complex of the ligand
derivative (BIMPNMes,Ad,Me)–, as this ligand was used more often for several reasons: Its
synthesis, especially in the interimly used tosylation route (section 2.1.4), is cleaner and
easier to handle; more importantly, the complexes of this ligand derivative crystallized
more readily, which is why the majority of the crystal structures explored in the following
chapters stems from this derivative.
When a complex differs only in its counter ion, the counter ion may be added in brackets.
Examples:
[(BIMPNMes,Ad,Me)Fe(Cl)] 1
[(BIMPNMes,tBu,tBu)Fe(Cl)] 1Mes,tBu,tBu
[(BIMPNMes,Ad,Me)Fe]BPh4 1[BPh4]
Complexes in which the (BIMPNR,R’,R”)– ligand has been modified covalently are denoted
by a star *.
A list of all numbered compounds and complexes is given in section 6 (p. 242).
2.1 Synthesis of Mixed Phenolate-Carbene N-Anchored Ligands
2.1.1 Motivation
The effort to create new hybrid ligands that combine NHC and phenolate binding sites in
an N-anchored tripodal ligand framework was driven mainly by two considerations: Firstly,
the reactive cavity of the TIMENR nitrido and imido complexes (and potential oxo
analogues) needed to be opened for side access of organic substrates to enable atom and
24
group transfer reactions. Secondly, the novel ligands would bridge the gap between the
tris(carbene) and the tris(phenolate) ligands employed in the Meyer group laboratories,
thereby providing a continuous ligand series from which to choose a ligand with the most
suited electronic and steric properties for a given task.
For the most part, the efforts described herein are confined to the
bis(carbene)-mono(phenolate) ligand (BIMPNR,R’,R”)– (anion of bis[2-(3-R-imidazol-2-
ylidene)ethyl-(3,5-R’,R”-2-hydroxyphenyl)methyl]amine). The synthesis of the
mono(carbene)-bis(phenolate) ligand (MIMPNR,R’,R”)2– (dianion of mono[2-(3-R-imidazol-
2-ylidene)ethyl-bis(3,5-R’,R”-2-hydroxyphenyl)methyl]amine) was developed in
collaboration with Johannes Hohenberger and elaborated on in his diploma thesis.[183]
2.1.2 The Basic Building Blocks: Imidazoles and Phenols
The ardent interest in NHC chemistry has fostered the development of viable synthetic
routes to substituted imidazoles. A common strategy for NHC formation is the
quaternization of the second nitrogen in the pentacycle, followed by deprotonation of the
positively charged imidazolium to the neutral carbene. Commonly, imidazoles are created
in 1-pot multicomponent syntheses (compare Scheme 18, p. 26) in a modified Debus-
Radziszewski reaction.[184-185] The drawback of this synthesis, especially for 1-aromatic
substituted imidazoles, is the low yield and therefore uneconomical production on higher
scale. Liu et al.[186] found that the yield of 1-aryl-imidazoles can be substantially increased
by a 1-pot-2-step procedure, in which the aromatic aniline is first stirred with the glyoxal at
RT to create in situ an imine. In the second step, the ring closure condensation with
ammonia and formaldehyde is done under reflux. For mesityl-imidazole (1-(2,4,6-
trimethylphenyl)imidazole) and xylyl-imidazole (1-(2,6-dimethylphenyl)imidazole), yields
in excess of 50 % were achieved in the course of this work.
While 2,4-di-tert-butylphenol is a cheap, commercially available starting material, phenols
with the sterically more demanding adamantyl ortho-substituent were synthesized using the
procedures outlined in Scheme 17: The 2-adamantyl-p-cresol (2-adamantyl-4-methyl-
phenol) was obtained by an electrophilic aromatic substitution, catalyzed by concentrated
sulphuric acid at room temperature,[187] 2-adamantyl-4-tert-butylphenol by a slightly
25
modified literature procedure[188-189] in which the pure, solvent-free starting materials are
stirred at elevated temperature.
OH
HO+
(CH2Cl2)
[H2SO4]
OH
OH
Cl+ii) 140°C, 14h
OHi) 100°C, 3h
Scheme 17. Syntheses of 2-adamantyl-p-cresol (top)[187] and 2-adamantyl-4-tert-butylphenol
(bottom).[188-189]
2.1.3 The Original Plan: Mannich-Chlorination-Route
The synthesis of the tris(carbene) TIMENR ligand is well established (Scheme 18) and, for
some substituents, like R = mesityl, 2,6-xylyl, the protonated ligand precursor can be
readily obtained on a multi-gram scale.[65, 71, 190] The tris(phenolate) ligand ((R,R’ArO)3N)3–
requires a very sensitive, acid catalyzed condensation (Scheme 19),[191] but for those
phenols mentioned in the last section it is also a well established ligand in the Meyer
group.[73-75] The syntheses of these C3-symmetric ligands are summarized in Schemes 18
and 19. The first draft for a synthesis of a mixed NHC/phenolate ligand was basically a
combination of these synthesis routes (Scheme 20). In the next section, the synthetic results
following this scheme towards (BIMPNR,R’,R’’)–, as well as the difficulties encountered, are
discussed.
26
R
NH2O
O
NH4
O
H
H
N
N
R
NOH
NCl
AcOH
(MeOH)
reflux
SOCl2(CH2Cl2)
+
(H3TIMENR)Cl3
NN
NR
3
Cl
TIMENR
NN
NR
3
KOtBu
(THF) - KCl
3
3
150 °C,
2d
Scheme 18. Synthesis of TIMENR.
(isopropanol, anhydrous)
[p-TsOH]
OH
R'
R
+ NN
N
NN
OH
R
R'3
(toluene)
NaOMeN
ONa
R
R'3
(R,R'ArOH)3N Na3((R,R'ArO)3N) Scheme 19. Synthesis of the tris(phenolate) ligand ((R,R’ArO)3N)3–.[191],[75]
Mannich reaction
OH
R''
R'
NN
NR
n
OH
R'
R''3-n
n = 2: (H3BIMPNR,R',R'')Cl2
n = 1: (H3MIMPNR,R',R'')Cl
NN
NR
n
OK
R'
R''3-n
n = 2: K(BIMPNR,R',R'')
n = 1: K2(MIMPNR,R',R'')
NOH
n NHO
n
OH
R'
R''3-n
NCl
n
OH
R'
R''3-n
SOCl2
chlorination
SN2
nN
N
R
KOtBu
deprot.
Cl
Scheme 20. Planned synthetic route towards mixed NHC/phenolate ligands (BIMPNR,R’,R’’)– and
(MIMPNR,R’,R’’)2–.
27
First Ligand Batches and Purification Problems
Several batches of ligand precursor (H3BIMPNR,R’,R’’)2+ were synthesized according to
Scheme 21. Derivatives with different substituents were created in the hope that
purification problems (vide infra) could be surmounted by altering solubility and
crystallization properties.
(MeOH / H2O)
OH
R''
R'
NN
NR
2
OH
R'
R''
(H3BIMPNR,R',R'')2+
HNOH
2 NHO
2
OH
R'
R''
NCl
2
OH
R'
R''
SOCl2
(CH2Cl2)
(MeCN)
> 2 eq.
N
N
R
CH2O
c1-R',R'' c2-R',R''
R' = R'' = tBu
R' = Ad, R'' = tBu, Me
R' = R'' = tBu
R' = Ad, R'' = tBu, Me
R' = R'' = tBu
R' = Ad, R'' = Me
R = Mes,
R' = R'' = tBu
R' = Ad, R'' = tBu, Me
R = Xyl,
Scheme 21. First synthetic route towards (BIMPNR,R’,R’’)– and combinations of substituents for
which the synthesis was carried out.
Compounds c1-R’,R” were synthesized in a Mannich type reaction following a slightly
modified procedure by Marinescue et al.[192] Transformation to the chlorinated compounds
c2-R’,R” with thionyl chloride went smoothly. Next, the imidazole moieties were attached
via an SN2 reaction in refluxing acetonitrile, which takes several days for completion. The
reaction can be monitored by 1H NMR spectroscopy, and is deemed complete when signals
of the mono-substituted intermediate – in which only one chloride has been replaced with
an imidazole – have vanished.
Several side products form during the reaction. Test reactions with different solvents or
temperatures gave equal or worse results, or, for lower temperatures, no notable
conversion. While the TIMENR ligand can be purified by salt metathesis and
recrystallization, crystallization has proven to be more intricate for the mixed ligands,
likely due to the change in solubility through the phenol group and/or the ambiphilic
28
behavior caused by the concurrence of the more lipophilic phenol and the hydrophilic
imidazolium groups.
Different solvents, methods (cooling, evaporation, layering technique, slow diffusion) and
counter-anions (PF6–, BPh4
–, ClO4–, –OTs) were tested for crystallization. In one instance, a
fraction of the compound (H3BIMPNXyl,tBu,tBu)2+ could be crystallized via addition of conc.
HCl in EtOAc in the presence of PF6– with subsequent cooling of the mixture. A change in
solubility hints that the crystallized product was most likely protonated at the amino-
function: The crystals were soluble only in alcohols and DMSO, whereas the pro-ligand,
prior to HCl-treatment, was also soluble in chlorinated solvents, THF, and EtOAc. Slightly
different 1H NMR chemical shifts corroborate this assessment, although the N–H proton
signal itself could not be observed due to rapid exchange with the protic solvent and/or
water impurities.3 However, this crystallization process proved unfit for steady ligand
precursor synthesis. Firstly, the amount of HCl the crystals contained did not appear to be
stoichiometric, causing difficulties in the deprotonation step. Efforts to remove the acid
after crystallization were hampered due to the water solubility of the cationic ligand
precursor (in the presence of the Cl– anion), which led to a great reduction in yield when
the ligand was washed with aqueous bases. Stirring the crystallized precursor in organic
solvent with solid K2CO3 resulted in low yields and/or ligand decomposition over time.
Secondly, crystallization of the precursor was difficult to reproduce. The procedure may be
highly sensitive to crystallization conditions, concentrations and/or amount of side
product(s) generated during the SN2 reaction.
NN
OH
Scheme 22. One of the side products formed in the reaction of c2-tBu,tBu with xylyl-imidazole.
3 The NMR solvents used in this case were not dried like the ones used for sensitive compounds under N2
atmosphere.
29
During one attempt at crystallization, one of the side products crystallized neatly and could
therefore be thoroughly analyzed (Scheme 22, for 1H NMR spectra, see p. 170).
Comparison of 1H NMR spectra revealed that this compound was one of the major side
products. It is formed by substitution of the amino group at the reactive benzylic position of
the phenol by an imidazole. While secondary amines are, in general, not favorable leaving
groups, the reaction might be facilitated by intra-molecular hydrogen bonding of the
hydroxyl-proton to the amino-nitrogen in a 6-membered ring conformation (Scheme 23).
This would pre-form the amino-leaving group. Also, hydrogen bonding would decrease the
electron density on the nitrogen, making it more electron-withdrawing and therefore
rendering the neighboring benzylic methylene group even more susceptible to a
nucleophilic attack. The same reaction also leads to mono- and bis(imidazolium)
substituted ethylamines (Scheme 24). (For deliberate synthesis of p2-R, see chapter 2.1.5,
p. 34; the N,N’-benzyl-aryl-imidazolium in Scheme 22 can be synthesized independently
by reacting the chloromethylated phenol p3-R’,R” (chapter 2.1.6, p. 39) with the
imidazole.)
O
R'
R''
H
N
N
R
NR1
R2
- HNR1R2
O
R'
R''
N
R
N
Scheme 23. Reaction pathway leading to cationic side products depicted in Scheme 22 and
Scheme 24.
HNN
NR
2NH
NN
R
Cl
p2-R Scheme 24. Mono- and bis-imidazolium side products of the SN2 reaction between c2-R’,R” and
N-R-imidazole.
30
While most neutral side products as well as excess starting material can be easily removed
by sonication of the crude product with solvents in which the ligand precursor is insoluble
(alkanes, Et2O, sometimes aromatics or EtOAc, depending on substituents and counter
ions), the great difficulty arose from separating the different imidazolium compounds, each
carrying one or two positive charges, from each other. Efforts to purify the ligand precursor
with chromatographic methods, testing different common column materials[193] as well as
reversed phase column chromatography, failed to give clean ligand precursor. While it
might be possible to separate the cationic compounds with special column material,[194] this
approach would be both cost- and time-intensive and thus impractical for multi-gram
batches of the desired compound for the subsequent coordination chemistry.
Protecting Groups
Since the formation of the inseparable, charged side products was rationalized with
hydrogen bonding (Scheme 23), it was reasoned that this side reaction might be suppressed
by attaching a protecting group (PG) on the phenol oxygen. This protecting group must be
cleavable under conditions that do not affect the ligand precursor or its further use. Basic
conditions should be either mild, so as not to deprotonate the imidazolium ring, or suitable
for deprotonation of the entire ligand, so that deprotonation and deprotection may be
carried out in one step prior to metal coordination. Acidic conditions may protonate the
nitrogen anchor, potentially causing the same problems as discussed for
(H3BIMPNXyl,tBu,tBu)2+ crystallized by HCl-addition (vide supra). Hydrogenation might
affect the double bond of the imidazolium rings, although Kariuki and coworkers have used
palladium catalyzed hydrogenation to cleave a benzyl group from bis(imidazolium)amine
compounds in good yields.[195-196]
According to these considerations, TBDMS (tert-butyldimethylsilyl) was chosen as a
suitable PG, since silyl-groups can be cleaved by a fluoride source, which should not affect
the rest of the ligand. However, standard procedures[197] for attaching the group with
TBDMS-Cl, with different bases, varying solvents, and reaction temperatures, showed no
conversion of c2-tBu,tBu according to TLC and 1H NMR.
Subsequently, the benzyl protecting group was tested for its applicability. While cleavage
of the benzyl group through hydrolysis involves the inherent risk of cleaving the benzylic
amine, no disruption of the C–N bond was observed by either Licini or Brown and
31
coworkers during similar reactions towards tris(phenolate) ligands.[198-199] The first
attempts to treat c2-tBu,tBu with benzyl bromide revealed slow conversion and a mixture
of unidentified products (1H NMR).
Next, protection of the phenol as a THP (tetrahydropyranyl) ether was tested. To save
c2-tBu,tBu material, the commercially available phenol was used for the test reaction. This
approach resulted, however, in ortho-substitution of the phenol with the THP group
(Scheme 25). In compound c2-R’,R”, the second ortho position is already blocked by the
nitrogen anchor, so the desired phenol-protection might occur instead. At this point,
however, the protecting group strategy was deferred in favor of other, simpler routes that
showed more promise.
OH
+
O(CH2Cl2)
[p-TsOH]
r.t.
OH
O
Scheme 25. Reaction of 2,4-di-tert-butylphenol with dihydropyran.
Bromination
Introduction of a bromide instead of a chloride leaving group did not effectively alter the
outcome of the SN2 reaction.[200]
Finkelstein Reaction
A different approach to avoid formation of the cationic side products during the SN2
reaction is to improve the quality of the leaving group on the ethyl arms, allowing the
selectivity to be high and/or reaction conditions to be mild enough to prevent substitution at
the benzylic position. An in situ Finkelstein reaction (Scheme 26) substitutes the chloride
with an iodide. For this, chlorinated compound c2-Ad,Me was dissolved in acetonitrile and
five equivalents of solid potassium iodide were added. The mixture was refluxed for three
hours to give intermediate c3-Ad,Me, before a solution of xylyl-imidazole in acetonitrile
was added and the mixture continued to reflux for several days. The reaction with xylyl-
imidazole to (H3BIMPNXyl,Ad,Me)2+, monitored by 1H NMR, occurred faster than without
the Finkelstein step, but the same amount of side products developed. Possibly, side
32
product formation might be suppressed by lowering the temperature after the in situ
Finkelstein reaction has occurred, and/or by adding the imidazole very slowly.
NCl
2
OH
R'
R''
c2-R',R''
KI
(MeCN or acetone)
NI
2
OH
R'
R''
c3-R',R'' Scheme 26. Finkelstein reaction of c2-R’,R” to c32-R’,R”.
It was also attempted to isolate the iodized intermediate. To this end, c2-Ad,tBu was
dissolved in acetone and refluxed with six equivalents of KI. However, subsequent reaction
of c3-Ad,tBu with xylyl-imidazole went exceedingly slow at low temperatures, while the
same product mixture as with the chloride compound c2-Ad,tBu was yielded at elevated
temperatures.
2.1.4 The Tosylation Route
An effective way to create excellent SN2-substrates from alkyl alcohols is to convert them
to alkyl sulfonates. The tosyl group (Ts, para-toluenesulfonyl) provides a particularly good
leaving group, since the emerging tosylate –OTs stabilizes the anionic charge through
delocalization into both the sulfonate and the aromatic ring. Reaction of c1-R’,R” with
TsCl in CH2Cl2 (Scheme 27) yielded the di-tosylated product c4-R’,R” alongside mono-
substituted intermediate c4a-R’,R” and tri-substituted, phenol-tosylated side-product
c4b-R’,R”. Reaction control by TLC can limit the amount of c4b-R’,R” by optimizing the
reaction time and slowly adding more TsCl when it is fully consumed. The amount of
c4b-R’,R” depends on the amount of TsCl that is effectively used, and on the steric
demand of the ortho-substituent R’ on the phenol. c4b-R’,R” formation is small for
R = Ad, larger for R = tBu, and finally for R = Me,[201] no disubstituted product could be
isolated.
Before c4-R’,R” can be used in an SN2 reaction to form (H3BIMPNR,R’,R’’)2+, all mono-
substituted c4a-R’,R” has to be removed, as it will lead to the corresponding mono-
imidazolium-compound and cause the same purification problems as discussed above
33
NHO
2
OH
R'
R''
2 eq. TsCl
(CH2Cl2)
c1-R',R''
0°C - r.t.
NTsO
2
OH
R'
R''
c4-R',R''
NTsO
OH
R'
R''
c4a-R',R''
NTsO
2
OTs
R'
R''
c4b-R',R''
++ Cl
Scheme 27. Tosylation of c1-R’,R”.
(pp. 27ff). The high reactivity renders c4-R’,R” very sensitive, so it has to be handled with
great care. c4-R’,R” has been observed to react even with free chloride anions, if those are
not removed after the tosylation reaction. Column chromatography with silica led to partial
decomposition, and experiments with ethyl acetate as eluent gave a secondary product
which can only have emerged from reaction with this solvent. With the right precautions,
c4-R’,R” can be purified by column chromatography with THF/hexane eluent mixtures on
neutral alumina. Ideally, c4-R’,R” is synthesized right before use, its workup done quickly,
and the compound never heated above room temperature.
While the high reactivity of c4-R’,R” encumbers handling and purification, it finally
yielded the desired results in the synthesis of (H3BIMPNR,R’,R’’)2+: The substitution with
imidazole can now be carried out at room temperature. Clean starting materials provided,
the reaction proceeds smoothly, and none of the plaguing cationic side products are
observed.
The tosylation route was also tried for synthesis of (MIMPNR,R’,R”)2–. It has to be pointed
out that for the bis(phenol) compound, recovery of the desired mono-tosylated product is
even more difficult since two phenols now compete with one alkyl alcohol for tosylation
(see Scheme 28). Therefore, while a first batch of (MIMPNR,R’,R”)2– could be obtained this
way, albeit not entirely pure, this was not deemed a convenient route for the bis(phenolate)
ligand.
34
NHO
OH
R'
R''
TsCl
(CH2Cl2)
0°C - r.t.
++
2
NTsO
OH
R'
R''2
NTsO
OTs
R'
R''2
N
OH
R'
R''
etc.
OTs
R'
R'' OTs
Scheme 28. Tosylation of a bis(phenol)-compound for synthesis of (MIMPNR,R’,R”)2–.
It may be possible to improve the Mannich–SN2 route by picking another, less tricky
leaving group, with a reactivity lying in between the chloride and tosylate leaving groups.
This new leaving group would have to be more stable than the tosylate, but still reactive
enough to maintain the positive effect of mild reaction conditions in the following SN2 step.
Protecting the phenol from tosylation might also be possible. For compound c1-R’,R”,
with its competing OH-groups, this would probably require a multi-PG strategy, adding at
least three extra steps to the overall ligand synthesis (protecting the alkyl alcohol with
selective PG1, protecting the phenol with PG2, removing PG1 to free the alcohol for
tosylation). Alternatively, the PG may be attached to the phenol prior to the Mannich
condensation, which, however, might in turn influence the Mannich reaction since the
electronic and steric properties of the phenol are altered.
2.1.5 The Bis(imidazolium) Routes
While the previous sections describe routes towards mixed NHC/phenol ligands in which
the phenol is attached first to the nitrogen anchor of the ligand, the complementary
“carbene first” routes are explored in the following.
Synthesis of Bis(imidazolium) Salt p2-R
Thionyl chloride chlorinates diethanolamine in near quantitative yield to
di(chloroethyl)ammonium chloride p1×HCl,[202] which reacts with excess aryl-imidazole to
35
the bis(imidazolium) salt p2-R×HCl in a slightly modified literature procedure[203] in
moderate yield (40 %, Scheme 29).
HNOH
2
SOCl2
(CH3Cl)
H2NCl
2
Cl
p1 x HCl
(MeCN)
> 2 eq.
N
N
R
H2NN
NR
2
3 Cl
p2-R x HClR = Mes, Xyl Scheme 29. Chlorination and SN2 reaction leading to bis(imidazolium) salt p2×HCl.
Yields of the protonated salt (H3TIMENR)Cl3 in the TIMENR synthesis (see p. 26) depend
strongly on whether or not the amine has been deprotonated beforehand. However, contrary
to the tertiary tri(chloroethyl)amine employed in the TIMENR synthesis, the deprotonated
secondary amine p1 is prone to polymerization. A portion of HCl-free
di(chloroethyl)amine left to stand at room temperature for a day will turn into a gooey,
yellowish solid, and even storage at low temperatures of the pure substance does not
eliminate this process. This explains why, at the elevated temperatures necessary for the
nucleophilic substitution at the chloride, only the imidazole starting material and an
insoluble residue could be retrieved when the di(chloroethyl)amine was deprotonated
beforehand.
This issue is even more pressing for the mono(halidoethyl)amine necessary for
(MIMPNR,R’,R”)2– synthesis, namely mono(bromodiethyl)amine-hydrobromide. The
imidazole alone is basic enough to sufficiently deprotonate this primary amine for
polymerization.[183, 204] The amine-hydrobromide is therefore added in small portions to a
solution of a four-fold excess of imidazole, to keep local concentration of the amine low
and suppress polymer formation.[183] A corresponding procedure might also increase yields
of p2-R.
Attaching the Phenol
Several procedures were tried for coupling the phenol to the anchoring unit of the
bis(imidazolium) salt p2. Mannich type reaction between phenol and p2×HCl with either
paraformaldehyde or an aqueous solution of formaldehyde showed no conversion
36
(Scheme 30). The protonated amine may not be able to condense with the formaldehyde to
form the imino-intermediate, however, addition of base (NEt3 or pyridine) in slight excess
did not further the reaction, nor did carbonate salts (NaHCO3 or K2CO3) that were stirred in
the reaction mixture.
OH
+CH2O
37% aq.+
H2NN
NXyl
2
3 Cl
p2-Xyl x HCl
(MeOH)
reflux, days
(MeOH)
reflux, days
1 eq. base(NEt3 or pyridine)
OH
+(CH2O)n
(solid)
H2NN
NXyl
2
3 Cl
p2-Xyl x HCl
(MeOH)
reflux, hours
(MeOH)
reflux, several days
monitored by 1H NMR Scheme 30. Attempts at attaching the phenol to the nitrogen anchor of p2-R×HCl by Mannich
condensation (examples).
Reductive amination has been used successfully to synthesize tris(phenolates)[198-199] and
asymmetric N-anchored ligands that combine for instance pyridyl or pyrazolyl groups with
phenolates.[205-206] Accordingly, bis(imidazolium) salt p2-Xyl×HCl and 2,4-di-tert-butyl-
salicyl aldehyde were treated with sodium cyanoborohydride (NaBH3(CN)) as reducing
agent in a MeOH / THF mixture. The protic solvent activates and enhances the reactivity of
the borohydride, while THF (or another non-chlorinated, suitable solvent) is necessary to
dissolve the salicyl aldehyde (Scheme 31). However, low conversions of p2-Xyl×HCl
were accompanied by reduction of the bulk of the salicyl aldehyde to the benzyl alcohol. In
order to prevent direct reduction of the aldehyde, different reaction conditions and
procedures were tested in an effort to enforce imine formation between the two starting
materials prior to addition of the reducing agent: p2-R×HCl and the aldehyde were stirred
37
for prolonged periods of time; water-removing agents, such as 3 Å molecular sieves, were
added to favorably shift the equilibrium and bases (NEt3 or pyridine) were added to
deprotonate the amine. None of these procedures substantially enhanced the reaction
outcome.
OH
+H2N
NN
Xyl
2
3 Cl
p2-Xyl x HCl
O i) 1h to days*, RT, **
ii) NaBH3(CN)OH OH
(MeOH / THF)
(H3BIMPNXyl,tBu,tBu)2+
+
(< 10%)
Scheme 31. Attempts at attaching the phenol to the nitrogen anchor of p2-R×HCl by reductive
amination (examples), *stirring time range was varied, **optional addition of molecular sieves
(3 Å) or base (Et3, pyridine).
Both Mannich type reaction and reductive amination may have failed because the amine
group in p2-R×HCl was protonated and its nucleophilic attack on a carbonyl to form the
imide/iminium ion-intermediates hindered. Since in situ addition of bases to the reaction
mixture was unsuccessful, it was tried to isolate the hydrochloride-free bis(imidazolium)
salt p2-R as starting material for these reactions.
p2-R×HCl was stirred with NaHCO3 in CH2Cl2, the idea being that while the hydro-
chloride adduct is only very moderately soluble in CH2Cl2, the favorable lattice energy of
NaCl and CO2-evolution would favorably shift the equilibrium, and free, CH2Cl2-soluble
p2-R could conveniently be filtered off the solids. This approach was only limitedly
successful: conversion was slow, and the extract contained side products apparently from
decomposition of the bis(imidazolium) amine.
When the bis(imidazolium) salt was stirred in aqueous NaHCO3 solution, any attempts to
extract it with organic solvents were unsuccessful due to the high solubility of both free
p2-R and its hydrochloride in water. When, however, a suitable counterion is added to the
mixture, it is possible to transfer p2-R into the organic phase. That way, it was possible to
extract HCl-free p2-R[PF6] with CH2Cl2 in 82 % yield (for R = xylyl) after the chloride
was exchanged by hexafluorophosphate (Scheme 32).
38
H2NN
NR
2
3 Cl
p2-R x HCl
i) NaHCO3 aq.
ii) > 2eq. NaPF6
ii) extraction with CH2Cl2 HNN
NR
2
2 PF6
p2-R[PF6] Scheme 32. Deprotonation of the N-anchor of bis(imidazolium) salt p2-R.
HCl-free p2-Xyl[PF6] was converted to (H3BIMPNXyl,tBu,tBu)2+ by reductive amination in
40 % yield (by 1H NMR), with 60 % direct reduction of the aldehyde to the benzylic alcohol
(see Scheme 33). Several possibilities remain to optimize the reaction: As mentioned, protic
solvents are necessary to activate the borohydride. Accordingly, the solvent (or mixture of
solvents) may be changed to adjust the reactivity (and selectivity) of the reducing agent.
Also, the exact pH value might be crucial. While HCl apparently hinders the reaction, acetic
acid has been used as catalyst for a similar reductive amination,[205-206] suggesting that a
milder acid may as well be able to further formation of the iminium intermediate. Finally,
other hydrogenation agents should be tested. In particular, sodium triacetoxyborohydride
NaBH(OAc)3 has been appraised as “a superior, convenient, and effective reducing agent for
reductive amination reactions”, which also “eliminate[s] the risk of residual cyanide, not only
in the product but also in the workup waste stream”.[207] It has been used in synthesis of
tris(phenolates)[198-199] and asymmetric glucose-containing ligands.[208]
The application of p2-R[PF6] in the Mannich reaction has not been tested, nor have
possible alternative reaction conditions and procedures for reductive amination been
exhaustively explored, since another, viable route towards the mixed NHC/phenolate
ligands emerged at the time.
OH
+HN
NN
Xyl
2
2 PF6
p2-Xyl[PF6]
O i) 1h, RT
ii) NaBH3(CN)OH OH
(MeOH / THF)
(H3BIMPNXyl,tBu,tBu)2+
+
(40%)
Scheme 33. Reaction of p2-Xyl[PF6] with di-tert-butyl salicyl aldehyde and NaBH3(CN).
39
2.1.6 Final Synthetic Route for (BIMPNR,R’,R”)– and (MIMPNR,R’,R”)2–:
Chloromethylation of the Phenol and SN2 Reaction with
Bis(imidazolium) Salt p2-R
Chloromethylphenol p3-R’,R” was generated via a Blanc reaction in a slightly modified
literature procedure (Scheme 34),[183, 209] in which gaseous HCl was substituted by HCl
conc. This change improves ease and safety of handling, while the reaction time increases
significantly (1 d for p3-tBu,tBu, > 2 d for p3-Ad,Me). Vigorous stirring is crucial for
conversion in the now biphasic reaction mixture. Great care must be taken during workup:
In the Blanc reaction, part of the formaldehyde can react to bis(chloromethyl)ether in
hydrochloric solutions. This substance is highly irritant as it decomposes to formaldehyde
and hydrochloric acid in wet air (e.g. inside the lungs), and its alkylating properties render
it one of the most potent synthetic carcinogens.[210-211]
OH
R''
R'HCl conc.,
CH2O
(toluene)
OH
R''
R'Cl
p3-R',R"
Scheme 34. Chloromethylation of the phenol to yield p3-R’,R”.
In the presence of base, p2-R×HCl reacts with p3-R’,R” to (H3BIMPNR,R’,R”)2+
(Scheme 35). The chloromethylated phenol can self-react intermolecularly to a spiro
compound (via HCl-elimination followed by Diels-Alder cycloaddition), therefore the
reaction vessel is first charged with the bis(imidazolium) salt and p3-R’,R” is added very
slowly and in small excess.[183, 212] The reaction can be tested for full conversion of
p2-R×HCl by 1H NMR spectroscopy, by evaporating a small sample of the reaction
mixture and dissolving the residue in DMSO-d6, since remaining bis(imidazolium) salt is
hardly removable from the ligand precursor (see pp. 27ff). After completion, the reaction
mixture is evaporated to dryness, the residue taken up in methanol, heated to reflux, and the
ligand precursor is precipitated by slow addition of a warm solution of > 2 eq. NaBPh4 in
methanol. The abundant, voluminous, white precipitate can be filtered off, further purified
through different filtration and washing steps if necessary (see experimental section), to
40
successfully give analytically pure (H3BIMPNR,R’,R”)(BPh4)2 in high to near quantitative
yields.
i) NEt3, reflux
ii)
(CH2Cl2)
OH
R''
R'Cl
H2NN
NR
2
3 Cl
p2-R x HCl
NN
NR
2
OH
R'
R''
(H3BIMPNR,R',R'')(BPh4)2(MeOH)
NaBPh4
2 BPh4
p3-R',R"
Scheme 35. Synthesis of the pro-ligand (H3BIMPNR,R’,R”)(BPh4)2 via SN2 reaction between
p2-R×HCl and p3-R’,R”.
Deprotonation of the Pro-Ligand
Several bases were tested for the deprotonation of the ligand precursor to the free, mono-
anionic ligand (BIMPNR,R’,R”)–. Commonly and conveniently, potassium tertiary-butoxide
(KOtBu) is used in THF, which provides the free ligand in near quantitative yields
(Scheme 36). After removing the solvent from the reaction mixture, the ligand can be
extracted from the residue with diethyl ether. The drawback of this method is that emerging
tBuOH perseveringly sticks to the product, turning the carbene into a glassy, sticky residue,
and sometimes causing difficulties during complexation to a metal center (see
chapter 2.2.1). If necessary, the tertiary alcohol can be removed by lyophilization from
benzene (several cycles are necessary; the tenacity of the alcohol to stay in the product may
be attributed to H-bonding to the phenolate).
KOtBuN
NN
R
2
OH
R'
R''
(H3BIMPNR,R',R'')(BPh4)2
2 BPh4
NN
NR
2
OK
R'
R''
K(BIMPNR,R',R'')
(THF)
Scheme 36. Deprotonation of the pro-ligand with KOtBu to the potassium salt of the free NHC/
phenolate ligand.
41
Benzyl potassium (BnK), prepared by metalation of toluene with Schlosser’s base
(n-butyllithium and KOtBu), also provides free ligand. The ligand solution or slurry in THF
has to be cooled before slow addition of BnK in small portions to ensure selectivity of the
deprotonation. Otherwise, the reaction proceeds markedly less cleanly than with KOtBu.
The other drawback in comparison with KOtBu is that BnK is not commercially available
but has to be freshly prepared since, due to its extremely high reactivity, it can only
limitedly be stored. The advantages on BnK lie in the bright orange color of the reagent,
which disappears upon reaction, conveniently pointing out when enough base has been
added (as soon as the orange color remains), and, above that, in the easy removal of the
generated toluene.
Deprotonation with sodium methanolate NaOMe leads to free carbene as well. The 1H NMR spectrum of the sodium salt Na(BIMPNR,R’,R”) differs from the spectrum of the
potassium salt, in that several chemical shifts are different and some signals broader. This
hints at some interaction of the ligand with the different alkali cations, which is
corroborated by the crystal structure of the potassium salt (vide infra). While the lower
boiling point/higher vapor pressure of methanol should render it more easily removable
than tBuOH, the H-bonding effects (vide supra) seem to play a role just as well.
Furthermore, it has been shown that the carbene can insert into the C–O bond of
methanol,[213] which may explain some of the impurities in the sodium salt 1H NMR
spectrum. The tertiary alcohol seems to be less prone to undergo this reaction. In summary,
methanol also stays tenaciously in the ligand substance, and leads to less pure ligand
batches, and therefore cannot be recommended.
Other bases that generate volatile side products include sodium hydride and alkali salts of
hexamethyldisilazane (HMDS). NaH generates H2, which would evaporate even during
deprotonation. Sadly, reaction of NaH with (H3BIMPNR,R’,R”)2+ (with –OTs of –BPh4
counter ion) gave low isolated yields of ligand with low purity. NaH was tested with and
without the use of DMSO as catalyst.[113] With NaHMDS, the reaction mixture turned into
a very sticky slurry, from which no free ligand could be extracted. Cooling of all reactants
before mixing and use of different solvents gave no better results. Therefore, on the whole,
KOtBu has been established as most efficient base, despite the persistent tert-butyl alcohol.
42
2.1.7 Crystal Structures of a (BIMPNR,R’,R”)– Ligand and its Protonated
Precursor
Colorless needles of (H3BIMPNMes,Ad,Me)(OTs)2 suitable for single crystal X-ray analysis
were obtained by slow evaporation of a solution of the compound in a CH2Cl2 /
ethyl acetate mixture (Figure 1).
Figure 1. Crystals of (H3BIMPNMes,Ad,Me)(OTs)2: Colorless needles grown in heaps upon slow
evaporation of a solution in a CH2Cl2 / ethyl acetate mixture, left and right: in daylight and under
the microscope.
The crystal structure of (H3BIMPNMes,Ad,Me)(OTs)2 is characterized by stacks of cations and
anions along the crystallographic b-axis (Figure 2). Within these stacks each imidazolium
cation forms two intermolecular C–H···O hydrogen bonds to one of the tosylate anions that
points towards the cavity of the ligand. The remaining O–H donor function of the
imidazolium cation forms an intramolecular O–H···N hydrogen bond with the anchor
nitrogen atom N1. The second tosylate anion forms separate stacks and does not show
pronounced interactions with the cations.
Crystals of the ligand were grown by cooling an ethereal solution of the ligand’s potassium
salt to –35 °C. The molecular structures of the cation of (H3BIMPNMes,Ad,Me)(OTs)2 and of
[K2(BIMPNMes,Ad,Me)2(C6H6)] are shown in Figures 3 and 4; selected geometrical structure
parameters are given in Table 22 (p. 224).
43
Figure 2. Packing in the crystals of (H3BIMPNMes,Ad,Me)(OTs)2 · 0.2 EtOAc along the
crystallographic b-axis.
In the crystal structure of [K2(BIMPNMes,Ad,Me)2(C6H6)] two potassium cations are
coordinated by two ligand molecules each. The phenolate oxygen atoms of both ligands
link the two potassium cations in a µ-η1-η1-fashion. The coordination sphere around K1 is
completed by the two anchoring N atoms and two carbene C atoms from two different
ligand molecules, thus forming a heavily distorted octahedron. In contrast to this, the
coordination around the second potassium K2 can be described as distorted tetrahedral.
This cation is coordinated by the two oxygen donors, one carbene donor atom, and a
solvent benzene molecule acting as a fourth donor in an η6 binding mode. The η6 benzene
turned out to be rotationally disordered with two preferred orientations.
The N–C bond distances and N–C–N angles within the protonated imidazole and
deprotonated imidazole-2-ylidene all agree well with literature values.[142] Upon
deprotonation, these bond lengths increase by an average 0.04 Å and the angles decrease by
about 7°.
44
Figure 3. Molecular structure of the (H3BIMPNMes,Ad,Me)2+ cation in crystals of
(H3BIMPNMes,Ad,Me)(OTs)2 · 0.2 EtOAc (50 % probability ellipsoids, the tosylate counter ions, co-
crystallized solvent molecules, and – with the exception of the phenol and imidazole H atoms –
hydrogen atoms are omitted for clarity.)
Figure 4. Molecular structure of [K2(BIMPNMes,Ad,Me)2(C6H6)] in crystals of
[K2(BIMPNMes,Ad,Me)2(C6H6)] · 3 Et2O. (50 % probability ellipsoids, co-crystallized solvent
molecules and hydrogen atoms are omitted for clarity.)
45
2.1.8 Summary of Ligand Synthesis and Outlook
The synthetic routes towards mixed NHC/phenolates that have been described in this
chapter can roughly be divided into two categories: On the one hand, the “phenol first”
routes, in which the phenol is attached to the N-anchor by Mannich condensation with
diethanolamine, followed by introduction of a leaving group to the compound that can be
replaced with the imidazole. On the other hand are the complementary “carbene first”
paths, in which the phenol is attached secondly to the bis(imidazolium-ethyl)ammonium
(p2-R, for (BIMPNR,R’,R”)–) or mono(imidazolium-ethyl)ammonium (for
(MIMPNR,R’,R”)2-).
The “phenol-first” routes often led to mixtures of cationic imidazolium compounds,
generated by nucleophilic attack of an imidazole at the benzylic position between N-anchor
and phenol. The different ionic imidazolium compounds are difficult to separate and purify.
This issue was resolved when tosylate was introduced as leaving group, which allows mild
reaction conditions in the SN2 step and therefore selective substitution on the ethylene
chains only. The high reactivity of the tosyl-group renders the intermediate compound
c4-R’,R” metastable and places high demands on its handling and purification. For steady
production of larger quantities of ligand it was therefore considered cumbersome and
further pathways were tested. However, the tosylation approach does open up a viable
route, especially for ligand derivatives with precious imidazoles, since the imidazole is
attached at a late stage in near quantitative yields, whereas the synthesis of the
(imidazolium-ethyl)ammonium compounds entails high losses of imidazole due to only
moderate yields.
The “carbene-first” route, on the other hand, avoids large-scale column chromatography
and metastable intermediates. Mannich condensation between the phenol and p2-R×HCl
has not yet yielded ligand precursor, and reductive amination reactions still have to be
thoroughly optimized to avoid direct reduction of a considerable part of the salicyl
aldehyde to the benzyl phenol. Superior outcomes were achieved by SN2 reaction of
p2-R×HCl with the chloromethylated phenol p3-R’,R”.
46
OH
R'
R''
H3-nNX
n
x HX
nN
N
R
H3-nNN
x HX
NR
n
X
(X = Cl, Br)
OH
R'
R''
Cl
NN
NR
n
OH
R'
R''3-n
(toluene)
HCl conc.
n = 2: (H3BIMPNR,R',R'')2+
n = 1: (H3MIMPNR,R',R'')+
NN
NR
n
OK
R'
R''3-n
n = 2: K(BIMPNR,R',R'')
n = 1: K2(MIMPNR,R',R'')
KOtBu
CH2O
Scheme 37. Final synthetic route for K(BIMPNR,R’,R’’) and K2(MIMPNR,R’,R’’).
Scheme 37 summarizes the total synthesis of the new mixed NHC/phenolate ligands. This
now established route gives good overall yields of pure ligand, finally providing an
excellent synthetic pathway for steady production of (H3BIMPNR,R’,R”)2+ and
(H3BIMPNR,R’,R”)+ with alkyl and aryl substituents on a multi-gram scale.
Outlook: Further Derivatives and Water Soluble Ligands.
The next chapters expand on complex synthesis and characterization with the bis(carbene)-
mono(phenolate) ligand (BIMPNR,R’,R’’)–, mainly with the derivatives (BIMPNR,tBu,tBu)– (R
= xylyl, mesityl) and (BIMPNMes,Ad,Me)–. In the future, a wide variety of substituents may
be combined on the ligand to fine-tune the complexes’ reactivity. In the meantime, the
tosylation approach is applied by Eva Zolnhofer towards potentially water-soluble ligands
synthesized with precious imidazoles bearing hydrophilic groups.
47
2.2 Iron Complexes of (BIMPNR,R’,R”)–
2.2.1 Iron(II) Complexes
Under inert atmosphere, treatment of (BIMPNMes,Ad,Me)– with one equivalent of anhydrous
ferrous chloride in diethyl ether and a small amount of pyridine at room temperature yields
the four-coordinate iron(II) complex [(BIMPNMes,Ad,Me)Fe(Cl)] (1) as yellow powder in
about 80 % yield (see Scheme 38). Pyridine is necessary unless all tBuOH generated during
ligand deprotonation has been removed completely (which is a tedious process, as
described in section 2.1.6). Otherwise, the tertiary alcohol in combination with the iron salt
creates acidic conditions capable of re-protonating the carbenes. Presumably, the alcohol’s
O-H-bond is activated by coordination of the oxygen to the iron ion, which acts as Lewis
acid (Scheme 39).
NN
NMes
2
OK
Ad
(py / Et2O)
FeCl2
- KCl
(py / Et2O)
FeBr2
- KBr
N
NC
NC
N
FeIIO
N
Br
N
NC
NC
N
FeIIO
N
BPh4N
NC
NC
N
FeIIO
N
NN
N
NaN3
(DMF)
-NaCl NaBPh4
- NaCl
(THF)
N
NC
NC
N
FeIIO
N
Cl
Scheme 38. Synthesis of various (BIMPNMes,Ad,Me))– iron(II) complexes.
48
FeII
N
NR
OH
Scheme 39. Re-protonation of the ligand in the presence of tBuOH.
Unlike the corresponding [(TIMENR)Fe(II)] complexes, the (BIMPNR,R’,R’’)– complexes do
not precipitate from neat pyridine, and therefore the mixture of pyridine and ether was
chosen, which facilitates isolation of the complex while still guaranteeing basic conditions.
Complex 1 is well soluble in pyridine, acetonitrile, DMSO and THF, less soluble in
chlorinated solvents, and insoluble in less polar solvents like diethyl ether and
hydrocarbons. While 1 is stable in dichloromethane, chloroform is capable of oxidizing the
Fe(II) complex, especially under elevated temperatures (vide infra). It is very sensitive
towards O2 and/or H2O, even in the solid state, and turns grayish brown upon exposure to
air. 1Mes,tBu,tBu and 1Xyl,tBu,tBu, which can be prepared analogously, show similar behavior,
only with somewhat higher solubilities in THF and chlorinated solvents. This could be
observed for all following complexes: In general, equivalent complexes of the derivatives
with tBu-substituents on the phenol are more readily soluble than their adamantyl-cresol
counterparts, and sometimes in less polar solvents (i.e. in a wider range of solvents). The
tBu-derivatives also tend to form gooey and sticky masses instead of powders or crystalline
material, and in crystallization setups, very often they form oily droplets rather than
crystals.
The corresponding complex [(TIMENMes)Fe(Cl)]Cl is insoluble in THF and MeCN, and as
a rule of thumb, for a given metal, less polar solvents are necessary to dissolve complexes
of (BIMPNR,R’,R’’)– than their TIMENR counterparts (R = Mes, Xyl) with the same NHC
substituents and counterion(s).
The paramagnetic 1H NMR spectra of 1 (MeCN-d3, CDCl3 or DMSO-d6) feature signals
ranging from 55 to –10 ppm, some of which are strongly broadened and, in part,
overlapping with each other, thus preventing unequivocal integration and signal
assignment. Regardless, the spectra – and especially two moderately sharp peaks around
49
50 ppm with relative intensities of 3 : 1 – are very characteristic and allow the
identification and determination of the purity of the complex. Traces of H2O lead to
additional signals in this region, hinting at the formation of an aqua or hydroxo complex,
since the oxidized iron(III) complex is considered NMR silent (vide infra).
While the solid state molecular structure, derived by X-ray crystallography, is of C1
symmetry (vide infra) with two complexes related by an inversion center in the unit cell,
the 1H NMR spectra in polar solvents suggest a dynamic behavior in solution, resulting in
Cs symmetry on the NMR time scale. The structure of the complex in solution also appears
to vary depending on the solvent’s ability for metal coordination and solvation of the
chloride counter ion, as is indicated by the markedly different signal distribution (and
higher overall number of signals) in the 1H NMR spectrum in THF-d8 compared to MeCN-
d3, CDCl3 or DMSO-d6.
50 45 40 35 30 25 20 15 10 5 0 -5Chemical Shift (ppm)
-3.0
4.7
5. 1
6.5
7.2
7.5
8.0
9. 2
10
.0
14
.2
35
.4
49
.9
51
.9
2H
6H
2H 6H 3H 3H
8H
8H
4H
6H
6H
12H
10H
THF-d8
toluene
BPh4
N
NC
NC
N
FeIIO
N
Figure 5. 1H NMR spectrum of 1[BPh4] in THF-d8. (The smaller solvent signals of pentane, hexane
and Et2O were already present in the THF-d8 before sample preparation; no further signals can be
observed in the range from 170 to –130 ppm.)
50
Anion exchange with NaBPh4 leads to [(BIMPNMes,Ad,Me)Fe]BPh4 (1[BPh4]). Its 1H NMR
spectra in MeCN-d3 and CDCl3 are virtually the same as for 1, except for the diamagnetic
signals of the (BPh4)– anion. Unlike for 1, the spectrum in THF-d8 (Figure 5) does not
significantly differ from those taken in the other solvents, another hint that the chloride of 1
is coordinated in the less polar solvent (while the bulky (weakly coordinating) BPh4-
counterion does not coordinate in any of the solvents).
Salt metathesis from chloride to azide affords [(BIMPNMes,Ad,Me)Fe(N3)] (2). The 1H NMR
spectra of 2 in polar solvents like CDCl3 and MeCN-d3 are practically identical to those of
1, while in THF-d8 the signals of both complexes differ significantly, clearly indicating that
the azide and chloride counterions are coordinated only in THF-solutions.
The IR spectrum of 1 features broad C–H bands (2902 and 2846 cm–1) and a fingerprint
region characteristic for divalent complexes of this ligand. 1[BPh4] adds the conspicuous
B-C stretching frequencies of its borane counter ion to the spectrum (734 and 706 cm–1;
the C-H frequencies of BPh4 fall together with those of the ligand’s phenol), while 2 is
identified by the intense and characteristic νas(N3) IR vibrational bands (in KBr: 2086, 2054
and 2001 cm–1) (Figure 6).4
Finally, [(BIMPNMes,Ad,Me)Fe(Br)] (1[Br]) can be prepared analogously to the chlorido by
reacting the free ligand with the bromide salt FeBr2.
4 For a discussion on the number of νas(N3) vibrational bands (namely three where only one is expected), see
the respective section for cobalt azide complex 7 on p. 83.
51
Figure 6. IR spectra of [(BIMPNMes,Ad,Me)Fe]Cl (1), [(BIMPNMes,Ad,Me)Fe]BPh4 (1[BPh4]), and
[(BIMPNMes,Ad,Me)Fe(N3)] (2) (KBr-pellets).
Crystal Structures of 1 and 2
Crystals of 1 suitable for X-ray crystallography were obtained as nearly colorless prisms by
diethyl ether diffusion into an acetonitrile solution of the complex. The molecular and
crystal structure reveals an acetonitrile molecule bound axially to the iron center (Figure 7).
The coordination geometry at the iron center can be described as nearly trigonal-
bipyramidal, with the iron atom positioned 0.113(1) Å above the plane defined by the two
carbene carbons and the oxygen atom, a near linear angle between the anchoring and
acetonitrile nitrogens ((NMeCN–Fe–Nanchor) = 177.76(5)°) and an Fe–Nanchor distance of
2.462(2) Å, indicating a weak bonding interaction between the nitrogen anchor and the iron
center. Table 1 summarizes bond lengths and angles of 1, as well as the out-of-plane-shift
doop, which is defined as the distance of the metal center from the least-squares plane
defined by the three coordinating atoms of the tripodal ligand; i.e., the carbene carbon and
phenolate oxygen atoms.
52
Figure 7. Molecular structure of the complex [(BIMPNMes,Ad,Me)Fe]Cl (1) with an acetonitrile
molecule coordinated to the iron center, in crystals of [(BIMPNMes,Ad,Me)FeII(NCMe)]Cl · MeCN
(50 % probability ellipsoids). Co-crystallized solvents and all hydrogen atoms except for the
acetonitrile’s are omitted for clarity. Selected bond distances and angles are collocated in Table 1.
In the molecular and crystal structure of the azide complex 2, the metal center is
coordinated in a trigonal pyramidal fashion with the carbene and phenolate C and O donor
atoms of the (BIMPNMes,Ad,Me)– ligand occupying the equatorial positions while the azide
resides in the axial position (Figure 8). Two crystallographically independent, but
structurally and metrically very similar, molecules are present within the unit cell. They
behave towards each other like two helical enantiomers, as viewed top-down along the
Nazide–Fe–Nanchor–axis, with the three ligand arms spiraling up from the anchor in opposite
sense of rotation. Table 1 summarizes values for one representative only, and the azide
complex [(TIMENMes)Fe(N3)]BPh4 for comparison; parameters for each independent
molecule of 2 are listed in Tables 23 and 24 (pp. 225 to 226).
53
Figure 8. Molecular structure of the complex [(BIMPNMes,Ad,Me)Fe(N3)] (2) in crystals of
[(BIMPNMes,Ad,Me)FeII(N3)] · 1.894 Et2O · 0.606 DMF (50 % probability ellipsoids). Co-crystallized
solvents and hydrogen atoms are omitted for clarity. Selected bond distances and angles are
collocated in Table 1.
Notably, the azide is coordinated in its preferred bent coordination mode
(2: Fe1-N6-N7 = 135.0(2)°), as opposed to the azide ligand in the corresponding
TIMENMes complex, which is forced into linear coordination by the mesityl substituents on
the NHCs ([(TIMENMes)Fe(N3)]+: 174.5(2)°).[67] Therefore, even with the bulky adamantyl
substituent on the phenol, the steric pressure at the metal center has already been
significantly decreased compared to the TIMENR-complexes by the substitution of one of
the NHCs with a phenol. The azide protrudes into the gap between the phenolate and one of
the carbene arms, thus widening the corresponding angle (CCarb.–M–O) to 138.31(9)° (see
Figure 9).
54
Figure 9. Space-filling representations of the molecular structure of 2 in crystals of
[(BIMPNMes,Ad,Me)FeII(N3)] · 1.894 Et2O · 0.606 DMF; side view (left) and top view (right)
along the Nazide–Fe–Nanchor–axis.
The azide’s linear coordination mode, as observed in the molecular structure of
[(TIMENMes)Fe(N3)]+, is accompanied by a Fe–Nα bond that is 0.1 Å shorter than the
corresponding bond in the (BIMPNR,R’,R”)– azide complex.[67] Additionally, this complex
exhibits a larger displacement of the metal ion above the plane of the three coordinating
carbenes of the TIMENMes ligand (doop). These observations indicate increased multiple
bond character between the metal center and the coordinating Nα nitrogen of the azide
ligand in complexes of the TIMENR ligand system, in which a tight cylindrical cavity
enforces the azide’s unsual linear coordination mode.
Nanchor
NNHC1
Ccarb2
Ccarb3
NNHC2
Ccarb.
Mes
Cph2Cph3
Cph4
Cph5
Cph6
Cph1
O
Ad
M
Laxial
2 Chart 2. Atom labels used in Table 1 (M = Fe), Table 3 (M = Mn), Tables 6 to 10 and 12 (M = Co)
as well as Tables 17 to 21 (Section Crystallographic Details).
55
Table 1. Selected bond distances [Å] and bond angles [°] for [(BIMPNMes,Ad,Me)FeII(NCMe)]Cl ·
2 MeCN (1(MeCN) · 2 MeCN), and [(BIMPNMes,Ad,Me)FeII(N3)] · 1.894 Et2O · 0.606 DMF
(2 · 1.894 Et2O · 0.606 DMF), as well as [(TIMENMes)Fe(N3)]BPh4 · 2.5 THF for comparison; see
Chart 2 for atom labeling. Further values and e.s.d.’s are listed in Tables 23 and 24.
Bond / Angle 1(MeCN) · 2 MeCN
2 · 1.894 Et2O · 0.606 DMF
[(TIMENMes)Fe(N3)]- BPh4 · 2.5 THF
M···Nanchor 2.461 2.591 3.243(3)
M–Laxial 2.229(2) 2.062(2) 1.947(3)
M–Ccarb. 2.100(2)
2.121(2)
2.144(3)
2.101(3)
2.104(3)
2.110(3)
2.110(3)
M–O 1.920(2) 1.939(2) -
Nα–Nβ - 1.175(3) 1.182(3)
Nβ–Nγ - 1.160(4) 1.156(3)
Nanchor-M– Laxial 177.76 170.2 177.5(1)
Ccarb.–M–Laxial 94.40(5)
94.63(5)
97.0(1)
105.4(1)
103.9(1)
105.1(1)
106.6(1)
O–M–Laxial 90.45(5) 92.74(9) -
M–Nα–Nβ - 135.0(2) 174.5(2)
Nα–Nβ–Nγ - 177.9(3) 179.6(3)
Ccarb.–M–C’carb. 120.82(6) 107.8(1) 115.3(1)
112.5(1)
112.18(9)
Ccarb.–M–O 117.61(6)
120.66(5)
138.31(9)
108.30(9)
-
NNHC1–Ccarb.–NNHC2 103.3(2)
103.1(2)
102.7(2)
103.0(2)
102.9(2)
102.8(2)
103.0(2)
doop 0.113(1) 0.271(2) 0.553(2)
56
Mößbauer Spectroscopy
The 57Fe Mößbauer spectra of all [(BIMPNR,R’,R”)FeII]+ complexes presented herein feature
doublets with isomer shifts, δ, in agreement with high spin Fe(II) complexes[214-215] (see
Table 2, and Figure 10 for an exemplary spectrum). The isomer shifts δ of analogous
TIMENMes complexes are lower than those of their (BIMPNMes,Ad,Me)– iron(II) counterparts
(grouped together in Table 2 by lines). This is most evident when comparing the complexes
with non-coordinating counterions, [(TIMENMes)Fe]OTf2 (δ = 0.59 mm · s–1)[216] and
1[BPh4] (δ = 0.66 mm · s–1, Figure 10), or the two azide complexes
[(TIMENMes)Fe(N3)]BPh4 and 2. The values for 1 and 1[Br] given in Table 2 stem from
samples that had been prepared from material that was filtered off from pyridine/ether
mixtures, so the chloride or bromide is in all likelihood not coordinated either. The values
given for 1Mes,tBu,tBu refer to a sample prepared by evaporating a THF solution of the
complex, so it can be supposed that the chloride was coordinated in the complex molecules
(vide supra), and the values are best compared to those of [(TIMENMes)FeCl]Cl.
Figure 10. Zero-field 57Fe Mößbauer spectrum of a microcrystalline sample of 1[BPh4] recorded at
77 K. The solid line represents the best fit obtained, with parameters given in Table 2.
57
The isomer shift for (BIMPNMes,Ad,Me)– is about 0.1 mm · s–1 higher than for TIMENMes,
which easily accounted for by the different electronic properties of the NHC and phenolate
ligands. While the σ-donating NHCs also engage in metal-to-ligand π-backbonding,[139] the
phenolates are σ and π donors. This leads to a higher d-electron density at the metal center,
which in turn increases the shielding of the s-orbitals vs. the nuclear charge. The s-orbitals
expand in volume and the electron density within the iron nucleus is effectively lowered,
increasing the isomer shifts, δ, in 57Fe Mößbauer spectroscopy.
The quadrupole splitting, ∆EQ, of the (BIMPNMes,Ad,Me)– iron(II) complexes is considerably
larger than that of the corresponding TIMENMes complexes, which may be explained by the
reduced symmetry around the iron center, resulting in a less symmetric distribution of the
electron density in the mixed NHC/phenolate complexes.
Table 2. Mößbauer parameters for several (BIMPNMes,Ad,Me)– and TIMENR iron(II) complexes.
Complex δδδδ
[mm · s–1
]
∆EQ
[mm · s–1
]
ΓΓΓΓFWHM
[mm · s–1
]
[(TIMENMes)FeII]OTf2 0.59(1) 1.02(1) 0.40(1)
[(BIMPNMes,Ad,Me)FeII]Cl (1) 0.68(1) 3.28(1) 0.27(1)
[(BIMPNMes,Ad,Me)FeII]Br (1[Br]) 0.68(1) 3.22(1) 0.37(1)
[(BIMPNMes,Ad,Me)FeII]BPh4 (1[BPh4]) 0.66(1) 3.42(1) 0.28(1)
[(TIMENMes)FeII(Cl]Cl 0.71(1) 1.78(1) 0.32(1)
[(BIMPNMes,tBu,tBu)FeII(Cl)] (1Mes,tBu,tBu) 0.84(1) 3.40(1) 0.30(1)
[(TIMENMes)FeII(N3)]BPh4[67] 0.687(1) 2.267(3) 0.48(1)
[(BIMPNMes,Ad,Me)FeII(N3)] (2) 0.83(1) 3.24(1) 0.31(1)
2.2.2 Essays at redox chemistry
Attempts to Reduce 1
The off-white [(TIMENR)FeII]2+ (R = Mes, Xyl) complexes are readily reduced to dark red
[(TIMENR)FeI]+ over sodium or sodium amalgam,[67, 71] and therefore, the same was
58
attempted with 1. When a solution of 1 in THF is stirred over sodium for 2 h, it gradually
turns from yellow to orange to dark red. Unlike 1, the reaction product dissolves in
aromatic hydrocarbons, and can be lyophilized from benzene solution to obtain a dark red
powder. When the reaction was performed with KC8 instead of sodium, the color changed
to a more brownish red, and yields after lyophilization were far lower, so this route was
considered less promising and abandoned. Reaction times may be reduced by using sodium
amalgam instead of neat sodium, to increase the effective surface of the solid reductant,
albeit the neat sodium avoids toxic mercury wastes.
The red powder’s Mößbauer spectrum (Figure 11) could be interpreted either as an
asymmetric doublet, or, as in Figure 11, as the sum of two subspectra. Either way, the
isomer shift lies roughly around 0.75 mm · s–1. According to literature values, this would be
in better agreement with another Fe(II) high spin species (expected range ≈ 0.6 to
Figure 11. Zero-field 57Fe Mößbauer spectrum of a solid sample of the dark red reaction product
obtained when a THF solution of 1 is stirred over sodium, recorded at 77 K. The solid line
represents the best fit obtained, which is the sum of the blue and red subspectra. Parameters: red
subspectrum (41 % relative area): δ = 0.72(1) mms–1, ∆EQ = 3.39(1) mms–1, ΓFWHM = 0.40(1); blue
subspectrum (59 % relative area): δ = 0.76(1) mms–1, ∆EQ = 2.83(1) mms–1, ΓFWHM = 0.70(1).
59
1.3 mm · s–1), rather than the expected Fe(I) high spin complex (expected range ≈ 1.7 to
1.8 mm · s–1).[215] However, as the TIMENMes complex series has shown, NHC complexes
can behave quite contrary to these expectations: It is generally applicable that a lower
oxidation number of the iron center leads to a higher isomer shift (for the same spin state),
since a higher d-electron count leads to a stronger shielding of the s-electrons, thereby
reducing the electron density at the nucleus. For TIMENR (R = Mes, Xyl), however,
reduction of the metal center from iron(II) to iron(I) (both high spin) lowers the isomer
shift from 0.71 to 0.64 mms–1. This was explained by the NHCs’ capacity for
π-backbonding.[71] The reduced iron center is more prone to form π-bonds to the three
carbenes, which leads to an overall lower d-electron density than in the iron(II) center.
Accordingly, in the (BIMPNMes,Ad,Me)– case, we may be faced with an in-between case,
where the isomer shift does increase upon reduction, but not as much as expected due to
π-bonding to the two remaining NHCs.
All essays at elemental analysis so far, however, have given lower C,H,N values than
calculated for [(BIMPNMes,Ad,Me)FeI]. Nonetheless, a SQUID magnetization measurement
was performed to further probe the product’s magnetism, and the data are displayed in
Figure 12. These data are to be seen only as preliminary, since no proof of the measured
samples’ purity can be given, but the calculated µeff (RT) of around 4 µB is closer to the
spin only value of 3.87 µB expected for a d7 high spin iron(I) system (S = 3/2) than for d6
high spin iron(II) (S = 2: µeff(s.o) = 4.90 µB).
Astonishingly, however, with EPR experiments at low temperature (helium cooling, frozen
toluene solution) no signal has been detected – even though one would expect a d7 iron(I)
complex to be EPR-active (be it high spin , S = 3/2 or low spin S = 1/2). Since attempts at
crystallization have not been fruitful, the final prove for successful reduction of 1 to an
iron(I) complex remains elusive.
60
Figure 12. Variable temperature SQUID magnetization measurements on a sample from the
reaction of 1 over sodium.
Attempts to Oxidize 1 and 2
So far, no TIMENR iron(III) complex has been isolated.5 Only once in the course of long
ongoing work, a single crystal of an [(TIMENR)FeIII]3+ complex was obtained,[217] but it has
not been possible to reproduce this serendipitous result, leave alone to isolate the
compound in bulk. This elusiveness of a TIMENR iron(III) species, together with the fact
that the iron(I) and iron(II) complexes (as well as the iron(IV) nitride) are thoroughly
studied, suggests that iron(III) is a highly unstable oxidation state within the TIMENR
framework even under inert conditions. The cyclic voltammograms of
[(BIMPNMes,Ad,Me)FeII]+ complexes 1 and 2 (see section 2.4) feature an FeII/FeIII redox
5 When [(TIMENR)FeII]2+ (R = Tol, 3,5-Xyl), which posses ortho-hydrogens on the NHCs’ aromatic
substituents, are reduced over sodium amalgam, the isolated products are iron(III) complexes where the
TIMENR-ligand has been metalated at said ortho-positions.[71-72] These interesting compounds are not counted
as „regular“ TIMENR iron(III) species in this context.
61
wave, therefore it seems likely that an iron(III) species should be isolable with the new
(BIMPNR,R’,R’’)– ligand system. Furthermore, in the tris(phenolate) system
((Ad,MeArO)3N)3-, iron(III) represents the most stable and readily synthesized oxidation
state, whereas iron(II) is extremely sensitive.[75] These practical observations suggest that
as one progresses along the ligand series from tris(carbene) to tris(phenolate), the more
stable oxidation state will shift from FeII to FeIII. This assumption is backed by the different
nature of the binding groups: Whereas NHCs are perfect for stabilizing high electron
counts (i.e. low oxidation states) with their ability for π-back-bonding, the σ- and π-
donating phenolates are more favorable for higher oxidations states.
The iron(II) complexes discussed herein are EPR silent, whereas for an iron(III) (d5)
system, signals are expected to be observable in the X-band region: with one g-value
around g = 2 for low spin (S = ½); three g-values at 4, 4, and 2 for intermediate spin
(S = 3/2), and also three g-values at 6, 6, and 2 for high spin (S = 5/2). Therefore, some
preliminary, in situ oxidation tests were carried out for EPR experiments at temperatures
around 10 K. For this, the oxidizing agents were added to solutions of the iron(II)
complexes in concentrations suitable for EPR spectroscopy.
Figure 13. EPR-spectrum of the in situ reaction product of 1 with AgSbF6 (helium cooling; frozen
CH2Cl2 solution); no further g-values were observed up to 1300 mT.
62
When 1 was treated with an excess of AgSbF6, only one, slightly asymmetric signal at
g ≈ 4.27 was observed (Figure 13). No additional signal at smaller g was visible even when
the measured range was extended up to 1300 mT (i.e. down to g ≈ 0.5). It may be
speculated that the additional g-value of around 2 of an anisotropic spectrum was too small
or broad to be observed. Furthermore, if the oxidation was successful, elemental silver
most likely precipitated from the solution before it was frozen, as no silver signal was
observed (For Ag0 (S = ½) a simple signal without coupling around g = 2.0 is expected[218-
219]). For 2, treatment with AgSbF6 resulted in an equal spectrum with a signal at g ≈ 4.26.
When the reactions with AgSbF6 were up-scaled for synthesis and isolation of an iron(III)
complex, the solutions of 1 or 2 (in either CH2Cl2 or THF) turned from yellow to dark
cherry red. For both starting materials, it was hard to obtain the product in other form than
as a sticky, slimy mass. Only with the product from 2 plus AgSb6 was it possible to receive
a powder after lyophilization of a Celite-filtered benzene solution. The IR spectrum of the
product is shown in Figure 14 (red line), with an overlay of the educt (black line). The
azide bands νas(N3) have shifted to 2114 and 2076 cm–1, and the counter ion’s Sb–F
stretching frequencies around 660 cm–1 are evident. It therefore seems the oxidation to
[(BIMPNMes,Ad,Me)FeIII(N3)]SbF6 was successful.
Figure 14. IR spectrum of the reaction product between [(BIMPNMes,Ad,Me)Fe(N3)] (2) and AgSbF6
(red line), with an overlay from the starting material 2 (black line) (KBr pellets).
63
EPR-scale experiments with NOBF4 invariably led to spectra like the one in Figure 15. The
observed g-values and line shape strongly suggest the presence of at least two EPR-active
species in the frozen solution. Possibly, one is the desired iron(III) complex
[(BIMPNMes,Ad,Me)FeIII(Cl)]BF4, while the other may be a complex with an NO ligand.
From micro-scale synthetic experiments, Mößbauer spectra were gained with a very broad
and asymmetric absorption signal (Figure 16). This kind of spectrum has also been
observed for [((Ad,MeArO)3N)FeIII(MeCN)]. The Mößbauer data also suggest the presence
of more than one species in the samples, none of them being the starting material 1 or any
other (BIMPNR,R’,R”)– iron(II) complex reported herein.
Figure 15. EPR-spectrum of the in situ reaction product(s) of 1 with NOBF4 (helium cooling;
frozen CH2Cl2 solution).
64
Figure 16. Mößbauer spectrum of two powdered samples (blue and black data points, respectively)
of the reaction product of 1 with NOBF4.
With neither oxidant was it possible to observe an NMR spectrum of any product. This is
not surprising, however, if one considers that d5 iron complexes, which are expected to be
high spin, are likely NMR silent, like their tris(phenolate) analogue
[((Ad,MeArO)3N)FeIII(MeCN)].[74-75]
While the iron(II) complexes are stable in CH2Cl2 – and this solvent can indeed greatly aid
in their purification – it has been observed that they are not stable over a longer time in
CHCl3. This may well be due to the chloroform’s higher oxidation potential, and therefore,
it was also tried to oxidize 1 by heating its chloroform solutions. Upon heating, the yellow
solution gradually turns red. By heating NMR samples of 1 or 1[BPh4] in CDCl3, the
reaction progress can be nicely monitored by the disappearance of the starting material’s
signals. In the case of 1[BPh4], only signals of the borate counter ion remain. If a well-
defined product evolves, however, it is again NMR silent. Bulk reactions have yielded
more of the red material. The next step would be to examine this by Mößbauer, helium-
temperature EPR – to compare it with the in situ reaction spectra with oxidizing agents
AgSbF6 and NOBF4 – and to monitor the reaction by UV/Vis. Final proof of formation of
65
an iron(III) complex, however, may only be possible through crystallographic analysis
and/or magnetization measurements.
2.2.3 Photolysis Experiments with [(BIMPNMes,Ad,Me)Fe(N3)] (2)
When solutions of the yellow azide complex 2 in THF are irradiated with a mercury vapor
lamp, its color turns to dark red (see Figure 17). When the same is done in acetonitrile, the
color changes only slightly, as does the 1H NMR spectrum. With the knowledge that the
azido ligand of 2 is not coordinated in acetonitrile solution, this is of course not surprising.
Further photolysis experiments were therefore carried out in THF.
Figure 17. Solutions of [(BIMPNMes,Ad,Me)Fe(N3)] (2, ≈ 12 mg / 20 mL) in THF (upper row) and
acetonitrile (lower row) before and after irradiation with a mercury vapor lamp.
When a sample of 2 in THF-d8 is irradiated, the reaction can be monitored via 1H NMR.
Over time, the starting material’s signals vanish completely, while new paramagnetic ones
arise (Figure 18). Should an iron nitrido species form during this time, it is apparently not
diamagnetic, as would be expected for an Fe(IV) nitride.[59, 67] Alternatively, it may be that
an emerging nitrido directly undergoes a follow-up reaction, for example with the solvent
66
(e.g. through H-abstraction from the THF), and/or an intramolecular insertion reaction, like
the one which has been observed upon oxidation of tetravalent [(TIMENMes)Fe(N)]+.[71], 6
45 40 35 30 25 20 15 10 5 0 -5Chemical Shift (ppm)
0.2
2.0
2.7
2.9
4.3
4.6
6.2
7.3
7.9
14
.0
37
.7
41
.1
55 50 45 40 35 30
Figure 18.
1H NMR spectrum of a sample of 2 photolyzed for 22 h in THF-d8 (ca. 17 mg in
0.8 mL). The inset depicts part of the spectrum with an overlay of the spectrum of the starting
material 2 (orange line), of which all signals have disappeared. No further product signals were
observed from 170 to –130 ppm.
While the 1H NMR (Figure 18) looks promisingly neat, giving reason to hope that the
sample contains only one (or one major) well-defined species, the only crystal ever
obtained from a photolyzed solution of 2 was again a crystal of the azido complex. Most
notably, this crystal grew from a fully photolyzed NMR sample, which exhibited a
spectrum that hardly showed any starting material peaks any more. Clearly, the azide
complex crystallizes far more readily than any potential irradiation product.
6 See also section 2.7.2 for the insertion reaction of the [(BIMPNMes,Ad,Me)Co(N3)] irradiation product.
67
Figure 19. Solutions of 2 in THF (left cuvette) and MeCN (right cuvette) before (A) and after (B)
27 h of irradiation with a mercury vapor lamp.
Further experiments may encompass monitoring the reaction by UV/Vis (Figure 19). The
safest way to ensure that it is in fact the azide ligand that is photolyzed may be by
photolysis of a KBr pellet of 2, monitored with IR spectroscopy to see the azide stretches
disappear.7 To ensure that the complex does not simply decompose by oxygen or air
moisture diffusing into the pellet during the time of irradiation, another KBr pellet may be
kept in the dark and measured as reference sample. However, in the end, with a
paramagnetic product, the most conclusive form of identification may remain single crystal
analysis.
7 For this experiment, see also section 2.7.2.
68
2.3 Manganese Complexes of (BIMPNR,R’,R”)–
Treatment of (BIMPNMes,Ad,Me)– with one equivalent of anhydrous manganese chloride in
an ether / pyridine mixture (15:1) at room temperature yields the four-coordinate
manganese(II) complex [(BIMPNMes,Ad,Me)Mn(Cl)] (3) as a white powder in 55 % yield
(Scheme 38). This solvent mixture was chosen for the same reasons as discussed for the
iron(II) complexes (see p. 47).
NN
NMes
2
OK
Ad
N
NC
NC
N
MnIIO
N
(py / Et2O)
MnCl2
- KCl
N
NC
NC
N
MnIIO
N
NN
N
NaN3
(MeCN)
-NaCl
Cl
Scheme 40. Synthesis of the (BIMPNMes,Ad,Me))– manganese(II) complexes [(BIMPNMes,Ad,Me)MnCl]
(3) and [(BIMPNMes,Ad,Me)Mn(N3)] (4).
The IR spectrum of 3 is very similar to that of iron complex 1. Salt metathesis from
chloride to azide affords colorless [(BIMPNMes,Ad,Me)Mn(N3)] (4), which is easily identified
by the intense and characteristic νas(N3) IR vibrational bands (in KBr: 2077 and 2056 cm-1,
Figure 20).8
8 For a discussion on the number of νas(N3) vibrational bands (two where one is expected), see the respective
section for cobalt azide complex 7 on p. 83.
69
Figure 20. IR spectrum of 4 (KBr pellet).
The UV/Vis-spectra of both complexes feature bands with maxima at 257 and 309 nm of
similar intensity (ε = 11300 and 5750 Μ−1 cm–1 for 3, 14900 and 7400 Μ−1 cm–1 for 4),
which are tentatively assigned to a π–π* transition in the phenolate and a charge-transfer
transition, respectively.[220] As expected, no d-d transitions can be observed as they are both
Laporte and spin forbidden.
Both manganese(II) complexes are NMR-inactive, which is to be expected from
manganese(II) d5 high spin systems. Variable temperature SQUID magnetization
measurement (2 – 300 K) confirms a high spin ground state for 3 (Figure 21). At room
temperature, 3 possesses a magnetic moment, µeff, of 5.82 µB (S = 5/2 ground state) that
decreases negligibly with decreasing temperature down to 10 K. From this temperature on
it drops to a value of 5.13 µB at 2 K. The simulation of the measurement returns a g-value
of 1.97 and a zero-field splitting |D| of 0.426 cm-1. Simulation of a variable field
measurement (Figure 22) confirms the zero-field splitting’s magnitude and gives it a
negative sign.
70
Figure 21. Variable temperature SQUID magnetization data (1 T) of [(BIMPNMes,Ad,Me)MnCl] (3).
Magnetic moment (µeff) plotted versus temperature (T). Data were corrected for diamagnetism, and
reproducibility was verified by measuring multiple independently synthesized samples. Parameters
are discussed in the text and listed in Table 16.
Figure 22. Temperature-dependent SQUID magnetization data of [(BIMPNMes,Ad,Me)MnCl] (3) at
variable field strength; magnetic moment (µeff) plotted versus temperature (T). Data were corrected
for diamagnetism. Parameters are discussed in the text and listed in Table 16.
71
Crystal Structure of 3
Crystals of 3 suitable for X-ray crystal structure determination were obtained as colorless
prisms by slow diethyl ether diffusion into a pyridine solution of the complex. The
molecular structure of this manganese complex (Figure 23 and Table 1) exhibits C1
symmetry. The manganese(II) central ion is coordinated by two NHC carbenes, the
phenolate oxygen, and the chloride in a distorted trigonal pyramidal geometry with the
manganese located above the plane of the three donor atoms O1, C3, and C8 (towards Cl1)
as is indicated by the corresponding out-of-plane shift doop of 0.362(2) Å. Notably, a non-
coordinated pyridine molecule is situated between the adamantyl and one of the mesityl
substituents. One of its C–H bonds is pointing towards the Mn-Cl bond at a distance of ca.
3 Å (see Figure 24), which does not suggest any bonding interaction, but the pyridine
effectively pushes apart two of the ligand arms increasing the angle between them to (Ccarb.–
M–O) = 130.69(8). This observation establishes the intended steric flexibility of the
(BIMPNR,R’,R”)– ligand to provide side access for possible substrates to the reactive center.
Figure 23. Molecular structure of [(BIMPNMes,Ad,Me)MnCl] (3) in crystals of
[(BIMPNMes,Ad,Me)MnCl] · 0.5 pyridine (50 % probability ellipsoids). Co-crystallized solvents and
hydrogen atoms are omitted for clarity. Selected bond distances and angles are collocated in
Table 1.
72
Table 3. Selected bond distances [Å] and bond angles [°] for [(BIMPNMes,Ad,Me)MnIICl] · 0.5
pyridine (3 · 0.5 py); see Chart 2 for atom labeling. Further values are listed in Tables 23 and 24.
Bond / Angle 3 · 0.5 py
M···Nanchor 2.696
M–Laxial 2.4221(7)
M–Ccarb. 2.227(3)
2.230(3)
M–O 2.001(2)
Nanchor–M– Laxial 170.25
Ccarb.–M–Laxial 104.79(7)
102.65(7)
O–M–Laxial 92.45(5)
Ccarb.–M–C’carb. 109.02(9)
Ccarb.–M–O 130.69(8)
111.61(8)
NNHC1–Ccarb.–NNHC2 103.1(2)
103.5(2)
doop 0.362(2)
Figure 24. Space-filling representations of the molecular structure of 3 in crystals of
[(BIMPNMes,Ad,Me)MnIICl] · 0.5 pyridine; side view (left and middle) and top view (right) down the
Mn–Cl axis, with and without the pyridine molecule situated between two ligand arms.
73
Outlook
The work on (BIMPNR,R’,R”)– manganese complexes was taken up by Eva Zolnhofer in her
master’s thesis, where she finished the full characterization of the azide complex 4,
including X-ray crystal structure analysis, EPR, and SQUID magnetization measurements.
These data have been published in a collaborative journal publication.[221]
The first photolysis experiments on the azide 4 were performed in the course of the
master’s thesis, with interesting preliminary results. Also, the anion exchange product
[(BIMPNMes,Ad,Me)Mn]BPh4 was fully characterized, and may serve as a more reactive
entrance molecule (compared to 3) for small molecule activation reactions.
74
2.4 Electrochemistry
To probe the redox-activity of the iron complexes [(BIMPNMes,Ad,Me)Fe]Cl (1) and
[(BIMPNMes,Ad,Me)Fe(N3)]Cl (2), as well as that of the cobalt complexes
[(BIMPNMes,Ad,Me)Co]X (X = Cl: 6, X = PF6: 6[PF6], and X = N3: 7) (see section 2.6.1),
electrochemical measurements (cyclic, linear sweep, and square wave voltammetry) were
performed. Linear sweep measurements were used to ascertain the identity of the processes
as oxidation- or reduction events. Square wave voltammetry was applied to discern
potentials that lie closely together. The results are summarized in Table 4 and explained in
the following. Cyclovoltammograms not shown here can be found in the experimental
section.
Table 4. Potentials of reversible redox processes for complexes 1, 2, 6, 6[PF6], and 7 from cyclic
voltammetry in acetonitrile; wave separations at scan rate 0.2 V/s. Potentials are given vs. Fc/Fc+.
compound
reversible
reduction
MI/M
II
E1/2 [V]
wave
separation
∆E [V]
reversible
oxidation
(MII
/MIII
and others)
E1/2 [V]
wave
separation
∆E [V]
[(BIMPNMes,Ad,Me)Fe]Cl 1 - - -0.31 0.6
[(BIMPNMes,Ad,Me)Fe(N3)] 2 - - -0.33 0.66
[(BIMPNMes,Ad,Me)Co]Cl 6 -2.0 0.07 -0.36
0.34
0.60
1.1
0.08
0.18
[(BIMPNMes,Ad,Me)Co]PF6
6[PF6] -2.0 0.07 -0.23
0.60
0.85
0.12
[(BIMPNMes,Ad,Me)Co(N3)] 7 -2.0 0.08 -0.43
0.32
0.60
1.1
0.07
0.08
75
As the chlorido and azido anions are not coordinated to the FeII and CoII complexes of
(BIMPNMes,Ad,Me)– in acetonitrile solution, the (BIMPNMes,Ad,Me)– complexes of the same
metal unsurprisingly give rise to very similar voltammograms in this solvent, regardless of
their “axial ligand” (i.e. counter ion in solution). Each voltammogram features a reversible
redox process assignable to the MII/MIII oxidation, with an unusually large peak separation
that can be explained by a rearrangement of the coordination sphere upon electron transfer
(Figure 25). This rearrangement could stem from coordination of either the counter ion (Cl–
or N3–), coordination of solvent molecule(s), or even a change in the coordination of the
(BIMPNMes,Ad,Me)– ligand.
Figure 25. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Fe]Cl (1) in MeCN: FeII/FeIII redox-wave at
different scan rates.
Cobalt complexes 6, 6[PF6], and 7 additionally show a metal-centered reversible reduction
(CoII/CoI) at –2.0 V, with a peak separation of 0.07 V (Figure 26). While the expected peak
separation ∆E for a reversible redox event is 59 mV in water (and other protic solvents), it
is usually larger in aprotic solvents, and thus ∆E = 0.07 V is common for a reversible
process in acetonitrile. Furthermore, the cobalt complexes’ cyclic voltammograms show
either one or two additional reversible oxidations depending on the counter ion (Figure 26
(left side) and Figure 27): 6 and 7 exhibit an oxidation around E1/2 = 0.3 V, again with a
small peak separation ∆E = 0.07 V (underlined values in Table 4), and all three have an
76
oxidation at E1/2 = 0.6 V. The latter may be assigned to a reversible ligand oxidation. Since
the cyclic voltammogram of the (protonated) ligand (H3BIMPNMes,Ad,Me)(BPh4)2 (see
Figure 65 p. 165)9 shows one irreversible oxidation at 0.72 V, this is in agreement with the
observation that the oxidation potentials of metal coordinated phenolates are generally
lower than those of free phenols.[170]
Figure 26. Voltammograms of 6[PF6]: Cyclic (green line) and linear sweep (maroon line)
voltammograms (left) and isolated CoI/CoII redox-wave at different scan rates (right).
Figure 27. Voltammograms of 6, 6[PF6], and 7 in MeCN.
9 For (H3BIMPNXyl,tBu,tBu)Cl2, the irreversible oxidation lies at 0.78 V (Figure 66 p. 167).
77
The exact mechanism to which the oxidation wave at E1/2 = 0.3 V of 6 and 7 can be
attributed has not been elucidated. What may be ruled out is that these are simply the
oxidations of the free Cl– and N3– anions: The N3
–/N3 potential has been reported to be
0.92 V vs. Fc/Fc+ in acetonitrile,[222] or 1.32 V vs. NHE in H2O,[223] and Cl– has a higher
potential still.
As a working hypothesis, the following tentative explanation may be given: Upon
oxidation, the Cl– and N3– (which had been solvatized counter ions) coordinate to the now
more positively charged metal center. Through mechanistics not yet understood, this
facilitates the oxidation of the counter ions, which now occur at 0.32 V. This oxidation is
reversible. The counter ion may stay coordinated until the cobalt center is reduced to CoII
again. This might also explain why the CoII/CoIII peak separation is somewhat smaller for
6[PF6] than for the other two: The coordinated counterion hinders the reduction to MII,
which then occurs at a lower potential, whereas the bulky PF6– cannot coordinate. (The
smaller wave separation for 6[PF6] is why the effectively observed E1/2 is less negative,
even though the oxidation half-wave is found at the same potential). However, with this
explanation it is on the other hand surprising that the oxidation at 0.6 V is identical in all
three voltammograms; it would mean that the oxidation of the phenolate ligand is not
influenced by a coordinated and oxidized chloride or azide.
78
2.5 Diamagnetic Complexes of (BIMPNR,R’,R”)–
While the isolated manganese(II) complexes of (BIMPNR,R’,R”)– are NMR silent, the
iron(II) complexes give paramagnetic 1H NMR spectra with paramagnetically shifted and
broadened signals that are diagnostic for the identity and purity of the respective
compounds, but not assignable to individual protons. It was therefore of interest to
synthesize at least one diamagnetic compound of the new tripodal ligand in order to receive
interpretable (1H and 13C) NMR spectra, which may also answer questions about the
complexes’ symmetry in solution. Furthermore, for future electrochemical studies, it may
be highly favorable to have diamagnetic reference complexes with a non-redox-active
metal center at hand. Provided the redox-waves are comparable, this may allow an
unambiguous assignment of metal-based and ligand-based redox processes in the
voltammograms of the redox-active complexes.
Accordingly, several metals were tested in closed-shell oxidation states that are expected to
lead to diamagnetic complexes with centers that are redox-inactive on a broad potential
range, including the main group elements Ga(III) and In(III) (Scheme 41) and transition
metal Ni.
GaCl3 A(BIMPNR,R',R")
A = Na, K
+
R = Xyl, R',R" = tBu
R = Mes, R' = Ad, R" = Me
(benzene
or Et2O)
" [(BIMPNR,R',R")M]2+ "
(H3BIMPNR,R',R")2+
+
M = Ga, In
(THF or
py/Et2O)in different ratios
InCl3 A(BIMPNR,R',R")
A = Na, K
+
R = Mes, R' = Ad, R" = Me
Scheme 41. Synthetic routes that were tried towards isolable gallium and indium complexes of
(BIMPNR,R’,R”)–.
Gallium trichloride salt was reacted with the potassium salt of the ligand. In 1H NMR
spectra of the crude product, new peaks are found in the diamagnetic region with a
distribution and coupling patterns that belong to neither a protonated form nor an alkali salt
of the ligand. Comparison with the spectra of the Zn complex (see below) provides further
evidence that these stem from ligand coordinated to the diamagnetic gallium metal center.
79
However, the samples also contained the protonated form of the ligand. Due to similar
solubilities of both the protonated ligand (H3BIMPNR,R’,R’’)2+ and the putative
[(BIMPNR,R’,R’’)Ga]2+, isolation of a pure Ga(III) complex has not been achieved.
The ligand may have been re-protonated, either by impurities in the M(III) trichloride salts,
which may contain traces of HCl, or by acidic conditions through interaction of tBuOH and
the metal salt, i.e., by the same phenomenon as in the iron complex synthesis (see p. 47). In
the first case, recrystallization of the metal salt may remove these impurities; in the latter,
protonation might be counteracted by the same practice as with iron, addition of pyridine to
the reaction mixture to ensure basic conditions.
With InCl3, complexation was carried out in neat THF and in a mixture of py/Et2O.
However, in THF, the same problem with re-protonated ligand arises. After synthesis with
pyridine, strangely enough, a small amount of re-protonated ligand was observed as well.
In addition, even after work-up in different solvents, the pyridine had not been removed
from the crude product. While it is conceivable that one pyridine molecule may be
coordinated to In, the pyridine signals’ integrated intensities strongly exceed 1 equivalent
of pyridine compared to the supposed complex signals. The pyridine signals also
superimpose several of the product signals in the aromatic region. However, NMR spectra
of the crude product still suggest complex formation: The ethylene-groups on the ligand
arms split into four signals, just like in the Zn complexes’ spectra (see below); for isolation
of the complex, however, the synthesis and/or work-up procedure has to be revisited.
Exposing the samples of “[(BIMPNR,R’,R’’)M]2+“ (M = Ga or In) to air did not change the 1H NMR spectra, which suggests that the metal complexes are, as expected, not susceptible
to oxidation by air.
Suggestions for future work on the group 13 compounds include the use of (more) pyridine
and/or a stronger base to avoid ligand protonation, as well as a 1-pot approach like for
[(BIMPNMes,Ad,Me)Zn]OTs (5[OTs], see below) to achieve direct coordination of the freshly
deprotonated ligand molecules.
With Ni(COD)2 (COD = cycloocta-1,4-diene), the free ligand reacts in THF to a light
orange compound. The field desorption mass spectrum (FD-MS) features a prominent peak
at 690 amu, which corresponds to a singly positively charged “[(BIMPNXyl,tBu,tBu)Ni]“.
80
However, further work on this compound has been deferred, since the work with Zn(II)
quickly lead to more promising results.
Experiments with anhydrous zinc chloride lead to readily isolable complexes (see
Scheme 42). Colorless [(BIMPNMes,Ad,Me)Zn]OTs (5[OTs]) was generated in a 1-pot
reaction between the three white solids (H3BIMPNMes,Ad,Me)(OTs)2, NaOMe, and ZnCl2 in
THF. Its 1H NMR spectrum substantiates that, on the NMR time scale, the complex has Cs
symmetry in solution: In the aliphatic region, four signals from 4.5 to 3.0 ppm couple with
each other (see Figure 28), as confirmed by H,H–COSY. These are assignable to the two
times four protons of the two ethylene bridges between N-anchor and NHC-rings, meaning
that the CH2 protons that were equivalent in the uncoordinated ligand are now
diastereotopic. The CH2 protons between anchor and phenolate are still equivalent
(enantiotopic), resulting in one singlet at 3.63 ppm. Furthermore, the two “sides” of one
mesitylene ring (o- and m-positions of the mesityl–substituent) are no longer equivalent,
and the adamantyl substituents give more intricate coupling patterns than in the free ligand.
Hence, neither substituent seems to rotate freely around its bond to the NHC- or phenolate-
ring (on the NMR time scale). The 31 carbon signals found in the 13C NMR spectrum of
5[OTs] (Figure 29) confirm Cs symmetry.
When the ligand’s potassium salt K(BIMPNMes,Ad,Me) is used instead of the 1-pot pathway,
the synthesis with ZnCl2 leads to the chloride complex [(BIMPNMes,Ad,Me)ZnCl] (5). Its 1H NMR spectrum is equivalent to the one to 5[OTs], naturally lacking the tosylate signals,
with only minor changes in chemical shifts and coupling constants.
(H3BIMPNR,R',R")OTs2
(THF)
[(BIMPNMes,Ad,Me)Zn]OTsNaOMe
ZnCl2
K(BIMPNR,R',R")
(THF)
[(BIMPNMes,Ad,Me)Zn(Cl)]
ZnCl2
Scheme 42. Synthesis of [(BIMPNMes,Ad,Me)Zn]+ complexes 5[OTs] (top row) and 5 (bottom row).
81
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5Chemical Shift (ppm)
2H 2H 2H
2+1 H
2+2 H
1H
3H
6H
3H
6H
6+3 H
9H
3H
CDCl3
2H2H 2H 2H 2H
N
NC
NC
N
ZnIIO
Na
OTs
d
a
a
b
c
c
c
c
bb
b
d
dd d de
e
f
f
g
g
g
gh
h
i
i
i
i
a
Figure 28. 1H NMR spectrum of [(BIMPNMes,Ad,Me)Zn]OTs (5[OTs]) in CDCl3.
170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
Chemical Shift (ppm)
17
. 11
7.4
20
.62
1. 0
21
. 3
29
. 1
36
. 73
6. 9
39
. 8
49
. 1
59
.7
65
. 4
121
. 91
22
. 21
23
.91
26
.11
27
.91
28
. 4
129
.31
34
.01
34
.41
35
. 41
38
. 71
38
.91
39
.31
44
.3
164
.5
171
.8
12
9.0
12
8. 6
12
2.5
CDCl3
TMS
Figure 29. 13C NMR spectrum of [(BIMPNMes,Ad,Me)Zn]OTs (5[OTs]) in CDCl3.
82
2.6 Cobalt Chemistry of (BIMPNR,R’,R”)–
2.6.1 Cobalt (II) Complexes
NN
NMes
2
OK
Ad
N
NC
NC
N
CoIIO
N
(benzene)
CoCl2
- KCl
Cl
N
NC
NC
N
CoIIO
N
PF6
N
NC
NC
N
CoIIO
N
NN
N
NaN3
(benzene, DMF)
-NaCl NaPF6
- NaCl
(benzene)
Scheme 43. Synthesis of the cobalt(II) complexes of (BIMPNMes,Ad,Me)–. All syntheses depicted can
be carried out likewise with the ligand derivatives (BIMPNMes,tBu,tBu)– and (BIMPNXyl,tBu,tBu)–.
The green cobalt complex [(BIMPNMes,Ad,Me)Co]Cl (6) was obtained by stirring an excess
of K(BIMPNMes,Ad,Me) and CoCl2 in benzene for at least 24 h, during which time the crude,
green complex precipitates (Scheme 43). As opposed to the iron and manganese complex
syntheses, ligand re-protonation was not observed, so non-basic solvents suffice. Impurities
of KCl can be gradually reduced by repeatedly re-dissolving the crude material in MeCN or
CH2Cl2, followed by CeliteTM filtration. The tenacity of KCl to remain in the bulk material
83
of all (BIMPNMes,Ad,Me)– chloride complexes (1, 3, and 6) suggests some interaction with
the complex, e.g. by coordination of K+ to the phenol-oxygens, as it has been observed in
the crystal structure of the ligand’s potassium salt (see Figure 1 p. 42). This kind of
interaction within a transistion metal complex has been observed in
Na[(tBu,tBuArO)3N)Co],[221] where the sodium cation is bound to two of the three aryloxides
just above the Co site.
Anion exchange of Cl– to PF6– or N3
– allows for a more straightforward work-up to gain the
analytically pure complexes [(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) and
[(BIMPNMes,Ad,Me)Co(N3)] (7), which are easily identifiable by their intense and
characteristic IR bands (in KBr: 843 cm–1 for PF6–, 2081, 2044 and 1999 cm–1 for the
coordinated azide in 7, Figure 30). The appearance of three bands for the azide centered at
around 2000 cm–1 is surprising.
For the coordinated azide ligand, a total of three absorption bands are expected in the IR
vibrational spectrum, two originating from the symmetric νs(N3) and the asymmetric
νas(N3) stretching vibrations at around 1300 and 2000 cm–1, respectively. The third
absorption is assigned to the deformation vibration δ(N3) at approx. 400 cm–1. Thus, strictly
speaking, for a mono-azide metal complex, only one νas(N3) absorption band is expected in
the 2000 cm–1 region. However, the bis(carbene)mono(phenolate) ligand in complexes 2, 4,
and 7 allows for a variety of M–N3 conformations, which likely results in a number of
different νas(N3) vibration frequencies and absorption bands. In addition, due to interactions
with the “matrix”, such as KBr in the solid state or a solvent molecule in solution, the
number of absorption bands can vary. In a simple, classic case of a coordinated azide, e.g,
TlN3, Dehnicke reports[224] two absorption bands centered at 2037 and 2000 cm–1 for
νas(N3) and two bands at 1328 and 1319 cm–1 for the νs(N3) vibration. Notably, the solution
IR spectrum of 7 also shows two vibrational bands at 2081 and 2041 cm–1 (see Figure 31).
84
Figure 30. IR spectra of [(BIMPNMes,Ad,Me)Co]Cl (6, black), [(BIMPNMes,Ad,Me)Co]PF6 (6[PF6],
green) [(BIMPNMes,Ad,Me)Co(N3)] (7, blue) (KBr pellets).
Figure 31. Frequencies of the azide vibrations observed in IR spectra 7: green line: solution
spectrum in THF, black line: KBr pellet.
85
Solutions of the blue azide complex 7 in MeCN display the same green color as the azide
free complexes 6 and 6[PF6] dissolved in MeCN. The blue color of 7 reappears upon
evaporation of the solvent. Similarly, the 1H NMR spectrum of the CoII-complexes 6,
6[PF6], and 7 in MeCN-d3 and CDCl3 feature the same signals (within the error margins of
the highly temperature dependent paramagnetic NMR shifts). In THF-d8, the signal
distribution of 6[PF6] remains the same, whereas 7 gives rise to a very different spectrum
with more signals.10 All of these observations lead to the conclusion that – in MeCN and
chloroform solutions – the divalent Co complexes [(BIMPNMes,Ad,Me)Co]+ exist in the
monocationic form with the anion not coordinated but solvated, whereas in THF solution,
the N3– anion coordinates to the metal centers.
When a sample of 6 is stored in a solution of CDCl3 at RT, in the course of a week, some
small new signals in the diamagnetic region (Figure 32) can be observed by 1H NMR
spectroscopy. As these are not signals of the free ligand, they may hint at the formation of a
d6 cobalt(III) complex. Conversion is slow, however, so overall the chlorido complex
possesses a limited stability in chloroform. In the solvents MeCN and THF, all cobalt(II)
complexes presented herein have proven stable even for a long time.
10 6 is not soluble enough in THF to yield an 1H NMR spectrum in THF-d8.
86
8 7 6 5 4 3 2 1 0 -1 -2 -3 -4
55 50 45 40 35 30 25 20 15 10 5 0 -5Chemical Shift (ppm)
CDCl3 TMS
CDCl3 TMS
benzene
Figure 32.
1H NMR spectrum of 6 in CDCl3 after 5 d at RT. Peaks that have arisen during this time
are highlighted in the inset.
In addition to the bands observed for the manganese(II) complexes, the UV/Vis spectra of
the (BIMPNR,R’,R”)- cobalt complexes exhibit metal-centered spin-forbidden d-d transitions
of low intensity (Figure 33, see experimental part p. 203 for more detailed parameters).
87
Figure 33. Magnified section of the electronic absorption spectrum of 6[PF6], recorded in THF,
featuring several low-intensity absorption bands assigned to d-d transitions.
Analogous complexes of the ligand derivatives (BIMPNMes,tBu,tBu)– and (BIMPNXyl,tBu,tBu)-
have been prepared along the same synthetic pathways. They display the same colors,
similar solubilities and behavior, with the notable difference that they are also soluble in
aromatic hydrocarbons. The trend that less polar solvents are necessary for ligand
derivatives with the tBu-substituted phenol instead of the adamantyl-cresol has been
established for the iron chemistry. Judging by their 1H NMR spectra, the small counter ions
are coordinated in benzene solution as well.
Magnetization Measurements
Variable temperature SQUID magnetization measurements (2 – 300 K) confirm d7
high spin ground states (S = 3/2) for 6[PF6] and 7 (Figure 34). While the µeff value of the
manganese 3 complex is found near the spin-only value (µs.o. = 5.92 µB for S = 5/2) at room
temperature, the magnetic moments of 4.28 and 4.42 µB for 6[PF6] and 7, respectively, are
significantly larger than the spin-only value for an S = 3/2 system (µ s.o. = 3.87 µB),
suggesting a considerable contribution of spin-orbit coupling. Also, the curve of 6[PF6]
suggests a large zero-field splitting: The magnetic moments of 6[PF6] decreases slightly
88
with decreasing temperature until it starts dropping at approx. 70 K to 3.15 µB at 2 K.
Azide complex 7 remains temperature independent for a larger temperature range,
indicative of a smaller zero-field splitting, though still larger by a magnitude than for the
manganese(II) complex 3 (p. 70). Simulations confirm this trend for the zero-field splitting
parameter D that is already evident by visual inspection of the curve progressions, namely
that D(3) << D(7) < D(6[PF6]) (Table 5).
Figure 34. Temperature-dependent SQUID magnetization data (1 T) for complexes
[(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) and [(BIMPNMes,Ad,Me)Co(N3)] (7). Magnetic moment (µeff)
plotted versus temperature (T). Data were corrected for diamagnetism, and reproducibility was
verified by measuring multiple independently synthesized samples.
Table 5. Effective magnetic moments µeff at room temperature derived from the magnetization
measurements and the corresponding parameters used for simulation of the experimental data.
Complex µeff (RT)
[µB] g-value |D| [cm
-1]
[(BIMPNMes,Ad,Me)MnCl] (3) 5.82 1.970 0.426
[(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) 4.28 2.202 26.837
[(BIMPNMes,Ad,Me)Co(N3)] (7) 4.42 2.266 7.086
89
Molecular Structures of Cobalt(II) Complexes 6, 6[PF6], 6Xyl,tBu,tBu
[PF6], and 7
Single crystals of 6 were obtained from an acetonitrile solution. While the manganese
center in 3 contains an axially coordinated chlorido ligand and the axial position in the
corresponding iron complex 1 is occupied by an acetonitrile solvate molecule, the cobalt
center in 6 is coordinated solely by the chelating (BIMPNMes,Ad,Me)– ligand in a distorted
trigonal pyramidal fashion (Figure 35). Thus, the cobalt ion prefers binding to the nitrogen
anchor over the chloride counterion or an acetonitrile solvent molecule. Consequently,
going along the transition metal series from Mn to Co, the out-of-plane shift doop decreases
from 0.362(2) Å (3) to 0.113(1) Å (1) above the plane to –0.262(1) Ǻ (6) below the plane
towards the N-anchor. Accordingly, d(M–Nanchor) decreases from 2.695(2) Ǻ (no
interaction in 3), to 2.461(2) Ǻ (weak interaction in 1), to 2.141(2) Ǻ (cobalt bound to
Nanchor in 6).
Figure 35. Molecular structure of the complex [(BIMPNMes,Ad,Me)Co]Cl (6) in crystals of
[(BIMPNMes,Ad,Me)Co]Cl · MeCN (50 % probability ellipsoids). Co-crystallized solvents and
hydrogen atoms are omitted for clarity. Selected bond distances and angles are summarized in
Table 6.
90
Crystals of 6[PF6] and 6Xyl,tBu,tBu
[PF6] were obtained from THF and benzene solutions,
respectively. The molecular structures of these two equivalent complexes of different
ligand derivatives possess, within the errors margins, the same bond lengths around their
cobalt metal cores, and virtually the same bond angles (Table 6, see Figure 36 for
6Xyl,tBu,tBu
[PF6]). The structures of the complex cations of 6 and 6[PF6] are likewise very
similar, the only small difference being a slightly shorter Co–Nanchor bond (by 0.05 Å) in
6[PF6] and less negative doop. This results in a slightly less obstructed metal core in the
structure of 6, whereas the mesityl substituents in the structure of 6[PF6] are folded
together to almost completely obscure the metal core (see space filling representations in
Figure 37). However, these small differences in structures of the same complex cation may
likely result from crystal packing effects. In the structure of 6Xyl,tBu,tBu[PF6], the core has as
stated the same geometry as 6[PF6]. It is only due to the different substituents on the NHC
and phenolate rings that the cavity remains opener.
Figure 36. Molecular structure of the complex cation of [(BIMPNXyl,tBu,tBu)Co]PF6 (6
Xyl,tBu,tBu[PF6])
in crystals of [(BIMPNXyl,tBu,tBu)CoII]PF6 · 2.5 C6H6 (50 % probability ellipsoids). Co-crystallized
solvents, PF6 anion and hydrogen atoms are omitted for clarity. Selected bond distances and angles
are summarized in Table 6.
91
Figure 37. Space-filling representations of the cation [(BIMPNR,R’,R”)CoII]+ in molecular structures
of 6 (left), 6[PF6] (middle), and 6Xyl,tBu,tBu
[PF6] (right), in crystals of
[(BIMPNMes,Ad,Me)CoII]Cl · MeCN, [(BIMPNMes,Ad,Me)CoII]PF6 · 3 THF, and
[(BIMPNXyl,tBu,tBu)CoII]PF6 · 2.5 C6H6, respectively; side views (top) and top views (bottom) along
the Co–Nanchor–axes.
Table 6. Selected bond distances [Å], bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)CoII]Cl · MeCN (6 · MeCN), [(BIMPNMes,Ad,Me)CoII]PF6 · 3 THF (6[PF6]
· 3 THF), and [(BIMPNXyl,tBu,tBu)CoII]PF6 · 2.5 C6H6 (6Xyl,tBu,tBu[PF6] · 2.5 C6H6). See Chart 2 for
atom labeling.
Bond / Angle 6 · MeCN 6[PF6] · 3 THF 6
Xyl,tBu,tBu[PF6] ·
2.5 C6H6
M···Nanchor 2.140(2) 2.099(2) 2.108(2)
M–Ccarb. 2.020(2)
2.028(2)
2.009(3)
2.023(3)
2.023(2)
2.025(2)
M–O 1.874(2) 1.889(2) 1.878(2)
Ccarb.–M–C’carb. 117.41(8) 133.3(2) 131.94(7)
Ccarb.–M–O 112.72(7)
124.64(7)
108.4(1)
115.2(1)
112.45(6)
113.35(6)
NNHC1–Ccarb.–NNHC2 103.9(2)
103.2(2)
104.0(2)
103.4(2)
103.8(2)
103.6(2)
doop –0.262(1) –0.195(2) –0.170
92
In the azide complexes’ molecular structure (7, Figure 38), the cobalt is coordinated in
trigonal pyramidal fashion with the carbene and phenolate CCarb and O atoms of the
(BIMPNMes,Ad,Me)– ligand occupying the equatorial positions while the azide resides in the
axial position. As in the crystal structure of iron azide 2, the unit cell contains two
crystallographically independent, but structurally and metrically very similar, molecules,
which behave towards each other like helical enantiomers in that their ligand arms spiral up
from the N-anchor in different sense of rotation. Table 7 summarizes the values of one
representative only, parameters for each molecule are found in Tables 26 and 27. As in the
case of iron azide complex 2, the azide is coordinated in its preferred bent coordination
mode ((M–Nα–Nβ) = 128.2(2)°), so that it protrudes into the gap between one carbene and
the phenolate, widening the angle between them to (Ccarb.–M–O) = 134.03(7)°. Again, the
corresponding TIMENR complexes feature a linearly bound azide (in
[(TIMENXyl)Co(N3)]+: (M–Nα–Nβ) = 166.3(2)°,[70] in [(TIMENMes)Co(N3)]
+ (see
section 2.7.3): (M–Nα–Nβ) = 175.4(2)°), pointing out the reduction of the steric pressure by
the introduction of a phenolate ligand arm.
Figure 38. Molecular structure of the complex [(BIMPNMes,Ad,Me)Co(N3)] (7) in crystals of
[(BIMPNMes,Ad,Me)CoII(N3)] · 2 THF · 0.5 C6H6 (50 % probability ellipsoids). Co-crystallized
solvents and hydrogen atoms are omitted for clarity. Selected bond distances and angles are listed
in Table 7.
93
Table 7. Selected bond distances [Å], bond angles [°], and doop [Å] for [(BIMPNMes,Ad,Me)CoII(N3)] ·
2 THF · 0.5 C6H6 (7 · 2 THF · 0.5 C6H6) and, for comparison, TIMENR azide complex
[(TIMENXyl)Co(N3)]BPh4 · MeCN. See Chart 2 for atom labeling and Tables 26 and 27 for more
parameters.
Bond / Angle 7 · 2 THF
· 0.5 C6H6
[(TIMENXyl)Co(N3)]BPh4
· MeCN
M···Nanchor 2.659 3.213
M–Laxial 2.039(2) 1.938(2)
M–Ccarb. 2.081(2)
2.060(2)
2.052(2)
2.049(2)
2.017(2)
M–O 1.931(2) -
Nα–Nβ 1.188(3) 1.161(3)
Nβ–Nγ 1.164(3) 1.169(3)
Nanchor–M– Laxial 168.7 174.27
Ccarb.–M–Laxial 102.23(8)
105.10(8)
102.17(8)
101.85(8)
110.48(8)
O–M–Laxial 89.93(7) -
M–Nα–Nβ 128.2(2) 166.3(2)
Nα–Nβ–Nγ 177.1(2) 178.3(2)
Ccarb.–M–C’carb. 106.95(8) 118.86(8)
111.91(8)
110.52(8)
Ccarb.–M–O 134.03(7)
112.31(7)
-
NNHC1–Ccarb.–NNHC2 102.8(2)
103.1(2)
103.2(2)
103.6(2)
103.3(2)
doop 0.297(2) 0.520
94
2.6.2 Co(I) Complexes and their Reactivity
One goal in the course of this work was to reproduce the synthesis of terminal cobalt imido
complexes demonstrated by Hu et al. with the TIMENR ligand (R = Xyl, Mes,
Scheme 44).[70] Hu’s imido complexes did not undergo the desired transfer reaction of the
imido group onto organic substrates, but instead the imido group inserted into the Co–
carbene bond. By virtue of a more accessible cobalt imido bond through the introduction of
the phenolate, the (BIMPNR,R’,R’’)– system was expected to provide a platform for
aziridination reactions.
As entrance molecule to the imido chemistry, a Co(I) complex was to be synthesized.
When different (BIMPNR,R’,R’’)– derivatives were reacted with the lime green Co(I)
precursor CoCl(PPh3)3 in benzene or THF, brown solutions formed, and 1H NMR spectra
N
N
N
CN
C
N
CoI NC
N
R
RR
+
R' N3
N
N
N
CN
C
N
CoIII NC
N
R
RR
N
+
_ 35°C, _ N2
R'
R = H, Me
R' = Me, MeO
for all reactions:
solvent, RT
N
N
NC
N
C
N
CoI NC
N
R
R
N
R
R'
N
N
NC
NC
N
CoII NC
N
R
R
N
R
R'
+ "Co(0) species"
+ organic residue
fast
disproportionation
insertion
2+
Scheme 44. Synthesis of terminal cobalt(III)-imido complexes by Hu et al. and subsequent
insertion into the cobalt-carbene bond.[70]
95
confirmed the formation of paramagnetic species. Product purification, however, proved
troublesome, particularly the removal of the nascent triphenyl phosphane. While the
TIMENR cobalt(I) complexes [(TIMENR)CoI]+ synthesized by Hu (R = Xyl, Mes)[70] and
Kropp (R = Mes, iPr)[225] precipitate from benzene during synthesis and can easily be
filtered off and washed, the (BIMPNR,R’,R’’)– Co(I) complexes are soluble in ether, and, for
R’ = R’’ = tBu, even in pentane. The presence of the triphenyl phosphane seems to increase
the complexes’ solubilities.
Product contamination with phosphane is not only a hindrance towards isolation and
characterization of a clean Co(I) product, but also highly adversary to the synthesis of the
Co-imido complexes: Phosphanes react quickly with organic azides (Staudinger reaction),
and a corresponding organic byproduct was observed by 1H NMR spectroscopy. Only with
an excess of organic azide, and after conversion of the entire phosphane, could the
remaining azide attack the cobalt(I)-complex, and the difficulty of product separation
remained. Therefore, another route to the cobalt(I)-complexes was sought.
When green cobalt(II) complex [(BIMPNMes,Ad,Me)Co]X (X = Cl, PF6) is suspended in
benzene and stirred with potassium graphite (KC8), a brown slurry forms. Filtration and
lyophilization yields a brown powder that gives the same paramagnetic 1H NMR signals
that were observed in the reaction between (BIMPNMes,Ad,Me)– and CoCl(PPh3)3.
The complex is soluble in both polar and aromatic solvents, very soluble in ethers and
even, to some extent, soluble in aliphatic hydrocarbons. Elemental analyses so far have
consistently given low C,H,N-values for the cobalt-(I)-complex. This may be due to
residual KCl that stayed in the material even after several cycles of extraction with ether or
benzene and evaporation/ lyophilization, through interactions e.g. of the potassium cation
with the complex phenolates (vide supra). Nevertheless, temperature dependent SQUID
magnetization measurements confirm successful reduction: The reproducibly obtained
value of µeff(RT) = 3.1 µB (see Figure 39) is far lower than the values of the d7 S = 3/2
cobalt(II) complexes 6[PF6] and 7. It is still higher than the spin-only value for a d8 S = 1
system (µs.o. = 2.83 µB), implying contribution of spin-orbit coupling. The identity of the
product as [(BIMPNMes,Ad,Me)CoI] (8) is furthermore established by its crystal structure and
by its follow-up chemistry (vide infra).
96
Figure 39. Temperature-dependent SQUID magnetization data (1 T) for five independently
synthesized samples of [(BIMPNMes,Ad,Me)CoI] (8). Magnetic moment (µeff) plotted versus
temperature (T). Data were corrected for diamagnetism.
Complex 8 is highly reactive towards oxygen and other oxidants, such as iodosylbenzene
(PhIO), which re-oxidize monovalent 8 to the cobalt(II) oxidation state, as can be observed
both by the color change back to green and by 1H NMR spectroscopy.11 It is however fairly
stable towards elevated temperatures, as even prolonged refluxing in acetonitrile did not
change either color or 1H NMR spectrum of the compound.
Crystal Structure of [(BIMPNMes,Ad,Me)CoI] (8)
Suitable crystals for X-ray crystal structure analysis were grown by cooling a saturated
ethereal solution to –35 °C. The crystal structure contains three crystallographically
independent molecules in the asymmetric unit, all of which are structurally and metrically
very similar. Table 8 lists parameters for one of these molecules, parameters for all three
are given in Tables 26 and 27.
11 Reaction of 8 with PhIO gave single crystals and the crystal structure of [(BIMPNMes,Ad,Me)CoII]I (6[I]
· 0.5 MeCN · 0.5 THF). Due to its high similarity with the structures of 6 and 6[PF6], the molecular structure
of 6[I] is not further discussed.
97
The metal center is coordinated solely by the tripodal ligand, and indeed no other
molecules are found in the crystal lattice. Compared to the cobalt(II) complexes 6, 6[PF6],
and 6Xyl,tBu,tBu[PF6], the Co–O and Co–Nanchor bonds in 8 are longer on average by 0.07 Å
and 0.08 Å, respectively, as is expected from σ-bonding ligands when the metal center
becomes more electron rich. Contrarily, the two Co–carbene bonds are shorter by about
0.1 Å, which is readily explained by a strengthened metal-to-ligand π-backbond. This has
also been observed for FeII-FeI complexes[226] and NiI-Ni0 complexes[66] of the TIMENR
ligand. This is accompanied by NCHN–CCarb. bonds that are about 0.02 Å longer in 8 than in
the cobalt(II) complexes.
Figure 40. Molecular structure of the complex [(BIMPNMes,Ad,Me)CoI] (8) (50 % probability
ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond distances and angles are listed in
Table 8.
98
Table 8. Selected bond distances [Å], bond angles [°], and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)CoII] (8). See Chart 2 for atom labeling.
Bond / Distance 8 Angle 8
M···Nanchor 2.194(2) Nanchor–M– Laxial -
M–Ccarb. 1.923(2)
1.937(2)
Ccarb.–M–C’carb. 111.60(7)
M–O 1.949(2) Ccarb.–M–O 122.58(6)
123.78(6)
doop –0.160 NNHC1–Ccarb.–NNHC2 102.1(2)
101.7(2)
2.6.3 Reactions of [(BIMPNMes,Ad,Me)CoI] (8) with Organic Azides
The reactivity the cobalt(I) complex 8 towards several organic azides was tested. Blank test
were conducted to ensure that conversion of the azide can only be attributed to complex 8:
ArN3 (Ar = Mes, Ph) was added to CoCl2 in THF or benzene and monitored over time.
Within 1 week, no conversion was observed. The test was repeated in the presence of PF6–,
to the same result.
When trityl azide (Ph3CN3) was reacted with 8 in THF, the brown solution immediately
turned blue; in benzene, blue solid immediately precipitated. The product was identified as
the azide complex 7, and in the reaction liquid remained the trityl radical or Gomberg’s
dimer, respectively (see Scheme 46, p. 109). The trityl substituent may be too bulky for
formation of an imido species within the reactive cavity of 8, and consequently it releases
its azide group instead.
When red-brown 8 is reacted with less bulky aryl azides (RN3 with R = Ph, Mes) in either
THF or benzene, the solution turns dark green. 1H NMR reveals total conversion of 8 after
addition of 1 eq of the azide; additional azide remains unreacted in the solution, so no
follow-up reactions occur with the azide. After work-up, the reactions yield green solids in
91 % and 35 % yield for PhN3 and MesN3, respectively.
99
Reaction with PhN3
In the reaction with PhN3, effervescence is observed. The products’ signals are distributed
around the chemical shift region of diamagnetic compounds, albeit no coupling patterns
can be distinguished and some signals are situated in the negative region. Elemental
analysis of the isolated green solid is in agreement with the sum formula for an imido
complex “[(BIMPNMes,Ad,Me)Co=NPh]”. However, variable temperature SQUID
magnetization measurement of this compound reveals a RT magnetic moment µeff(RT) =
4.10 µB close to the values for the (BIMPNR,R’,R”)– cobalt(II) complexes (Figure 41),
whereas the cobalt-imido complex would be expected to possess a cobalt(III) center, like
the diamagnetic d6 cobalt(III) imido species of Hu and Meyer. In the insertion reaction
observed by Hu et.al., the initial insertion product would be cobalt(I), whereas all crystal
structures of the insertion products revealed cobalt(II) centers. Presumably, the cobalt(I)
species undergoes a disproportionation reaction to the cobalt(II) species and elemental
Co(0) (see Scheme 44).
Figure 41. Temperature-dependent SQUID magnetization data (1 T) of the green product from the
reaction between [(BIMPNMes,Ad,Me)CoI] (8) and PhN3. Magnetic moment (µeff) plotted versus
temperature (T). Data were corrected for diamagnetism.
Growth of single crystals of the green compound has not been achieved; however, a few
violet crystals suitable for X-ray crystallographic structure analysis were obtained by
100
diffusion of pentane into a benzene solution of the green compound. The molecular
structure derived from these violet crystals revealed a dinuclear cobalt-complex
[bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] (9*, Figure 42 and Table 9). The dinuclear
complex is situated on a crystallographic inversion center of the space group (P1̄ ) and
exhibits Ci molecular symmetry. Each cobalt center is coordinated in distorted tetrahedral
geometry by one NHC carbene atom and three oxygens, namely the phenolate and the two
bridging OH groups. The other carbene carbon atom has reacted with the phenyl azide to
form an imine. Most plausibly, this reaction proceeded in the same fashion as demonstrated
for the TIMENR species, i.e. imido complex formation followed by insertion of the imido-
group into the metal-carbene bond. The imino-NHC group thus formed has detached itself
from the metal center. The bond length suggest a fragment of the form Cphenyl–N=CCarbene,
i.e., with stronger double bond character between the nitrogen and the carbene carbon atom.
Figure 42. Molecular structure of the complex [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] (9*) in
crystals of [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] · C6H6 (50 % probability ellipsoids).
Co-crystallized solvent and hydrogen atoms except for those of the bridging OH groups are omitted
for clarity. Selected bond distances and angles are summarized in Table 9.
101
Table 9. Selected bond distances [Å], angles [°], and doop [Å] with e.s.d.’s in parentheses for
[bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] · C6H6 (9* · C6H6). See Figure 42 for atom labeling.12
Bond / Angle # Bond / Angle 9* · C6H6
M···Nanchor Co1···N1 2.510
Co1–O2
Co1–O2A
(= Co-µ-O)
1.995(2)
2.052(2)
M–Ccarb. Co1–C3
Co1–C8
2.066(2)
-
M–O Co1–O1 1.938(2)
N7–C8 12 1.297(3)
N7–C47Phenyl 1.399(3)
C8–N7–C47Phenyl 126.0(2)
O2–Co1–O2A 81.15(7)
Co1–O2–Co1A 98.85(7)
O1–Co1–O2
O1–Co1–O2A
116.89(7)
98.84(7)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
110.35(8)
-
C3–Co1–O2
C3–Co1–O2A
130.83(8)
104.89(8) #corresponding bond or angle in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand.
Assuming that the bridging OH-groups stem from water, the most likely source seems
moisture in the glovebox atmosphere. This would be in keeping with the extended time
necessary of crystallization for this compound13 – if the water were in the solvent, it would
be available at once, whereas atmospheric moisture may diffuse into the solution over time
– and also the very small quantity that was obtained of this violet compound. On the other
hand, it might be that the green compound reacts only slowly with the available moisture.
In any case, this dinuclear compound is, obviously, vastly more prone to crystallization
than the green compound.
12 No nitrogen atom in this structure was labeled N6. 13 The crystal structure was reproduced, and in both instances crystal growth took several months.
102
The release of the imino-NHC arm from the metal center is probably induced by the
coordination of the oxygens. It is not clear whether the insertion reaction into the Co–carbene
bond was also induced by formation of the new Co–O bond(s); however, the SQUID and
NMR data on the green compound suggest that the insertion reaction took place beforehand.
Reaction with MesN3
In the reaction of 8 with MesN3, no gas evolution is observed. Judging by the abundance of
signals, and also by their integrals compared to those of the Co(I) complex before the
reaction (versus solvent peak), more than one species is observed within minutes of azide
addition. Also, the spectrum of samples kept at RT changes within the course of the next
days. Therefore, no meaningful assignment of the spectra has been possible.
Green, block-shaped single crystals suitable for X-ray single crystal analysis were obtained
by layering a benzene solution with pentane and hexane. The crystals contained complex
[(BIMPNMes,Ad,Me*N3Mes)CoII] (10*, Figure 43), in which the cobalt center is coordinated
in distorted tetrahedral geometry by the nitrogen anchor, the phenolate oxygen, one NHC
carbene carbon atom, and an abnormal carbene: the second NHC ring has rotated around its
Figure 43. Molecular structure of 10* in crystals of [(BIMPNMes,Ad,Me*N3Mes)CoII] · 0.461 C6H6 ·
0.539 n-pentane (30 % probability ellipsoids). Co-crystallized solvent molecules and hydrogen
atoms are omitted for clarity. Selected bond distances and angles are summarized in Table 10.
C3
C47
103
own axis, and is now coordinated to the cobalt center with the atom CCarb2, the carbon atom
of the double bond which is nearer the nitrogen anchor. The mesityl azide unit is still intact
and its terminal nitrogen atom bound to the former (normal) carbene carbon atom. The zig-
zag shaped N3-unit’s bond lengths suggest the following mode of binding: C3Carb.=N6–
N7=N8–C47Phenol, i.e., alternating single and double bonds.
Table 10. Selected bond distances [Å], angles [°], and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me*N3Mes)CoII] · 0.461 C6H6 · 0.539 n-pentane (10* · 0.461 C6H6 · 0.539 n-C5H12).
See Figure 43 for atom labeling.
Bond / Angle # Bond / Angle 10* · 0.461 C6H6
· 0.539 n-C5H12
M···Nanchor Co1···N1 2.108(2)
M–Ccarb. Co1–C3
Co1–C8
-
1.985(2)
Co1–C4 1.972(2)
M–O Co1–O1 1.892(2)
C3–N6 1.363(3)
N6–N7 1.337(3)
N7–N8 1.279(3)
N8– C47Phenol 1.437(3)
C3–N6–N7 107.7(2)
N6–N7–N8 112.2(2)
N7–N8–C47Phenol 107.6(2)
Ccarb.–M–CCarb. C4–Co1–C8 117.36(8)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
-
118.18(7)
C4–Co–O1 118.06(7)
NNHC1–Ccarb.–NNHC2 N2–C3–N3
N4–C8–N5
106.1(2)
104.3(2)
doop doop
–0.287 ##
#corresponding bond or angle in complexes of (BIMPNMes,Ad,Me) with unaltered tripodal ligand. ## with C4 (=CCarb2) instead of C3 (=CCarb.) as third atom to define the plane.
104
The molecular structure of 10* explains why no effervescence is observed during this
reaction, since contrary to expectations, no dinitrogen was released from MesN3. The
reason for the very different behavior of 8 towards MesN3 vs. PhN3 may be found in the
azides’ different steric demand. In general, an organic azide may coordinate to a metal
center either linearly through its terminal nitrogen or in bent form through the substituted
nitrogen (Scheme 45). Nitrogen can only be released in the latter case, and the ortho-
methyl-groups of MesN3 may be too sterically demanding for this form of coordination
within the reactive cavity of [(BIMPNMes,Ad,Me)Co]. This explanation is in agreement with
the observation that neither azide employed by Hu et al.[70] for the synthesis of TIMENR
cobalt imido complexes – para-tolyl- and para-anisyl-azide[227] – bears ortho-substituents.
Co
N
N
N
Co
NN
N
Scheme 45. Linear (left) and bent (right) coordination modes of the aryl azide.
Outlook: Suggestions for Further Experiments
Insights gained from the two crystal structures of [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2]
(9*) and [(BIMPNMes,Ad,Me*N3Mes)CoII] (10*), combined with mechanistic considerations,
have revealed that only organic azides that are sterically less demanding than MesN3 may
form a Co=NR imido group within the reactive cavity of the ligand scaffold
(BIMPNMes,Ad,Me)–. Promising candidates for other aryl azides to be tested are those
employed by Hu[70, 227] and further aryl azides with no ortho-substituents. Interesting would
also be the reaction of 8 with alkyl azides such as AdN3 and iPrN3. Alternatively,
(BIMPNMes,Ad,Me)– derivatives with less bulky substituents, especially on the phenolate’s
ortho-position (R’), may be tested.
105
Addition of styrene to 8 followed by addition of MesN3, at RT or at low temperatures, has
been tested with the intention to transfer an imido group onto the organic substrate, before
it became known that this azide does not form an imido species with the (BIMPNMes,Ad,Me)–
complexes. This experiment may be repeated with PhN3, with either stoichiometric
amounts of styrene or another alkene, or in neat cyclohexane as both solvent and reactant.
106
2.7 Towards a Cobalt Nitride
2.7.1 Introduction
To the best of my knowledge, no mononuclear, terminal cobalt nitride has been
characterized and published. Isolated examples of group 9 nitrido complexes include
iridium[228-229] complexes that feature tridentate, meridional donor ligands, resulting in an
overall square planar geometry. In 2010, Chirik and coworkers reported the formation of
cobalt amide and imine complexes after photolysis of a square planar cobalt azides.[230]
This was rationalized by C–H activation of the ligand by a putative intermediate cobalt
nitrido complex, and the mechanism was elucidated by experiments with deuterated ligand.
However, the putative cobalt nitride could neither be isolated nor was any spectroscopic
evidence provided.
As the ligand field splitting (see Scheme 7, p. 11) suggests, and as the history of the iron
nitrido complexes has shown, it is plausible that the cobalt nitrido group may be well-
stabilized within a tripodal ligand field.
2.7.2 Photolysis of [(BIMPNR,R’,R”)Co(N3)] (7)
As stated in section 2.6.1, in solutions of polar solvents, divalent cobalt(II) complexes of
(BIMPNMes,Ad,Me)– exist in monocationic form. Therefore, it is not surprising that attempts
to photolyze (or thermolyze) the azide complex 7 in MeCN, chloroform, or DMSO lead to
no conversion, even upon prolonged irradiation: The solutions’ color and the 1H NMR
spectra of irradiated samples remain unchanged, since the azide is not coordinated to the
cobalt center. Complex 7 is insoluble in less polar solvents like benzene, therefore THF
remains as the only solvent for photolysis experiments. In the following, if not otherwise
noted, all photolysis experiments are carried out with a Heraeus 150 W mercury vapor
lamp.
107
Bulk and 1H NMR Experiments
Thermolysis of 7 is not feasible, since the complex is stable in THF even upon prolonged
heating, and naturally also in more polar solvents. When THF solutions of the sapphire
blue complex 7 are irradiated, they turn dark green (Figure 44). Upon irradiation of 1H NMR samples of 7 in THF-d8, the azide complexes’ signals vanish, while no distinctive
new signals emerge.
Figure 44. A much diluted solution of 7 (ca. 5 mg) in THF (ca. 8 mL), from left to right: before,
after 1 h, and after 2 h of irradiation.
When an NMR sample was irradiated at 309 nm (± 10 nm) with a LOT 1000 W Xe-OF arc
lamp, some small, paramagnetic peaks formed from 42 to –12 ppm. However, the relative
intensities (determined using the solvent signal height as internal standard for integration)
were about 1/10th of the starting complex at most, meaning that even if those signals can be
attributed to a well-defined photolysis product, the yield of this product would be very
small.
Neither from NMR experiments nor from several bulk photolyses at either RT or with
water or methanol cooling (ca. 10 °C and –10 °C, respectively) was it possible to isolate or
crystallize a new compound.
Trapping experiments
Since the direct product of irradiation remained elusive, several reagents were tested in
NMR and/or vial (20 mL) scale photolyses to trap a possible nitride intermediate. The
electronic structure of the nitride decides which sort of reagent is suitable for trapping
reactions: Closed-shell nitrides, which can be nucleophilic or electrophilic in nature, may
108
be captured by Lewis acids or bases respectively,[230-231] while radicals may be more
suitable for open-shell nitrides.[229] Furthermore, H-atom donors may stabilize a nitride in
the form of an imide or amide, or, if the nitride undergoes insertion into the metal-carbene
bond as was observed by Vogel,[226] may further the reaction towards this product.
Table 11 summarizes the results from photolysis experiments on the NMR scale and/or in
vials with a diverse range of trapping reagents, which are explored in depths in the
following.
Table 11. Trapping reagents used in photolysis experiments with 7.
1H NMR observations upon photolysis:
reagent#
immediate
reaction conversion
of reagent further comments
B(C6F5)3 yes - no further change
TMS-I yes - (immediate precipitation from THF)
nBu3P no no (see Figure 44)
Ph3P no no (see Figure 44)
(Ph3C)2 no no (Gomberg’s dimer, see Scheme 46)
styrene no no -
TEMPO no yes some small new peaks
tBuNC no yes some small new peaks
tBu3-phenol no yes well-defined paramagnetic product
# see text for abbreviations.
Photolysis in the presence of tris(n-butyl)- or triphenyl phosphane (nBu3P and Ph3P),
Gomberg’s dimer ((Ph3C)2, Scheme 46)[232], 14 or styrene lead to no conversion of the
respective reagent despite full conversion of 7. These reagents did not seem to have an
influence on the outcome of the photolysis, even though the presence of the phosphanes
leads to brownish colors of the photolyzed solutions (see Figure 48).
14 As shown in section 2.6.2, reaction of 8 with trityl-azide (Ph3C-N3) gives 7. When done in THF, this leads
to a solution containing 7 and the trityl radical / Gomberg’s dimer, ready for photolysis.
109
C2 CC
Scheme 46. Solution equilibrium of trityl radicals and their dimerization product (Gomberg’s
Dimer).
Figure 45. Solutions of 7 in THF after 4 h of irradiation without reagent (left) and with Ph3P
(middle: 1 eq, right: 16 eq).
When tris(pentafluorophenyl)borane (B(C6F5)3) is added to a THF solution of 7, the color
immediately changes to a brownish green, the 1H NMR spectrum becomes practically
identical to that of 6[PF6], and irradiation leads to no further change. Most likely, the
borane seizes the N3– molecule and forms an anionic Lewis acid / base adduct, leaving the
cobalt(II) complex cation behind (Scheme 47, top row). On addition of trimethylsilane-
iodide (TMS-I) to the THF solution of 7, a light green solid precipitates, which re-dissolves
in MeCN-d3 to give a spectrum akin to 6. Presumably, the TMS-I leads to ion exchange,
leaving the cobalt(II) complex with an iodide counter ion, [(BIMPNMes,Ad,Me)Co]I, which is
insoluble in THF just like its chloride congener (Scheme 47, bottom row).
110
N
NC
NC
N
CoIIO
N
NN
N
+
N
NC
NC
N
CoIIO
N
I
N
NC
NC
N
CoIIO
N
blue
(THF)
(THF)
B(B6F5)3
TMS - I
B(B6F5)3N3
+ TMS - N3
brownish green
light green precipitate Scheme 47. Reactions of 7 with B(C6F5)3 and TMS-I.
If 7 is irradiated in THF solution in the presence of tert-butyl-isocyanide (tBuNC) or
tetramethylpiperidinoxyl (TEMPO), the reagent is consumed while the azide diminishes.
With tBuNC, some new signals grow in the diamagnetic region, though their broadness and
lack of observable coupling declare that they belong to a paramagnetic species. Their
integrals are smaller compared to the original azide signals. The reaction towards the
compound(s) generating these signals seems either very unselective, or most of the
product’s signals are not observable. Some new signals mainly in the diamagnetic region
emerge also in the presence of TEMPO, again with comparably small integrals, but less
broadened.
2,4,6-Tris-tert-butylphenol (tBu3-phenol) can serve as H-atom donor, since the
corresponding phenoxyl radial is fairly stable.[233-234] Upon irradiation of 7 in the presence
of tBu3-phenol, the reagent’s signals are without exception shifted slightly up-field and
broadened, indicating the generation of the phenoxyl radical. Undoubtedly, a new
paramagnetic species emerges (Figure 46). Its signals grow in the range from 53 to
111
-10 ppm and their integrals are comparable to those of the starting material, i.e. the
reaction towards this compound appears to be fairly selective, promising high isolable
yields. In addition, it is noteworthy that the irradiated solutions become emerald green,
whereas solutions that lack the trapping reagent often turn a brownish or grayish hue of
green. The spectrum of the tBu3-phenol “trapping product” is tentatively evaluated and
integrated in the experimental section (p. 208).
Several vial- and larger-scale photolyses were run in an effort to isolate this compound.
The work-up has been optimized by Eva Zolnhofer, who has further characterized the
product.
45 40 10 5 0 -5 -1035 30 25 15Chemical Shift (ppm)
20
Figure 46. 1H NMR spectra of 7 in THF-d8, before (blue), after 2h (orange), and after 6h (green) of
irradiation with a mercury vapor lamp.
Crystal Structure of the Insertion Product 11*
Single crystals of the photolysis product gained in the presence of the phenol were obtained
as green needles from a THF solution. X-ray single crystal structure analysis revealed that
the crystals contained indeed an insertion product of the formula
[(BIMPNMes,Ad,Me*NH)CoII]Cl (11*): An imino-complex, wherein a hydrogen bearing
nitrogen is bound to one carbene carbon and the cobalt center. The crystal was composed
of two different species with a majority of 93.5(2) % of the NH insertion product and
6.5(2) % of the minor component [(BIMPNMes,Ad,Me)Co]Cl (6), and a total of one molecule
of THF and 0.16 molecules of H2O per formula unit. The source for both 6 and H2O is
unclear. It may be that the water molecules got into the crystallization setup through not
perfectly dried solvent or through moisture in the glovebox’s atmosphere. The chloride
complex 6 may either have been in the 7 azide batch used in the photolysis – although it
was not observed in the 1H NMR spectrum – or may have formed through residual
112
inorganic chloride salt contained in the azide batch. Either way, some surplus chloride must
have been present in the crystallization setup as the cationic insertion product is also paired
off with a chloride. The hydrogen atom bound to the inserted nitrogen presumably stems
from the phenol, although the crystals surprisingly contained H2O as another possible
proton source.
Figure 47 depicts the molecular structure of the insertion product and a schematic rendering
of its core structure; selected bond lengths and angles are summarized in Table 12; a more
complete list can be found in Tables 32 and 33 (pp. 234 and 235).
The cobalt(II) metal center is coordinated in distorted tetrahedral geometry by four different
binding groups: carbene, phenolate, amine (Nanchor), and imine. The bond lengths between the
first three groups and the cobalt center are comparable to those in 6[PF6]. The imine nitrogen
is bound to the cobalt more closely than the nitrogen anchor (1.979(2) vs. 2.098(2)), and the
nitrogen-carbene bond length (d(C3-N6) = 1.323(3)) suggest a bond order between 1 and 2.
Figure 47. Molecular structure of the insertion product [(BIMPNMes,Ad,Me*NH)Co]Cl (11*) in
crystals of 0.935 [(BIMPNMes,Ad,Me*NH)Co]Cl · 0.065 [(BIMPNMes,Ad,Me)Co]Cl · THF · 0.16 H2O
(50 % probability ellipsoids); inset: schematic representation of the complexes’ core structure.
Co-crystallized solvents, chloride complex 6, and hydrogen atoms except for the imine hydrogen
are omitted for clarity. Selected bond distances and angles are summarized in Table 12.
113
Table 12. Selected bond distances [Å] and angles [°] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me*NH)Co]Cl · THF · 0.16 H2O (with the actual compound distribution in the
crystal: 0.935 [(BIMPNMes,Ad,Me*NH)Co]Cl · 0.065 [(BIMPNMes,Ad,Me)Co]Cl · THF · 0.16 H2O;
denoted as 11* · THF · 0.16 H2O). See Figure 47 for atom labeling.
Bond / Angle# Bond / Angle 11*
· THF · 0.16 H2O
M···Nanchor Co1···N1 2.098(2)
M–Ccarb. Co1–C3
Co1–C8
-
1.989(3)
Co1–N6 1.979(2)
M–O Co1–O1 1.887(2)
N6–C3 1.323(3)
C3–N6–Co1 130.4(2)
Ccarb.–M–C’carb. N6–Co1–C8 109.9(2)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
-
118.5(1)
N6–Co–O1 118.91(9)
NNHC1–Ccarb.–
NNHC2
N2–C3–N3
N4–C8–N5
106.5(2)
103.7(2)
doop doop
## –0.404
#corresponding bond or angle in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand. ## with N6 (= inserted nitrogen) instead of C3 (= CCarb.) as third atom to define the plane.
IR Experiments
KBr pellets of the azide complex 7 were irradiated and the changes followed over time by
IR spectroscopy. The majority of the IR features of the cobalt(II) complex is conserved,
with the following exceptions: The azide stretch diminishes, and a new, broad band grows
with its absorption maximum at 1605 cm-1, as well as one at 3337 cm–1 (Figure 48). These
wavenumbers are assignable to an imine bond (C=N) and an N–H stretching frequency,
respectively, so the spectra surely show the gradual conversion of 7 to 11*.
114
3000 2000 1000
0 h
1 h
3 h
5 h
18 h
Tra
nsm
itta
nce
/ %
Wavenumber / cm-1
Figure 48. Photolysis of 7 in a KBr pellet, followed by IR spectroscopy.15
A 15N labeled batch of 7 was prepared with Na15N3, wherein one terminal nitrogen in each
azide molecule is of the heavier isotope. Consequently, in statistically 50 % of the azide
complex, the 15N atom is coordinated directly to the cobalt, which, upon release of
dinitrogen, would result in a 50 % labeled “cobalt-nitrido” or follow-up product. In the IR
spectrum of the labeled compound 7, the azide stretches are shifted slightly towards smaller
wavenumbers (in KBr: 2070, 2035, and 1989 cm–1 vs. 2081, 2044 and 1999 cm–1,
respectively).16 When a KBr pellet of 15N labeled 7 is irradiated, the newly arising bands
are shifted almost negligibly, to 1603 cm–1 (transmission minimum) and 3332 cm–1 (see
spectra p. 209).
This is in line with the assignments of the bands to the cobalt-imino species 11*: The
reduced mass calculation for a harmonic oscillator would predict a higher shift for a
terminal Co-nitride (86 cm–1), for example, but the cobalt-bound imido stretch would shift
15 No smoothing or baseline correction was performed during spectra processing to allow an undistorted
evaluation of the bands’ relative transmittances over time. 16 Instead of two distinctive sets of bands, for the two isotopomers in the batch of 7, all three bands are shifted
entirely since the symmetric and asymmetric stretches involve all three azide nitrogen atoms, not just one
terminal atom of the azide.
115
less since the N is bound between the cobalt and carbene, and therefore the reduced mass
effect is much smaller.
EPR Measurements
While the isolation and characterization of a secondary product following the photolysis
provides strong evidence for the generation of an intermediate cobalt nitride, spectroscopic
confirmation of its generation is desirable to complement the evidence. The direct
photolysis product of a d7 Co(II) azide is expected to be a Co(IV) nitride, a d5 electron
system with presumably an S = ½ ground state. On that premise, EPR spectroscopy can be
a powerful tool to characterize this compound. In addition, 59Co, the only natural isotope,
has a nuclear spin of 7/2, lending a distinctive hyperfine coupling pattern of 8 lines to most
cobalt EPR spectra. Finally, if superhyperfine coupling to one N nucleus could be
observed, this would help to finally identify the species as the nitride.
The following X-band EPR measurements were, if not otherwise mentioned, carried out in
a helium-cooled EPR cavity at temperatures between 8 and 15 K with a LOT 150 W
Xe-OF arc lamp.
The EPR spectrum of the azide complex 7 is of nearly axial symmetry, with effective
g-values around 4.6, 4, and 1.8 – typical of a d7, S = 3/2 system, although no cobalt-
coupling was observed. The sample was irradiated in situ, that is, within the EPR cavity
and under constant cooling by liquid helium. Over time a pronounced, fairly symmetrical
signal with distinct hyperfine coupling evolved around g = 2, indicating a new species with
an S = ½ spin state (Figure 49). This is accompanied by a color change of the irradiated
part of the sample from blue to green (Figure 50).
116
Figure 49. EPR spectra of a sample of 7 (ca. 1.5 mM in frozen methyl-THF) before (black line) and
after (blue line) irradiation with a Xe arc lamp at 10 K. On the right hand side, the progression over
time (in minutes) is shown.
Figure 50. EPR sample of 7 in methyl-THF (ca. 1.5 mM), still frozen, after photolysis at 10 K
within the EPR cavity. The photolyzed part in the middle has turned green, the part that is still blue
was not within the cavity’s quartz glass window and consequently shielded from the lamps’
radiation.
117
The experiment could be reproduced on several different batches of 7 (Figure 51). As soon
as a sample is de-frosted, even for a few seconds, the new signal disappears, leaving behind
only the rhombic signal of (one or more) S = 3/2 species.
Figure 51. EPR spectra of photolyzed samples of 7 (ca. 1.5 mM in methyl-THF) from different
batches of regular 7 (black and blue lines) and one batch of 15N labeled azide (orange line).
Furthermore, it was attempted to reduce the experimental effort and expenditures by
running the photolysis with nitrogen cooling only, keeping the sample at temperatures
around 80 to 88 K, either within the EPR cavity or by photolyzing outside thereof in a
nitrogen-cooled Quartz Dewar (see photo in Figure 107 p. 210). In either case, the spectra
obtained differed significantly from the original one. Not only can the cobalt(II) species not
be observed at 80 K and not only is the signal-to-noise ratio smaller, both of which was to
be expected, but photolysis at 80 K apparently results in more than one species with
g-values around 2 (Figure 52).
118
Figure 52. Samples of 7 (ca. 1.5 mM) in frozen methyl-THF irradiated and measured at 10 K (blue
line) and at 80 K (red and brown lines).
Returning to the 10 K spectra, the main, enveloping eight-lines coupling pattern clearly
demonstrates that the unpaired electron spin must be coupling with one cobalt nucleus. The
smaller features in the center of the spectrum may arise from different sources: A) nitrogen
coupling, which would pinpoint the generation of a Co–N species, B) anisotropic cobalt-
coupling with large and small coupling constants, or C) an underlying second S = ½
species. An aspect in disfavor of option C) is the accurate reproducibility of the experiment
demonstrated in Figure 51: the different species would have to be generated in precisely the
same ratio every time, for different samples and irradiation times.
Simulation of the S = ½ spectrum has proven intricate. One of the best fits obtained so far
(Figure 53) uses cobalt coupling only, speaking for option B) (no N-coupling observable).
However, even in this fairly good fit, not all of the finer features of the spectrum are
reproduced exactly.
119
Figure 53. EPR spectrum (black) and simulation (red) with parameters.
To further examine whether the smaller hyperfine coupling patterns arises from nitrogen
coupling, photolysis experiments were carried out with 15N labeled azide complex.
However, as the labeled azide bears only one 15N isotope at one end of the N3– ligand, and
coordination of the cobalt to the azide would be random to either end, only 50 % of a
generated Co-nitride would be 15N labeled. Furthermore, the nuclear spins of the two
nitrogen isotopes (1 and ½) would cause 3 + 2 lines that would densely overlay each other.
So, all in all, only small changes in the spectrum are expected to begin with.
The spectrum obtained with the 15N labeled sample remained nearly identical to the ones
from non-labeled batches (see Figure 51). Therefore, option B) (only cobalt alone
responsible for the spectral features) seems more likely to be true than option A) (nitrogen
coupling).
Computational Study
To better understand the spectroscopic results and possibly improve the simulated fit of the
EPR spectrum, DFT calculations were performed by Dr. Marat Khusniyarov. The crystal
structure of 7 served as a basis for the calculation, for which an N2 unit was removed from
120
the azide and an S = ½ ground state was assumed. Table 13 summarizes the calculated
structural parameters of the putative nitride complex “[(BIMPNMes,Ad,Me)CoIV(N)]”,
Table 14 lists the calculated EPR parameters and those from the fit in Figure 53. The spin
density map (Figure 54) obtained from these calculations reveals that a large portion of the
unpaired spin is localized on the nitrogen atom. This puts into question the assignment of
the species as a “Co(IV) nitride”, vs. a “Co(III) nitridyl radical”, in analogy to Schneider’s
“iridium(III) nitridyl” complex.[229]
Table 13. Principal bond distances [Å] and an angle [º] of the optimized structure of
“[(BIMPNMes,Ad,Me)CoIV(N)]” obtained from the spin-unrestricted BP-DFT calculations.
Bond / Angle “[(BIMPN
Mes,Ad,Me)Co
IV(N)]”
S = 1/2
Co···Nanchor 3.393
Co–N 1.586
Co–CCarb. 1.913
1.906
Co–O 1.996
N–Co–Nanchor 170.2
121
Table 14. Calculated EPR parameters for “[(BIMPNMes,Ad,Me)CoIV(N)]” (S = 1/2)# and, for
comparison, those of the simulated spectrum in Figure 53; ACo: hyperfine coupling with a 57Co
nucleus; AN: hyperfine coupling with a 14N nitride nucleus. Hyperfine coupling constants are given
in units of MHz.
“[(BIMPNMes,Ad,Me
)CoIV
(N)]”
S = 1/2
simulation parameters
(Figure 53)
g1 1.911 1.992
g2 1.990 2.085
g3 2.008 2.038
giso 1.970
ACo1 –109 80
ACo2 +160 80
ACo3 +545 272
ACo
iso +199
AN1 –18 -
AN2 +28 -
AN3 +48 -
AN
iso +19
# The parameters have been obtained from the spin-unrestricted ZORA-B3LYP-DFT calculations.
Figure 54. Spin density map for “[(BIMPNMes,Ad,Me)CoIV(N)]” (S = 1/2) obtained from the spin-
unrestricted B3LYP-DFT calculations; side view (left) and top view (right) of the complex along
the N–Co–Nanchor axis.
122
As can be easily recognized, the calculated coupling parameters are too large to fit the
experimental spectrum: The cobalt coupling of 545 MHz alone would render the simulated
spectrum too broad. Nevertheless, the result that the nitride coupling is one order of
magnitude smaller than the cobalt one would be in agreement with the assumption that the
nitride coupling is not resolved in the experimental spectrum.
ENDOR Spectroscopy
Since it appears that the nitrogen coupling of the putative CoIV nitrido (or CoIII nitridyl)
species cannot be observed in the EPR experiment, it was tried to determine the nitrogen
coupling constants AN by ENDOR spectroscopy (electron-nuclear double resonance). This
analytical method combines the sensitivity of EPR with the high resolution of NMR
spectroscopy.
A sample of 7 was irradiated in frozen methyl-THF matrix with a Xe flash lamp. During
irradiation, the sample temperature was kept below 10 K. It was then cooled to 5 K and the
spectrum in Figure 55 was taken. The spectrum shows coupling of the unpaired electron to
hydrogen (which may be ligand or solvent hydrogens) and the smallest Co coupling. In
between these two, four lines assignable to N coupling emerge. They are not yet distinctive
enough to confidently evaluate them, but this experiment lays a sound basis for future
experiments.
Still more time needs to be invested to optimize the conditions of photolysis for the
ENDOR measurement. The difficulty is also to find a suitable ENDOR instrument with
both a window in the cavity for light and the potential for liquid helium cooling. Probably,
light source and/or wavelength need to be optimized as well. When the sample was further
irradiated with the Xe flash lamp after the ENDOR spectrum had been taken, the
concentration of the observed “nitrido species” seemed to diminish again.
123
Figure 55. ENDOR spectrum taken at 5 K of a sample of 7 that was irradiated by a xenon flash
lamp.
2.7.3 Photolysis of [(TIMENMes)Co(N3)]+
In the course of his work on TIMENR cobalt imido complexes,[70, 190] Xile Hu treated
[(TIMENXyl)CoI]Cl with several organic azides. When the Co(I) complex was reacted with
TMS-azide in the hope of creating a TMS-imido complex, the one-electron oxidized azido
complex [(TIMENXyl)Co(N3)]Cl was obtained instead.[70] While this complex was
characterized with 1H NMR and IR spectroscopy, elemental and X-ray single crystal
analysis (in crystals of [(TIMENXyl)Co(N3)]BPh4 · MeCN, see Table 7. p. 93), no mention
is made in any of Hu’s works about photolysis experiments on the azide complex, nor any
other means or ways of creating a cobalt nitride.[190] Therefore, after the (BIMPNR,R’,R”)–
azide complexes showed promising potential in this area, photolysis of the TIMENR cobalt
azide complexes and their possible transformation to nitrides was studied.
Synthesis and Characterization of [(TIMENMes
)Co(N3)]+
While Hu’s synthesis of [(TIMENXyl)Co(N3)]Cl involved the sensitive Co(I) complex and
TMS-N3, a more convenient pathway was found: As for the (BIMPNR,R’,R”)– congener, the
azide can synthesized from the well-known Co(II) chlorido complex. And while the
H
Co
124
TIMENR Co(II) chlorido complexes (R = Xylyl, Mesityl) have hitherto been synthesized
by oxidizing the corresponding Co(I) complex, e.g. by benzyl chloride or CH2Cl2,[69, 225] it
has now been found that it can be obtained directly by reacting the free TIMENR ligand
with CoCl2. Also, a direct 1-pot synthesis to the azido complex starting directly with the
free TIMENMes ligand, CoCl2 and sodium azide proved equally successful (Scheme 48).
Through these synthetic pathways, the blue azide complex was synthesized with two
different counter anions, as [(TIMENMes)Co(N3)]OTs (12[OTs]) and
[(TIMENMes)Co(N3)]PF6 (12[PF6]).
NN
NMes
3
CoCl2
ex. NaN3
- NaCl
N
N
NC
NC
N
CoII NC
N
Cl
in all reactions:
X = OTs or PF6
N
N
NC
NC
N
CoII NC
N
N
N
N
1 eq NaX (THF)
(THF)
CoCl2, ex. NaN3, 1 eq NaX
(THF)
1-pot route
Cl
X
Scheme 48. Synthesis of [(TIMENMes)Co(N3)]
+ complexes (12+) in a two-step route via the
dichloride complex or a 1-pot synthesis directly from the free ligand.
125
The 1H NMR spectrum of 12[X] (X = OTs, PF6) in MeCN-d3 features 9 paramagnetically
shifted and broadened signals from 76 to –5 ppm (Figure 56), with a tenth signal of an
intensity of three protons either broadened into the baseline or lying underneath the residual
solvent signal.17 This is in agreement with C3 symmetry of the complex in solution.18
Around 3000 cm–1 and in the fingerprint region, the IR spectrum displays the same features
as other complexes with the TIMENMes ligand framework, and in addition the characteristic
νas(N3) IR vibrational band (in KBr: 2092 cm–1; for comparison, the same band of
[(TIMENMes)Fe(N3)]+: 2094 cm–1).
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
Chemical Shift (ppm)
-4. 1
1. 1
1.9
2.3
2. 6
5. 6
7. 1
7.6
7.6
8. 9
9. 6
32
. 333
.6
75
.7
NN
NC
NC
N
CoII NC
N
N
N
NOTs
3H
2H2H
3H 3H 3H3H
3H 3H
3H
18H
9H
MeCN-d3
Figure 56. 1H NMR spectrum of 12[OTs] in MeCN-d3.
17 For the iron congener [(TIMENMes)Fe(N3)]
+, nine such signals are observed, with two signals of (3+3)
protons intensity probably broadened into the baseline. 18 One signal of 18 H intensity is very broad and probably stems from the merging signals of all ortho-
methyl-groups of the mesityl substituents.
126
Crystal Structure of [(TIMENMes
)Co(N3)]OTs (12[OTs])
Suitable crystals of 12[OTs] for X-ray single crystal analysis were recovered as blue blocks
by cooling an acetonitrile solution to -35 °C. The molecular structure is shown in
Figure 57. The azide complex cations’ structure is very similar to that described by Hu,[70]
with one small but notable difference: The azide in [(TIMENMes)Co(N3)]+ (12
+) is
coordinated more linearly ((Co–Nα–Nβ) = 175.4(2)° in 12+ vs. 166.3(2)° in
[(TIMENXyl)Co(N3)]+), accompanied by a longer Nα–Nβ bond and a shorter Nβ-Nγ bond
(each by 0.1 Å), which again demonstrates the connection between the (linear)
coordination mode of the azide and its activation towards dinitrogen loss. This difference is
naturally more pronounced when compared to the azide complex 7, in which the Co–Nα
difference is also longer by about 0.1 Å.
Figure 57. Molecular structure of the complex [(TIMENMes)Co(N3)]OTs (12[OTs]) in crystals of
[(TIMENMes)Co(N3)]OTs · 3 MeCN (50 % probability ellipsoids). Co-crystallized solvents and
hydrogen atoms are omitted for clarity. Selected bond distances and angles are summarized in
Table 15.
127
Table 15. Selected bond distances [Å], bond angles [°], and doop [Å] with e.s.d.’s in parentheses for
[(TIMENMes)Co(N3)]OTs · 3 MeCN (12[OTs] · 3 MeCN), and, for comparison,
[(TIMENXyl)Co(N3)]BPh4 · MeCN and [(BIMPNMes,Ad,Me)Co(N3)] · 2 THF · 0.5 C6H6 (7 · 2 THF
· 0.5 C6H6). See Chart 2 for atom labeling.
Bond / Angle 12[OTs] · 3 MeCN [(TIMENXyl)Co(N3)]BPh4
· MeCN
7 · 2 THF
· 0.5 C6H6
M···Nanchor 3.234 3.213 2.659
M–Laxial = M–Nα 1.939(2) 1.938(2) 2.039(2)
M–Ccarb. 2.053(2)
2.058(2)
2.062(2)
2.052(2)
2.049(2)
2.017(2)
2.081(2)
2.060(2)
M–O - - 1.931(2)
Nα–Nβ 1.171(3) 1.161(3) 1.188(3)
Nβ–Nγ 1.157(3) 1.169(3) 1.164(3)
Nanchor–M–Laxial
= Nanchor–M–Nα
178.5 174.27 168.7
Ccarb.–M–Laxial
= Ccarb.–M–Nα
103.44(9)
104.83(9)
106.02(9)
102.17(8)
101.85(8)
110.48(8)
102.23(8)
105.10(8)
O–M–Laxial - - 89.93(7)
M–Nα–Nβ 175.4(2) 166.3(2) 128.2(2)
Nα–Nβ–Nγ 179.5(3) 178.3(2) 177.1(2)
Ccarb.–M–C’carb. 113.62(9)
110.29(8)
117.30(9)
118.86(8)
111.91(8)
110.52(8)
106.95(8)
Ccarb.–M–O - - 134.03(7)
112.31(7)
doop 0.524 0.520 0.297(2)
128
Photolysis of [(TIMENMes
)Co(N3)]+ (12
+)
Complex 12[X] (X = OTs, PF6) is soluble in polar solvents like DMSO and MeCN, but not
in THF and less polar solvents. All photolysis experiments were therefore carried out in
MeCN.19 Upon irradiation, the color of the solutions changes from blue to a dark, brownish
green (Figure 58).
Figure 58. A solution of 12[OTs] in MeCN before (left), after 2 h (middle), and after 20 h (right)
of irradiation.
When NMR samples of 12[X] (X = OTs, PF6) in MeCN-d3 are irradiated, the solutions’
color changes to a more brownish-lilac hue, though the difference in color may stem from
either different concentrations or from residual unreacted azide complex in the case of
photolysis in the vial. In the NMR experiment, full conversion is easily verified by a
proton NMR spectrum. Over time, the 1H NMR signals of 12+ diminish and disappear
completely, and new ones arise (Figure 59). When the product of vial-scale photolysis is
extracted, its 1H NMR spectrum features the same signals, another argument for the
premise that the different hues of the solutions stems merely from differences in
concentration of minor impurities.
19 As established for the TIMENR Fe and Mn azides, and contrary to the (BIMPNR,R’,R’’)– congener 7, the
azide anion of 12+ is coordinated in MeCN solution, as proven by the crystal structure of 12[OTs].
129
80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
Chemical Shift (ppm)
12 11 10 9 8 7 6 5 4 3 2 1 0 -1
OTs
3H
3H
1H 1H 1H 1H 1H1H 1H
1H
1H
1H 1H1H
3H
1H1H1H 3H
1H
2H
3H
2H
1H
1H1H
4H
THF THF
MeCN-d33H
2H
2H
2H 3H
Figure 59. 1H NMR spectrum of a fully photolyzed sample of 12[OTs] in MeCN-d3.
Integral values could be assigned by comparison with the intensities of the –OTs anion’s
signals, which remain the same throughout the photolysis (with the residual solvent signal
serving as stable reference for intensity and chemical shift).
The great increase in signal number going from 12+ (9 signals for the monocation, one is
hidden) to the photolysis product (29 signals, more might be hidden)20 clearly points at a
reduction in symmetry. Possibly the azide loses dinitrogen, and the emerging nitride
complex undergoes an insertion reaction of the nitride atom into one cobalt–carbene bond,
resulting in a complex similar to 11*.
Contrary to the photolysis of 7, addition of 2,4,6-tris-tert-butyl-phenol to the samples prior
to irradiation did not alter the outcome of the experiment: The product signals remained the
same, the only difference seems to be that conversion is slower, i.e. longer irradiation times
20 Plus three signals for the tosylate anion when 12[OTs] is photolyzed.
130
are needed for full conversion, which may be attributed to a loss of light intensity due to
absorption by the phenol. If the photolysis product truly is an insertion product, it is either
an imide (without hydrogen atom on the nitrogen), or the TIMENR complex is capable of
scavenging hydrogens from the solvent. Alternatively, it may be that the ligand is activated
at another site. What is certain, though, is that the observed reaction is markedly more
selective than for 7.
EPR Experiment
A 2 mM solution of 12[PF6] in frozen MeCN was irradiated at 12 K inside the EPR cavity
with a LOT 150 W Xe-OF arc lamp. The starting material displays a nearly axial-
symmetric spectrum typical of a Co(II) compound, albeit without distinctive cobalt
hyperfine coupling (Figure 60). During irradiation, only minor changes to the spectrum
were observed. Possibly, a small part of those spectral changes may or may not be
attributable to temperature fluctuations, although the temperature never rose above 15 K.
The expected “nitride” signal, analogous to the one observed for 7, did not appear. The
color of the frozen sample changes in the irradiated region from blue to a lighter, brownish-
violet hue.
Figure 60. EPR spectrum of 12[PF6] in frozen MeCN at 12 K.
131
Apparently, judging by EPR, the reaction goes from one cobalt(II) species to another
cobalt(II) compound, that is, no S = ½ intermediate is observable within the time scale of
the EPR measurement.
The second cobalt(II) species observed in the EPR experiment may be the paramagnetic
photolysis product of 12+ that has been observed reproducibly by 1H NMR. Whether it is
an insertion product – even though it would be astonishing if this happened within minutes
even at temperatures around 12 K – or a wholly different species remains, at this point,
speculation. Its intriguing identity remains to be elucidated.
2.7.4 Outlook and Suggestions for Further Experiments
The low-temperature EPR spectrum gained upon irradiation of azido complex
[(BIMPNMes,Ad,Me)Co(N3)] (7), together with the isolation and characterization of the
insertion product 11*, provide strongest evidence of an intermediate nitride complex.
The yield of secondary product, be it 11* or another, may be optimized by altering
photolysis conditions, namely temperature, concentration, and irradiation wavelength(s).
The irradiation time may also be important, as the experience with the ENDOR experiment
has shown, where the concentration of the “nitrido” species seemed to diminish again with
extensive irradiation. This issue might also be brought under control by choosing the
optimal light source and/or irradiation wavelength.
The molecular structures of 11* showed that the complex cation was paired with a chloride
anion, the source of which has not been determined yet. Hence, deliberate supply of
sufficient chloride may also increase the compounds’ yield. For this, however, a chloride
source that is well-soluble in THF would be advantageous. Common chloride salts such as
alkali salts or PNPCl are therefore not purposeful; possibly, an organic ammonium chloride
such as n-Bu4NCl can be used. Alternatively, another anion may contribute to product
isolation, provided it does not absorb too strongly in the crucial area of the electromagnetic
spectrum and it does not cause salt metathesis in 7, i.e. does not eliminate the azide from
the complex. Also, a higher quality crystal for single crystal analysis may be obtained.
It may also be worthwhile to examine the tBu3-phenol’s contribution to the reaction. While
it has been assumed that the phenol provides the hydrogen atom to the imino group, this is
132
still open to discussion. Although by 1H NMR, the phenol does greatly influence the
reactions’ outcome, the phenol may stabilize an intermediate in other ways. Other possible
H-atom sources include the solvent or the ligand. Deuteration experiments may lead to new
insights. Chirik and coworkers proved through deuteration experiments that, in the imine
complex which they obtained upon photolysis of their square planar cobalt azide, the
nitrogen’s proton stems from the ligand.[230]
Furthermore, the essays with other trapping reagents, particularly tert-butyl-isocyanide
(tBuNC), hint that other trapping products may be at hand. The isolobal relationship
between the isocyanide and carbon monoxide suggests that photolysis under a carbon
monoxide (instead of dinitrogen) atmosphere might lead to yet another isolable follow-up
product.
The behavior of other (BIMPNR,R’,R’’)– cobalt azide complexes, 7Xyl,tBu,tBu or 7
Mes,tBu,tBu,
may also be studied. On the one hand, the experience that crystals are far more difficult to
obtain with the tBu-substituted ligand derivatives is somewhat deterring. On the other
hand, these azide complexes may be soluble in aromatic solvents, like their chloride and
PF6 counterparts, and therefore be photolyzed in benzene solution. This could shed a light
on the question whether the solvent influences the reaction – e.g. through THF radicals.
Last but not least, in-depths computational studies may provide crucial additional insights
into the reaction pathways. Comparative computational studies with the TIMENR analogue
12+ may also elucidate whether this compound undergoes the same process, i.e. also forms
an imino insertion product. Comparison of reaction barriers may answer the question
whether the paramagnetic photolysis product of 12+ observed by 1H NMR and EPR can
indeed be a species analogous to 11*.
It is highly doubtable that a cobalt nitrido species can ever be isolated itself, but
spectroscopic evidence of its intermediacy, complemented with computational studies, and
supplemented with isolation and characterization of a secondary product, will provide all
the necessary evidence to prove its fleeting existence. The work is being carried on by Eva
Zolnhofer.
133
3 Summary in English and German
3.1 Summary
Tripodal ligands provide a powerful platform for small molecule activation. The ligand
field splitting resulting from their trigonal coordination environment is suitable for the
stabilization of highly unusual metal-ligand multiple bonds, even for relatively electron
rich mid to late transition metals. Such species can serve as model complexes for
intermediates that have been either postulated or spectroscopically observed in biocatalytic
reactions and industrial catalytic processes, and therefore aid in the elucidation of reaction
mechanisms and in our understanding of chemical transformations in nature. This, in turn,
may aid the development of new catalysts. Furthermore, said species may themselves be
used in atom- and group-transfer chemistry and catalytic transformations, e.g., imido
complexes may be used for aziridination reactions.
The N-anchored, tripodal ligands TIMENR (tris[2-(3-R-imidazol-2-ylidene)ethyl]amine,
R = alkyl, aryl) and ((R,R’ArO)3N)3– (trianion of tris[(3,5-R,R’-2-hydroxyphenyl)methyl]-
amine, R, R’ = alkyl) have been employed in the Meyer group’s laboratories for transition
metal and small molecule activation chemistry. In this work, synthetic routes were
developed towards the mixed bis(carbene)mono(phenolate) ligand (BIMPNR,R’,R’’)–
(anion of bis[2-(3-R-imidazol-2-ylidene)ethyl-(3,5-R’,R”-2-hydroxyphenyl)methyl]amine).
Together with its counterpart, the mono(carbene)bis(phenolate) ligand (MIMPNR,R’,R’’)2–
(dianion of mono[2-(3-R-imidazol-2-ylidene)ethyl]-bis[(3,5-R’,R”-2-hydroxyphenyl)-
methyl]amine), this hybrid ligand bridges the gap between TIMENR and ((R,R’ArO)3N)3–,
creating a complete an N-anchored ligand series with donor functionalities ranging from
tris(carbene) to tris(phenolate) (Chart 3).
NN
N
3
R
TIMENR
N
O
3
((R,R'ArO)3N)3-(BIMPNR,R',R'')-
NN
N
2
RO
(MIMPNR,R',R'')2-
NN
NR
O
2
R'
R'' R''
R' R
R'
Chart 3. Series of tripodal N-anchored ligands from tris(carbene) (left) to tris(phenolate) (right).
134
One synthetic route towards (BIMPNR,R’,R’’)–, in which the phenol is attached first to the
nitrogen anchor, involves a tosylation reaction in order to create a highly reactive leaving
group for the following nucleophilic substitution with the imidazole (Scheme 49). This
route may be employed particularly for (BIMPNR,R’,R’’)– derivatives in which the imidazole
building block is more precious than the phenolate.
Mannich reaction
OH
R''
R'
NN
NR
2
OH
R'
R''
NN
NR
2
OK
R'
R''
HNOH
2 NHO
2
OH
R'
R''
ClOTs
tosylation
SN2
2N
N
R
KOtBu
deprot.
OTs
NTsO
2
OH
R'
R''
K(BIMPNR,R',R'')(H3BIMPNR,R',R'')(OTs)2 Scheme 49. Synthetic route for K(BIMPNR,R’,R’’)– in which the phenol is attached first to the
N-anchor.
Another route with better overall yields and easier workup was developed in collaboration
with Johannes Hohenberger, who researches the (MIMPNR,R’,R”)2– ligand and its
coordination chemistry. This route is used for more easily prepared imidazoles such as
xylyl- and mesityl-imidazoles. Scheme 50 summarizes this synthesis for both new, hybrid
ligand types (BIMPNR,R’,R”)– and (MIMPNR,R’,R”)2–: In a first step, the carbene units are
attached to the nitrogen anchor by SN2 reaction of a halido-ethylamine and the substituted
imidazol. The phenol is chloromethylated via a Blanc reaction, after which the amine
nitrogen nucleophilically attacks the benzylic position to form the ligand precursors
(H3BIMPNR,R’,R”)2+ and (H3MIMPNR,R’,R”)+, respectively. These ligand precursors are
deprotonated with KOtBu in THF to give the potassium salts of the free ligands in near
quantitative yield.
135
OH
R'
R''
H3-nNX
n
x HXn
N
N
R
H3-nNN
x HX
NR
n
X
(X = Cl, Br)
OH
R'
R''
Cl
NN
NR
n
OH
R'
R''3-n
(toluene)
HCl conc.
n = 2: (H3BIMPNR,R',R'')2+
n = 1: (H3MIMPNR,R',R'')+
NN
NR
n
OK
R'
R''3-n
n = 2: K(BIMPNR,R',R'')
n = 1: K2(MIMPNR,R',R'')
KOtBu
CH2O
Scheme 50. Synthetic route for K(BIMPNR,R’,R’’) and K2(MIMPNR,R’,R’’).
Both the protonated ligand precursor and the ligand’s potassium salt have been
characterized thoroughly, not only by elemental analysis, 1H, 13C, and 2D NMR as well as
IR spectroscopy, but also by X-ray single crystal analysis.
The complete ligand series offers great tunability of the electronic and steric environment
around the metal center, allowing adjustment of their complexes’ reactivity. Additionally,
the modular synthesis of the mixed ligands allows combination of different substituents on
the NHC and phenolate moieties.
The coordination of the novel, chelating tripodal ligands to Mn, Fe and Co was explored
and a range of divalent complexes of Mn, Fe, and Co was synthesized and characterized by 1H NMR, IR and UV/Vis spectroscopy as well as single crystal X-ray diffraction. Variable
temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed
high spin ground states for the divalent complexes. 57Fe Mößbauer spectroscopy of Fe(II)
complexes 1, 1[Br], 1[BPh4], 1Mes,tBu,tBu and 2 in comparison with data of corresponding
TIMENR complexes revealed an increase of the isomer shifts δ of about 0.1 mm · s–1
caused by the substitution of one (σ-donating/ π-backbonding) NHC with one (σ- and
π-donating) phenolate.
136
Figure 61. Molecular structures of divalent complexes of (BIMPNR,R’,R’’); top row: chlorido
complexes of the formula [(BIMPNMes,Ad,Me)MIICl], bottom row: azide complexes of iron and cobalt
and [(BIMPNXyl,tBu,tBu)Co]PF6 (6Xyl,tBu,tBu[PF6]). The iron complex 1 is coordinated by an
acetonitrile molecule (1(MeCN)) after crystals were grown from an acetonitrile solution.
The complexes’ crystal structures (Figure 61) reveal the different steric demand of the new
ligand. Particularly, the molecular structure of 3 – in which a pyridine molecule is situated
next to the Mn–Cl bond – and those of azide complexes 2 and 7, in which the azide is
coordinated in its preferred bent coordination mode, demonstrate the mixed ligand’s
flexibility, and, in contrast to the corresponding TIMENR ligands, their potential to allow
side access to the reactive center for e.g. organic substrates.
Cyclovoltammograms on the divalent complexes feature an unusually large wave
separation for the redox-wave assigned to the metal centered MII/MIII oxidation, which is
explainable with a rearrangement of the coordination sphere upon electron transfer. The
isolated and spectroscopically characterized diamagnetic complex [(BIMPNMes,Ad,Me)Zn]X
137
(5 and 5[OTs], X = Cl, OTs) may serve as a reference complex with non-redox-active
metal center for further electrochemical studies.
In place of Hu’s way of synthesizing TIMENR Co(I) complexes treating the precursor
CoCl(PPh3)3 with the free ligand, it was found that the reduction of the cobalt(II) complex
was a much more favorable route for the (BIMPNR,R’,R’’)– ligand system. Reaction of the
reddish-brown cobalt(I) complex [(BIMPNMes,Ad,Me)CoI] (8) (Scheme 51) with organic
azides led to different outcomes depending on the size of the azide’s substituent: With trityl
azide, 8 reacted to 7, demonstrating that the trityl substituent is too large for the reactive
cavity, and rather releases its N3– group instead. Treatment of 8 with mesityl azide (MesN3)
led to the green compound [(BIMPNMes,Ad,Me*N3Mes)CoII] (10*), in which the whole
MesN3 unit is bound to one of the ligand’s NHC arms at the former carbene carbon atom.
The NHC ring is flipped around and is now coordinated as an alkenyl, sometimes referred
to as “abnormal carbene”. With phenyl azide (PhN3), effervescence is observed, and a
green compound is obtained. In the violet crystals grown from solutions of this green
compound, the dinuclear complex [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] (9*) was found.
Mechanistic considerations led to the conclusion that solely those aryl azides without
ortho-substituents are capable of binding to the cobalt center in 8 through the aryl-bound
nitrogen atom, i.e., in the “bent” coordination mode necessary for release of dinitrogen.
138
Scheme 51. Reaction of cobalt(I) complex 8 with aryl azides: Mesityl azide leads to abnormal
carbene 10*, phenyl azide to a putative, intermediate imido complex and ultimately to the dinuclear
bis-µ-hydroxo complex 9*.
In the search for an isolable cobalt nitride complex, the photolysis behavior of the blue
azide complexes [(BIMPNMes,Ad,Me)Co(N3)] (7) and [(TIMENMes)Co(N3)]X (12[X], X =
OTs, PF6) was studied. When 7 is photolyzed in frozen solution at very low temperatures
(10 K), an X-band EPR signal emerges around g = 2 with distinctive hyperfine coupling
pattern, indicative of an S = ½ cobalt species (Figure 62). Irradiation at liquid nitrogen
temperatures (80 – 86 K) leads to less well-defined spectra which apparently originate from
at least two species, and the signal vanishes entirely if the solution is defrosted even for a
second. All of this proves the extremely low stability and/or high reactivity of the observed
species. Experiments with trapping reagents were conducted in an effort to isolate a
secondary product following the photolysis of 7. Photolysis in the presence of 2,4,6-tris-
tert-butylphenol led to the green imino complex [(BIMPNMes,Ad,Me*NH)CoII]Cl (11*)
139
(Figure 47), which corroborates the assignment of the S = ½ EPR spectrum to a cobalt-
nitrido species. The proposed reaction mechanism towards 11* and intermediacy of the
nitride species is supported by IR, 1H NMR and preliminary ENDOR data.
Figure 62. X-band EPR signal obtained after photolysis of 7 at 10 K in frozen methyl-THF
solution, left: spectrum (black line) and simulated fit (red line) with parameters, right: growth of the
signal with irradiation time (given in minutes).
Figure 63. Molecular structure of the insertion product [(BIMPNMes,Ad,Me*NH)Co]Cl (11*).
140
Hu’s synthesis of [(TIMENXyl)Co(N3)]Cl involved the highly reactive TIMENXyl cobalt(I)
complex and TMS-N3. For the TIMENMes azide complex 12[X], two more convenient
routes were developed: Firstly, salt metathesis from the corresponding chloride complex,
and secondly, direct synthesis by reaction of the free ligand with CoCl2, an excess of NaN3,
and 1 equivalent of NaX (X = OTs, PF6). Photolysis of 12[X] in acetonitrile solution
apparently led to a well-defined paramagnetic species (see Figure 64). However, EPR
experiments at low temperatures have not permitted the observation of an intermediate
S = ½ species. The elucidation of this photolysis product’s identity is intriguing both in
itself and with regard to formation of 11*, i.e. to see in how far the photolytic behavior of
12[X] is analogous to or different from the photolytic behavior of 7.
80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
Chemical Shift (ppm)
3H
1H 1H 1H 1H 1H1H 1H
1H
1H
1H 1H1H
3H
Figure 64. 1H NMR spectrum of an irradiated sample of 12[OTs] in acetonitrile-d3 after full
conversion of the azide complex.
141
3.2 Zusammenfassung
Tripodale Liganden bieten eine leistungsstarke Plattform zur Aktivierung kleiner Moleküle.
Ihre trigonale Koordinationssphäre führt zu einer Ligandenfeldaufspaltung, die zur
Stabilisierung ungewöhnlicher Metall-Ligand-Mehrfachbindungen selbst bei relativ
elektronenreichen, mittleren bis späten Übergangsmetallen genutzt werden kann. Solche
Spezies dienen als Modellkomplexe von Intermediaten, welche in biokatalytischen
Reaktionen ebenso wie in industriellen Anwendungen entweder postuliert oder
spektroskopisch beobachtet wurden. Somit können sie zur Aufklärung von Reaktions-
mechanismen beitragen und unser Verständnis über chemische Umwandlungen in der
Natur erweitern. Dies kann wiederum die Entwicklung neuer Katalysatoren voranbringen.
Ferner können besagte Spezies selbst in Atom- und Gruppen-Transferreaktionen eingesetzt
werden, beispielsweise werden Imidokomplexe zu Aziridinierungen genutzt.
Die stickstoffgeankerten, tripodalen Liganden TIMENR (Tris[2-(3-R-imidazol-2-yliden)-
ethyl]amin, R = Alkyl, Aryl) und ((R,R’ArO)3N)3– (Trianion von Tris[(3,5-R,R’-2-
hydroxyphenyl)methyl]amin, R, R’ = Alkyl) werden in den Laboren des Arbeitskreises
Prof. Meyer für Übergangsmetallchemie und zur Aktivierung kleiner Moleküle eingesetzt.
In dieser Arbeit wurden Synthesewege hin zum gemischten Bis(carben)mono(phenolat)-
Liganden (BIMPNR,R’,R’’)– (Anion von Bis[2-(3-R-imidazol-2-yliden)ethyl]-[(3,5-R’,R”-2-
hydroxyphenyl)methyl]amin) entwickelt. Zusammen mit seinem Gegenstück, dem
Mono(carben)bis(phenolat)-Liganden (MIMPNR,R’,R’’)2– (Dianion von Mono[2-(3-R-
imidazol-2-yliden)ethyl]-bis[(3,5-R’,R”-2-hydroxyphenyl)methyl]amin), schließt dieser
Hybridligand die Lücke zwischen TIMENR und ((R,R’ArO)3N)3– und schafft somit eine
vollständige Ligandenserie mit Donor-Gruppen von Tris(carben) bis Tris(phenolat)
(Schema 1).
NN
N
3
R
TIMENR
N
O
3
((R,R'ArO)3N)3-(BIMPNR,R',R'')-
NN
N
2
RO
(MIMPNR,R',R'')2-
NN
NR
O
2
R'
R'' R''
R' R
R'
Schema 1. Serie tripodaler, stickstoffgeankerter Liganden von Tris(carben) (links) bis
Tris(phenolat) (rechts).
142
In einer der (BIMPNR,R’,R’’)– Syntheserouten, in welcher das Phenol als erstes an den
Stickstoffanker gekoppelt wird, wird durch einen Tosylierungsschritt um eine hochreaktive
Abgangsgruppe geschaffen für die darauffolgende nukleophile Substitution durch das
Imidazol (Schema 2). Dieser Weg ist besonders für den Einsatz bei (BIMPNR,R’,R’’)-
Derivaten mit wertvollen Imidazolen geeignet.
Mannich- Reaktion
OH
R''
R'
NN
NR
2
OH
R'
R''
NN
NR
2
OK
R'
R''
HNOH
2 NHO
2
OH
R'
R''
ClOTs
Tosylierung
SN2
2N
N
R
KOtBu
Deprot.
OTs
NTsO
2
OH
R'
R''
K(BIMPNR,R',R'')(H3BIMPNR,R',R'')(OTs)2 Schema 2. Syntheseroute für K(BIMPNR,R’,R’’), in welcher das Phenol zuerst am Stickstoffanker
angebracht wird.
Eine weitere Route mit besserer Gesamtausbeute und einfacherer Aufarbeitung wurde in
Zusammenarbeit mit Johannes Hohenberger entwickelt, welcher das (MIMPNR.R’,R’’)2–
System erforscht. Diese Route wird bei leichter herzustellenden Imidazolen wie jenen mit
Xylyl- und Mesityl-Substituenten verwendet. Schema 3 fasst diese Synthese für beide
neuen Ligandentypen, (BIMPNR.R’,R’’)– und (MIMPNR.R’,R’’)2–, zusammen: Im ersten Schritt
werden die späteren Carben-Einheiten am Stickstoffanker fixiert durch SN2-Reaktion
zwischen halogeniertem Ethylamin und Imidazol. In einer Blanc Reaktion wird das Phenol
chlormethyliert, woraufhin der Aminstickstoff die benzylische Position nukleophil
angreifen kann, um zu den Ligandenvorstufen (H3BIMPNR.R’,R’’)2+ und (H3MIMPNR.R’,R’’)+
zu gelangen. Diese werden in THF mit KOtBu deprotoniert, woraufhin man das
Kaliumsalz der freien Liganden in nahezu quantitativer Ausbeute erhält.
Beide protonierten Ligandenvorstufen sowie das Kaliumsalz des Liganden wurden
eingehend charakterisiert, nicht nur mittels Elementaranalyse, 1H, 13C und 2D NMR- sowie
IR-Spektroskopie, sondern auch durch Röntgenstrukturanalyse am Einkristall.
143
OH
R'
R''
H3-nNX
n
x HXn
N
N
R
H3-nNN
x HX
NR
n
X
(X = Cl, Br)
OH
R'
R''
Cl
NN
NR
n
OH
R'
R''3-n
(toluene)
HCl conc.
n = 2: (H3BIMPNR,R',R'')2+
n = 1: (H3MIMPNR,R',R'')+
NN
NR
n
OK
R'
R''3-n
n = 2: K(BIMPNR,R',R'')
n = 1: K2(MIMPNR,R',R'')
KOtBu
CH2O
Schema 3. Syntheseroute für K(BIMPNR,R’,R’’) und K2(MIMPNR,R’,R’’).
Die elektronische und sterische Umgebung am Metallzentrum kann durch die komplette
Ligandenserie einfach eingestellt werden, wodurch die Reaktivität der resultierenden
Komplexe an den jeweiligen Zweck angepasst werden kann. Zusätzlich erlaubt die
bausteinartige Synthese der gemischten Liganden freie Kombination verschiedener
Substituenten an den NHC- und Phenolat-Ringen.
Die Koordination der neuartigen Chelat-Liganden an Mn, Fe und Co wurde untersucht und
eine Reihe von zweiwertigen Komplexen dieser Metalle synthetisiert und mittels 1H NMR-, IR- und UV/Vis-Spektroskopie sowie Einkristall-Röntgenstrukturanalyse
charakterisiert. SQUID Magnetisierungsmessungen im Temperaturbereich von 2 bis 300 K
bestätigten high spin Grundzustände für die zweiwertigen Komplexe. Der Vergleich der 57Fe Mößbauer Spektren der Eisen(II)-Komplexe 1, 1[Br], 1[BPh4], 1
Mes,tBu,tBu und 2 mit
jenen der entsprechenden TIMENR-Komplexe zeigte, dass sich die Isomerieverschiebung δ
um etwa 0.1 mm · s–1 erhöht, hervorgerufen durch den Austausch eines (σ-donierenden, π-
rückbindenen) NHCs durch ein (σ- und π-donierendes) Phenolat.
144
Abbildung 1. Molekularstrukturen einiger zweiwertiger (BIMPNR,R’,R’’)–-Komplexe; obere Reihe:
Chlorido-Komplexe der Formel [(BIMPNMes,Ad,Me)MIICl], untere Reihe: Azid-Komplexe von Eisen
und Cobalt sowie [(BIMPNXyl,tBu,tBu)Co]PF6 (6Xyl,tBu,tBu[PF6]). Ein Acetonitril-Molekül ist am
Eisenkomplex 1 koordiniert (1(MeCN)), nachdem die Kristalle in Acetonitril-Lösung gezüchtet
wurden.
Die Kristallstrukturen der Komplexe (Abbildung 1) zeigen den unterschiedlichen sterischen
Anspruch der Liganden auf. Die Flexibilität der gemischten Liganden wird besonders
hervorgehoben durch die Strukturen von 3 – in welchem ein Pyridin-Molekül sich
zwischen zwei der Ligandenarme geschoben hat – sowie von den Azid-Komplexe 2 und 7.
Im Gegensatz zu den entsprechenden TIMENR–Liganden bieten die neuen Liganden
offenbar seitlichen Zugang zum reaktiven Metallzentrum, z.B. für organische Substrate.
Cyclovoltammogramme der zweiwertigen Komplexe weisen eine ungewöhnlich große
Differenzspannung auf zwischen der Oxidations- und der Reduktions-Welle, welche der
metallzentrierten Oxidation MII/MIII zugeschrieben werden. Erklärbar ist dies durch eine
durch den Elektronentransfer hervorgerufene Umordnung der Koordinationssphäre. Der
isolierte und spektroskopisch charakterisierte diamagnetische Komplex
145
[(BIMPNMes,Ad,Me)Zn]X (5 und 5[OTs], X = Cl, OTs) mag in zukünftigen
elektrochemischen Studien als Referenz mit redox-inaktivem Metallzentrum dienen.
Anstelle von Hu’s Synthese der TIMENR Co(I) Komplexe, bei welcher die Vorstufe
CoCl(PPh3)3 mit dem freiem Liganden umgesetzt wird, ist es beim (BIMPNR,R’,R’’)–
Ligandensystem wesentlich vorteilhafter, den Cobalt(II)-Komplex zu reduzieren. Reaktion
des rotbraunen Cobalt(I)-Komplexes [(BIMPNMes,Ad,Me)CoI] (8) (Schema 4) mit
organischen Aziden führte abhängig von der Größe des organischen Restes zu
unterschiedlichen Resultaten: Mit Tritylazid reagierte 8 zu 7, somit ist der Trityl-
Substituent zu groß für den Zugang zum Metallzentrum und setzt stattdessen die N3–
Gruppe frei. Umsetzung von 8 mit Mesitylazid (MesN3) führte zur grünen Verbindung
[(BIMPNMes,Ad,Me*N3Mes)CoII] (10*), in welcher die gesamte MesN3-Einheit an einen der
NHC-Ligandenarme gebunden ist, und zwar am ehemaligen Carben-Kohlenstoffatom. Der
NHC-Ring ist um 180° gedreht und nun als Alkenyl, auch „abnormales Carben“ genannt,
koordiniert. Mit Phenylazid (PhN3) reagiert 8 unter Gasentwicklung zu einer grünen
Verbindung. Aus Lösungen dieser grünen Verbindung wurden, vermutlich nach Reaktion
mit Feuchtigkeit aus einer kontaminierten Glovebox-Atmosphäre, violette Kristalle
erhalten, die den zweikernigen Komplex [Bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] (9*)
enthielten. Mechanistische Überlegungen führten zu dem Schluss, dass nur Arylazide ohne
ortho-Substituenten in der Lage sind, mit dem arylgebundenen Stickstoffatom am Cobalt-
Zentrum von 8 zu koordinieren. Dieser „gewinkelte“ Koordinationsmodus ist
Voraussetzung für die Freisetzung von Distickstoff.
146
Schema 4. Reaktion von Cobalt(I)-Komplex 8 mit Arylaziden: Mesitylazid führt zum abnormalen
Carben 10*, Phenylazide zum mutmaßlichen Imidokomplex als Zwischenprodukt und schließlich
zum zweikernigen Bis-µ-hydroxo-Komplex 9*.
Auf der Suche nach isolierbaren Cobalt-Nitriden wurde das Photolyse-Verhalten der blauen
Azidkomplexe [(BIMPNMes,Ad,Me)Co(N3)] (7) und [(TIMENMes)Co(N3)]X (12[X], X = OTs,
PF6) untersucht. Bei Photolyse von 7 in einer gefrorenen Methyl-THF-Matrix bei sehr
niedrigen Temperaturen (10 K) entsteht ein Signal um den g-Wert 2 mit deutlichem
Hyperfein-Kopplungsmuster, welches auf eine S = ½ Cobalt-Spezies hinweist
(Abbildung 2). Bestrahlung bei Flüssigstickstoff-Temperaturen (80 – 86 K) führt dagegen
zu weniger wohlgeformten Spektren, die augenscheinlich von mindestens zwei Spezies
herrühren, und das Signal verschwindet gänzlich sobald die Lösung auch nur einen
Moment aufgetaut wird. All dies beweist eine extrem niedrige Stabilität bzw. hohe
Reaktivität der beobachteten Spezies. Experimente mit Abfangreagenzien wurden mit dem
Ziel durchgeführt, ein Folgeprodukt der Photolyse von 7 zu isolieren. In Anwesenheit von
147
2,4,6-Tris-tert-butylphenol führt die Photolyse zum grünen Komplex
[(BIMPNMes,Ad,Me*NH)CoII]Cl (11*) (Abbildung 3), was die Zuordnung des S = ½ EPR-
Spektrums zu einer Cobalt-Nitrid-Verbindung untermauert. Der vorgeschlagene
Reaktionsmechanismus zur Bildung von 11* und das Auftreten des Nitrids als
Zwischenstufe werden durch IR-, 1H NMR- und vorläufige ENDOR-Daten gestützt.
Abbildung 2. X-Band EPR-Signal nach Photolyse von 7 bei 10 K in gefrorener Methyl-THF-
Lösung; links: Spektrum (schwarze Linie) und simulierter Fit (rote Linie) mit
Simulationsparametern; rechts: Wachstum des Signals im Laufe der Belichtungszeit (in Minuten).
148
Abbildung 3. Molekülstruktur des Insertionsproduktes [(BIMPNMes,Ad,Me*NH)Co]Cl (11*).
Hu’s Synthese von [(TIMENXyl)Co(N3)]Cl benötigt den hochreaktiven TIMENXyl
Cobalt(I)-Komplex und TMS-N3. Für den TIMENMes Azidkomplex 12[X] wurden zwei
bequemere Synthesen entwickelt: zum Einen durch Salz-Metathese aus dem Chlorido-
Komplex, und zum Anderen Direktsynthese durch Umsetzung des freien Liganden mit
CoCl2, einem Überschuss an NaN3 und 1 Äquivalent NaX (X = OTs, PF6). Wird 12[X] in
Acetonitril photolysiert, entsteht anscheinend eine wohldefinierte paramagnetische
Verbindung (siehe Abbildung 4). In EPR-Experimenten bei niedrigen Temperaturen konnte
jedoch keine intermediäre Spezies mit S = ½ beobachtet werden. Die Identität des
Photolyseproduktes aufzudecken wäre sowohl an sich faszinierend, als auch in Hinblick
auf die Bildung von 11*: Es könnte verglichen werden, inwieweit die Photolyse von 12[X]
analog oder verschieden zu der von 7 verläuft.
80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10
chemische Verschiebung (ppm)
3H
1H 1H 1H 1H 1H1H 1H
1H
1H
1H 1H1H
3H
Abbildung 4. 1H NMR Spektrum einer bestrahlten Probe von 12[Ots] in Acetonitril-d3 nach
vollständigem Umsatz des Azidkomplexes.
149
4 Experimental Part
4.1 Methods, Procedures and Starting Materials
4.1.1 General
All air- and moisture-sensitive experiments were performed under dry nitrogen atmosphere
using standard Schlenk techniques or an MBraun inert-gas glovebox containing an
atmosphere of purified dinitrogen.
Solvents for air- and moisture-sensitive experiments were purified using a two-column
solid-state purification system (Glasscontour System, Irvine, CA) and transferred to the
glovebox without exposure to air. NMR solvents were obtained packaged under argon and
stored over activated molecular sieves and sodium (where appropriate) prior to use.
4.1.2 Starting Materials
Manganese(II) chloride and iron(II) chloride, each anhydrous 99.9 %, cobalt(II) chloride,
anhydrous 97 %, and PNPCl 97 % were purchased from Sigma-Aldrich and used as
received. Sodium azide 97 % and sodium hexafluorophosphate were purchased from
ACROS Organics, as were any organic chemicals for ligand synthesis, and used without
further purification. KOtBu 98+% was obtained from ACROS Organics and purified by
sublimation under reduced pressure before it was transferred into the glovebox. NaOMe
97 % was purchased from Merck and used as received.
Imidazoles[186], 2-adamantyl-para-cresol[187], aryl azides[235] and the cobalt(I) precursor
CoCl(PPh3)3[236] were synthesized according to literature procedures; some other steps in
ligand syntheses were modified from literature procedures which are referenced in the
respective section.
4.1.3 Analytical Methods
1H-NMR spectra were recorded on JEOL 270 and 400 MHz instruments, operating at
respective frequencies of 269.714 and 400.178 MHz with a probe temperature of 23 ºC. 13C NMR spectra were recorded on JEOL 270 and 400 MHz instruments, operating at
respective frequencies of 67.82 MHz and 100.624 MHz with a probe temperature of 23 ºC.
150
Chemical shifts are reported relative to the peak for SiMe4, using 1H (residual) chemical
shifts of the solvent as a secondary standard[237] and are reported in ppm.
Elemental analysis results were obtained from the Analytical Laboratories at the
Friedrich-Alexander-University Erlangen-Nuremberg (Erlangen, Germany).
FD mass spectra were recorded on a Jeol MSTATION 700 mass spectrometer.
Infrared spectra were recorded on a Shimadzu IRAffinity-1 system in KBr pellets.
Electronic absorption spectra were recorded with a S600 UV-Vis Spectrophotometer
from Jena Analytik, a UV-Vis Spectrophotometer from Shimadzu (UV-2450), and a UV-
Vis-NIR Spectrophotometer from Shimadzu (UV-3600). 57
Fe Mößbauer spectra were recorded on a WissEl Mößbauer spectrometer (MRG-500) at
77 K in constant acceleration mode. 57Co/Rh was used as radiation source. WinNormos for
Igor Pro software was used for the quantitative evaluation of the spectral parameters (least-
squares fitting to Lorentzian peaks). The minimum experimental line widths were
0.21 mm s−1. The temperature of the samples was controlled by an MBBC-HE0106
MÖSSBAUER He/N2 cryostat within an accuracy of ±0.3 K. Isomer shifts were
determined relative to α-iron at 298 K.
Magnetism data of crystalline, finely powdered samples (15 – 30 mg) restrained within a
polycarbonate gel capsule were recorded with a Quantum Design MPMS-XL SQUID
magnetometer. DC susceptibility data were collected in the temperature range of 2 – 300 K
under a DC field of 1 T. Values of the magnetic susceptibility were corrected for core
diamagnetism of the sample estimated using tabulated Pascal’s constants.[238] Samples used
for magnetization measurement were checked for chemical composition and purity by
elemental analysis (C, H, and N) and, for the Fe and Co complexes, 1H NMR spectroscopy.
Data reproducibility was carefully checked on independently synthesized samples. The
program julX written by E. Bill was used for the simulation and analysis of magnetic
susceptibility data.[239]
EPR measurements were performed in quartz tubes with J. Young valves. Frozen solution
EPR spectra were recorded on a JEOL continuous wave spectrometer JESFA200 equipped
with an X-band Gunn diode oscillator bridge, a cylindrical mode cavity, and a helium
cryostat. The spectra were simulated using the W95EPR program.[240]
151
Electrochemical measurements were carried out at room temperature under inert
atmosphere with an Autolab potentiostat. They were recorded in acetonitrile solutions
containing 0.1 M TBAPF6 (tetra-n-butylammonium hexafluorophosphate) using a glassy
carbon working electrode and Pt as counter and pseudo-reference electrodes. Ferrocene
was added as internal standard and all measurements were referenced to E(Fc/Fc+) =
0.41 V vs. NHE.
4.2 Synthetic Details
4.2.1 Ligand Syntheses
4.2.1.1 Imidazoles and Phenols
Mesityl-Imidazole (1-(2,4,6-trimethylphenyl)-imidazole)
N
HN
After a slightly modified literature procedure:[186]
2,4,6-trimethylaniline (28.2 mL, 27.0 g, 200 mmol, 1 eq) is dissolved in methanol (50 mL)
and added dropwise to a solution of 40% aq. glyoxal (24.0 mL, 210 mmol, 1.05 eq) in
methanol (100 mL). The yellowish reaction mixture is stirred for 16 h at RT, then diluted
with methanol (200 mL), treated with 37% aq formaldehyde (15 mL, 200 mmol, 1 eq) and
heated to 70 °C. On slow, dropwise addition of NH4OAc (15.4 g, 200 mmol, 1 eq,
dissolved in 50 mL H2O and 25 mL methanol), the mixture turns to dark brown. Acetic
acid (200 mL) is added and the reaction is refluxed for at least 30 h. After removal of the
solvent, the dark residue is poured onto ice and neutralized with aq NaOH until pH 9. The
resulting mixture is extracted with CH2Cl2 (3 × 200 mL). The combined organic layers are
dried over Na2SO4 and evaporated. The crude product is purified by Kugelrohr distillation
to give a white solid (21.7 g, 116 mmol, 58 %).
152
If the product is not pure after Kugelrohr distillation, it can be stirred and heated to reflux
in a small amount of hexane until all impurities have dissolved and, after cooling to RT, a
white solid can be filtered off.
1H NMR (270 MHz, CDCl3, ppm): δ = 7.43 (t, 3JHH = 1.1 Hz, 1 H, NIm-CH- NIm),
7.23 (t, 3JHH = 1.1 Hz, 1 H, NIm-CH-CH- Nanchor), 6.97 (s, 2 H, Mes-H),
6.89 (t, 3JHH = 1.3 Hz, 1 H, NIm-CH-CH-Nanchor), 2.34 (s, 3 H, Mes-CH3),
1.98 (s, 6 H, Mes-CH3).
13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 139.0, 137.6, 135.6, 133.5, 129.7, 129.1,
120.1, 21.15 (Mes-CH3), 17.45 (Mes-CH3).
EA (%) calcd. for C12H14N2: C 77.38, H 7.58, N 15.04;
found: C 77.11, H 7.80, N 15.16.
Xylyl-Imidazole (1-(2,6-dimethylphenyl)-imidazole)
N
HN
2,6-dimethylaniline (24.7 mL, 24.2 g, 200 mmol, 1 eq) is used in the above synthesis to
give a white solid (18.5 g, 56 %).
1H NMR (270 MHz, CDCl3, ppm): δ = 7.45 (t, 3JHH = 1.1 Hz, 1 H, N-CH-N), 7.25
(t, 3JHH = 7.6 Hz, 1 H, p-Ar-H), superimposed with 7.24 (t, 3JHH = 1.1 Hz,
1 H, N-CH-CH-N), 7.15 (d, 3JHH = 7.6 Hz, 2 H, m-Ar-H), 6.91 (t, 3JHH = 1.1 Hz, 1 H, N-CH-CH-N), 2.02 (s, 6 H, CH3).
13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 137.2, 135.7, 129.6, 128.9, 128.3, 119.7,
17.4.
EA (%) calcd. for C11H12N2: C 76.71, H 7.02, N 16.27;
found: C 76.56, H 7.05, N 15.96.
153
2-Adamantyl-p-Cresol (2-adamant-1-yl-4-methyl-phenol)[187]
OH
To a solution of p-cresol (11.1 g, 104 mmol, 1 eq) in CH2Cl2 (90 mL) is added 1-
adamantanol (16.42 g, 108 mmol, 1.1 eq). The solution is stirred and 18 M H2SO4 (6.0 mL)
is added dropwise over 20 min. The biphasic mixture is stirred for 20 min and H2O
(100 mL) is added. The mixture is neutralized to pH = 9 by addition of NaOH (4M). The
mixture is extracted with CH2Cl2 (3 × 100 mL). The combined organics are washed with
brine (150 mL), dried over Na2SO4, filtered and evaporated. The residue is dissolved in
MeOH (100 mL) and the mother liquor concentrated. The abundant white precipitate is
filtered off, washed with cool MeOH and dried i. vac. to give a white solid (13.4 g,
55.1 mmol, 53 %, Lit.: 78 %).
1H NMR (270 MHz, CDCl3, ppm): δ = 7.03 (d, 4JHH = 2.1 Hz, 1 H, Ar-H), 6.88
(dd, 3JHH = 7.9 Hz, 4JHH = 2.1 Hz, 1 H, Ar-H), 6.56 (d, 3JHH = 7.9 Hz,
1 H, Ar-H), 4.60 (s, 1 H, Ar-OH), 2.28 (s, 3 H, Ar-CH3), 2.13 – 2.09
(m(br) , 6+3 H, Ad-H superimposed by Ad-H2) 1.79 (s, 6 H, Ad-H2). 13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 152.0, 136.0, 129.7, 127.6, 127.0, 116.6,
40.51, 37.03, 36.51, 29.00, 20.81.
EA (%) calcd. for C17H22O: C 84.25, H 9.15.
found: C 84.31, H 9.39.
2-Adamantyl-4-tert-Butylphenol (2-adamant-1-yl-4-tert-butylphenol)
OH
After a slightly modified literature procedures:[188-189]
1-Chloroadamantane (34.0 g, 200 mmol, 1 eq.) and 4-tert.-butylphenol (30.0 g, 200 mmol,
1 eq.) were heated to 100 °C for 3 h, then to 140 °C overnight. The crude product was
154
heated under vacuum to 90 °C in the Kugelrohr apparatus until no more starting material
evaporates from the sample flask and then recrystallized from a hexane / benzene mixture
(3:1) to give white crystals (43.8 g, 154 mmol, 77 %).
1H NMR (270 MHz, benzene-d6, ppm): δ = 7.42 (d, 4JHH = 2.1 Hz, 1 H, Ar-H),
7.00 (dd, 3JHH = 8.2 Hz, 4JHH = 2.1 Hz, 1 H, Ar-H), 5.99 (d, 3JHH = 8.2 Hz, 1 H, Ar-H), 3.84 (s, 1 H, Ar-OH), 2.31 (m, 6 H, Ad-H2),
2.08 (m, 3 H, Ad-H), 1.81 (m, 6 H, Ad-H2), 1.31 (s, 3 H, Ar-C(CH3)3).
4.2.1.2 Tosylation Route
2,2'-(3-adamantyl-5-methyl-2-hydroxybenzylazanediyl)diethanol (c1-Ad,Me)
Ad
OH
NHO
2
c1-Ad,Me
The synthesis for c1-tBu,tBu (see below) can be simplified for c1-Ad,Me due to different
solubilities:
Adamantylcresol (22.9 g, 94.5 mmol, 1 eq) and diethanolamine (13.5 mL, 14.9 g,
142 mmol, 1.5 eq) are dissolved in methanol (65 mL) in a 250-mL round-bottom flask.
Formaldehyde (28.3 mL of a 37% aq. sol., 378 mmol, 4 eq) is added while stirring. The
reaction is refluxed for overnight. After cooling to RT., the abundant cream-colored
precipitate is filtered off, washed with small portions of methanol and ether and dried i.vac.
to give a white powder (25.2 g, 70.1 mmol, 74 %).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 6.94 (s, 1H, Ar-H), 6.66 (s, 1H, Ar-H),
3.80 (2, 2H, Ar-CH2-N), 3.74 (t, 3JHH = 4.8 Hz, 2H, NCH2CH2OH), 2.77
(t, 3JHH = 4.8 Hz, 2H, NCH2CH2OH), 2.23 (s, 3H, Ar-CH3), 2.12 (s (br),
6H, Ad-H2), 2.04 (s (br), 3H, Ad-H), 1.76 (m (br), 6H, Ad-H2).
EA (%) calcd: C, 73.50; H, 3.90; N, 9.25.
found: C, 73.63; H, 4.09; N, 9.33.
155
2,2'-(3-adamantyl-5-tert-butyl-2-hydroxybenzylazanediyl)diethanol (c1-Ad,tBu)
Ad
OH
NHO
2
c1-Ad,tBu
To a solution of 2-adamantyl-4-tert-butylphenol (2.00 g, 7.03 mmol, 1 eq) in methanol
(15 mL) were added formaldehyde (2.2 mL of a 37% aq. sol., 28.1 mmol, 4 eq) and
diethanolamine (1.02 mL, 1.11 g, 10.6 mmol, 1.5 eq). The reaction mixture was stirred at
reflux for 2 d. The solvent was evaporated, the residue taken up in Et2O and HCl conc.
(1.10 mL) was added. The precipitating solid was filtered off, washed with a small amount
of Et2O, then dissolved in CH2Cl2 (40 mL) and washed 2 M NaOH (2 x 40 mL) and H2O
(50 mL). The organic layer was dried over NaSO4 and evaporated to give a white solid
(0.80 g, 1,99 mmol, 28 %).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 7.16 (d, 4JHH = 2.4 Hz, 1 H, p-Ar-H),
6.83 (d, 4JHH = 2.4 Hz, 1 H, o-Ar-H), 3.84 (s, 2 H, Ar-CH2-N), 3.75 (t, 3JHH = 10.3 Hz, 4 H, N-CH2-CH2-OH) 2.78 (t, 3JHH = 10.3 Hz, 4 H, N-
CH2-CH2-OH), 2.15 (m, 6 H, Ad-H2), 2.06 (m, 3 H, Ad-H), 1.77 (m, 6 H,
Ad-H2), 1.28 (s, 9 H, CH3).
2,2'-(3,5-di-tert-butyl-2-hydroxybenzylazanediyl)diethanol (c1-tBu,tBu)[192]
OH
NHO
2
c1-tBu,tBu
The literature procedure[192] ends after the HCl-precipitate is filtered off; for tosylation to
c4-tBu,tBu, the deprotonation step is necessary, for chlorination to c3-tBu,tBu the
precipitate may be used as received.
156
2,4-Di-tert-butylphenol (20 g, 97 mmol, 1 eq), formaldehyde (30 mL of a 37% aq. sol.,
388 mmol, 4 eq) and diethanolamine (14 mL, 15.3 g, 145 mmol, 1.5 eq) are dissolved in
methanol (60 mL) and heated to reflux overnight. All volatiles are removed i.vac.. The oily
residue is dissolved in Et2O (150 mL) and treated with HCl conc. (14 mL). The white
precipitate is filtered off, washed with Et2O (80 mL), and H2O (80 mL). The solid is
extracted with CH2Cl2 (100 mL) against aqueous NaOH (2 M, 100 mL), the organic phase
is washed again with H2O (100 mL), dried over Na2SO4 and evaporated to give an oil
(22 g, 68 mmol, 70 %).
See literature[192] for the 1H NMR spectrum.
((3-(adamantan-1-yl)-2-hydroxy-5-methylbenzyl)azanediyl)bis(ethane-2,1-diyl)bis(4-
methylbenzenesulfonate) (c4-Ad,Me)
Ad
OH
NTsO
2
c4-Ad,Me
c1-Ad,Me (10.0 g, 27.8 mmol, 1 eq) and pyridine (11.2 mL, 11.0 g, 139 mmol, 5 eq) are
dissolved in anhydrous CH2Cl2 (40 mL) in a 250-mL round-bottomed flask and cooled to
0 °C. TsCl (11.1 g, 58.4 mmol, 2.1 eq) is dissolved in CH2Cl2 (30 mL) and slowly added
dropwise during 3 h. The reaction is stirred at RT for another hour, quenched with H2O
(50 mL), washed with sat. aq. NH4Cl (60 mL), sat. aq. NaHCO3 (60 mL) and brine
(80 mL), dried over Na2SO4 filtered and evaporated. The crude product (ca. 18 g of yellow
oil) is purified by flash column chromatography (gradient hexane / THF, 600 g neutral
aluminium oxide (Brockman activity III) to give c4-Ad,Me as a white solid (5.8 g,
8.68 mmol, 31%).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 9.23 (s, 1H, NH), 7.78 (d, 3JHH = 8.1 Hz,
4H, O3S-Ar-H), 7.33 (d, 3JHH = 8.1 Hz, 4H, O3S-Ar-H), 6.92 (d, 3JHH = 1.5 Hz, 1H, Ad-Ar-H), 6.53 (d, 3JHH = 1.5 Hz, 1H, Ad-Ar-H), 4.05
(t, 3JHH = 5.5 Hz, 4H, NCH2CH2O), 3.67 (s, 2H, Ar-CH2-N), 2.88 (t,
157
3JHH = 5.5 Hz, 4H, NCH2CH2O), 2.44 (s, 6H, O3S-Ar-CH3), 2.21 (s, 3H,
Ad-Ar-CH3), 2.07 (m, 9H, Ad-CH2 and Ad-CH), 1.77 (m, 6H, Ad-CH2). 13C{1H} NMR (100.53 MHz, CDCl3, ppm): δ = 154.0, 145.1, 136.9, 132.5, 129.9, 128.0,
127.7, 127.3, 127.1, 120.9, 66.7, 58.1, 52.0, 40.2, 37.1, 36.6, 29.1, 21.7,
20.7.
(2-hydroxy-2,5-di-tert-butylbenzyl)azanediyl)bis(ethane-2,1-diyl)bis(4-methylbenzene-
sulfonate) (c4-tBu,tBu)
OH
NTsO
2
c4-tBu,tBu
c1-tBu,tBu is used instead of c1-Ad,Me in the above synthesis.
1H NMR (399.78 MHz, CDCl3, ppm): δ = 9.30 (s, 1 H, NH), 7.79 (d,
3JHH = 7.9 Hz, 4 H, O3S-Ar-H), 7.33 (d, 3JHH = 8.1 Hz, 4 H, O3S-Ar-H),
7.20 (d, 3JHH = 1.5 Hz, 1H, Ar-H), 6.73 (d, 3JHH = 1.5 Hz, 1H, Ar-H),
4.07 (t, 3JHH = 5.3 Hz, 4 H, NCH2CH2O), 3.71 (s, 2 H, Ar-CH2-N), 2.91
(t, 3JHH = 5.3 Hz, 4 H, NCH2CH2O), 2.44 (s, 6 H, O3S-Ar-CH3), 1.34 (s,
9 H, C(CH3)3), 1.25 (s, 9 H, C(CH3)3).
(H3BIMPNMes,Ad,Me
)(OTs)2
NN
N
2
OH
2 OTs
c4-Ad,Me (5.80 g, 8.68 mmol, 1 eq) and mesityl-imidazole (3.32 g, 17.8 mmol, 2.05 eq)
are dissolved in acetonitrile (30 mL) and stirred at RT for one week. The reaction is
158
evaporated, sonicated with Et2O and filtered. The filtercake is washed several times with
Et2O and dried i. vac. to give a white solid (6.97 g, 79%).
If the reactions becomes “foggy”, the mixture it is filtered before evaporization. The
reaction is essentially quantitative (by 1H NMR), losses are due to work-up or side-
products which are filtered off.
1H NMR (399.78 MHz, CDCl3, ppm): δ = 9.64 (s(t), 2H, NIm-CH-NIm), 8.59
(Ph-OH), 8.24 (s(t), 2H, NIm-CH-CH-NIm), 7.61 (d, 3JHH = 8.2 Hz, 2H, -OTs), 7.01 (d, 3JHH = 8.2 Hz, 2H, –OTs), 6.98 (s(t), 2H, NIm-CH-CH-
NIm), 6.93 (s, 4H, m-Mes-H), 6.79 (d, 4JHH = 1.3 Hz, Ar-H), 4.44 (t, 3JHH = 6.8 Hz, 4H, N-CH2-CH2-N), 4.05 (s, 2H, Ar-CH2-N), 3.39 (t, 3JHH = 6.8 Hz, 4H, N-CH2-CH2-N), 2.32 (s, 6H, –OTs), 2.29 (s, 6H,
p-Mes-CH3), 2.19 (s, 3H, Ar-CH3), 2.01 (m, 6H, Ad-H2), 1.97 (s, 12H,
o-Mes-CH3), 1.93 (m, 3H, Ad-H), 1.67 (tm, 6H, Ad-H2); one aromatic
proton from the phenol ring is superimposed by the signal of m-Mes-H,
as confirmed by H,H-COSY NMR spectroscopy. 13C{1H} NMR (100.624 MHz, CDCl3, ppm): δ = 153.3 (NIm-CH-NIm), 143.6 (Cquart.),
140.9 (Cquart.), 139.2 (Cquart.), 138.2 (Cquart.), 137.0 (Cquart.), 134.3 (Cquart.),
130.9 (Cquart.), 129.7 (Cmesityl-H), 128.8 (Cphenol-H), 128.6 (–OTs: CH),
128.3 (Cquart.), 127.4 (Cphenol-H), 125.8 (–OTs: CH), 124.7 (NIm-CH-CH-
NIm), 123.1 (NIm-CH-CH-NIm), 58.8 (Ar-CH2-H), 53.9 (N-CH2-CH2-N),
47.4 (N-CH2-CH2-N), 40.5 (CAd-H2), 37.1 (CAd-H2), 36.7 (CAd,quart.), 29.1
(CAd-H), 21.4 (p-Mes-CH3), 21.2 (–OTs: CH3), 20.9 (phenol-CH3), 17.5
(o-Mes-CH3); assignments were verified through H,H-COSY and C,H-
correlation NMR spectra.
159
(H3BIMPNMes,tBu,tBu
)(OTs)2
NN
N
2
OH
2 OTs
c4-tBu,tBu is used in the above synthesis in lieu of c4-Ad,Me.
1H NMR (399.78 MHz, CDCl3, ppm): δ = 9.68 (s(t), 2 H, NIm-CH-NIm), 8.26
(Ph-OH), 8.26 (s(t), 2 H, NIm-CH-CH-NIm), 7.67 (d, 3JHH = 7.9 Hz, 2 H, -OTs), 7.01 (d, 3JHH = 7.9 Hz, 2 H, -OTs), 7.01 (s(t), 2 H, NIm-CH-CH-
NIm), 6.95 (s, 4 H, m-Mes-H), 4.47 (s(br), 4 H, N-CH2-CH2-N), 4.20 (s,
2 H, Ar-CH2-N), 3.49 (s(br), 4 H, N-CH2-CH2-N), 2.33 (s, 6 H, –OTs),
2.30 (s, 6 H, p-Mes-CH3), 1.99 (s, 12H, o-Mes-CH3), 1.35 (s, 9 H,
C(CH3)3), 1.26 (s, 9 H, C(CH3)3).
4.2.1.3 Final (BIMPNR,R’,R”
)– Synthesis Route
Di(chloroethyl)amine hydrochloride[202]
p1×HCl
H2NCl
2
Cl
p1×HCl
After a slightly modified literature procedure:[202]
To a stirred solution of diethanolamine (50 g, 0.48 mol, 1 eq) in chloroform (150 mL) is
added a solution of thionyl chloride (130 mL, 1.79 mol, 3.7 eq) in chloroform (130 mL),
cautiously at first and then more rapidly. The reaction mixture is then heated to reflux and
stirred vigorously. After 1 h, the cloudy mixture is cooled in an ice bath. The precipitating
solid is filtered off, washed with chloroform (2x) and diethyl ether (2x) and dried in
vacuum to give white crystals (80.7 g, 0.452 mol, 94 % (Lit.: 77 %)).
160
1H NMR (399.78 MHz, DMSO-d6, ppm): δ = 9.60 (s, 2 H, NH2), 3.95 (t, 3JHH = 6.4 Hz, 4 H, CH2), 3.36 (t, 3JHH = 6.4 Hz, 4 H, CH2).
EA (%) calcd. for C4H10Cl3N: C, 26.92; H, 5.65; N, 7.85.
found: C, 27.06; H, 5.73; N, 7.50.
Bis(mesitylimidazolium-ethyl)amine (3,3'-(ammonium-bis(ethan-2,1-diyl))bis(1-(2,4,6-
trimethylphenyl)-imidazolium)trichloride, p2-Mes×HCl)
3 Cl
H3NN
N
2
p2-Mes×HCl
After a slightly modified literature procedure:[203]
1-(2,4,6-Trimethylphenyl)-imidazole (17.00 g, 91.2 mmol, 2 eq) and bis-2-
(chloroethyl)amine hydrochloride (8.16 g, 45.6 mmol, 1 eq) are dissolved in MeCN
(50 mL). The solution is refluxed for 48 h. After cooling to 0 °C, the abundant white
precipitate is filtered off, washed with cold MeCN and dried in vacuum to give
bis(mesitylimidazolium-ethyl)amine as white solid (8.94 g, 16.2 mmol, 36 %).
The solid is at first insoluble in chlorinated solvents. Upon storage, possibly due to air
moisture, it can become soluble in chlorinates.
1H NMR (399.78 MHz, DMSO-d6): δ = 10.91 (s (br), 2 H, -+NH2-) 9.71 (t,
3JHH = 1.5 Hz, 2 H, N-CH-N), 8.24 (t, 3JHH = 1.6 Hz, 2 H, N-CH-CH-N),
7.95 (t, 3JHH = 1.6 Hz, 2 H, N-CH-CH-N), 7.12 (s, 4 H, Mes-H), 4.79 (s
(t), 4 H, Nim-CH2-CH2-N), 3.64 (s (t), 4 H, Nim-CH2-CH2-N), 2.32 (s, 6 H,
p-Mes-CH3), 2.09 (s, 12 H, o-Mes-CH3). 1H NMR (270 MHz, CDCl3, ppm): δ = 10.1 (s, 2 H, N-CH-N), 8.64 (t,
3JHH = 1.5 Hz, 2 H, N-CH-N), 7.13 (t, 3JHH = 1.6 Hz, 2 H, N-CH-CH-N),
6.94 (d, 3JHH = 1.6 Hz, 4 H, Mes-H), 5.27 (s(br), 2 H, Nim-CH2-CH2-N,
2H), 4.05 (s(br), 2 H, Nim-CH2-CH2-N), 2.31 (s, 6 H, p-Mes-CH3), 2.13
(s, 12 H, o-Mes-CH3).
161
13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 141.1, 138.4, 134.7, 131.0, 129.9, 125.0,
123.3, 47.69 (NIm-CH2-CH2-N), 46.49 (NIm-CH2-CH2-N), 21.19 (Mes-
CH3), 18.22 (Mes-CH3).
EA (%) calcd. for C28H38Cl3N5: C, 61.04; H, 6.95; N, 12.71.
found: C, 60.57; H, 6.93; N, 12.27.
Bis(xylylimidazolium-ethyl)amine (3,3'-(ammonium-bis(ethan-2,1-diyl))bis(1-(2,6-
dimethylphenyl)-imidazolium)trichloride, p2-Xyl×HCl)
3 Cl
H2NN
N
2
p2-Xyl×HCl
The compound is synthesized according to the above procedure with 1-(2,6-
dimethylphenyl)-imidazole (xylyl-imidazole) instead of 1-(2,4,6-Trimethylphenyl)-
imidazole (mesityl-imidazole).
1H NMR (399.78 MHz, DMSO-D6): δ = 10.95 (s (br), 2 H, -+NH2-) 9.78 (t,
3JHH = 1.6 Hz, 2 H, N-CH-N), 8.28 (t, 3JHH = 1.7 Hz, 2 H, N-CH-CH-N),
8.00 (t, 3JHH = 1.7 Hz, 2 H, N-CH-CH-N), 7.43 (t, 3JHH = 7.5 Hz, 1 H, p-
Ar-H), 7.31 (d, 3JHH = 7.5 Hz, 2 H, m-Ar-H), 4.83 (t, 3JHH = 5.5 Hz, 4 H,
Nim-CH2-CH2-N), 3.69 (t, 3JHH = 5.5 Hz, 4 H, Nim-CH2-CH2-N), 2.15 (s,
12 H, o-Ar-CH3).
2-(Adamant-1-yl)-4-methyl-6-(chloromethyl)phenol (p3-Ad,Me)
OH
Cl
p3-Ad,Me
2-Adamantyl-p-cresol (18.8 g, 77.3 mmol, 1 eq) is dissolved in toluene (200 mL) and
treated with HCl 37 % aq. (30 mL, 384 mmol, 5 eq). While stirring strongly,
162
paraformaldehyde (3.03 g, 100 mmol, 1.3 eq) is then added. The reaction mixture is stirred
vigorously overnight. The organic phase is separated, dried over Na2SO4, filtered and
evaporated. The crude product is purified by crystallization from toluene to give the
product as a white solid (10.5 g, 36.1 mmol, 47 %).
1H NMR (270 MHz, CDCl3, ppm): δ = 7.07 (d, 4JHH = 2.1 Hz, 1 H, Ar-H), 6.91 (d,
4JHH = 2.0 Hz, 1 H, Ar-H), 5.36 (s, 1 H, Ar-OH), 4.66 (s, 2 H, Ar-CH2-
Cl), 2.28 (s, 3 H, Ar-CH3), 2.13 – 2.10 (m (br), 6+3 H, Ad-H
superimposed by Ad-H2), 1.79 (s, 6 H, Ad-H2). 13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 152.0, 138.0, 129.9, 129.2, 128.2, 123.9,
44.80, 40.89, 37.15, 36.94, 29.14, 20.89.
EA (%) calcd. for C18H23ClO: C, 74.34; H, 7.97.
found: C, 74.58; H, 8.13.
2,4-di-tert-butyl-6-(chloromethyl)phenol (p3-tBu,tBu)
OH
Cl
p3-tBu,tBu
After a literature procedure[209] modified by Johannes Hohenberger:[183]
2,4-Di-tert-butylphenole (8.25 g, 40.0 mmol, 1 eq) was dissolved in toluene (30 mL) and
treated with HCl (37%, 15 mL, 180 mmol, 4.5 eq). Paraformaldehyde (1.57 g, 52.0 mmol,
1.3 eq) was added and the mixture was stirred overnight at room temperature. The organic
phase was separated, dried over Na2SO4 and the solvent was removed via rotary
evaporation to yield the product as yellow, viscous oil (9.23 g, 36.2 mmol, 91%).
1H-NMR (270 MHz, CDCl3, ppm): δ = 7.39 (d, 4J = 2.4 Hz, 2 H, CH), 7.12 (d, 4J =
2.7 Hz, 2 H, CH), 5.44 (s, 1 H, OH), 4.73 (s, 2 H, CH2Cl), 1.47 (s, 9 H,
C(CH3)3), 1.34 (s, 9 H, C(CH3)3).
163
13C{1H} NMR (67.8 MHz, CDCl3, ppm): δ = 151.77 (COH), 143.00 (CC(CH3)3), 137.22
(CC(CH3)3), 125.61(CH), 124.79 (CH), 123.17 (CCH2Cl), 45.18 (CH2),
35.06 (C(CH3)3), 34.45 (C(CH3)3), 31.69 (C(CH3)3), 30.03 (C(CH3)3).
(H3BIMPNMes,Ad,Me)(BPh4)2
NN
N
2
OH
2 BPh4
On addition of triethylamine (3.28 g, 4.52 mL, 32.6 mmol, 2.2 eq), a suspension of
bis(mesitylimidazoliumethyl)amine hydrochloride (9.00 g, 14.6 mmol, 1 eq) in CH2Cl2
(80 mL) becomes a clear solution. After heating to reflux, a solution of 2-(adamant-1-yl)-4-
methyl-6-(chloromethyl)phenol (5.70 g, 16.3 mmol, 1.1 eq) in CH2Cl2 (40 mL) is added
dropwise over 3 h. The reaction mixture is refluxed overnight. After cooling to RT, the
reaction mixture is filtered and evaporated. The residue is dissolved in MeOH (80 mL) and
heated to reflux. While stirring, a solution of NaBPh4 (10.0 g, 29.2 mmol, 2 eq) in MeOH
(20 mL) is added dropwise. The abundant white precipitate is filtered off and washed with
cool MeOH. The dried crude product is sonicated in ether (100 mL), filtered off and dried.
Benzene (80 mL) is added and the emerging biphasic mixture is strongly stirred for 2 h.
The supernatant liquid is decanted and the low-viscous residue is evaporated and dried in
vacuum to give the pro-ligand as a white solid (11.7 g, 8.75 mmol, 60 %).
Crystals suitable for X-ray single crystal analysis were obtained as colorless needles by
slow evaporation of a solution of the compound in a CH2Cl2 / ethyl acetate mixture after
anion exchange to tosylate (–OTs).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 8.37 (s, 1 H, Ar-OH), 7.33 (m, 16 H,
BPh4), 6.99 (s, 4+1 H, m-Mes-H superimposing Ar-H), 6.85 (tm, 3JHH =
7.1 Hz, 16 H, BPh4), 6.77 (tm, 3JHH = 7.1 Hz, 8 H, BPh4), 6.62 (d, 3JHH =
1.1 Hz, Ar-H), 5.96 (m, 2 H, NIm-CH-CH-NIm), 5.82 (m, 2 H, NIm-CH-
CH-NIm), 5.70 (s(br), 2H, NIm-CH-NIm), 3.35 (s, 2 H, Ar-CH2-N), 3.04
164
(m(t), 4 H, NIm-CH2-CH2-N), 2.38 (s, 6 H, p-Mes-CH3), 2.29 (m(t), 4 H,
NIm-CH2-CH2-N), 2.23 (s, 3 H, Ar-CH3), 2.10 (m, 6 H, Ad-H2), 2.08 (m,
3 H, Ad-H), 1.78 (m, 6 H, Ad-H2), 1.75 (s, 12 H, o-Mes-CH3). 13C{1H} NMR (100.624 MHz, CDCl3, ppm): δ = 164.7 (BPh4: Cquart.), 164.2 (BPh4:
Cquart.), 163.7 (BPh4: Cquart.), 163.2 (BPh4: Cquart.), 153.2 (Cquart.), 141.6
(Cquart.), 137.0 (Cquart.), 136.1 (BPh4: o-CH), 135.6 (NIm-CH-NIm), 134.0
(Cquart.), 130.4 (Cquart.), 129.8 (Cmesityl-H), 129.1 (Cquart.), 128.5 (Cquart.),
128.1 (Cphenol-H), 127.8 (Cphenol-H), 126.1 (BPh4: m-CH), 124.0 (NIm-CH-
CH-NIm), 122.4 (BPh4: p-CH), 121.9 (NIm-CH-CH-NIm), 119.9 (Cquart.),
58.1 (Ar-CH2-Nanchor), 53.0 (N-CH2-CH2-N), 45.8 (N-CH2-CH2-N), 40.6
(CAdH2), 37.2 (CAdH2), 29.2 (CAdH), 21.3 (p-Mes-CH3), 21.0 (phenol-
CH3), 18.0 (o-Mes-CH3); signal assignments of –B(Ph4)2 agree with
spectra in the SDBS database;[241] all assignments were verified through
H,H-COSY and C,H-correlation NMR spectra. 1H NMR (399.78 MHz, DMSO-d6, ppm): δ = 9.47 (t, 3JHH = 1.5 Hz, 2 H, N-CH-
N), 8.94 (s, 1 H, Ar-OH), 8.04 (t, 3JHH = 1.5 Hz, 2 H, N-CH-CH-N), 7.95
(t, 3JHH = 1.5 Hz, 2 H, N-CH-CH-N), 7.18 (m, 16 H, BPh4), overlapping
with 7.15 (s, 4 H, m-Mes-H), 6.92 (tm, 3JHH = 7.2 Hz, 16 H, BPh4), 6.79
(tm, 3JHH = 7.2 Hz, 8 H, BPh4), 6.86 (d, 4JHH = 1.6 Hz, 1 H, Ar-H), 4.49
(t, 3JHH = 6.2 Hz, 4 H, N-CH2-CH2-N), 3.90 (s, 2 H, Ar-CH2-N), 3.17 (t, 3JHH = 6.2 Hz, 4 H, N-CH2-CH2-N), 2.33 (s, 6 H, p-Mes-CH3), 2.17 (s,
3 H, Ar-CH3), 2.02 (s, 12 H, o-Mes-CH3), 1.98 (s (m), 9 H, Ad-H2 and
Ad-H), 1.66 (m, 6 H, Ad-H2); one aromatic proton is most likely
superimposed by one of the BPh4-signals.
EA (%) calcd. for C94H99B2N5O: C, 84.48; H, 7.47; N, 5.24.
found: C, 84.53; H, 7.72; N, 5.19.
IR (KBr, cm-1): ν~ = 3122 (m, C-H), 3089 (m, C-H), 3055 (s, C-H), 3037 (s,
C-H), 2998 (m, C-H), 2984 (m, C-H), 2904 (s, C-H), 2849 (m, C-H),
1545 (m, C-H), 1479 (s, C-H), 1199 (m, C-H), 852 (m, C-H), 734 (s, C-
H), 705 (s, C-H), 612 (m, C-H).
165
Figure 1. IR spectrum of (H3BIMPNMes,Ad,Me)(BPh4)2 (KBr pellet).
Figure 65. Cyclic voltammogram (green line) and linear sweep measurement (maroon line) of
(BIMPNMes,Ad,Me)(BPh4)2 in MeCN; the irreversible oxidation lies at 0.72 V.
166
(H3BIMPNMes,tBu,tBu
)(BPh4)2
NN
N
2
OH
2 BPh4
In the synthesis for (H3BIMPNMes,Ad,Me)(BPh4)2, 2,4-di-tert-butyl-6-(chloromethyl)phenol
is used in lieu of 2-(adamant-1-yl)-4-methyl-6-(chloromethyl)phenol.
1H NMR (399.78 MHz, DMSO-d6, ppm): δ = 9.44 (t, 3JHH = 1.5 Hz, 2 H, N-CH-
N), 9.19 (s, 1 H, Ar-OH), 8.05 (t, 3JHH = 1.5 Hz, 2 H, N-CH-CH-N), 7.97
(t, 3JHH = 1.5 Hz, 2 H, N-CH-CH-N), 7.18 (m, 16 H, BPh4), overlapping
with 7.15 (s, 4 H, m-Mes-H), 7.13 (d, 4JHH = 1.6 Hz, 1 H, Ar-H), 6.96 (d, 4JHH = 1.6 Hz, 1 H, Ar-H), 6.92 (tm, 3JHH = 7.2 Hz, 16 H, BPh4), 6.78
(tm, 3JHH = 7.2 Hz, 8 H, BPh4), 4.52 (t, 3JHH = 6.2 Hz, 4 H, N-CH2-CH2-
N), 3.96 (s, 2 H, Ar-CH2-N), 3.20 (t, 3JHH = 6.2 Hz, 4 H, N-CH2-CH2-N),
2.33 (s, 6 H, p-Mes-CH3), 2.02 (s, 12 H, o-Mes-CH3), 1.27 (s, 9 H,
C(CH3)3), 1.23 (s, 9 H, C(CH3)3).
(H3BIMPNXyl,tBu,tBu
)(BPh4)2
NN
N
2
OH
2 BPh4
In the synthesis for (H3BIMPNMes,Ad,Me)(BPh4)2, 2,4-di-tert-butyl-6-(chloromethyl)phenol
is used in lieu of 2-(adamant-1-yl)-4-methyl-6-(chloromethyl)phenol and
bis(xylylimidazoliumethyl)amine (p2-Xyl×HCl) in lieu of bis(mesitylimidazoliumethyl)-
amine (p2-Mes×HCl).
167
FD-MS (m/z) for C41H55N5O calcd. 634 (M+); obsd. 369 ((M+Cl)+, 100%), 172 ((2,6-xylyl-
imidazole)+, 64 %).
Figure 66. Cyclic voltammogram of (BIMPNXyl,tBu,tBu)Cl2 in MeCN; the irreversible oxidation lies
at 0.78 V.
K(BIMPNMes,Ad,Me
)
NN
N
2
O
K
A solution of (H3BIMPNMes,Ad,Me)(BPh4)2 (450 mg, 373 µmol, 1 eq) in THF (8 mL) and a
solution of potassium tert-butoxide (117 mg, 1045 µmol, 3.1 eq) are both cooled to –35 °C.
Both solutions are combined. The reaction mixture is stirred at RT for 2 h, evaporated and
extracted with Et2O to give the potassium salt of the ligand as a dark green solid (quant.).
Crystals suitable for X-ray single crystal analysis were obtained as colorless blocks by
cooling a saturated solution of diethyl ether, containing a small amount of benzene, to
–35 °C.
168
1H NMR (399.78 MHz, C6D6, ppm): δ = 6.97 (d, 4JHH = 1.3 Hz, Ar-H), 6.74 (s,
4 H, m-Mes-H), 6.50 (s, 2 H, NIm-CH-CH-NIm), 6.23 (s, 2 H, NIm-CH-
CH-NIm), 3.81 with a broad downfield shoulder (s(t) + s(br), 4+2 H, N-
CH2-CH2-N and Ar-CH2-N), 2.94 (s(t), 4 H, N-CH2-CH2-N), 2.57 (s, 3 H,
Ar-CH3), 2.33 (m, 6 H, Ad-H2), 2.17 (s, 6 H, p-Mes-H), 1.97 (m, 3 H,
Ad-H), 1.92 (s, 12 H, o-Mes-CH3), 1.80 (m, 6 H, Ad-H2); one Ar-H
signal is superimposed by the residual solvent signal, as established by
H,H-COSY NMR. 13C{1H} NMR (100.624 MHz, C6D6, ppm): δ = 208.8 (CCarbene), 167.7 (C-O), 138.7
(Cquart.), 137.8 (Cquart.), 137.6 (Cquart.), 135.5 (Cquart.), 131.0 (Cphenolate-H),
129.2 (Cmesityl-H), 127.6 (Cphenolate-H), 125.1 (Cquart.), 120.5 (CNHC-H),
120.3 (CNHC-H), 115.5 (Cquart.), 59.8 (Ar-CH-Nanchor), 57.4 (N-CH2-CH2-
N), 50.1 (N-CH2-CH2-N), 41.2 (CAdH2), 38.4 (CAdH), 37.7 (CAd,quart.),
30.1 (CAdH2), 21.8 (phenolate-CH3), 21.1 (p-Mes-CH3), 18.0 (o-Mes-
CH3); assignments were verified through H,H–COSY and C,H–
correlation NMR spectra.
K(BIMPNMes,tBu,tBu
)
NN
N
2
O
K
The deprotonation of the ligand precursor is carried out as described above for
(H3BIMPNMes,Ad,Me)(BPh4)2.
1H NMR (399.78 MHz, C6D6, ppm): δ = 7.13 (d, 4JHH = 1.3 Hz, Ar-H), 6.95 (d,
4JHH = 1.3 Hz, Ar-H), 6.74 (s, 4 H, m-Mes-H), 6.51 (s, 2 H, NIm-CH-CH-
NIm), 6.21 (s, 2 H, NIm-CH-CH-NIm), 3.89 (s(t), 4 H, N-CH2-CH2-N), 3.82
(s, 2H, N-CH2-Ar), 2.88 (s(t), 4 H, N-CH2-CH2-N), 2.13 (s, 6 H, p-Mes-
169
H), 1.91 (s, 12 H, o-Mes-CH3), 1.60 (s, 9 H, C(CH3)3), 1.56 (s, 9 H,
C(CH3)3).
K(BIMPNXyl,Ad,Me
)
NN
N
2
O
K
The deprotonation of the ligand precursor is carried out as described above for
(H3BIMPNMes,Ad,Me)(BPh4)2.
1H NMR (399.78 MHz, C6D6, ppm): δ = 7.42 (s(d), Ar-H), 7.05 (s(d), Ar-H), 6.90
(t, 3JHH = 6.8 Hz, 4 H, m-Xyl-H), 6.85 (d, 3JHH = 6.8 Hz, 2 H, p-Xyl-H),
6.48 (s, 2 H, NIm-CH-CH-NIm), 6.12 (s, 2 H, NIm-CH-CH-NIm), 3.88 (s(t),
4 H, N-CH2-CH2-N), 3.80 (s, 2H, N-CH2-Ar), 2.90 (s(t), 4 H, N-CH2-
CH2-N), 2.56 (s, 3 H, Ar-CH3), 2.31 (m, 6 H, Ad-H2), 1.97 (m, 3 H, Ad-
H), 1.91 (s, 12 H, o-Xyl-CH3), 1.80 (m, 6 H, Ad-H2).
K(BIMPNXyl,tBu,tBu
)
NN
N
2
O
K
The deprotonation of the ligand precursor is carried out as described above for
(H3BIMPNMes,Ad,Me)(BPh4)2.
1H NMR (399.78 MHz, C6D6, ppm): δ = 7.48 (s(d), Ar-H), 7.11 (s(d), Ar-H), 6.97
(t, 3JHH = 6.8 Hz, 4 H, m-Xyl-H), 6.90 (d, 3JHH = 6.8 Hz, 2 H, p-Xyl-H),
170
6.52 (s, 2 H, NIm-CH-CH-NIm), 6.14 (s, 2 H, NIm-CH-CH-NIm), 4.01 (s(t),
4 H, N-CH2-CH2-N), 3.81 (s, 2H, N-CH2-Ar), 2.87 (s(t), 4 H, N-CH2-
CH2-N), 1.90 (s, 12 H, o-Xyl-CH3), 1.65 (s, 9 H, C(CH3)3), 1.52 (s, 9 H,
C(CH3)3).
4.2.1.4 N,N’-benzyl-aryl-imidazolium
NN
OH
Cl
This compound is a side product in the chlorination route (see Scheme 22, p. 28). It can be
synthesized directly by treating the corresponding phenyl chloride with the imidazole.
1H NMR (399.78 MHz, CDCl3, ppm): δ = 10.16 (s(t), 1 H, N-CH-N), 8.79 (s, 1 H,
OH), 7.34 (t, 3JHH = 7.7 Hz, 1 H, p-Ar-H), superimposed with 7.37 (s(t),
1 H, N-CH-CH-N) and 7.33 (d, 4JHH = 2.3 Hz, 1 H, Ar-H), 7.19 (d, 3JHH = 7.7 Hz, 2 H, m-Ar-H), 7.12 (d, 4JHH = 2.3 Hz, 1 H, Ar-H), 7.07
(s(t), 1 H, N-CH-CH-N), 6.11 (s, 2 H, N-CH2-Ar), 2.10 (s, 6 H, CH3),
1.35 (s, 9 H, C(CH3)), 1.30 (s, 9 H, C(CH3)3).
171
4.2.2 Iron Complexes
4.2.2.1 [(BIMPNMes,Ad,Me
)FeII
]Cl (1)
K(BIMPNMes,Ad,Me) (317 mg, 432 µmol, 1 eq) is dissolved in Et2O (19 mL) and added to a
slurry of anhydrous FeCl2 (54.7 mg, 432 µmol, 1 eq) in pyridine (1 mL). The mixture is
stirred at RT overnight. The precipitated solid is filtered off, washed thoroughly with ether,
dried over vacuum and then stirred in benzene for several hours. The solid is filtered off
again, washed with benzene, pentane and dried to give the crude product as yellow solid
(285 mg, 362 µmol, 85 % if compound is considered pure; 329 µmol, 75 % accounting for
1 eq KCl per ligand molecule / 1.25 eq KCl per complex molecule).
KCl can be removed by repeated dissolution of the complex in MeCN or THF, filtration
through a plug of Celite and evaporation. For synthesis of 1[BPh4] and 2 the compound
was used as received.
The complex is well soluble in pyridine, acetonitrile, DMSO and THF, less soluble in
chlorinated solvents, insoluble in less polar solvents like hydrocarbons and diethyl ether.
Crystals suitable for X-ray crystallography were obtained as nearly colorless prisms by
slow diffusion of diethyl ether into an acetonitrile solution of 1.
1H NMR (399.78 MHz, CDCl3, ppm): δ = 52.87 (s, 6 H), 50.43 (s, 2 H), 35.76 (s,
2 H), 15.23 (s (vbr), 6 H), 9.70 (s, 3 H), 8.97 (s, 3 H), 7.3 – 6.4 (s (vbr),
10 H), 5.34 (s, 6 H), 4.62 (s, 12 H), -4.01 (s (vbr), 6 H); some signals are
not unequivocally integratable, due to their strong broadening; the signals
around 50 ppm are, however, very characteristic. 1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 52.02 (s, 6 H), 47.32 (s, 2 H), 40.29
(s, 2 H), 12.28 (s (br), 4 H), 11.5 – 6.8 (s (vbr), 12 H), 3.86 (s (br), 12 H),
3.43 (s (vbr), 4 H), 3.01 (s, 6 H), -3.3 – -7.2 (s (vbr), 8 H); all
paramagnetic signals in the range from 15 to -10 ppm are so broadened
and, in part, overlapping, that integration is not unequivocal. 1H NMR (399.78 MHz, THF-d8, ppm): δ = 53.58 (s, 6 H), superimposed signals:
52.4 – 49.2 (s (vbr)) and 51.01 (s), (together 6 H, probably 4 + 2 H),
46.38 (s, 2 H), 41.15 (s (br), 1 H), 27.53 (s, 1 H), 13.28 (s, 1 H), 11.77 (s
172
(br), 4 H), 9.04 (s (br), 1 H), 7.81 (s (vbr), 1 H), 6.14 (s (br), 12 H), 0.69
(s, 3 H), -0.57 (s, 3 H), -1.83 (s, 3 H), -2.62 (s, 3 H), -10.06 (s (vbr), 2 H),
-28.87 (s (vbr), 1 H); signal broadening and, in some cases, overlap,
impedes unequivocal integration, esp. in the range from 15 to -4 ppm; the
signals around 50 ppm are, however, very characteristic for the complex.
Elemental analysis is calculated for 1.25 eq KCl per complex molecule, which is in keeping
with the yield of the complex vs. amount of KCl precipitating during the synthesis. KCl
content varies for different batches depending on the yield.
EA (%) calcd.: C, 62.81; H, 6.43; N, 7.96.
found: C, 62.69; H, 6.44; N, 7.99.
IR (KBr, cm-1): ν~ = 3022 (m, C-H), 2901 (s, C-H), 2845 (s, C-H), 1491 (s),
1462 (s), 1287 (s), 1265 (s), 850 (m), 733 (m).
Figure 67. IR spectrum of 1 (KBr pellet).
57Fe Mößbauer (solid state, 77 K): δ = 0.68(1) mm s−1, ∆EQ = 3.28(1) mm s−1, ΓFWHM =
0.27(1) mm s-1.
(solid state, 293 K): δ = 0.57(1) mm s−1, ∆EQ = 3.16(1) mm s−1, ΓFWHM =
0.29(1) mm s-1.
173
Figure 68. Zero-field 57Fe Mößbauer spectrum of a solid sample of 1 recorded at 77 K. The solid
line represents the best fit obtained, with parameters given above.
Figure 69. Zero-field 57Fe Mößbauer spectrum of a solid sample of 1 recorded at 298 K. The solid
line represents the best fit obtained, with parameters given above.
174
Figure 70. Cyclic voltammogram (green line) and linear sweep measurement (maroon line) of
[(BIMPNMes,Ad,Me)Fe]Cl (1) in MeCN.
Figure 71. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Fe]Cl (1) in MeCN: FeII/FeIII redox-wave at
different scan rates.
175
4.2.2.2 [(BIMPNMes,tBu,tBu
)FeII
]Cl (1Mes,tBu,tBu
)
1Mes,tBu,tBu is prepared from K[BIMPNMes,tBu,tBu] analogously to 1 in about 50 % yield.
It was dissolved in THF and filtered over a plug of Celite. To the filtrate was added
benzene, and the THF was evaporated until a yellow-white precipitate remained in the
benzene. The precipitate was filtered off, and from this filter cake a Mößbauer sample was
prepared. 57Fe Mößbauer (solid state, 77 K): δ = 0.84(1) mm s−1, ∆EQ = 3.40(1) mm s−1, ΓFWHM =
0.30(1) mm s-1.
Figure 72. Zero-field 57Fe Mößbauer spectrum of a solid sample of 1Mes,tBu,tBu recorded at 77 K. The
solid line represents the best fit obtained, with parameters given above.
4.2.2.3 [(BIMPNXyl,tBu,tBu)FeII]Cl (1Xyl,tBu,tBu)
1Mes,tBu,tBu is prepared from K[BIMPNMes,tBu,tBu] analogously to 1. Mößbauer samples were
prepared directly from the pyridine/ether filter cake.
57Fe Mößbauer (solid state, 77 K): δ = 0.85(1) mm s−1, ∆EQ = 3.06(1) mm s−1,
ΓFWHM(1) = 0.57(1) mm s-1, ΓFWHM(2) = 0.46(1) mm s-1.
176
Figure 73. Unusual zero-field 57Fe Mößbauer spectrum of a solid sample of 1Xyl,tBu,tBu recorded at
77 K, featuring an asymmetric doublet. Both lines possess equal intensities but different line
widths. The solid line represents the best fit obtained, with parameters given above.
4.2.2.4 [(BIMPNMes,Ad,Me
)FeII
]Br (1[Br])
1[Br] is prepared from K[BIMPNMes,tBu,tBu] analogously to 1 with FeBr2 instead of FeCl2,
giving a salmon colored solid (79 %).
Elemental analysis is calculated for 1.25 eq KBr per complex molecule, which is in
keeping with the yield of the complex vs. amount of KCl precipitating during the synthesis.
KCl content varies for different batches depending on the yield.
EA (%) calcd.: C, 56.40; H, 5.77; N, 7.15.
found: C, 56.47; H, 5.83; N, 7.08. 57Fe Mößbauer (solid state, 77 K): δ = 0.68(1) mm s−1, ∆EQ = 3.22(1) mm s−1, ΓFWHM =
0.37(1) mm s-1.
177
Figure 74. Zero-field 57Fe Mößbauer spectrum of a solid sample of 1[Br] recorded at 77 K. The
solid line represents the best fit obtained, with parameters given above.
4.2.2.5 [(BIMPNMes,Ad,Me
)FeII
]BPh4 (1[BPh4])
[(BIMPNMes,Ad,Me)FeII]Cl (1, crude product containing KCl) (45 mg, 57 µmol, 1 eq) and an
excess of NaBPh4 (98 mg, 286 µmol, 5eq) were stirred at RT in THF (6 mL) overnight.
The precipitated white solid is filtered off, the yellow filtrate is evaporated and the residue
extracted with CH2Cl2 to give [(BIMPNMes,Ad,Me)FeII]BPh4 as a yellow solid (53 mg,
49 µmol, 87%).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 53.19 (s, 6 H), 50.67 (s, 2 H), 35.92 (s,
2 H), 17 – 13 (s (vbr), 6 H), 9.78 (s, 3 H), 8.96 (s, 3 H), 8.32 (m, 8 H,
BPh4), 7.71 (m, 8 H, BPh4), 7.44 (m, 4 H, BPh4), 7.1 – 6.4 (s (br), 10 H),
5.23 (s, 6 H), 4.68 (s, 12 H), -1.5 – -5.0 (s (vbr), 6 H); the complexes’
limited solubility in CDCl3 results in relatively weak, hard-to-integrate
signals. The signals around 50 ppm are, however, very characteristic. 1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 51.90 (s, 6 H), 47.24 (s, 2 H), 40.21
(s, 2 H), 12.33 (s (vbr), 6 H), 11.5 – 7.0 (s (vbr), 12 H), 7.27 (m, 8 H,
178
BPh4), 6.99 (tm, 3JHH = 7.35 Hz, 8 H, BPh4), 6.83 (tm, 3JHH = 7.35 Hz,
4 H, BPh4), 3.88 (s (vbr), 12 H), 3.43 (s (vbr), 4 H), 3.02 (s (vbr), 6 H), -
3.5 – -7.0 (s (vbr), 6 H); all paramagnetic signals in the range from 15 to -
10 ppm are so broadened and, in part, overlapping, that integration is not
unequivocal. 1H NMR (399.78 MHz, THF-d8, ppm): δ = 51.93 (s, 6 H), 49.94 (s, 2 H), 35.45 (s,
2 H), 16.5 – 12.5 (s (vbr), 6 H), 9.95 (s, 3 H), 9.22 (s, 3 H), 8.04 (m, 8 H,
BPh4), 7.48 (m, 8 H, BPh4), 7.25 (m, 4 H, BPh4), 7.1 – 6.2 (s (br), 10 H),
5.14 (s, 6 H), 4.74 (s, 12 H), -1.0 – -5.0 (s (vbr), 6 H); all paramagnetic
signals in the range from 17 to -6 ppm are so broadened and, in part,
overlapping, that integration is not unequivocal.
IR (KBr, cm-1): ν~ = 3053 (m, C-H), 2903 (s, C-H), 2846 (s, C-H), 1456 (s),
1285 (m), 1263 (s), 852 (m), 734 (s, B-P), 706 (s, B-P), 612 (m).
Figure 75. IR spectrum of 1[BPh4] (KBr pellet).
57Fe Mößbauer (solid state, 77 K): δ = 0.66(1) mm s−1, ∆EQ = 3.42(1) mm s−1, ΓFWHM =
0.28(1) mm s-1.
179
Figure 76. Zero-field 57Fe Mößbauer spectrum of a solid sample of 1[BPh4] recorded at 77 K. The
solid line represents the best fit obtained, with parameters given above.
4.2.2.6 [(BIMPNMes,Ad,Me
)FeII
(N3)] (2)
[(BIMPNMes,Ad,Me)FeII]Cl (1, crude product containing KCl) (213 mg, 270 µmol, 1 eq) and
an excess of NaN3 (88 mg, 1.35 µmol, 5 eq) are stirred in a small amount of DMF (2 mL)
for at least one day. The mixture is filtered through a plug of Celite, layered with a large
amount of Et2O (18 mL) and the mixture is left to stand at low temperature until the
solvents have mixed entirely. The solid is filtered off, washed with Et2O and dried. The
filtercake is dissolved in THF, filtered through a plug of Celite into benzene, and the
mixture concentrated until all THF has evaporated and the precipitate can be filtered off to
give 2 as a light yellow solid (198 mg, 250 µmol, 93 %).
The complex is well soluble in pyridine, acetonitrile, DMSO, THF, and chlorinated
solvents, insoluble in less polar solvents like hydrocarbons and diethyl ether.
Layering a DMF solution of 2 with Et2O at –35 °C afforded light orange, nearly colorless
blocks suitable for X-ray single crystal analysis.
180
1H NMR (399.78 MHz, CDCl3, ppm): δ = 52.65 (s, 6 H), 50.22 (s, 2 H), 35.72 (s,
2 H), 15.22 (s (vbr), 6 H), 9.61 (s, 3 H), 8.88 (s, 3 H), 7.4 – 6.4 (s (vbr),
10 H), 5.35 (s, 6 H), 4.61 (s, 12 H), -4.25 (s (vbr), 6 H); some signals are
not unequivocally integratable, due to their strong broadening; the signal
distribution is, however, very characteristic for a [(BIMPNMes,Ad,Me)FeII]-
complex. 1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 51.65 (s, 6 H), 47.09 (s, 2 H), 39.92
(s, 2 H), 12.09 (s (br), 4 H), 11.0 – 6.0 (s (vbr), 12 H), 3.90 (s (br), 16 H),
3.03 (s (br), 6 H), -5.15 (s (vbr), 8 H); the signals in the range from 15 to
-10 ppm are so broadened and, in part, overlapping, that integration is not
unequivocal. 1H NMR (399.78 MHz, THF-d8, ppm): δ = 80.94 (s(br), 1 H), 55.91 (s(br), 2 H),
52.68 (s, 6 H), 51.24 (s(br), 2 H), 49.73 (s, 2 H), 46.88 (s(br), 1 H), 45.58
(s, 2 H), 28.69 (s(br), 2 H), 12.57 (s(br), 1 H), overlapping with 10.73
(s(vbr), 4 H), overlapping with 9.39 (s(br), 1 H), 5.3 (s(vbr), 9 H), 0.84 (s,
6 H), 0.34 (s, 6 H), -1.38 (s(br), 6 H), -9.02 (s(br), 2 H), -27.50 (s(br,
1 H); unequivocal integration is hampered by signal broadening.
EA (%) calcd.: C, 69.69; H, 7.12; N, 14.13.
found: C, 69.55; H, 7.26; N, 13.54.
IR (KBr, cm-1): ν~ = 2900 (s, C-H), 2845 (s, C-H), 2086 (s, N3), 2054 (s,
N3), 2001 (s, N3), 1489 (m), 1465 (s), 1403 (m), 1321 (s), 1296 (m), 1270
(m), 848 (m), 736 (m), 580 (w), 525 (w).
181
Figure 77. IR spectrum of 2 (KBr pellet).
57Fe Mößbauer (solid state, 77 K): δ = 0.83(1) mm s−1, ∆EQ = 3.24(1) mm s−1, ΓFWHM =
0.31(1) mm s-1.
Figure 78. Zero-field 57Fe Mößbauer spectrum of a microcrystalline, finely ground sample of 2
recorded at 77 K. The solid line represents the best fit obtained, with parameters given above.
182
Figure 79. Cyclic voltammogram (green line) and linear sweep measurement (maroon line) of
[(BIMPNMes,Ad,Me)Fe(N3)] (2) in MeCN.
Figure 80. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Fe(N3)] (2) in MeCN: FeII/FeIII redox-wave
at different scan rates.
183
4.2.2.7 Redox-Chemistry with [(BIMPNMes,Ad,Me
)FeII
]Cl (1)
Reduction of [(BIMPNMes,Ad,Me)FeII]Cl (1) with sodium:
A yellow solution of [(BIMPNMes,Ad,Me)FeII]Cl (1) (30 mg, 38 µmol, 1 eq) in THF (3 mL) is
stirred at RT over an excess of sodium, which has been dispersed with a spatula in order to
increase its surface. After 2 h, dark red solution is filtered through a plug of Celite,
evaporated, and the residue is extracted with benzene. Lyophilization from benzene yields
a dark red powder (28 mg, 36.7 µmol for “[(BIMPNMes,Ad,Me)FeI]”, 96 %)
57Fe Mößbauer (solid state, 77 K):
subspectrum 1 ( (41 % relative area): δ = 0.72(1) mms–1, ∆EQ = 3.39(1)
mms–1, ΓFWHM = 0.40(1);
subspectrum 2 (59 % relative area): δ = 0.76(1) mms–1, ∆EQ = 2.83(1)
mms–1, ΓFWHM = 0.70(1).
Figure 81. Zero-field 57Fe Mößbauer spectrum of a solid sample of the dark red reaction product
obtained when a THF solution of 1 is stirred over sodium; recorded at 77 K. The black solid line
represents the best fit obtained, which is the sum of the two subspectra 1 (red line) and 2 (blue line),
with parameters given above.
184
Oxidation of [(BIMPNMes,Ad,Me
)FeII
(N3)] (2) with AgSbF6:
AgSbF6 (7 mg, 20 µmol, 1.5 eq) is dissolved in THF (1.5 mL) and added dropwise to a
solution of [(BIMPNMes,Ad,Me)Fe(N3)] (2) (11.3 mg, 14 µmol, 1 eq) in THF (1.5 mL) which
has been cooled to –35 °C. The reaction mixture turns cherry red. After stirring for ten
minutes, the mixture is filtered through a plug of Celite, which retains some black solids,
and the filtrate is evaporated. The residue is extracted with benzene and the benzene filtrate
is lyophilized to give a dark, cherry red powder (9 mg, 9 µmol for
“[(BIMPNMes,Ad,Me)Fe(N3)]SbF6”, 62 %).
The product’s IR and EPR spectra are shown and discussed in section 2.2.2.
Oxidation of [(BIMPNMes,Ad,Me
)FeII
]Cl (1) with NOBF4:
NOBF4 (11 mg, 94 µmol, 1.8 eq) is added to a yellow solution of [(BIMPNMes,Ad,Me)FeII]Cl
(1) (40.3 mg, 51.3 µmol, 1 eq) in MeCN (8 mL) and the mixture is stirred at RT overnight.
The cherry red mixture is filtered, the filtrate evaporated and the residue extracted with
THF to give a dark cherry red solid (44.5 mg, 51 µmol for “[(BIMPNMes,Ad,Me)FeCl]BF4”,
99 %; the solid may still contain traces of THF).
The Mößbauer spectrum is shown and discussed in section section 2.2.2.
4.2.3 Manganese Complexes
4.2.3.1 [(BIMPNMes,Ad,Me
)MnII
Cl] (3)
K(BIMPNMes,Ad,Me) (137 mg, 187 µmol, 1 eq) is dissolved in ether (15 mL) and added to a
slurry of anhydrous MnCl2 (23.5 mg, 187 µmol, 1 eq) in pyridine (1 mL). The mixture is
stirred at RT overnight. The precipitated solid is filtered off, washed thoroughly with ether,
and dried in vacuum. KCl is removed by repeated dissolution of the complex in THF,
filtration through a plug of CeliteTM and evaporation. After stirring in benzene for several
hours, the solid is filtered off again, washed with benzene, pentane and dried to give 3 as
white solid (81 mg, 103 µmol, 55 %).
185
The complex is well soluble in acetonitrile, DMSO and THF, less soluble in chlorinated
solvents, insoluble in less polar solvents, like hydrocarbons and diethyl ether.
Layering a pyridine solution of 3 with ether afforded colorless, prism-shaped crystals
suitable for X-ray diffraction.
1H NMR silent.
Elemental analysis was performed on crystalline material and is calculated for 1 pyridine
per 2 complex molecules, in accordance with the results from X-ray single crystal analysis.
EA (%) calcd.: C 70.60; H 7.17; N 9.34.
found: C 71.16; H 7.42; N 9.07.
IR (KBr, cm-1): ν~ = 3119 (w), 2899 (s (br), C–H), 2841 (s, C–H), 1468 (s),
1273 (m).
Figure 82. IR spectrum of 3 (KBr pellet).
UV/Vis (THF, nm (Μ−1 cm-1)) : λ (ε) = 257 (11333), 309 (5750).
186
Figure 83. Electronic absorption spectrum of 3, recorded in THF.
SQUID (χdia = 3.30 ⋅ 10–4 cm3mol–1, RT) µeff = 5.82 µB
Figure 84. Variable temperature SQUID magnetization measurements on three samples from
independently synthesized batches of complex 3.
187
4.2.3.2 [(BIMPNMes,Ad,Me
)MnII
N3] (4)
[(BIMPNMes,Ad,Me)MnIICl] (3) (60 mg, 76 µmol, 1 eq) and an excess of NaN3 (5.4 mg,
84 µmol, 1.1 eq) are stirred in acetonitrile overnight. The mixture is filtered and
evaporated. The yellow residue is stirred in benzene (10 mL) for 2 hours and filtered. The
filter cake is washed with benzene and Et2O and dried in vacuum to give 4 as a white solid
(35 mg, 44 µmol, 63 %).
The complex is well soluble in acetonitrile, DMSO and THF, less soluble in chlorinated
solvents, insoluble in less polar solvents, like hydrocarbons and diethyl ether.
Slow diffusion of ether into a pyridine solution of 4 afforded colorless, needle-shaped
crystals suitable for X-ray diffraction.
1H NMR silent.
EA (%) calcd.: C, 69.59; H, 7.36; N, 14.11.
found: C, 68.92; H, 7.04; N, 13.80.
IR (KBr, cm-1): ν~ = 2900 (s, C–H), 2844 (s, C–H), 2077 (s, N3), 1466 (s).
Figure 85. IR spectrum of 4 (KBr pellet).
188
UV/Vis (THF, nm (Μ−1 cm-1)) : λ (ε) = 257 (14889), 309 (7444).
Figure 86. Electronic absorption spectrum of 4, recorded in THF.
4.2.4 Diamagnetic Complexes
4.2.4.1 Indium
InCl3 (64.6 mg, 221 µmol, 1 eq) is suspended in pyridine (2 mL). K(BIMPNMes,Ad,Me)
(214 mg, 292 µmol, 1eq) is dissolved in diethyl ether (8 mL) and added. The reaction
mixture is stirred overnight, the precipitate is filtered off, washed several times with diethyl
ether and dried to give the crude product as a salmon-colored solid (159 mg).
The 1H NMR spectra (in CDCl3 or MeCN-d3) show signals in the aliphatic range similar to
the spectra of 5 (vide infra), suggesting complex formation. However, pyridine signals
superimpose product signals in the aromatic region; also, some signals of re-protonated
ligand are observed.
189
4.2.4.2 Nickel
A solution of K(BIMPNXyl,tBu,tBu) (285 mg, 425 µmol, 1eq) in THF (5 mL) and a
suspension of Ni(COD)2 (118 mg, 425 µmol, 1 eq) in THF (5 mL) are cooled to –35 °C
before the solution of the ligand is added to the suspension. The mixture is stirred at RT for
3 h, during which time all solids dissolve. The reaction liquid is laced with a small amount
of pentane (< 1 mL), filtered through a plug of Celite® and concentrated. The product is
precipitated with pentane, filtered off, washed first with cold diethyl ether, then pentane,
and dried to give a dark yellow solid (167 mg, 229 µmol, 54 %).
In order to exchange K+ for PNP+, the yellow solid is stirred with an excess PNPCl in
MeCN. After evaporation, the residue is extracted with THF. Evaporation and extraction
are repeated to remove KCl and excess PNPCl.
FD-MS (m/z) for C41H52N5NiO calcd. 689 (M+); obsd. 389 (M+, 29 %), 539 (PNP+, 100 %).
4.2.4.3 [(BIMPNMes,Ad,Me
)Zn]OTs (5[OTs])
Three white solids, (H3BIMPNMes,Ad,Me)(OTs)2 (200 mg, 192 µmol, 1 eq), NaOMe
(31.5 mg, 586 µmol, 3.05 eq), and ZnCl2 (26.2 mg, 192 µmol, 1 eq), are suspended in THF
(7 mL) and stirred at RT for 4 h (or longer as convenient). The milky–white suspension is
filtered, the filtercake washed with THF, Et2O (2x) and dried in vacuo. The filtercake is
extracted with CHCl3 and the colorless filtrate evaporated to give a white solid (118 mg,
127 µmol, 66 %).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 7.83 (d, 3JHH = 8.0 Hz, 2H, –OTs), 7.49
(s(br), 2H, NIm-CH-CH-NIm), 7.14 (d, 3JHH = 8.0 Hz, 2H, –OTs), 6.98 (d,
2H, m-Mes-H), superimposing (1H, Ar-H), 6.81 (s(br), 2H, NIm-CH-CH-
NIm), overlapping with 6.80 (d, 4JHH = 1.4 Hz, 2H, m-Mes-H), 6.66 (d, 4JHH = 1.9 Hz, 1H, Ar-H), 4.35 (s(br) or m, 2H, N-CHH-CHH-N), 4.13
(ddm, 3JHH = 14.1 Hz, 3JHH = 6.6 Hz, 2H, N-CHH-CHH-N), 3.63 (s, 2H,
Ar-CH2-N), 3.47 (ddm, 3JHH = 14,1 Hz, 2H, N-CHH-CHH-N), 3,16 (ddm,
190
3JHH = 6,6 Hz, 2H, N-CHH-CHH-N), 2.33 (s, 3H, –OTs), 2.32 (s, 6H,
p-Mes-CH3), 2.18 (s, 3H, Ar-CH3), 1.91 (s, 6H, o-Mes-CH3), 1.85 (s, 6H,
o-Mes-CH3 and m, 3H, Ad-Hx), 1.66 – 1.53 (m, 9H, Ad-Hx), 1.48 – 1.41
(m, 3H, Ad-H); some couplings between protons in the aromatic region
are not observed due to superposition of signals; some of the coupling
constants of the protons on the ethylene arms are not resolved;
assignments are confirmed by H,H-COSY NMR spectroscopy. 13C{1H} NMR (100.624 MHz, CDCl3, ppm): 171.8, 164.5, 144.3, 139.3, 138.9, 138.7,
135.4, 134.4, 134.0, 129.3, 129.0, 128.6, 128.4, 127.9, 126.1, 123.9,
122.5, 122.2, 121.9, 65.4, 59.7, 49.1, 39.8, 36.9, 36.7, 29.1, 21.3, 21.0,
20.6, 17.4, 17.1; the 13C NMR spectrum shows the right number of
signals, in a reasonable distribution with regard to expected signals by
tabulated increments and also by comparison to the spectra of the
protonated ligand or its potassium salt. However, assignments are not
given here as they have not been confirmed by C,H-correlation NMR.
4.2.4.4 [(BIMPNMes,Ad,Me
)ZnCl] (5)
K(BIMPNMes,Ad,Me) (275 mg, 375 µmol, 1 eq) is dissolved in diethyl ether (10 mL) and
added to a suspension of ZnCl2 in pyridine (1 mL). The slurry is stirred overnight, filtered,
and the filtercake is washed thoroughly with diethyl ether and dried in vacuum. The
filtercake is extracted with CH2Cl2, the extracted filtrate concentrated, and the product is
precipitated with diethyl ether, filtered off and dried to give a white solid (170 mg,
214 µmol, 57 %).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 7.61(s, 2H, NIm-CH-CH-NIm), 6.86 (2H,
m-Mes-H), superimposing (1H, Ar-H), 6.82 (d, 4JHH = 1.0 Hz, 2H,
m-Mes-H), 6.81 (s, 2H, NIm-CH-CH-NIm), 6.67 (d, 4JHH = 1.8 Hz, 1H,
Ar-H), 4.44 (s(br) or m, 2H, N-CHH-CHH-N), 4.25 (m, 2H, N-CHH-
CHH-N), 3.63 (s, 2H, Ar-CH2-N), 3.47 (ddm, 3JHH = 13.9 Hz, 3JHH = 6.8 Hz, 2H, N-CHH-CHH-N), 3.22 (ddm, 3JHH = 13,6 Hz, 2H, N-
CHH-CHH-N), 2.32 (s, 6H, p-Mes-CH3), 2.18 (s, 3H, Ar-CH3), 1.91 (s,
191
6H, o-Mes-CH3), 1.85 (s, 6H, o-Mes-CH3 and m, 3H, Ad-Hx), 1.65 – 1.54
(m, 9H, Ad-Hx), 1.49 – 1.42 (m, 3H, Ad-H); some couplings between
protons in the aromatic region are not observed due to superposition of
signals and some of the coupling constants of the protons on the ethylene
arms are not resolved. Assignments are confirmed by H,H-COSY NMR
spectroscopy.
4.2.5 Cobalt Complexes
4.2.5.1 [(BIMPNMes,Ad,Me)CoII]Cl (6)
K(BIMPNMes,Ad,Me) (382 mg, 519 µmol, 1.1 eq) is dissolved in benzene and added to a
suspension of CoCl2 (61 mg, 472 µmol, 1 eq) in benzene. The reaction mixture is stirred at
RT overnight and filtered. The green filtercake is washed thoroughly with benzene and
pentane and dried over vacuum (283 mg, 359 µmol, 76 % if compound is considered pure;
314 µmol, 67 % accounting for 1 eq KCl per ligand molecule).
The crude product can be used as obtained for salt metathesis to 6[PF6] or 7. For removal
of KCl, the crude product can be repeatedly dissolved in THF or CH2Cl2, filtered through
CeliteTM and evaporated.
The complex is well soluble in pyridine, acetonitrile, and DMSO, less soluble in
chlorinated solvents, insoluble in THF and less polar solvents like hydrocarbons.
Crystals suitable for X-ray single crystal analysis were obtained as brown platelets by slow
evaporation of an acetonitrile solution of 6.
1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 54.43 (s, 2 H), 52.48 (s, 6 H), 35.11
(s, 3 H), 26.99 (s (vbr), 2 H), 21.64 (s, 2 H), 8.64 (s, 9 H), 1.68 (s, 6 H), -
0.50 (s, 6 H), -1.70 (s, 12 H), -2.85 (s, 6 H); no further signals are
observed from 170 to -130 ppm; some broad singlets may be
superimposed and the signals of two protons are broadened into the
baseline. 1H NMR (399.78 MHz, CDCl3, ppm): δ = 55.41 (s, 2 H), 52.48 (s, 6 H), 32.91 (s
(vbr)), overlapping with 28.08 (s (vbr), together 5 H), 22.00 (s, 2 H), 5.38
(s (vbr), 9 H), 0.66 (s, 6 H), -1.28 (s, 6 H), -2.15 (s (br), 12 H), -4.11 (s,
192
6 H); no further signals are observed from 170 to -130 ppm; some broad
singlets may be superimposed and the signals of two protons are
broadened into the baseline.
IR (KBr, cm-1): ν~ = 3025 (m, C-H), 2903 (s, C-H), 2846 (s, C-H), 1491 (m),
1455 (s), 1282 (s), 1263 (s), 851 (w), 805 (w), 731(w).
Figure 87. IR spectrum of 6 (KBr pellet).
Figure 88. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Co]Cl (6) in MeCN.
193
Figure 89. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Co]Cl (6) in MeCN: CoI/CoII redox-wave at
different scan rates.
4.2.5.2 [(BIMPNMes,tBu,tBu)CoII]Cl (6Mes,tBu,tBu)
K(BIMPNMes,Ad,Me) (350 mg, 501 µmol, 1.1 eq) is dissolved in benzene and added to a
suspension of CoCl2 (65 mg, 501 µmol, 1 eq) in benzene. The reaction mixture is stirred at
RT overnight, whereupon it turns into a dark green slurry, then filtered through a plug of
CeliteTM and the filtrate is lyophilized. The residue is washed thoroughly with pentane and
dried over vacuum to give a dark green powder (178 mg, 242 µmol, 66 %).
The complex is soluble in THF, acetonitrile, DMSO, and aromatic hydrocarbons, insoluble
in aliphatic hydrocarbons.
1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 55.19 (s, 2 H), 40.19 (s, 3 H), 24.51
(s, 2 H), 23.11 (s, 2 H), 22.25 (s, 1 H), 10.36 (s, 12 H), 6.35 (s, 3 H), 4.44
(s, 3 H), 3.12 (s, 16 H), -0.98 (s, 9 H), -2.40 (s, 1 H), -12.29 (s, 1 H),
-13.19 (s, 1 H); due to signal broadening, integration not unequivocal and
tentative at best; further signals may be broadened into the baseline. 1H NMR (399.78 MHz, C6D6, ppm): a number of broadened signals in the region
from 7 to 1 ppm so strongly overlap each other, that no proper account of
194
these signals can be given, but the following signals downfield and
upfield of that reagion may be used to identify the product: δ = 54.08 (s,
2 H), 53.67 (s, 1 H), 28.80 (s(vbr, by its shape most likely two signals
merging into one), 6 H), 22.84 (s, 2 H), 20.29 (s, 1 H), -0.76 (s, 1 H),
-1.63 (s, 9 H), -2.58 (s, 2 H), -3.00 (s, 1 H), -4.10 (s, 1 H); no further
signals are obseved from 170 to –130 ppm, although some may be
broadened into the baseline.
4.2.5.3 [(BIMPNXyl,tBu,tBu
)CoII
]Cl (6Xyl,tBu,tBu
)
6Xyl,tBu,tBu is synthesized from K(BIMPNXyl,Ad,Me) analoguously to 6Mes,tBu,tBu and obtained
as a dark green powder in about 80 % yield.
The complex is soluble in THF, acetonitrile, DMSO, and aromatic hydrocarbons, slightly
soluble in diethyl ether, insoluble in aliphatic hydrocarbons.
1H NMR (399.78 MHz, C6D6, ppm): a number of broadened signals in the region
from 7 to 1 ppm so strongly overlap each other, that no proper account of
these signals can be given, but the following signals downfield and
upfield of that reagion may be used to identify the product: δ = 54.31 (s,
4 H), 53.71 (s, 1 H), 33.97 (s (by its shape most likely two signals
merging into one), 2 H), 28.10 (s, 16 H), 22.92 (s, 4 H), 19.31 (s, 3 H); no
further signals are obseved from 170 to –130 ppm, although some may be
broadened into the baseline.
Elemental analysis is calculated for 1.25 eq KCl per complex molecule, which is in keeping
with the yield of the complex vs. amount of KCl precipitating during the synthesis. KCl
content varies for different batches depending on the yield.
EA (%) calcd.: C, 60.16; H, 6.42; N, 8.56.
found: C, 60.35; H, 6.15; N, 8.30.
195
4.2.5.4 [(BIMPNMes,Ad,Me
)CoII
]PF6 (6[PF6])
[(BIMPNMes,Ad,Me)CoII]Cl (crude 6, which may still contain KCl, 127 mg, 161 µmol, 1 eq)
and NaPF6 (32.4 mg, 193 µmol, 1.2 eq) are stirred in benzene overnight. The green slurry
is filtered and the filtercake is extracted with CH2Cl2, whereby all other salts are removed
via filtration through a plug of CeliteTM. After addition of some hexane, the filtrate is
evaporated to give 6[PF6] as a dark green solid (132 mg, 146 µmol, 91 %).
The complex is well soluble in polar solvents such as acetonitrile, DMSO and THF, less
soluble in chlorinated solvents, insoluble in less polar solvents like hydrocarbons.
Green needles suitable for X-ray single crystal analysis can be obtained by cooling a THF
solution of 6[PF6] to –35 °C.
1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 53.64 (s, 2 H), 51.70 (s, 6 H), 34.63
(s (br), 3 H), 26.52 (s (vbr), 2 H), 21.45 (s, 2 H), 8.55 (s (vbr), 9 H), 1.68
(s, 6 H), -0.46 (s, 6 H), -1.61 (s, 12 H), -2.77 (s, 6 H); no further signals
are observed from 170 to -130 ppm; some broad singlets may be
superimposed and the signals of two protons are broadened into the
baseline. 31P{1H} NMR (161.83 MHz, MeCN-d3, ppm): δ = -143.9 (sept, 1JPF = 706 Hz).
1H NMR (399.78 MHz, CDCl3, ppm): δ = 56.64 (s, 2 H), 54.40 (s, 6 H), 28.12 (s
(vbr), 5 H), 22.09 (s, 2 H), 5.31 (s (vbr), 9 H), 0.66 (s, 6 H), -1.28 (s,
6 H), -2.17 (s (br), 12 H), -4.11 (s, 6 H); no further signals are observed
from 170 to -130 ppm; some broad singlets may be superimposed and the
signals of two protons are broadened into the baseline. 31P{1H} NMR (161.83 MHz, CDCl3, ppm): δ = -141.7 (sept, 1JPF = 711 Hz).
1H NMR (399.78 MHz, THF-d8, ppm): δ = 54.07 (s, 2 H), 52.41 (s, 6 H), 32.66
(s(vbr), overlapping with 28.65 (s(vbr), together 5 H), 21.63 (s, 2 H), 5.61
(s(vbr), 9 H), 0.74 (s, 6 H), -1.14 (s, 6 H), -1.79 (s(br), 12 H), -3.83 (s,
6 H); no further signals are observed from 170 to -130 ppm; some broad
196
singlets may be superimposed and not distinguishable as different peaks,
the signals of two protons are broadened into the baseline. 31P{1H} NMR (161.83 MHz, THF-d8, ppm): δ = -142.8 (sept, 1JPF = 709 Hz).
EA (%) calcd.: C, 61.47; H, 5.83; N, 7.79.
found: C, 61.73; H, 6.04; N, 7.94.
IR (KBr, cm-1): ν~ = 3169 (w), 2901 (s (br), C–H), 2847 (s, C–H), 1465 (s),
1263 (m), 843 (s (br), PF6), 557 (s).
Figure 90. IR spectrum of 6[PF6] (KBr pellet).
UV/Vis (THF, nm (Μ−1 cm-1)) : λ (ε) = 246 (13743), 287 (6346), shoulder around
340 (ca. 1900), 435 (1665), shoulder around 516 (ca. 600), 606 (270), 684
(623).
197
Figure 91. Electronic absorption spectrum of 6[PF6], recorded in THF.
Figure 92. Magnified section of the electronic absorption spectrum of 6[PF6], recorded in THF.
SQUID (χdia = 4.90 ⋅ 10–4 cm3mol–1, RT) µeff = 4.28 µB.
198
Figure 93. Variable temperature SQUID magnetization measurements on three samples from
independently synthesized batches of complex 6[PF6].
Figure 94. Cyclic voltammogram (green line) and linear sweep measurement (maroon line) of
[(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) in MeCN.
199
Figure 95. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Co]PF6 (6[PF6]) in MeCN: CoI/CoII redox-
wave at different scan rates.
4.2.5.5 [(BIMPNXyl,tBu,tBu
)CoII
]PF6 (6Xyl,tBu,tBu
[PF6])
[(BIMPNXyl,tBu,tBu)CoII]Cl (crude 6Xyl,tBu,tBu, which still contains KCl, 30 mg, 41 µmol,
1 eq) is stirred with NaPF6 (35 mg, 21 µmol, 5eq) at RT in benzene (4 mL) overnight. The
mixture is filtered through CeliteTM, spiked with some hexane and evaporated. The residue
is extracted with benzene, lyophilized, washed with pentane and dried in vacuum to give a
dark green powder (33 mg, 40 µmol, 96 %).
Crystals suitable for X-ray single crystal analysis were obtained as green platelets from a
saturated solution of 6Xyl,tBu,tBu[PF6] in benzene.
1H NMR (399.78 MHz, C6D6, ppm): δ = 55.09 (s, 2 H), 29.39 (s, 3 H), 27.87 (s,
3 H), 22.75 (s, 2 H), 3.77 (s, 12 H), 3.16 (s, 15 H), -0.26 (s, 3 H); further
signals may be broadened into the baseline. 31P{1H} NMR (161.83 MHz, CDCl3, ppm): δ = -138.6 (sept, 1JPF = 712 Hz).
200
4.2.5.6 [(BIMPNMes,Ad,Me
)CoII
(N3)] (7)
[(BIMPNMes,Ad,Me)CoII]Cl (309 mg, 391 µmol, 1 eq) and an excess of NaN3 (180 mg,
2,77 mmol, 7 eq) are suspended in benzene (8 mL), a small amount of DMF is added and
the suspension is stirred overnight. The blue solid is filtered off, washed thoroughly with
benzene, and extracted with THF. The THF solution is concentrated and cooled to -35 °C
to give the product as blue needles (230 mg, 289 µmol, 74 %).
The complex is well soluble in polar solvents such as acetonitrile, DMSO and THF, also
soluble in chlorinated solvents, insoluble in less polar solvents like hydrocarbons. Its
solubility in THF lies around 10 mg/ mL.
Cooling a saturated THF solution of 7 containing a small amount of benzene afforded blue
prisms suitable for X-ray diffraction.
1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 53.30 (s, 2 H), 51.32 (s, 6 H), 34.33
(s (br), 3 H), 26.40 (s (vbr), 2 H), 21.34 (s, 2 H), 8.26 (s (vbr), 9 H), 1.63
(s, 6 H), -0.48 (s, 6 H), -1.59 (s, 12 H), -2.79 (s, 6 H); no further signals
are observed from 170 to -130 ppm; some broad singlets may be
superimposed and the signals of two protons broadened into the baseline. 1H NMR (399.78 MHz, CDCl3, ppm): δ = 55.65 (s, 2 H), 54.88 (s, 6 H), 33.1 (s
(vbr), 2 H), 28.4 (s (vbr), 3 H), 22.19 (s, 2 H), 5.50 (s (vbr), 9 H), 0.63 (s,
6 H), -1.34 (s, 6 H), -1.98 (s, 12 H), -4.20 (s, 6 H); no further signals are
observed from 170 to -130 ppm; some broad singlets may be
superimposed and the signals of two protons broadened into the baseline. 1H NMR (399.78 MHz, THF-d8, ppm): δ = 91.90 (s, 1 H), 73.29 (s, 1 H), 57.44 (s,
1 H), 42.65 (s, 1 H), three overlapping signals: 40.22, 39.78, 39.23
(s+s+s, 3+1+2 H), two overlapping signals: 29.76 with upfield shoulder
(s+s, 1+1 H), 19.19 (s, 1 H), 12.71 (s, 1 H), overlapping peaks: 7.38 and
6.95, 6.32, 4.89 (all s(br), 5+2+3 H), broad signal(s) from 4.0 to 2.8
(12 H), broad signals from 2.8 to 2.0 with maxima at 2.54 and 2.41
(12 H), -2.24 (s, 3 H), -4.08 (s, 3 H), -6.10 (s, 1 H), -9.58 (s, 1 H); some
signals are very hard to integrate unequivocally due to their strong
201
broadening and/or overlap with each other and the residual solvent
signals, especially in the area from 8 to 2 ppm; signals further up- or
downfield are, however, very characteristic in their distribution and
relative intensity for this compound.
Elemental analysis is calculated for crystals containing 0.5 THF molecules per complex
molecule, from which solvent it was recrystallized. This has reproducibly given the right
analytical values, and is also in agreement with X-ray crystallography and SQUID data
analysis.
EA (%) calcd.: C, 69.28; H, 7.28; N, 13.47.
found: C, 69.42; H, 7.38; N, 13.31.
IR (KBr, cm-1): ν~ = 2901 (s, C–H), 2846 (s, C–H), 2081 (s, N3), 2044 (s,
N3), 1999 (s, N3), 1489 (m), 1460 (s), 1407 (m), 1321 (s), 1284 (m), 849
(m), 810 (m), 763 (m), 737 (m), 730 (m), 600(w), 580 (w), 520 (w).
Figure 96. IR spectrum of 7 (KBr pellet).
202
Figure 97. IR spectrum of 7 in THF soluion.
Figure 98. Magnified section of the IR spectrum of 7 in THF soluion.
203
UV/Vis (THF, nm (Μ−1 cm-1)) : λ (ε) = 253 (12540, shoulder), 310 (6795), small
shoulder, 388 (2130), ca. 565 (ca. 435, shoulder), 606 (659), 652 (722),
855 (125).
Figure 99. Electronic absorption spectrum of 7, recorded in THF.
Figure 100. Magnified section of the electronic absorption spectrum of 7, recorded in THF.
204
SQUID (χdia = 4.45 ⋅ 10–4 cm3mol–1, RT) µeff = 4.42 µB.
Figure 101. Variable temperature SQUID magnetization measurements on three samples from
independently synthesized batches of complex 7.
Figure 102. Cyclic voltammogram (green and blue lines) and linear sweep measurement (maroon
line) of [(BIMPNMes,Ad,Me)Co(N3)] (7) in MeCN.
205
Figure 103. Cyclic voltammogram of [(BIMPNMes,Ad,Me)Co(N3)] (7) in MeCN: CoI/CoII redox-wave
at different scan rates.
4.2.5.7 [(BIMPNMes,Ad,Me
)CoI] (8)
[(BIMPNMes,Ad,Me)Co]Cl (8) is suspended in benzene and stirred with an excess of KC8 at
room temperature, until all green solid has dissolved. The brown suspension is filtered over
a plug of CeliteTM. The filtrate is lyophilized to give the product as a red-brown solid in
quantitative yield.
The resulting brown powder is soluble in benzene and THF, soluble in diethyl ether, less
soluble in lower alkanes. Its solubility in benzene is approx. 10 mg/ mL.
Brown block-shaped crystals suitable for X-ray single crystal analysis were obtained by
cooling a diethyl ether solution of 8 to –35 °C.
1H NMR (399.78 MHz, benzene-d6, ppm): δ = 25.0 (s (vbr), 2 H), 18.9 (s (vbr),
1 H), 13.15 (s, 5 H), 12.82 (s, 2 H), 8.98 (s, 2 H), 3.5 (s (vbr), 5 H), 2.1
(s (vbr), 9 H), 0.56 (s, 6 H), 0.51 (s, 12 H), 0.23 (s, 6 H), –1.19 (s, 6 H).
Signal integration is not unequivocal.
EA (%) calcd.: C, 73.28; H, 7.49; N, 9.29.
found: C, 73.02; H, 7.40; N, 8.69.
206
Elemental analysis calculated for 0.5 eq. KCl per complex molecule (see p. 95):
EA (%) calcd.: C, 69.82; H, 7.15; N, 8.85.
found: C, 69.79; H, 6.98; N, 8.60.
SQUID (χdia = 4.25 ⋅ 10–4 cm3mol–1, RT) µeff = 3.12 µB.
Figure 104. Temperature-dependent SQUID magnetization data (1 T) for five independently
synthesized samples of [(BIMPNMes,Ad,Me)CoI] (8). Magnetic moment (µeff) plotted versus
temperature (T). Data were corrected for diamagnetism.
4.2.5.8 Reactions of [(BIMPNMes,Ad,Me)CoI] (8) with aryl azides
In a typical NMR-scale experiment, about 6-8 mg of 8 were dissolved in either benzene-d6
or THF-d8. The azide was either added as solid with a spatula (trityl azide and AdN3) or by
dipping a glass pipette into the viscous azide (MesN3 and PhN3) and flushing the azide into
the NMR-tube with a small amount of NMR solvent.
Bulk reaction of [(BIMPNMes,Ad,Me
)CoI] (8) with phenyl azide PhN3:
To a solution of [(BIMPNMes,Ad,Me)CoI] (8) (52 mg, 69 µmol, 1 eq) in benzene (6 mL) is
added PhN3 (8.2 mg, 69 µmol, 1 eq) via a glass pipette. The solution’s color turns from
brown to green, and effervescence can be observed well if the solution is not stirred during
addition. The solution is stirred overnight, filtered through CeliteTM, and lyophilized. The
207
residual green powder is re-dissolved in a small amount of benzene. The product is
precipitated with pentane, filtered off and dried over vacuum to give a green solid (52 mg,
63 µmol, 91 % for “[(BIMPNMes,Ad,Me)Co=NPh]”).
The green solid is soluble in THF and benzene, less soluble in diethyl ether, and insoluble
in alkanes.
1H NMR (399.78 MHz, benzene-d6, ppm): δ = 13.16 (s, 2 H), 13.01 (s, 1 H), 12.03
(s, 1 H), 11.92 (s, 1 H), 11.20 (s, 1 H), 10.73 (s, 1 H), 6.60 (s, 1 H), 6.42
(s, 1 H), 5.84 (s, 2 H), 5.40 (s, 2 H), 4.38 (s, 2 H), 3.48 (s, 1 H), 2.47 (s,
3 H), 1.59 (s, 3 H), 0.61 (s, 3 H), -0.15 (s, 3 H), -0.84 (s, 3 H), -1.14 (s,
2 H), -1.29 (s, 3 H), -2.38 (s, 2 H), -3.82 (s, 6 H), -5.89 (s, 2 H). Signal
integration is not always unequivocal, and further signals may be
broadened into the baseline.
Elemental analysis calculated for “[(BIMPNMes,Ad,Me)Co=NPh]” (C52H61CoN6O):
EA (%) calcd.: C, 73.91; H, 7.28; N, 9.95.
found: C, 73.38; H, 6.90; N, 9.98.
SQUID (χdia = 4.85 ⋅ 10–4 cm3mol–1, RT) µeff = 4.10 µB.
Figure 105. Temperature-dependent SQUID magnetization data (1 T) of the green product from the
reaction between [(BIMPNMes,Ad,Me)CoI] (8) and PhN3. Magnetic moment (µeff) plotted versus
temperature (T). Data were corrected for diamagnetism.
208
Violet crystals suitable [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] (9*) for single crystal
X-ray diffraction analysis were grown by diffusion of pentane into a benzene solution of
the green product. Violet crystals were also obtained by diffusion of pentane into a THF
solution of the green product
Bulk reaction of [(BIMPNMes,Ad,Me
)CoI] (8) with mesityl azide MesN3:
Reaction and work-up are carried out analogously to the reaction with PhN3 and yields a
green solid (yields around 35% when calculated for [(BIMPNMes,Ad,Me*N3Mes)Co] (10*)).
The green solid is soluble in THF and benzene and insoluble in diethyl ether and alkanes.
Green, block-shaped crystals suitable for single crystal X-ray analysis were grown by
layering a benzene solution in an NMR-tube with pentane and hexane.
Elemental analysis calculated for [(BIMPNMes,Ad,Me*N3Mes)CoII] (C55H67CoN8O):
EA (%) calcd.: C, 72.19; H, 7.38; N, 12.24.
found: C, 71.62; H, 7.33; N, 10.23.
4.2.5.9 Photolysis Experiments with [(BIMPNMes,Ad,Me
)Co(N3)] (7)
Prolonged heating to reflux does not change the 1H NMR spectra of 7 in THF-d8 or
MeCN-d3.
NMR-scale photolysis of 7 in THF-d8:
In a typical experiment, 5 mg of 7 are dissolved in THF-d8 (ca. 0.7 mL) and the sample is
irradiated and 1H NMR spectra are taken at regular intervals.
For trapping experiments, the trapping reagent is added to the sample stoichiometrically or
in small excess.
For 1H NMR experiments vs. time, one spectrum is taken before irradiation (t = 0) to
determine conversion by comparing signal integrals with the residual solvent signal.
Product spectrum for photolysis of 7 in the presence of tBu3-phenol: 1H NMR (399.78 MHz, THF-d8, ppm): δ = 48.45 (s, 3 H), 46.49 (s, 2 H), 26.07 (s,
1 H), 24.17 (s, 1 H), 18.27 (s, 1 H), 12.72 (s, 2 H), 9.27 (s, 1 H), 5.8 to
209
5.0 (s, 6 H), 4.38 (s, 2 H), 2.7 to 2.2 (s, 3 H), -0.15 (s, 3 H), -0.63 (s, 3 H),
-1.32 (s, 3 H), -2.56 (s, 3 H), 9.64 (s, 3 H); further signals are in all
probability not observed due to paramagnetic broadening.
IR pellet photolysis of 7:
A KBr pellet of 7 was irradiated with a mercury vapor lamp and the reaction monitored
over time by IR spectroscopy.
As reference, another KBr pellet of 7 was left in the dark. No change was observed, except
for a slight growth of the OH stretching band, confirming that the observed changes in the
irradiated KBr pellet do not stem from reaction with air (oxygen or moisture).
The overlay of the photolysis spectra was optimized for the region 3300 to 2500 cm–1, in
which no change was expected, by the least squares method, using two parameters to adjust
offset and transmission intensities of the graphs.
Figure 106. Photolysis of 15N labeled 7 in a KBr pellet, followed by IR spectroscopy.
210
EPR experiments.
EPR samples of 7 in methyl-THF with concentrations of ca. 1.5 mM (prepared by
dissolving 6 mg in 5 mL) were frozen in liquid nitrogen, inserted into the EPR cavity and
therein cooled to photolysis temperature. The samples were irradiated through a quartz
glass window in the EPR cavity with a LOT 150 W Xe-OF arc lamp, using a glass pane as
filter.
For photolysis outside the EPR cavity with liquid nitrogen cooling, a quartz glass Dewar
was used (Figure 107). Care was taken that the sample was frozen at all times until
measurement, which included intermittent irradiation when no filter was used on the lamp,
and fast transfer to the measurement cavity.
Figure 107. Quartz Dewar used for photolysis with liquid nitrogen cooling outside of the EPR
cavity.
211
4.2.6 TIMENMes Cobalt Chemistry
4.2.6.1 [(TIMENMes)Co(Cl)]Cl
This complex can be synthesized by letting the corresponding Co(I) complex stand in
CH2Cl2 overnight[225] or by reacting the free ligand TIMENMes[70-71, 225] (1 eq) with a
substoichiometric amount of CoCl2 (0.9 eq) in THF, from where the product precipitates.
For characterization, see the Diplomarbeit by Kropp.[225]
4.2.6.2 [(TIMENMes
)Co(N3)]OTs (12[OTs])
Path A (2-step synthesis): [(TIMENMes)CoII(Cl)]Cl (235 mg, 300 µmol, 1 eq), an excess of
NaN3 (120 mg, 1.85 mmol, 6.1 eq), and NaOTs (58.3 mg, 300 µmol, 1 eq) are stirred in
THF for at least 24 h. Reaction work-up is carried out as described in Path B.
Path B (1-pot synthesis): TIMENMes[70-71, 225] (500 mg, 765 µmol, 1.11 eq), CoCl2 (89.4 mg,
689 µmol, 1 eq), an excess of NaN3 (269 mg, 4.13 mmol, 6 eq), and NaOTs (133.7 mg,
689 µmol, 1 eq) are stirred in THF for at least 24 h. The precipitate is filtered off onto a
plug of Celite, washed thoroughly with THF, and extracted with MeCN. If the MeCN
filtrate is not clear, it should be filtered through Celite until a clear solution remains which
is evaporated to give the product as blue crystals (424 mg, 458 µmol, 66 %).
If necessary, the product can be recrystallized from MeCN to increase purity.
The compound is light sensitive; the vessel for synthesis should therefore be wrapped in
tinfoil to exclude light and the compound be stored in the dark. It is soluble in polar
solvents such as DMSO and MeCN and in chlorinated solvents, insoluble in THF, Et2O and
hydrocarbons.
Single crystals were obtained as blue blocks by cooling a MeCN solution of 12[OTs] to
–35 °C.
1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 75.73 (s, 3 H), 33.56 (s, 3 H), 32.35
(s, 3 H), 9.58 (s(br), 3 H), 8.94 (s, 3 H), 7.61 (d, 3JHH = 7.4 Hz, 2 H, –OTs: Ar-H), 7.14 (d, 3JHH = 7.4 Hz, 2 H, –OTs: Ar-H), 5.65 (s, 3 H), 2.61
(s, 9 H), 2.32 (s, 3 H, –OTs: Ar-CH3), 1.24 (s(vbr), 18 H), -4.04 (s, 3 H);
no further signals were observed from 170 to –130 ppm; 1 signal of 3 H
212
may either be swallowed by the residual solvent signal at 1.94 ppm or
broadened into the baseline.
EA (%) calcd.: C, 63.55; H, 6.31; N, 15.13; S 3.46.
found: C, 62.83; H, 6.30; N, 15.04; S 3.20.
IR (KBr, cm-1): ν~ = 2092 (s, N3).
Figure 108. IR-spectrum of [(TIMENMes)Co(N3)]OTs (12[OTs], KBr-pellet).
4.2.6.3 [(TIMENMes)Co(N3)]PF6 (12[PF6])
12[PF6] was synthesized and purified like 12[OTs] (path B), using NaPF6 (1 eq) instead of
NaOTs.
As expected, the 1H NMR spectrum of 12[PF6] is practically identical to that of 12[OTs]
except of course for the three signals of the –OTs anion: 1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 75.58 (s, 3 H), 33.56 (s, 3 H), 32.37
(s, 3 H), 9.61 (s(br), 3 H), 8.94 (s, 3 H), 5.65 (s, 3 H), 2.61 (s, 9 H), 1.13
(s(vbr), 18 H), -4.06 (s, 3 H); no further signals were observed from 170
to –130 ppm; 1 signal of 3 H may either be swallowed by the residual
solvent signal at 1.94 ppm or broadened into the baseline.
213
31P{1H} NMR (161.83 MHz, MeCN-d3, ppm): δ = -143.9 (sept, 1JPF = 706 Hz).
EA (%) calcd.: C, 56.06; H, 5.71; N, 15.57.
found: C, 55.26; H, 5.60; N, 15.76.
4.2.6.4 Photolysis Experiments with [(TIMENMes
)Co(N3)]+
In a typical NMR-scale experiment, 7 mg of 12[OTs] (or equivalent amount of 12[PF6]) is
dissolved in MeCN-d3 (ca. 0.7 mL) and irradiated with a mercury vapor lamp. The reaction
is monitored by 1H NMR, and total conversion can be observed after about 2 h of
irradiation.
In a typical vial-scale experiment, 50 mg of 12[OTs] (or equivalent amount of 12[PF6]) is
dissolved in MeCN (20 mL) and irradiated with a mercury vapor lamp. For work-up, the
solution is evaporated and the residue extracted with benzene.
1H NMR of the photolysis product: 1H NMR (399.78 MHz, MeCN-d3, ppm): δ = 78.73 (s, 1 H), 62.30 (s, 1 H), 52.21
(s, 1 H), 44.26 (s, 1 H), 29.60 (s, 1 H), 29.04 (s, 1 H), 27.06 (s, 1 H), 24.4
(s(vbr), 1 H), 21.49 (s, 1 H), 20.06 (s(br), 1 H), 16.2 (s(vbr), 1 H), 11.71
(s, 1 H), 9.80 (s, 1 H), 9.59 (s, 1 H), 8.98 (s, 3 H), 8.46 (s, 1 H), 7.55 (d, 3JHH = 7.3 Hz, 2 H, –OTs: Ar-H), 7.13 (d, 3JHH = 7.3 Hz, 2 H, –OTs: Ar-
H), 6.46 (s, 3 H), 6.27 (s, 1 H), 5.0 (s(vbr), 1 H), 3.91 (s, 1 H), 2.40 (s,
3 H), 2.34 (s, 4 H), 2.32 (s, 3 H, –OTs: Ar-CH3), 1.33 (s, 2 H), 1.06 (s(br),
2 H), 0.48 (s, 2 H), -0.72 (s, 3 H), -2.76 (s, 3 H), -4.64 (s, 1 H), -7.47 (s,
3 H); no further signals were observed from 170 to –130 ppm. Integration
in the region 4.5 to –0 ppm is difficult due to partially overlapping signals
(compound signals and solvent signals), the intensity of four protons may
therefore be hidden due to three possibilities:
- too small integral assignment in the region 10 to 0 ppm
- overlap with solvent signal(s)
- broadening into the baseline
214
4.3 Magnetism Data and Simulation Parameters
Table 16. Experimental and simulation parameters of the variable temperature SQUID
magnetization measurements.
complex µeff (RT)
[µB]
µeff (2 K)
[µB] g-value
| D |
[cm-1
] |E/D| comment#
[(BIMPNMes,Ad,Me)MnCl] 3 5.82 5.13 1.970 0.426 0.000 0.5 pyridine
[(BIMPNMes,Ad,Me)MnCl] 3
variable field measurement 5.81 (variable) 1.968
D =
- 0.426 0.03 0.5 pyridine
[(BIMPNMes,Ad,Me)Co]PF6
6[PF6] 4.28 3.15 2.202 26.837 0.000 -
[(BIMPNMes,Ad,Me)Co(N3)] 7 4.42 3.32 2.266 7.086 0.305 0.5 THF
“[(BIMPNMes,Ad,Me)Co=NPh]”
(see p. 100) 4.10 2.42 2.123 29.776 0.300 -
# In agreement with elemental analysis of the sample, these diamagnetic impurities were included
for the simulation.
215
4.4 Crystallographic Details
4.4.1 X-ray Crystal Structure Determination Details
Colorless needles of (H3BIMPNMes,Ad,Me)(OTs)2 were obtained by slow evaporation of a
solution of the compound in a dichloromethane / ethyl acetate mixture after anion exchange
from BPh4– to tosylate (OTs–). Colorless blocks of K(BIMPNMes,Ad,Me) were obtained by
cooling a saturated solution of diethyl ether, containing a small amount of benzene, to
-35 °C. Nearly colorless prisms of 1 were obtained by slow diffusion of diethyl ether into
an acetonitrile solution. Layering a DMF solution of 2 with Et2O at –35 °C afforded light
orange, nearly colorless blocks. Layering a pyridine solution of 3 with ether afforded
colorless, prism-shaped crystals. Slow diffusion of benzene into an acetonitrile solution of
4 afforded colorless, needle-shaped crystals. Brown platelets of 6 were obtained by slow
evaporation of an acetonitrile solution. Green needles of 6[PF6] were obtained by cooling a
THF solution to –35 °C. Green platelets of 6Xyl,tBu,tBu[PF6] were obtained from a saturated
benzene solution. Cooling a saturated THF solution of 7 containing a small amount of
benzene afforded blue prisms. Brown blocks of 8 were obtained by cooling a diethyl ether
solution to –35 °C. Slow diffusion of pentane into a benzene solution of the green reaction
product between 8 and PhN3 yielded violet platelets of compound 9*. Green blocks of 10*
were obtained by layering a benzene solution with pentane and hexane. Green needles of
the insertion product 11* grew from a THF solution that was cooled down from 30 °C to
23 °C. Blue blocks of 12[OTs] were recovered from an acetonitrile solution that was
cooled to -35 °C.
Suitable single crystals of the investigated compounds were embedded in protective
perfluoropolyalkyether oil and transferred to the cold nitrogen gas stream of the
diffractometer. Intensity data for (H3BIMPNMes,Ad,Me)(OTs)2 · 0.2 EtOAc were collected
using CuKα radiation (λ = 1.54178 Å) on a Bruker Kappa APEX 2 IµS Duo diffractometer
equipped with QUAZAR focusing Montel optics and for 8 using MoKα radiation, (λ =
0.71073 Å) on a Bruker Smart APEX2 diffractometer equipped with a graphite
monochromator. All other intensity data were collected using MoKα radiation (λ = 0.71073
Å) either on a Bruker-Nonius Kappa CCD equipped with a graphite monochromator
(2 · 0.61 DMF · 1.89 Et2O, 3 · 0.5 pyridine, 6 · MeCN, 12[OTs] · 3 MeCN) or on a Bruker
216
Kappa APEX 2 IµS Duo diffractometer equipped with QUAZAR focusing Montel optics
([K2(BIMPNMes,Ad,Me)2(C6H6)] · 3 Et2O · 0.5 C6H6, 2MeCN · 2 MeCN, 4 · 3 C6H6,
6[PF6] · 3 THF, 6Xyl,tBu,tBu[PF6] · 2.5 C6H6, 7 · 2 THF · 0.5 C6H6, 9* · C6H6, 10* · 0.461 C6H6
· 0.539 n-C5H12, 11*· THF · 0.16 H2O). Data were corrected for Lorentz and polarization
effects, semiempirical absorption corrections were performed on the basis of multiple scans
using SADABS.[242] The structures were solved by direct methods and refined by full-matrix
least-squares procedures on F2 using SHELXTL NT 6.12.[243]
All non-hydrogen atoms were refined with anisotropic displacement parameters. The
positions of the O bound hydrogen atom in 9* and of the water hydrogen atom in 11* were derived
from a difference Fourier synthesis and were refined. All other hydrogen atoms were placed in
positions of optimized geometry. The isotropic displacement parameters of all H atoms were tied to
those of their corresponding carrier atoms by a factor of 1.2 or 1.5. Crystallographic data, data
collection, and structure refinement details are given in Tables 17 – 21. Selected bond
distances and angles are listed in Tables 22 – 35.
(H3BIMPNMes,Ad,Me)(OTs)2 crystallized with 0.2 molecules of EtOAc per formula unit. This
solvent molecule was situated on a crystallographic inversion centre (on atom C62) and it
was disordered. SIMU, FLAT and DFIX restraints were applied for this molecule and no
hydrogen atoms were included.
In the crystal of K(BIMPNMes,Ad,Me), one benzene molecule was coordinated to one of the
potassium cations (K2). This benzene was disordered. Two alternative orientations were
refined resulting in site occupancies of 53.4(7) and 46.6(7) % for the atoms C93 – C98 and
C93A – C98A, respectively. Another solvate benzene molecule was situated on a
crystallographic inversion centre. The compound crystallized with a total of three diethyl
ether molecules in its asymmetric unit. These solvent molecules were disordered over six
different sites. The resulting site occupancies were 77.4(8) % for C201 – C205, 35.0(7) %
for C211 – C215, 27.8(8) % for C301 – C305, 65.8(8) % for C311 – C315, 39.0(9) % for
C401 – C405, and 54.8(11) % for C411 – C415. SAME, SIMU, and ISOR restraints were
applied in the refinement of the disordered atoms. Apparent short intermolecular H...H
contacts were due to the heavy disorder of the diethyl ether solvent molecules.
In the crystal of 1, the chloride anion was disordered over two different sites with the
occupancy of the major component being 93.6(2) % for Cl1 and of the minor component
217
being 6.4(2) % for Cl1A. The compound crystallized with one molecule of acetonitrile per
formula unit.
Compound 2 crystallized with two symmetrically independent molecules of the complex
and a total of 3.788 molecules of diethyl ether and 1.212 molecules of DMF per
asymmetric unit. The solvent molecules were subjected to disorder and in part two different
molecules shared one crystallographic site. A number of restraints were applied in the
refinement of the disordered solvent molecules including SIMU, ISOR, FLAT and one
DFIX restraint.
Compound 3 crystallized with 0.5 molecules of pyridine per formula unit. This solvent
molecule was disordered on a crystallographic inversion centre. ISOR restraints were
applied in the refinement of the solvent molecule.
Compound 6 crystallized with one molecule of MeCN per formula unit.
Compound 6[PF6] crystallized with three molecules of THF. SIMU restraints were applied
in the refinement of one of the THF solvent molecules.
6Xyl,tBu,tBu
[PF6] crystallized with a total of 2.5 molecules of benzene. One of these solvent
molecules was situated on a crystallographic inversion centre.
Complex 7 crystallized with two independent molecules of the complex and a total of four
molecules of THF and one molecule of benzene in the asymmetric unit. Two of the THF
solvent molecules were disordered. Two alternative orientations were refined in each case
resulting in site occupancies for the affected atoms of 47.7(9) and 52.3(9) % for C202 and
C212 in one case and of 70.6(4) and 29.4(4) % for O300 – C304 and O310 – C314,
respectively, in the second case. SAME, DFIX, SIMU, and ISOR restraints were applied in
the refinement of the solvent molecules.
The crystal structure of 8 contains three independent molecules in the asymmetric unit.
The dinuclear Co complex 9* is situated on a crystallographic inversion centre of the space
group and exhibits Ci molecular symmetry. The compound crystallized with one molecule
of benzene per dinuclear complex. This solvent molecule was situated on a crystallographic
inversion centre.
In one of the mesityl groups of 10*, one of the methyl groups is disordered with two
positions slightly deviating from the mesityl plane being refined (site occupancies: 54(3)
and 46(3) % for C53 and C53A). The compound crystallized with a total of 0.461(6)
218
molecules of benzene and 0.539(1) molecules of n-pentane being disordered on one single
crystallographic site. DFIX, SAME, SIMU, and ISOR restraints were applied in the
refinement of the disorder.
The crystal of 11* was composed of two different species with a majority of 93.5(2) % of
the NH insertion product and 6.5(2) % of the minor component [Co(BIMPNMes,Ad,Me)]Cl.
The compound crystallized with a total of one molecule of THF and 0.16 molecules of H2O
per formula unit. The THF molecule was disordered. Two different orientations were
refined with resulting site occupancies of 48.8(6) and 51.2(6) % for O100 – C104 and
O110 – C114, respectively. The water molecule was situated on a crystallographic 2-fold
axis and only partially occupied. SAME and SIMU restraints were applied in the
refinement of the disordered THF.
12[OTs] crystallized with three molecules of MeCN per formula unit. SIMU restraints
were applied for one of the solvent molecules (N300 – C302).
219
4.4.2 Crystal Data, Data Collection, and Structure Refinement Details:
Table 17. Crystallographic data, data collection and refinement details of
(H3BIMPNMes,Ad,Me)(OTs)2 · 0.2 EtOAc and [K2(BIMPNMes,Ad,Me)2(C6H6) ]· 3 Et2O · 0.5 C6H6.
[H3BIMPNMes,Ad,Me]
(OTs)2 · 0.2 EtOAc
[K2(BIMPNMes,Ad,Me)2
(C6H6)]
· 3 Et2O · 0.5 C6H6
Empirical formula C60.8H74.6N5O7.4S2 C113H151K2N10O5
Mol. weight 1057.97 1807.64
Crystal size [mm] 0.19×0.08×0.04 0.20×0.18×0.14
Temperature [K] 100(2) 100(2)
Crystal system monoclinic triclinic
Space group C2/c P-1
a [Å] 51.022(3) 15.904(2)
b [Å] 8.3174(4) 18.674(3)
c [Å] 36.5011(18) 19.304(3)
α [°] 90 72.310(2)
β [°] 132.078(2) 69.182(2)
γ [°] 90 76.540(3)
V [Å3] 11497.2(10) 5055.3(12)
Z 8 2
ρ [g cm3] (calc.) 1.222 1.188
µ [mm1] 1.292 0.152
F (000) 4525 1954
Tmin; Tmax 0.753; 0.640 0.746; 0.619
2Θ interval [ °] 4.66 ≤ 2Θ ≤ 136.70 4.12 ≤ 2Θ ≤ 52.74
Coll. refl. 36905 71769
Indep. refl.; Rint 10253; 0.0321 20502; 0.0406
Obs. refl. F0 ≥ 4σ(F) 8902 13685
No. ref. param. 729 1357
wR2 (all data) 0.0994 0.2317
R1 (F0 ≥ 4σ(F)) 0.0369 0.0768
GooF on F2 1.027 1.261
∆ρmax/min 0.612; –0.372 1.621; –0.942
220
Table 18. Crystallographic data, data collection and refinement details of
[(BIMPNMes,Ad,Me)FeII(NCMe)]Cl · 2 MeCN (1MeCN · MeCN), [(BIMPNMes,Ad,Me)FeIIN3] · 1.894 Et2O
· 0.606 DMF (2 · 1.894 Et2O · 0.606 DMF), and [(BIMPNMes,Ad,Me)MnCl] · 0.5 pyridine (3 · 0.5
pyridine).
1MeCN
· MeCN 2 · 1.894 Et2O
· 0.606 DMF 3 · 0.5 pyridine
Empirical formula C50H62ClFeN7O C55.39H79.18FeN8.61O3.5 C48.5H58.5ClMnN5.5O
Mol. weight 868.37 977.52 824.90
Crystal size [mm] 0.20×0.14×0.10 0.28×0.22×0.11 0.35×0.30×0.20
Temperature [K] 100(2) 100(2) 150(2)
Crystal system triclinic monoclinic monoclinic
Space group P-1 P21/c P21/c
a [Å] 10.1155(11) 24.081(5) 16.416(2)
b [Å] 11.4124(12) 16.3320(15) 14.879(2)
c [Å] 20.714(2) 35.883(2) 18.5420(10)
α [°] 89.033(2) 90 90
β [°] 85.620(2 131.205(11) 105.713(7)
γ [°] 79.081(2) 90 90
V [Å3] 2341.2(4) 10618(2) 4359.7(8)
Z 2 8 5
ρ [g cm 3] (calc.) 1.232 1.223 1.257
µ [mm1] 0.423 0.336 0.407
F (000) 924 4206 1752
Tmin; Tmax 0.746; 0.690 0.964; 0.661 0.757; 0.922
2Θ interval [ °] 3.64 ≤ 2Θ ≤ 57.42 6.46 ≤ 2Θ ≤ 54.20 6.22 ≤ 2Θ ≤ 54.2
Coll. refl. 44305 162616 59072
Indep. refl.; Rint 12099; 0.0286 23379; 0.0789 9582; 0.0835
Obs. refl. F0 ≥ 4σ(F) 10385; 17692 6368
No. ref. param. 560 1391 538
wR2 (all data) 0.1005 0.1538 0.1161
R1 (F0 ≥ 4σ(F)) 0.0395 0.0547 0.0483
GooF on F2 1.069 1.052 1.076
∆ρmax/min 0.554; –0.466 1.252; –0.728 0.528; -0.491
221
Table 19. Crystallographic data, data collection and refinement details of
[(BIMPNMes,Ad,Me)CoII]Cl · MeCN (6 · MeCN), [(BIMPNMes,Ad,Me)CoII]PF6 · 3 THF (6[PF6] · 3
THF), and [(BIMPNXyl,tBu,tBu)CoII]PF6 · 2.5 C6H6 (6Xyl,tBu,tBu[PF6] · 2.5 C6H6).
6 · MeCN 6[PF6] · 3 THF 6
Xyl,tBu,tBu[PF6]
· 2.5 C6H6
Empirical formula C48H59ClCoN6O C58H80CoF6N5O4P C56H67CoF6N5OP
Mol. weight 830.39 1115.17 1030.05
Crystal size [mm] 0.38×0.20×0.05 0.18×0.15×0.07 0.16×0.10×0.04
Temperature [K] 150(2) 100(2) 100(2)
Crystal system orthorhombic triclinic triclinic
Space group P212121 P-1 P-1
a [Å] 9.7911(3) 9.655(2) 9.9376(6)
b [Å] 11.4573(7) 13.960(3) 13.8685(9)
c [Å] 40.340(3) 21.756(4) 21.1882(13)
α [°] 90 100.652(4) 71.2240(13)
β [°] 90 93.970(4) 77.0860(14)
γ [°] 90 103.686(4) 72.1710(14)
V [Å3] 4525.3(5) 2780.5(10) 2606.6(3)
Z 4 2 2
ρ [g cm 3] (calc.) 1.219 1.332 1.312
µ [mm1] 0.480 0.408 0.425
F (000) 1764 1182 1084
Tmin; Tmax 1.0; 0.750 0.746; 0.657 0.746; 0.697
2Θ interval [ °] 4.80 ≤ 2Θ ≤ 54.18 4.38 ≤ 2Θ ≤ 52.74 4.54 ≤ 2Θ ≤ 55.76
Coll. refl. 59683 38039 41714
Indep. refl.; Rint 9979; 0.0565 11318; 0.0573 12398; 0.0404
Obs. refl. F0 ≥ 4σ(F) 8677 8747 9645
No. ref. param. 522 683 641
wR2 (all data) 0.0810 0.1366 0.0966
R1 (F0 ≥ 4σ(F)) 0.0344 0.0544 0.0406
GooF on F2 1.041 1.058 1.019
Absolute struct.
param.[244] 0.045(9) - -
∆ρmax/min 0.287; –0.265 0.922; –0.599 0.576; –0.393
222
Table 20. Crystallographic data, data collection and refinement details of
[(BIMPNMes,Ad,Me)CoII(N3)] · 2 THF · 0.5 C6H6 (7 · 2 THF · 0.5 C6H6), [(BIMPNMes,Ad,Me)CoI] · (8),
and [bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] · C6H6 (9* · C6H6).
7 · 2 THF · 0.5 C6H6 8 9* · C6H6
Empirical formula C114H150Co2N16O6 C46H56CoN5O C110H130Co2N12O4
Mol. weight 1958.36 753.89 1802.12
Crystal size [mm] 0.16×0.14×0.12 0.50×0.45×0.30 0.18×0.08×0.03
Temperature [K] 150(2) 100(2) 120(2)
Crystal system triclinic monoclinic triclinic
Space group P-1 P21/n P-1
a [Å] 16.2835(14) 31.8655(4) 11.6660(13)
b [Å] 16.5632(14) 12.0863(2) 13.8435(16)
c [Å] 23.247(2) 32.2012(4) 15.4554(18)
α [°] 83.346(2) 90 72.736(2)
β [°] 71.426(2) 106.4750(10) 81.026(3)
γ [°] 60.9094(19) 90 77.339(3)
V [Å3] 5187.8(8) 11892.7(3) 2314.5(5)
Z 2 12 1
ρ [g cm 3] (calc.) 1.254 1.263 1.293
µ [mm1] 0.383 0.475 0.420
F (000) 2096 4824 960
Tmin; Tmax 0.746; 0.692 0.746; 0.693 0.987; 0.807
2Θ interval [ °] 4.10 ≤ 2Θ ≤ 54.20 2.12 ≤ 2Θ ≤ 57.80 4.32 ≤ 2Θ ≤ 54.20
Coll. refl. 93209 245524 37987
Indep. refl.; Rint 22882; 0.0333 27406; 0.0461 10196; 0.585
Obs. refl. F0 ≥ 4σ(F) 18519 20831 7813
No. ref. param. 1301 1453 587
wR2 (all data) 0.1274 0.0962 0.1332
R1 (F0 ≥ 4σ(F)) 0.0455 0.0389 0.494
GooF on F2 1.045 1.030 1.023
Absolute struct.
param.[244] - - -
∆ρmax/min 1.081; –0.802 0.619; –0.409 1.076; –0.554
223
Table 21. Crystallographic data, data collection and refinement details of
[(BIMPNMes,Ad,Me*N3Mes)Co] · 0.461 C6H6 and 0.539 n-pentane (10* · 0.461 C6H6 · 0.539 n-C5H12),
[(BIMPNMes,Ad,Me*NH)Co]Cl · THF · 0.16 H2O (with the actual compound distribution in the
crystal: 0.935 [(BIMPNMes,Ad,Me*NH)Co]Cl · 0.065 [(BIMPNMes,Ad,Me)Co]Cl · THF · 0.16 H2O;
denoted as 11* · THF · 0.16 H2O), and [(TIMENMes)Co(N3)]OTs · 3 MeCN (12[OTs] · 3 MeCN).
10* · 0.461 C6H6
· 0.539 n-C5H12
11* · THF · 0.16 H2O 12[OTs] · 3 MeCN
Empirical formula C60.46H75.24CoN8O C50H65.25ClCoN5.935O2.16 C55H67CoN13O3S
Mol. weight 988.94 878.34 1049.21
Crystal size [mm] 0.18×0.1×0.05 0.18×0.06×0.05 0.35×0.25×0.20
Temperature [K] 120(2) 120(2) 150(2)
Crystal system triclinic orthorhombic triclinic
Space group P-1 Ccca P-1
a [Å] 13.2709(14) 19.0807(18) 10.9838(7)
b [Å] 13.8125(14) 42.120(5) 14.9918(18)
c [Å] 17.3710(18) 22.668(2) 18.5158(14)
α [°] 102.6621(19) 90 66.775(8)
β [°] 90.2478(20) 90 83.296(6)
γ [°] 117.7995(17) 90 84.437(7)
V [Å3] 2727.2(5) 18217(3) 2778.4(4)
Z 2 16 2
ρ [g cm3] (calc.) 1.204 1.281 1.254
µ [mm1] 0.362 0.482 0.401
F (000) 1058 7489 1110
Tmin; Tmax 0.746; 0.692 0.746; 0.650 0.746; 0.662
2Θ interval [ °] 5.20 ≤ 2Θ ≤ 57.76 4.02 ≤ 2Θ ≤ 54.32 5.84 ≤ 2Θ ≤ 54.20
Coll. refl. 41657 73400 72482
Indep. refl.; Rint 12987; 0.0325 10057 12230; 0.0440
Obs. refl. F0 ≥ 4σ(F) 10800 0.0729 9883
No. ref. param. 697 611 671
wR2 (all data) 0.1384 0.1188 0.1470
R1 (F0 ≥ 4σ(F)) 0.0509 0.0589 0.0478
GooF on F2 1.037 1.672 1.069
∆ρmax/min 0.880; –0.479 0.565; –0.477 1.617; –0.611
224
4.4.3 Geometrical Structure Parameters
Table 22. Selected bond distances [Å] and angles [°] with e.s.d.’s in parentheses for
(H3BIMPNMes,Ad,Me)(OTs)2 · 0.2 EtOAc (A) and [K2(BIMPNMes,Ad,Me)2(C6H6) ] · 3 Et2O · 0.5 C6H6 (B
and B’). See Chart 2 for atom labeling.
Bond / Angle A B B’#
NNHC1–Ccarb. 1.328(2)
1.328(2)
1.367(4)
1.377(4)
1.375(4)
1.363(5)
NNHC2–Ccarb. 1.328(2)
1.342(2)
1.364(4)
1.364(5)
1.377(5)
1.369(4)
NNHC1–Ccarb2 1.378(2)
1.381(2)
1.389(4)
1.379(4)
1.380(5)
1.386(4)
Ccarb2–Ccarb3 1.348(2)
1.346(2)
1.341(5)
1.339(5)
1.345(5)
1.343(5)
NNHC2–Ccarb3 1.328(2)
1.342(2)
1.390(4)
1.391(4)
1.378(4)
1.389(5)
Cph1–O 1.369(2) 1.312(4) 1.315(4)
Cph1– Cph2 1.412(2) 1.448(5) 1.449(5)
Cph2– Cph3 1.400(2) 1.390(5) 1.397(5)
Cph3– Cph4 1.391(2) 1.399(5) 1.394(5)
Cph4– Cph5 1.389(2) 1.376(5) 1.388(5)
Cph5– Cph6 1.390(2) 1.391(5) 1.389(5)
Cph1– Cph6 1.406(2) 1.440(5) 1.427(5)
NNHC1–Ccarb.–NNHC2 108.3(2)
108.1(2)
101.6(3)
101.6(3)
101.4(3)
101.7(3) # 2nd independent molecule in unit cell.
225
Table 23. Selected bond distances [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)FeII(NCMe)]Cl · 2 MeCN (1MeCN · MeCN), [(BIMPNMes,Ad,Me)FeIIN3]
· 1.894 Et2O · 0.606 DMF (2 · 1.894 Et2O · 0.606 DMF), and [(BIMPNMes,Ad,Me)MnCl] · 0.5 pyridine
(3 · 0.5 pyridine). See Chart 2 for atom labeling.
Bond 1MeCN ·
MeCN
2 · 1.894
Et2O ·
0.606 DMF
2’ · 1.894
Et2O ·
0.606 DMF#
3 · 0.5
pyridine
M···Nanchor 2.461 2.591 2.731 2.696
M–Laxial 2.229(2) 2.062(2) 2.057(2) 2.4221(7)
M–Ccarb. 2.100(2)
2.121(2)
2.144(3)
2.101(3)
2.131(3)
2.107(2)
2.227(3)
2.230(3)
M–O 1.920(2) 1.939(2) 1.930(2) 2.001(2)
Nα–Nβ - 1.175(3) 1.189(3) -
Nβ–Nγ - 1.160(4) 1.167(3) -
NNHC1–Ccarb. 1.353(2)
1.359(2)
1.360(3)
1.359(3)
1.365(3)
1.353(3)
1.365(3)
1.356(3)
NNHC2–Ccarb. 1.363(2)
1.363(2)
1.364(3)
1.364(3)
1.361(3)
1.364(3)
1.364(3)
1.363(3)
NNHC1–Ccarb2 1.389(2)
1.383(2)
1.383(3)
1.384(3)
1.384(4)
1.390(3)
1.385(3)
1.380(3)
Ccarb2–Ccarb3 1.350(2)
1.346(2)
1.343(4)
1.344(4)
1.334(4)
1.337(4)
1.338(4)
1.345(4)
NNHC2–Ccarb3 1.386(2)
1.387(2)
1.382(3)
1.390(3)
1.389(4)
1.395(3)
1.387(3)
1.393(3)
Cph1–O 1.339(2) 1.327(3) 1.325(3) 1.319(3)
Cph1– Cph2 1.419(2) 1.428(4) 1.423(4) 1.416(3)
Cph2– Cph3 1.398(2) 1.394(4) 1.396(4) 1.389(4)
Cph3– Cph4 1.399(2) 1.396(4) 1.396(4) 1.390(4)
Cph4– Cph5 1.385(2) 1.384(4) 1.383(4) 1.398(4)
Cph5– Cph6 1.395(2) 1.393(4) 1.398(3) 1.390(4)
Cph1– Cph6 1.413(2) 1.412(4) 1.417(3) 1.433(3)
# 2nd independent molecule in unit cell.
226
Table 24. Selected bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)FeII(NCMe)]Cl · 2 MeCN (1MeCN · MeCN), [(BIMPNMes,Ad,Me)FeIIN3]
· 1.894 Et2O · 0.606 DMF (2 · 1.894 Et2O · 0.606 DMF), and [(BIMPNMes,Ad,Me)MnCl] · 0.5 pyridine
(3 · 0.5 pyridine). See Chart 2 for atom labeling.
Angle 1MeCN ·
MeCN
2 · 1.894
Et2O ·
0.606 DMF
2’ · 1.894
Et2O ·
0.606 DMF#
3 · 0.5
pyridine
Nanchor–M– Laxial 177.76 170.2 170.6 170.25
Ccarb.–M–Laxial 94.40(5)
94.63(5)
97.0(1)
105.4(1)
99.05(9)
107.25(9)
104.79(7)
102.65(7)
O–M–Laxial 90.45(5) 92.74(9) 95.24(8) 92.45(5)
M–Nα–Nβ - 135.0(2) 132.8(2) -
Nα–Nβ–Nγ - 177.9(3) 177.5(3) -
Ccarb.–M–C’carb. 120.82(6) 107.8(1) 107.97(9) 109.02(9)
Ccarb.–M–O 117.61(6)
120.66(5)
138.31(9)
108.30(9)
133.31(9)
109.66(9)
130.69(8)
111.61(8)
NNHC1–Ccarb.–NNHC2 103.3(2)
103.1(2)
102.7(2)
103.0(2)
102.7(2)
103.4(2)
103.1(2)
103.5(2)
doop 0.113(1) 0.271(2) 0.351(2) 0.362(2)
# 2nd independent molecule in unit cell.
227
Table 25. Selected bond distances [Å], bond angles [°], and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)CoII]Cl · MeCN (6 · MeCN), [(BIMPNMes,Ad,Me)CoII]PF6 · 3 THF (6[PF6]
· 3 THF), and [(BIMPNXyl,tBu,tBu)CoII]PF6 · 2.5 C6H6 (6Xyl,tBu,tBu[PF6] · 2.5 C6H6). See Chart 2 for
atom labeling.
Bond / Angle 6 · MeCN 6[PF6]
· 3 THF
6Xyl,tBu,tBu[PF6]
· 2.5 C6H6
M···Nanchor 2.140(2) 2.099(2) 2.108(2)
M–Laxial - - -
M–Ccarb. 2.020(2)
2.028(2)
2.009(3)
2.023(3)
2.023(2)
2.025(2)
M–O 1.874(2) 1.889(2) 1.878(2)
NNHC1–Ccarb. 1.356(2)
1.359(2)
1.357(4)
1.358(4)
1.358(2)
1.355(2)
NNHC2–Ccarb. 1.356(2)
1.361(2)
1.357(4)
1.362(4)
1.357(2)
1.356(2)
NNHC1–Ccarb2 1.378(3)
1.385(3)
1.384(4)
1.378(4)
1.381(2)
1.380(2)
Ccarb2–Ccarb3 1.344(3)
1.349(3)
1.343(4)
1.347(4)
1.342(3)
1.342(3)
NNHC2–Ccarb3 1.390(3)
1.385(2)
1.386(4)
1.389(4)
1.383(2)
1.382(2)
Cph1–O 1.346(2) 1.337(3) 1.337(2)
Cph1– Cph2 1.419(3) 1.425(4) 1.411(3)
Cph2– Cph3 1.393(3) 1.396(4) 1.388(3)
Cph3– Cph4 1.383(3) 1.388(4) 1.392(3)
Cph4– Cph5 1.394(3) 1.387(4) 1.399(3)
Cph5– Cph6 1.396(3) 1.395(4) 1.393(3)
Cph1– Cph6 1.412(3) 1.413(4) 1.419(2)
Nanchor–M– Laxial - - -
Ccarb.–M–C’carb. 117.41(8) 133.3(2) 131.94(7)
Ccarb.–M–O 112.72(7)
124.64(7)
108.4(1)
115.2(1)
112.45(6)
113.35(6)
NNHC1–Ccarb.–NNHC2 103.9(2)
103.2(2)
104.0(2)
103.4(2)
103.8(2)
103.6(2)
doop –0.262(1) –0.195(2) –0.170
228
Table 26. Selected bond distances [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)CoII(N3)] · 2 THF · 0.5 C6H6 (7 · 2 THF · 0.5 C6H6) and
[(BIMPNMes,Ad,Me)CoI] (8). See Chart 2 for atom labeling.
Bond 7 · 2 THF
· 0.5 C6H6
7’ · 2 THF
· 0.5 C6H6#
8 8’ # 8” ##
M···Nanchor 2.659 2.677 2.194(2) 2.189(2) 2.206(2)
M–Laxial 2.039(2) 2.037(2) - - -
M–Ccarb. 2.081(2)
2.060(2)
2.067(2)
2.051(2)
1.923(2)
1.937(2)
1.936(2)
1.920(2)
1.921(2)
1.942(2)
M–O 1.931(2) 1.929(2) 1.949(2) 1.950(2) 1.948(2)
Nα–Nβ 1.188(3) 1.193(3) - - -
Nβ–Nγ 1.164(3) 1.162(3) - - -
NNHC1–Ccarb. 1.357(3)
1.354(3)
1.358(3)
1.362(3)
1.374(2)
1.378(2)
1.372(2)
1.369(2)
1.374(2)
1.378(2)
NNHC2–Ccarb. 1.361(3)
1.360(3)
1.364(3)
1.362(3)
1.379(2)
1.376(2)
1.378(2)
1.382(2)
1.378(2)
1.378(2)
NNHC1–Ccarb2 1.377(3)
1.378(3)
1.375(3)
1.392(3)
1.379(2)
1.376(2)
1.378(2)
1.391(2)
1.389(2)
1.377(2)
Ccarb2–Ccarb3 1.338(3)
1.340(3)
1.334(4)
1.332(4)
1.337(3)
1.338(2)
1.338(3)
1.339(3)
1.343(3)
1.334(2)
NNHC2–Ccarb3 1.385(3)
1.383(3)
1.377(3)
1.392(3)
1.387(2)
1.391(2)
1.394(2)
1.389(2)
1.391(2)
1.390(2)
Cph1–O 1.319(3) 1.321(3) 1.320(2) 1.320(2) 1.318(2)
Cph1– Cph2 1.428(3) 1.432(3) 1.416(2) 1.417(2) 1.421(2)
Cph2– Cph3 1.391(3) 1.396(3) 1.390(2) 1.390(2) 1.388(2)
Cph3– Cph4 1.392(4) 1.390(3) 1.387(2) 1.384(2) 1.387(2)
Cph4– Cph5 1.381(4) 1.386(3) 1.396(2) 1.399(2) 1.396(3)
Cph5– Cph6 1.389(3) 1.396(3) 1.397(2) 1.394(2) 1.394(2)
Cph1– Cph6 1.411(3) 1.410(3) 1.429(2) 1.426(2) 1.428(2)
# 2nd independent molecule in unit cell. ## 3rd independent molecule in unit cell.
229
Table 27. Selected bond angles [°], and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me)CoII(N3)] · 2 THF · 0.5 C6H6 (7 · 2 THF · 0.5 C6H6), and
[(BIMPNMes,Ad,Me)CoI] (8). See Chart 2 for atom labeling.
Angle 7 · 2 THF
· 0.5 C6H6
7’ · 2 THF
· 0.5 C6H6#
8
8’ # 8” ##
Nanchor–M– Laxial 168.7 169.5 - - -
Ccarb.–M–Laxial 102.23(8)
105.10(8)
100.93(8)
112.04(8)
- - -
O–M–Laxial 89.93(7) 91.93(7) - - -
M–Nα–Nβ 128.2(2) 124.1(2) - - -
Nα–Nβ–Nγ 177.1(2) 177.6(2) - - -
Ccarb.–M–C’carb. 106.95(8) 107.06(8) 111.60(7) 113.12(7) 112.24(7)
Ccarb.–M–O 134.03(7)
112.31(7)
133.45(8)
112.04(8)
122.58(6)
123.78(6)
120.75(6)
123.95(6)
120.66(6)
125.18(6)
NNHC1–Ccarb.–NNHC2 102.8(2)
103.1(2)
102.8(2)
103.5(2)
102.1(2)
101.7(2)
101.8(2)
102.1(2)
102.1(2)
101.8(2)
doop 0.297(2) 0.313(2) –0.160 –0.166 –0.155
# 2nd independent molecule in unit cell. ## 3rd independent molecule in unit cell.
230
Table 28. Selected bond distances [Å] with e.s.d.’s in parentheses for
[bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] · C6H6 (9* · C6H6). See Figure 42 for atom labeling.21
Bond# Bond 9* · C6H6
M···Nanchor Co1···N1 2.510
M–Laxial Co1–O2
Co1–O2A
(= Co-µ-O)
1.995(2)
2.052(2)
M–Ccarb. Co1–C3
Co1–C8
2.066(2)
-
M–O Co1–O1 1.938(2)
N7–C8 21 1.297(3)
N7–C47 1.399(3)
NNHC1–Ccarb. N2–C3
N4–C8
1.359(3)
1.380(3)
NNHC2–Ccarb. N3–C3
N5–C8
1.370(3)
1.389(3)
NNHC1–Ccarb2 N2–C4
N4–C9
1.376(3)
1.382(3)
Ccarb2–Ccarb3 C4–C5
C9–C10
1.339(3)
1.328(3)
NNHC2–Ccarb3 N3–C5
N5–C10
1.382(3)
1.397(3)
Cph1–O C13–O1 1.320(3)
Cph1– Cph2 C12– C13 1.414(3)
Cph2– Cph3 C12– C17 1.390(3)
Cph3– Cph4 C16– C17 1.386(3)
Cph4– Cph5 C15– C16 1.389(3)
Cph5– Cph6 C14– C15 1.402(3)
Cph1– Cph6 C13– C14 1.431(3)
#corresponding bond in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand.
21 No nitrogen atom in this structure was labeled N6.
231
Table 29. Selected bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] · C6H6 (9* · C6H6). See Figure 42 for atom labeling.22
Angle# Angle 9* · C6H6
C8–N7–C47Phenyl 126.0(2)
O2–Co1–O2A 81.15(7)
Co1–O2–Co1A 98.85(7)
O1–Co1–O2
O1–Co1–O2A
116.89(7)
98.84(7)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
110.35(8)
-
C3–Co1–O2
C3–Co1–O2A
130.83(8)
104.89(8)
NNHC1–Ccarb.–
NNHC2
N2–C3–N3
N4–C8–N5
102.7(2)
104.9(2)
doop doop
-
#corresponding angle in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand.
22 No nitrogen atom in this structure was labeled N6.
232
Table 30. Selected bond distances [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me*N3Mes)CoII] · 0.461 C6H6 · 0.539 n-pentane (10* · 0.461 C6H6 · 0.539 n-C5H12).
See Figure 43 for atom labeling.
Bond# Bond
10*
· 0.461 C6H6 ·
0.539 n-C5H12
M···Nanchor Co1···N1 2.108(2)
M–Ccarb. Co1–C3
Co1–C8
-
1.985(2)
Co1–C4 1.972(2)
M–O Co1–O1 1.892(2)
C3–N6 1.363(3)
N6–N7 1.337(3)
N7–N8 1.279(3)
N8–C47 1.437(3)
NNHC1–Ccarb. N2–C3
N4–C8
1.345(3)
1.351(2)
NNHC2–Ccarb. N3–C3
N5–C8
1.358(3)
1.354(2)
NNHC1–Ccarb2 N2–C4
N4–C9
1.420(3)
1.384(3)
Ccarb2–Ccarb3 C4–C5
C9–C10
1.349(3)
1.349(3)
NNHC2–Ccarb3 N3–C5
N5–C10
1.404(3)
1.388(3)
Cph1–O C13–O1 1.337(2)
Cph1– Cph2 C12– C13 1.412(3)
Cph2– Cph3 C12– C17 1.394(3)
Cph3– Cph4 C16– C17 1.386(3)
Cph4– Cph5 C15– C16 1.388(3)
Cph5– Cph6 C14– C15 1.400(3)
Cph1– Cph6 C13– C14 1.427(3)
#corresponding bond in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand.
233
Table 31. Selected bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me*N3Mes)CoII] · 0.461 C6H6 · 0.539 n-pentane (10* · 0.461 C6H6 · 0.539 n-C5H12).
See Figure 43 for atom labeling.
Angle# Angle 10* · 0.461 C6H6 ·
0.539 n-C5H12
C3–N6–N7 107.7(2)
N6–N7–N8 112.2(2)
N7–N8–CPhenol47 107.6(2)
C4–Co1–C8 117.36(8)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
-
118.18(7)
C4–Co–O1 118.06(7)
NNHC1–Ccarb.–NNHC2 N2–C3–N3
N4–C8–N5
106.1(2)
104.3(2)
doop doop
–0.287 ##
# corresponding angle in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand. ## with C4 (=CCarb2) instead of C3 (= CCarb.) as third atom to define the plane.
234
Table 32. Selected bond distances [Å] with e.s.d.’s in parentheses for [(BIMPNMes,Ad,Me*NH)Co]Cl
· THF · 0.16 H2O (with the actual compound distribution in the crystal:
0.935 [(BIMPNMes,Ad,Me*NH)Co]Cl · 0.065 [(BIMPNMes,Ad,Me)Co]Cl · THF · 0.16 H2O; denoted as
11* · THF · 0.16 H2O). See Figure 47 for atom labeling.
Bond# Bond 11*
· THF · 0.16 H2O
M···Nanchor Co1···N1 2.098(2)
M–Ccarb. Co1–C3
Co1–C8
-
1.989(3)
Co1–N6 1.979(2)
M–O Co1–O1 1.887(2)
N6–C3 1.323(3)
NNHC1–Ccarb. N2–C3
N4–C8
1.353(3)
1.354(3)
NNHC2–Ccarb. N3–C3
N5–C8
1.361(3)
1.356(3)
NNHC1–Ccarb2 N2–C4
N4–C9
1.395(4)
1.381(3)
Ccarb2–Ccarb3 C4–C5
C9–C10
1.328(4)
1.347(4)
NNHC2–Ccarb3 N3–C5
N5–C10
1.406(3)
1.382(3)
Cph1–O C13–O1 1.346(3)
Cph1– Cph2 C12– C13 1.409(4)
Cph2– Cph3 C12– C17 1.384(3)
Cph3– Cph4 C16– C17 1.385(4)
Cph4– Cph5 C15– C16 1.378(4)
Cph5– Cph6 C14– C15 1.396(4)
Cph1– Cph6 C13– C14 1.426(4)
#corresponding bond in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand.
235
Table 33. Selected bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[(BIMPNMes,Ad,Me*NH)Co]Cl · THF · 0.16 H2O (with the actual compound distribution in the
crystal: 0.935 [(BIMPNMes,Ad,Me*NH)Co]Cl · 0.065 [(BIMPNMes,Ad,Me)Co]Cl · THF · 0.16 H2O;
denoted as 11* · THF · 0.16 H2O). See Figure 47 for atom labeling.
Angle# Angle 11*
· THF · 0.16 H2O
C3–N6–Co1 130.4(2)
Ccarb.–M–C’carb. N6–Co1–C8 109.9(2)
Ccarb.–M–O C3–Co1–O1
C8–Co1–O1
-
118.5(1)
N6–Co–O1 118.91(9)
NNHC1–Ccarb.–NNHC2 N2–C3–N3
N4–C8–N5
106.5(2)
103.7(2)
doop doop
–0.404 ##
#corresponding angle in complexes of (BIMPNMes,Ad,Me)– with unaltered tripodal ligand. ## with N6 (= inserted nitrogen) instead of C3 (= CCarb.) as third atom to define the plane.
236
Table 34. Selected bond distances [Å] with e.s.d.’s in parentheses for [(TIMENMes)Co(N3)]OTs
· 3 MeCN (12[OTs] · 3 MeCN). See Chart 2 for atom labeling. For swift and easy comparison,
values of [(BIMPNMes,Ad,Me)Co(N3)] (7, see above) and [(TIMENXyl)Co(N3)]BPh4 · MeCN are also
tabulated here.
Bond 7 · 2 THF
· 0.5 C6H6
12[OTs]
· 3 MeCN
[(TIMENXyl)Co(N3)]BPh4
· MeCN
M···Nanchor 2.659 3.234 3.213
M–Laxial 2.039(2) 1.939(2) 1.938(2)
M–Ccarb. 2.081(2)
2.060(2)
2.053(2)
2.058(2)
2.062(2)
2.052(2)
2.049(2)
2.017(2)
M–O 1.931(2) - -
Nα–Nβ 1.188(3) 1.171(3) 1.161(3)
Nβ–Nγ 1.164(3) 1.157(3) 1.169(3)
NNHC1–Ccarb. 1.357(3)
1.354(3)
1.357(3)
1.363(3)
1.358(3)
1.357(3)
1.356(3)
1.358(3)
NNHC2–Ccarb. 1.361(3)
1.360(3)
1.361(3)
1.364(3)
1.366(3)
1.368(3)
1.364(3)
1.361(3)
NNHC1–Ccarb2 1.377(3)
1.378(3)
1.385(3)
1.385(3)
1.387(3)
1.384(3)
1.385(3)
1.382(3)
Ccarb2–Ccarb3 1.338(3)
1.340(3)
1.343(4)
1.342(4)
1.338(4)
1.336(3)
1.339(3)
1.335(3)
NNHC2–Ccarb3 1.385(3)
1.383(3)
1.384(3)
1.391(3)
1.389(3)
1.384(3)
1.382(3)
1.383(3)
237
Table 35. Selected bond angles [°] and doop [Å] with e.s.d.’s in parentheses for
[(TIMENMes)Co(N3)]OTs · 3 MeCN (12[OTs] · 3 MeCN). See Chart 2 for atom labeling. For swift
and easy comparison, values of [(BIMPNMes,Ad,Me)Co(N3)] (7, see above) and
[(TIMENXyl)Co(N3)]BPh4 · MeCN are also tabulated here.
Angle 7 · 2 THF
· 0.5 C6H6
12[OTs]
· 3 MeCN
[(TIMENXyl)Co(N3)]BPh4
· MeCN
Nanchor–M– Laxial 168.7 178.5 174.27
Ccarb.–M–Laxial 102.23(8)
105.10(8)
103.44(9)
104.83(9)
106.02(9)
102.17(8)
101.85(8)
110.48(8)
O–M–Laxial 89.93(7) - -
M–Nα–Nβ 128.2(2) 175.4(2) 166.3(2)
Nα–Nβ–Nγ 177.1(2) 179.5(3) 178.3(2)
Ccarb.–M–C’carb. 106.95(8) 113.62(9)
110.29(8)
117.30(9)
118.86(8)
111.91(8)
110.52(8)
Ccarb.–M–O 134.03(7)
112.31(7)
- -
NNHC1–Ccarb.–
NNHC2
102.8(2)
103.1(2)
103.1(2)
103.2(2)
103.1(2)
103.2(2)
103.6(2)
103.3(2)
doop 0.297(2) 0.524 0.520
238
4.4.4 Crystal Structure of [(BIMPNMes,Ad,Me)Co]PF6 (6[PF6])
Figure 109. Molecular structure of 6[PF6] in crystals of [(BIMPNMes,Ad,Me)Co]PF6 · 3 THF
(50 % probability ellipsoids, hydrogen atoms and solvent molecules omitted for clarity).
239
5 Symbols and Abbreviations
× adduct with
Ad Adamantyl
aq. aqueous
Ar aryl
(BIMPNR,R’,R’’)– anion of bis[2-(3-R-imidazol-2-ylidene)ethyl]-[(3,5-R’,R”-2-hydroxy-
phenyl)methyl]amine
Bn benzyl
br broad
calcd. calculated
δ a) NMR chemical shift
b) Mößbauer chemical isomer shift
d doublet
D zero field splitting parameter
doop
out of plane shift (distance of the metal center from the least-squares
plane defined by the three coordinating atoms of the tripodal ligand;
i.e., the carbene carbon and phenolate oxygen atoms)
∆EQ Mößbauer quadrupole splitting parameter
e.s.d. estimated standard deviation (standard uncertainty)
EA elemental analysis
EPR Electron Paramagnetic Resonance
eq equivalent(s)
Et ethyl
Et2O diethyl ether
FD-MS field desorption mass spectrometry
FWHM full-width at half-maximum
ΓFWHM line width; FWHM = full-width at half-maximum
g EPR g-value
HOMO Highest Occupied Molecular Orbital
240
i. vac. in vacuo (in/ under vacuum)
iPr iso-Propyl
IR infrared
KOtBu potassium tert-butanolate
LUMO Lowest Unoccupied Molecular Orbital
µB Bohr magneton
µeff effective magnetic moment
µmol micromole
m multiplet
M molar
Me methyl
Mes mesityl
(MIMPNR,R’,R’’)2– dianion of mono[2-(3-R-imidazol-2-ylidene)ethyl]-bis[(3,5-R’,R”-2-
hydroxyphenyl)methyl]amine
min. minute(s)
mM millimolar
mmol millimole
MS mass spectrometry
NHC N-heterocyclic carbene
NHE normal hydrogen electrode
NMR Nuclear Magnetic Resonance
obsd. observed
–OTs tosylate = anion of para-toluene-sulfonic acid
PG protecting group
Ph phenyl
py pyridine
quant. quantitative yield
quart. quaternary
RT room temperature
s singlet
241
S total spin angular momentum
sat. saturated
sept septet
SN2 bimolecular nucleophilic substitution
SQUID superconducting quantum interference device
t triplet
tBu tert-butyl
tBu3-phenol 2,3,6-tris-tert-butylphenol
TEMPO tetramethylpiperidinoxyl
tert tertiary
TIMENR tris[2-(3-R-imidazol-2-ylidene)ethyl]amine
TLC thin layer chromatography
TMS trimethylsilyl
Tol tolyl
trityl triphenylmethyl
Ts tosyl = para-toluene-sulfonyl
UV/Vis Ultra-Violet/Visible
ν~ as asymmetric vibration
ν~ wave number
vbr very broad
Xyl xylyl
3,5-Xyl 3,5-xylyl
242
6 Numbered Compounds
Organic compounds
R'
R''
OH
NHO
2
R'
R''
OH
NCl
2
c1-R’,R’’ c2-R’,R’’
R'
R''
OH
NI
2
R'
R''
OH
NTsO
2
c3-R’,R’’ c4-R’,R’’
H2NCl
2
Cl 3 Cl
H3NN
NR
2
p1×HCl p2-R×HCl
HNN
NR
2
2 PF6
OH
R''
R'Cl
p2-R[PF6] p3-R’,R’’
243
Complexes
[(BIMPNMes,Ad,Me)Fe(Cl)] 1
[(BIMPNMes,tBu,tBu)Fe(Cl)] 1Mes,tBu,tBu
[(BIMPNMes,tBu,tBu)Fe(Cl)] 1Xyl,tBu,tBu
[(BIMPNMes,Ad,Me)Fe(Br)] 1[Br]
[(BIMPNMes,Ad,Me)Fe]BPh4 1[BPh4]
[(BIMPNMes,Ad,Me)Fe(N3)] 2
[(BIMPNMes,Ad,Me)Mn(Cl)] 3
[(BIMPNMes,Ad,Me)Mn(N3)] 4
[(BIMPNMes,Ad,Me)Zn(Cl)] 5
[(BIMPNMes,Ad,Me)Zn]OTs 5[OTs]
[(BIMPNMes,Ad,Me)Co]Cl 6
[(BIMPNMes,tBu,tBu)Co]Cl 6Mes,tBu,tBu
[(BIMPNXyl,tBu,tBu)Co]Cl 6Xyl,tBu,tBu
[(BIMPNMes,Ad,Me)Co]PF6 6[PF6]
[(BIMPNXyl,tBu,tBu)Co]PF6 6Xyl,tBu,tBu
[PF6]
[(BIMPNMes,Ad,Me)Co(N3)] 7
[(BIMPNMes,Ad,Me)CoI] 8
[bis-µ-OH{Co(BIMPNMes,Ad,Me*NPh)}2] 9*
[(BIMPNMes,Ad,Me*N3Mes)CoII] 10*
[(BIMPNMes,Ad,Me*NH)CoII]Cl 11*
[(TIMENMes)Co(N3)]OTs 12[OTs]
[(TIMENMes)Co(N3)]PF6 12[PF6]
244
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