solvent-free synthesis of bisferrocenylimines
DESCRIPTION
sfsTRANSCRIPT
SOLVENT-FREE SYNTHESIS OF BISFERROCENYLIMINES
AND THEIR COORDINATION TO RHODIUM(I)
PHUMELELE ELDRIDGE KLEYI
Submitted in partial fulfilment of the requirements for the degree of
MAGISTER SCIENTIAE in the Faculty of Science at the Nelson Mandela
Metropolitan University
Supervisor: Prof. Christopher Imrie
Co-Supervisors: Prof. T. I. A. Gerber
Prof. C. W. McCleland
January 2009
i
ACKNOWLEDGEMENTS
The author would like to express his gratitude to the following, for the contributions
made in the thesis:
1. Dr. Christopher Imrie for guidance, support and encouragement throughout
the project.
2. Prof. C. W. McCleland for help with NMR problems.
3. Prof. T. I. A. Gerber assistance with the project and thesis.
4. Dr. P. Mallon from the University of Stellenbosch for the help with Gel
Permeation Chromatography.
5. Mr. Harold Marchand, Mr H. Schalekamp and Mr. J. Booi for technical
assistance.
6. Mr. Irvin Booysen for assistance with UV-vis, CV and conductometry.
7. Mr. M. Mtyopo and Mr. B. Mpuhlu for assistance with GC.
8. Dr. E. R. T. Elago, Dr. V. O. Nyamori, Dr. Z. Tshentu and Mr. P. Hlangothi for
friendly advice.
9. My colleagues, Mr. D. Onyancha and Mrs. N. Adams for friendship and
support. My brother, Ayanda, for being there when I needed him most.
10. NRF and NMMU for financial support towards my studies.
11. Last but not least, The Almighty God for making my dreams come true.
ii
ABSTRACT
Solvent-free reactions possess advantages compared to the solvent route, such as
shorter reaction times, less use of energy, better yields, etc. Herein, the synthesis
and characterization of bisferrocenylimines and arylbisamines are described.
Reduction of the above compounds with LAH resulted in the formation of
bisferrocenylamines and arylbisamines, respectively. The coordination chemistry of
all the above compounds to rhodium(I) is also discussed in the prepared complexes
[Rh(COD)(NN)]ClO4, where NN = bisferrocenylimines, and [Rh(COD)(NN)]BF4,
where NN = bisferrocenylamines and arylbisamines. X-ray crystal structures of the
complexes [Rh(COD)(NN)]ClO4 ([3.2] and [3.3]) have been obtained. Complexes of
the type [Rh(COD)(NN)]BF4 were characterized with IR and UV-vis spectroscopy,
cyclic voltammetry and conductometry. The catalytic activity of the complexes was
also investigated: [Rh(COD)(NN)]ClO4 for the polymerization of phenylacetylene and
[Rh(COD)(NN)]BF4 for the hydroformylation of styrene.
Keywords: bisferrocenylimines, coordination chemistry, rhodium(I).
iii
PRESENTATIONS AND PUBLICATIONS
Publications
• Further solvent-free reactions of ferrocenylaldehydes: Synthesis of 1,1’-
ferrocenyldiimines and ferrocenylacrylonitrile, C. Imrie, P. Kleyi, V. O.
Nyamori, T. I. A. Gerber, D. C. Levendis and J. Look, J. Organomet. Chem.,
692 (2007) 3443-3454.
Conference proceedings
• Solvent-free synthesis and coordination chemistry of
diferrocenyldiazaalkanes, P. Kleyi, C. Imrie and C. W. McCleland, 37th
International Conference on Coordination Chemistry (ICCC37), Cape Town,
South Africa, August 2006.
• Ferrocenylnitrogen-donor ligands for homogeneous catalysis, P. Kleyi, D.
Saku, C. Imrie and C. W. McCleland, 15th International Symposium on
Homogeneous Catalysis (ISHCXV), Sun City, South Africa, August 2006.
• Synthesis and use of bisferrocenylimines as new catalysts for olefin
polymerization, P. Kleyi, C. Imrie and C. W. McCleland, Inorganic Chemistry
Conference (INORG007), Club Mykonos, Western Cape, South Africa, July
2007.
iv
CONTENTS
Page
ACKNOWLEDGEMENTS ........................................................................................... i
ABSTRACT ................................................................................................................ ii
PRESENTATIONS AND PUBLICATIONS ................................................................ iii
LIST OF FIGURES................................................................................................... viii
LIST OF SCHEMES ................................................................................................... x
LIST OF TABLES ..................................................................................................... xii
ABBREVIATIONS.................................................................................................... xiii
CHAPTER 1 ........................................................................................................... 1
INTRODUCTION ........................................................................................................ 1
1.1 Solvent-free synthesis ...................................................................................... 1
1.1.1 Background................................................................................................... 1
1.2 FERROCENES ................................................................................................... 5
1.2.1 Solvent-free synthesis of ferrocenes ............................................................. 5
1.2.1.1 Synthesis of ferrocenylenones ............................................................... 6
1.2.1.2 Reaction of ferrocenecarboxaldehyde with methylene active compounds ............................................................................................................................ 7
1.2.1.3 Reaction of ferrocenecarboxaldehyde with an ylid ................................. 9
1.2.1.4 Synthesis of ferrocenyl-1,5-diketone derivatives .................................. 10
1.2.1.5 Synthesis of ferrocenoate esters .......................................................... 10
1.2.1.6 Synthesis of ferrocenylimines ............................................................... 11
1.3 SOLVENT-FREE SYNTHESIS OF LIGAND SYSTEMS ................................... 12
1.4 NITROGEN-DONOR LIGAND CHEMISTRY .................................................... 17
1.4.1 Polymerization reactions ............................................................................. 19
1.4.2 Cross-coupling reactions ............................................................................ 22
1.4.2.1 Heck reactions ..................................................................................... 22
1.4.2.2 Suzuki cross-coupling reactions ........................................................... 24
v
1.4.3 Epoxidation reactions ................................................................................. 26
1.4.4 Asymmetric allylic substitution reactions ..................................................... 29
1.4.5 Ring-opening metathesis polymerization (ROMP) ...................................... 30
1.5 FERROCENYL-NITROGEN DONOR LIGAND CHEMISTRY........................... 32
1.5.1 Ferrocenyl-pyridine ligands ......................................................................... 32
1.5.2 Ferrocenyl-Schiff base ligands .................................................................... 36
1.6 OBJECTIVES OF THE PROJECT .................................................................... 40
1.7 REFERENCES .................................................................................................. 41
CHAPTER 2 ......................................................................................................... 50
RESULTS AND DISCUSSION ................................................................................ 50
2.1 SOLVENT-FREE SYNTHESIS OF BISFERROCENYLIMINES........................ 50
2.1.1 Introduction ................................................................................................. 50
2.1.2 Synthesis and characterization of bisferrocenyimines ................................ 51
2.1.3 Solvent-free synthesis of arylbisimines ....................................................... 57
2.2 REDUCTION REACTION OF BISFERROCENYLIMINES .............................. 60
2.2.1 Reduction of bisferrocenylimines ................................................................ 60
2.2.2 Reduction of arylbisimines .......................................................................... 63
2.3 ELECTRONIC SPECTROSCOPY .................................................................... 63
2.4 CYCLIC VOLTAMMETRY ................................................................................ 66
2.5 EXPERIMENTAL .............................................................................................. 68
2.5.1 Purification procedures ............................................................................... 68
2.5.2 Instrumentation ........................................................................................... 69
2.6 SYNTHESIS OF BISFERROCENYLIMINES AND ARYLBISIMINES .............. 70
2.6.1 General procedure for the synthesis of bisferrocenylimines ....................... 70
2.6.2 Reduction of bisferrocenylimines and arylbisimines ................................... 75
2.7 REFERENCES .................................................................................................. 79
vi
CHAPTER 3 ......................................................................................................... 80
RESULTS AND DISCUSSION ................................................................................ 80
3.1 SYNTHESIS OF CATIONIC RHODIUM(I) COMPLEXES ................................. 80
3.1.1 Rhodium(I) complexes containing bisferrocenylimines ............................... 80
3.1.2 X-ray Crystallography ................................................................................. 84
3.1.3 Rhodium(I) complexes containing bisferrocenylamines .............................. 89
3.2 ELECTRONIC SPECTROSCOPY .................................................................... 93
3.3 CYCLIC VOLTAMMETRY ................................................................................ 95
3.4 EXPERIMENTAL .............................................................................................. 97
3.4.1 Purification procedures ............................................................................... 97
3.4.2 Instrumentation ........................................................................................... 97
3.5 SYNTHESIS OF RHODIUM(I) COMPLEXES ................................................... 98
3.5.1 Rhodium(I) complexes containing bisferrocenylimines ............................... 98
3.5.1.1 General procedure3 .............................................................................. 98
3.5.2 Rhodium(I) complexes containing bisferrocenylamines ............................ 100
3.6 REFERENCES ................................................................................................ 102
CHAPTER 4 ....................................................................................................... 103
RESULTS AND DISCUSSION .............................................................................. 103
4.1 POLYMERIZATION OF PHENYLACETYLENE ............................................. 103
4.1.1 Introduction ............................................................................................... 103
4.1.2 Polymer characterization .......................................................................... 104
4.2 CATALYTIC POLYMERIZATION STUDIES .................................................. 105
4.2.1 Spectroscopic properties of polymers ....................................................... 106
4.2.2 Thermal analysis ....................................................................................... 108
4.2.3 Mechanistic pathways for polymerization of phenylacetylene ................... 109
vii
4.3 HYDROFORMYLATION OF STYRENE ......................................................... 111
4.3.1 Introduction ............................................................................................... 111
4.3.2 Catalytic hydroformylation studies ............................................................ 113
4.3.3 Mechanism for hydroformylation of styrene .............................................. 115
4.4 EXPERIMENTAL ............................................................................................ 117
4.4.1 Purification procedures ............................................................................. 117
4.4.2 Instumentation .......................................................................................... 117
4.4.3 Polymerization of phenylacetylene ........................................................... 117
4.4.4 Hydroformylation of styrene ...................................................................... 118
4.5 REFERENCES ................................................................................................ 118
CHAPTER 5 ....................................................................................................... 121
CONCLUSION ....................................................................................................... 121
5.1 Conclusion ...................................................................................................... 121
viii
LIST OF FIGURES
Page
Figure 1.1: DSC analysis of ferrocenecarboxaldehyde ............................................ 6
Figure 1.2: Some N-donor ligands with sp2-hybridised nitrogen atoms .................. 18
Figure 1.3: Bis(alkylphenylaminopyridinato) titanium complexes ........................... 20
Figure 1.4: Pyridyl-imine complexes of iron (Fe) and palladium (Pd) ..................... 21
Figure 1.5: Examples of Pd-pyridyl complexes used for the Heck reactions .......... 23
Figure 1.6: Examples of Ni(0) complexes used for cross-coupling of arylchlorides.
................................................................................................................................. 24
Figure 1.7: Palladium imine and amine complexes for coupling of aryl bromides .. 26
Figure 1.8: Three isomers of Ru(pap)2Cl2 .............................................................. 27
Figure 1.9: Mn(III) Schiff-base complexes for electrocatalytic epoxidation of olefins
................................................................................................................................. 28
Figure 1.10: The cis-β isomer of the binaphthyl-bridged Schiff base titanium
complex .......................................................................................... 28
Figure 1.11: Example of chiral bidentate thiazolyl-pyridine ligands………………30 Figure 1.12: Chiral diimine palladium(II) catalyst for asymmetric alkylation ........ 30
Figure 1.13: The reaction pathway of ROMP ...................................................... 31
Figure 1.14: Re and Pt complexes with ferrocenylpyridine ligands ..................... 33
Figure 1.15: Tungsten complexes with ferrocenylpyridine ligands ...................... 33
Figure 1.16: Examples of ferrocenylpyridyl and pyrimidyl complexes ................. 34
Figure 1.17: Pd and Pt complexes of 1,1’-bis(2-pyridyl)ferrocene ....................... 34
Figure 1.18: Palladium and nickel complexes ..................................................... 35
Figure 1.19: Dimeric cyclopalladated ferrocenylimine complex for catalytic Heck
reaction ........................................................................................... 36
Figure 1.20: Cyclopalladated ferrocenylimine for Mirozoki-Heck reaction ........... 37
Figure 1.21: 1,1’-N-substituted ferrocenediyl Pd(II) complex for Suzuki cross-
coupling reaction ............................................................................. 37
Figure 1.22: Dimeric cyclopalladated ferrocenylketimine complexes for Suzuki
reaction ........................................................................................... 38
Figure 1.23: Chiral ferrocenylphosphine-imine ligand ......................................... 38
Figure 1.24: Cationic Rh(I) and Ir(I) complex for olefin polymerization ................ 39
ix
Figure 1.25: Cationic Rh(I) complexes for hydroformylation reactions. ................ 40
Figure 2.1: General structure of bisferrocenylimines to be synthesized ............. 51
Figure 2.2: The pictorial stages of the solvent-free synthesis of N,N’-
octylenebis(ferrocenylmethylidine)amine ........................................ 52
Figure 2.3: IR spectrum of [2.5].......................................................................... 54
Figure 2.4: 1H NMR spectrum of [2.5] ................................................................. 55
Figure 2.5: 13C NMR spectrum of [2.8] ................................................................ 60
Figure 2.6: UV-vis spectrum of unsubstitued ferrocene in dichloromethane. ...... 64
Figure 2.7: UV-vis spectra of bisferrocenylimines in dichloromethane. ............... 65
Figure 2.8: UV-vis spectra of bisferrocenylamines in dichloromethane. .............. 65
Figure 2.9: Cyclic voltammogram of ferrocene in acetonitrile. ............................. 67
Figure 2.10: Cyclic voltammograms of [2.2], [2.5], [2.12] and [2.14]. ................... 67
Figure 3.1: 1H NMR spectra of [3.2] (top) and [3.3] (bottom) in CDCl3................ 83
Figure 3.2: ORTEP diagram of [3.2]. ................................................................... 85
Figure 3.3: Crystal packing of [3.2], projection viewed along [100] ..................... 87
Figure 3.4: ORTEP drawing of [3.3] .................................................................... 88
Figure 3.5: Crystal packing of [3.3], projection viewed along [100]. .................... 88
Figure 3.6: Cationic rhodium(I) diamine complexes with the [Rh(COD)Cl2]- anion.
................................................................................................................................. 89
Figure 3.7: Cationic rhodium(I) complexes ........................................................... 90
Figure 3.8: IR spectrum of [3.5] ........................................................................... 91
Figure 3.9: IR spectrum of [3.8] ........................................................................... 93
Figure 3.10: UV-vis spectra of [3.1], [3.2] and [3.3]. .............................................. 94
Figure 3.11: UV-Vis spectra of [3.4], [3.5] and [3.6] .............................................. 94
Figure 3.12: UV-vis spectra of [3.7] and [3.8]. ....................................................... 95
Figure 3.13: Cyclic voltammograms of [3.2], [3.4], [3.5] and [3.6] ......................... 96
Figure 4.1: Stereoisomers of polyphenylacetylene ............................................. 104
Figure 4.2: 1H NMR spectrum of PPA, catalyzed by [3.2]. ................................. 107
Figure 4.3: IR spectrum of PPA prepared using [3.2]......................................... 108
Figure 4.4: TGA and DSC curves of PPA obtained with [3.1]. ............................ 109
Figure 4.5: 1H NMR spectrum of the products of hydroformylation of styrene
catalyzed by [3.6] .............................................................................. 114
x
LIST OF SCHEMES
Page
Scheme 1.1: Examples of solvent-free reactions .................................................... 3
Scheme 1.2: Solvent-free synthesis of 3-carboxycoumarins ................................... 3
Scheme 1.3: Photoirradited solvent-free dimerization of cholest-4-en-3-one .......... 4
Scheme 1.4: Solvent-free synthesis of ferrocenylenones ........................................ 7
Scheme 1.5: Solvent-free Knoevenagel condensation reaction .............................. 8
Scheme 1.6: Solvent-free Wittig reaction of ferrocenecarboxaldehyde ................... 9
Scheme 1.7: Solvent-free synthesis of ferrocenyl 1,5-diketone derivatives ........... 10
Scheme 1.8: Solvent-free synthesis of ferrocenoate esters .................................. 11
Scheme 1.9: Solvent-free reactions of ferrocenylaldehydes with aromatic amines 11
Scheme 1.10: Synthesis of binaphthol .................................................................... 12
Scheme 1.11: Solvent-free palladium-catalyzed phosphination reaction................. 13
Scheme 1.12: Solvent-free oxidation of thiols to disulfides ...................................... 13
Scheme 1.13: A mechanism for the synthesis of unsymmetrical sulfides from thiols
and alkyl halides using hydrotalcite clays .......................................... 14
Scheme 1.14: Solvent-free metal mediated synthesis of homoallyl alcohols ........... 14
Scheme 1.15: Microwave assisted solvent-free synthesis of β-aminoalcohols........ 15
Scheme 1.16: Sc(OTf)3 catalyzed solvent-free synthesis of β-aminoalcohols ......... 15
Scheme 1.17: Solvent-free synthesis of tetrasubstituted imidazoles on silica gel
support ............................................................................................ 16
Scheme 1.18: Solvent-free synthesis of tetrasubstituted imidazoles on SiO2/NaHSO4
support .............................................................................................. 16
Scheme 1.19: Solvent-free synthesis of 2,4,6-triarylpyridines ................................. 17
Scheme 1.20: Solvent-free synthesis of Schiff bases .............................................. 17
Scheme 1.21: Synthesis of bis(phenoxyketimine) zirconium complexes ................. 20
Scheme 1.22: Synthesis of silica-supported imine palladacycles. ........................... 25
Scheme 1.23: Suzuki cross-coupling of aryl bromides with phenylboronic acid. ..... 26
Scheme 1.24: Allylic alkylation of 1,3-di[henyl-2-enyl acetate with dimethyl malonate
................................................................................................................................. 29
Scheme 1.25: Synthesis of Ru complexes derived from 1st generation Grubbs
catalyst. ........................................................................................... 32
xi
Scheme 1.26: Synthesis of N-ferrocene salicylaldimine ligand ............................... 39
Scheme 2.1: Solvent-free synthesis of ferrocenylimines ......................................... 50
Scheme 2.2: Solvent-free synthesis of bisferrocenylimines ..................................... 51
Scheme 2.3: Solvent-free synthesis of arylbisimines in the presence of a catalyst. 57
Scheme 2.4: Solvent-free synthesis of arylbisimines............................................... 58
Scheme 2.5: Hydrogenation of ferrocenylbisimines. ............................................... 61
Scheme 3.1: Procedure for the synthesis of cationic rhodium(I) complexes. .......... 81
Scheme 4.1: Polymerization of phenylacetylene with Rh(I) catalysts. ................... 105
Scheme 4.2: Insertion mechanism for polymerization of phenylacetylene ............ 110
Scheme 4.3: Metallacyclic mechanism for the polymerization of phenylacetylene 110
Scheme 4.4: Hydroformylation of olefins. .............................................................. 111
Scheme 4.5: Synthesis of precursor to the indolizidine alkaloid ............................ 113
Scheme 4.6: Hydroformylation of styrene catalyzed by [3.4]-[3.8] ........................ 113
Scheme 4.7: Possible mechanism for hydroformylation of styrene catalyzed by [3.4]-
[3.8] ................................................................................................... 116
xii
LIST OF TABLES
Page
Table 2.1: Yields of bisferrocenylimines from the solvent-free reaction of diamines
and ferrocenecarboxaldehyde ............................................................... 53
Table 2.2: Chemical shifts for protons on the carbon directly bonded to nitrogen
group. .................................................................................................... 56
Table 2.3: Yields of arylbisimines from a reaction of substituted benzaldehyde and
ethylenediamine .................................................................................... 59
Table 2.4: Yields of bisferrocenylimines .................................................................. 62
Table 2.5: UV-vis data for ferrocene, [2.1]-[2.5] and [2.10]-[2.14]. ......................... 66
Table 2.6: Half-wave potentials of [2.1]-[2.5] and [2.12]-[2.14]. .............................. 68
Table 3.1: The summarized NMR data for [3.1]-[3.3]. ............................................. 82
Table 3.2: Crystal data and structure refinement of [3.2] and [3.3]. ....................... 84
Table 3.3: Selected bond distances, bond angles and torsion angles of [3.2] ......... 86
Table 3.4: Selected bond distances, bond angles and torsion angles of [3.3] ......... 89
Table 3.5: Table of yields and conductivity measurements ...................................... 92
Table 3.6: UV-vis data for complexes [3.1]-[3.8]. .................................................... 95
Table 3.7: Half-wave potentials of [3.1]-[3.6] ........................................................... 96
Table 4.1: Polymerization of phenylacetylene with Rh(I) complexes. .................... 106
Table 4.2: Determination of cis-content of polymers .............................................. 108
Table 4.3: Hydroformylation of styrene catalyzed by rhodium(I) complexes .......... 115
xiii
ABBREVIATIONS
Å Angstrom
ca. approximately
cat catalyst
COD 1,5-cyclooctadiene
Cp cyclopentadienyl, C5H5
CDCl3 deuterated chloroform
˚ degree
DSC differential scanning calorimetry
ε extinction coefficient
EI electron impact
FAB fast atom bombardment
J coupling constant
Lit. literature
M+ parent molecular ion
MeOH methanol
M.p. melting point
NMR nuclear magnetic resonance
Ph phenyl
PP phenylpropanal
PPA poly(phenylacetylene)
ppm parts per million
RT room temperature
1
CHAPTER 1
INTRODUCTION
1.1 Solvent-free synthesis
1.1.1 Background
Chemistry has played a leading role in changing people’s lives, due to its impact in
areas such as agrochemicals, the clothing industry, food technology, energy and
transport, the pharmaceutical industry and most recently in the manufacture of
electronic devices. However, discoveries about ecotoxic effects such as endocrine
disruption1 indicated that synthetic chemicals released into the environment have a
negative impact on the world ecosystem. Industrial incidents involving explosions at
a major South African petrochemical company,2,3 the discovery of persistent organic
pollutants and the global warming are examples of chemical disasters. It is because
of this reason that chemists are compelled to shoulder the responsibility for the
consequences and thus develop new synthetic protocols that are environmentally
benign. These new synthetic protocols should comply with green chemistry
principles.4 As an alternative to organic solvents, chemists should employ other
strategies to perform chemical reactions, namely ionic liquids, supercritical fluids,
water as a solvent and solvent-free conditions.
Conventionally, chemical transformations have been carried out in the presence of a
solvent to provide a homogeneous medium for the reagents to interact effectively as
well as for the isolation and purification of the desired product.5,6 It was believed that
in solution the reagents have higher mobility, hence increased molecular collisions
leading to faster chemical reactions. Unfortunately, organic solvents are high on the
list of toxic or hazardous compounds because of large volumes used in industry, and
difficulties in containing volatile organic compounds (VOC’s).7
2
The development of solvent-free organic synthetic procedures has become an
important and popular research area.8 Chemical reactions under solvent-free
conditions have been practised for many years. It has been reported that the first
written document on a solvent-free reaction came from a book ‘De Lapidibus’ by
Theophrastus (371-286 B.C).9 He reported that when cinnabar (HgS) was ground in
a brass motor with a brass pestle in the presence of vinegar, metallic mercury was
obtained. Afterwards, reports on chemical transformations under solvent-free
medium were rare, until the work of Carey Lea10,11 as well as Ling and Baker at the
end of the 19th century.12 Ling and Baker were successful in preparing quinhydrone
by grinding together two solids. It was again only more than half a century later that
researchers became interested in chemical transformations in the absence of a
solvent medium.
In the 1960’s, Rastogi et al.13-15 carried out detailed investigations about factors
governing the reaction between two solids, such as mobility on the interface, kinetics
and mechanism. In 1984, Patil et al.16 reported on the successful solvent-free
synthesis of unsymmetrically substituted quinhydrones, which was an alternative to
the problematic solution procedure due to self-oxidation reactions. In 1987, Toda et
al.17 introduced the concept of ‘host-guest’ complex formation in the solid state. It
was proposed that the quinine sublimes and its vapour attacks certain hydroquinone
sites. Toda et al.18,19 went on to report on their successful Grignard reaction and
aldol condensation under solvent-free conditions.
Owing to these successes, many researchers began to recognise the advantages of
carrying out chemical reactions in the absence of solvents. In the 1990’s,
researchers became strongly involved in carrying out studies on different types of
reactions without the use of classical solvents. Examples were the work of Kaupp et
al. (1993)20 (Scheme 1.1a), Villemin et al. (1995)21 (Scheme 1.1b) and Ranu et al.
(1997)22 (Scheme 1.1c) as well as other researchers.23-25 Ranu and coworkers were
successful in synthesizing a Michael addition type product under microwave-assisted
solvent-free conditions.
3
R CHO +
NH
N
O
NH
CH3
Solvent-free
MWNH
N
O
NH
CH3
H
R
R =
R R'
O O
+n
O
Al2O3
MW
n
O
R
R'
O
O
(i): R =CH3; R' = OEt; n = 1, 2(ii): R, R' = CH3; n = 1,2
O
O ;
Cl
;O
NH
N
S
O
R
R'
+ N
R''
solvent-free NN N
R'
O
SR''
HH
(i) R = R' = R'' = H(ii) R = R' = H, R" = CH3(iii) R = R'' = H, R' = Ph
a)
b)
c)
100%
84-91%
78-90%
Scheme 1.1: Examples of solvent-free reactions20-22
In 2000, Scott and Raston26 reported on the synthesis of 3-carboxycoumarins, via
the Knoevenagel condensation reaction, by gently grinding the starting materials in a
mortar with a pestle (Scheme 1.2).
R
OH
CHO
+
O
O
O
OR
OH
O
O
O
O
R
OH
O
OH
O
O
NH4+MeCO2
-
Scheme 1.2: Solvent-free synthesis of 3-carboxycoumarins26
4
DellaGreca et al27 reported, a year later, that it is possible to dimerize cholest-4-en-
3one under photoirradiation. Powdered cholest-4-en-3-one was simply irradiated
with a UV lamp to yield the desired product (Scheme 1.3).
O
hv
OH
O
+
O OH
Scheme 1.1: Photoirradited solvent-free dimerization of cholest-4-en-3-one27
Solvent-free reactions are of utmost interest from the ecological point of view, and
they offer advantages, such as reduced reaction times, increased product yields,
reduced environmental pollution, simple equipment (lab scale), increased selectivity,
and low cost compared with reactions carried out in solvents.4 The formation of hot
spots (solvents can act as heat sinks) and the prospects of runaway reactions is one
of the few disadvantages of the solvent-free reactions. Another disadvantage is the
difficulty in designing suitable reactors for these reactions to be applied in the
industrial scale.4 However, these problems can be solved by using engineering
reactor technology.7
5
1.2 Ferrocenes
1.2.1 Solvent-free synthesis of ferrocenes The discovery and characterization of the structure of ferrocene or cyclopentadienyl
iron, Fe(C5H5)2 in the early 1950’s,28 led to an explosion of interest in d-block metal-
carbon bonds and stimulated the development of organometallic chemistry.29-33
Ferrocene derivatives are extremely important since they can be used in a variety of
functions, such as the synthesis of non-linear optical materials, organometallic
complexes, and in catalysis.34-37 However, the preparation of ferrocene derivatives
has usually been performed under homogeneous conditions in the presence of
classical solvents. With the current upsurge of interest in performing chemical
transformations under solvent-free reactions, researchers have successfully
prepared ferrocene derivatives under solvent-free conditions.
In general, the reaction is induced by a method of mechanochemical activation.38-40
This merely means mechanical mixing (grinding or stirring) of the chemical reagents
to bring about the reaction at room temperature. In some cases, this is coupled with
microwave irradiation,41 especially to accelerate reactions that are very slow at room
temperature. A mortar and pestle set is used for simple mechanical mixing and a
commercial microwave oven for irradiation.
It is worth noting that after reviewing the literature, it is apparent that most of the
solvent-free reactions of ferrocenes reported are carbonyl condensation reactions,
particularly the two compounds, ferrocenecarboxaldehyde35-37,41-44 and
acetylferrocene.41,43 These two compounds are important precursors for numerous
ferrocene derivatives, and have been shown to be particularly amenable to reactions
under solvent-free conditions.41-44,47 Differential scanning calorimetry (DSC) analysis
(Figure 1.1)42 indicates that ferrocenecarboxaldehyde exhibits a phase transition,
called a ‘plastic crystal phase’, at 45 °C and runs to its melting point at 120 °C. This
phase transition is considered to a point where a reaction takes place.
Acetylferrocene is expected to be less reactive due to the lack of this phase
transition and also due to steric hindrance by the methyl group.
6
Figure 1.1: DSC analysis of ferrocenecarboxaldehyde
We shall now consider the solvent-free reactions of ferrocenes and comparisons with
other synthetic methods will be provided where possible.
1.2.1.1 Synthesis of ferrocenylenones
The synthesis of ferrocenylenones can be performed by a variety of methods, such
as Claisen-Schmidt41 and aldol reactions43,44 under homogeneous conditions in
ethanol. Other methods that have been reported include the use 18-crown ether as a
phase-transfer catalyst (PTC) and solvent-free aldol reactions using pulverized
potassium hydroxide.41,43 In 1994, Villemin and co-workers41 described the solvent-
free synthesis of ferrocenylenones under microwave irradiation conditions.
Acetylferrocene was stirred with an aromatic aldehyde and ferrocenecarboxaldehyde
was stirred with a ketone, in the presence of a base or a phase-transfer catalyst
(Aliquat 336), or both, under microwave irradiation, to achieve the corresponding
ferrocenylenone (Scheme 1.4).
7
Fe
C
+ Ar
O
R
O
KOH, A336
RT or MW Fe
CCH
O
H
HC Ar
R = H, CH3
Scheme 1.4: Solvent-free synthesis of ferrocenylenones41
However, this method suffers from a serious disadvantage,41,43 since it was found
that the ferrocene derivatives combust easily in the microwave oven. Consequently,
Liu et al.43 and Méndez et al.44 described an improved method for the preparation of
ferrocenylenones via aldol condensation under solvent-free conditions. Méndez and
co-workers performed the aldol reaction by adding powdered base to a stirred
mixture of the reagents mixture in the presence of a PTC (Aliquat 336) while Liu et
al. managed to achieve excellent yields of the products, in the absence of the PTC,
by grinding a ketone and base using a mortar and pestle. Thereafter, the appropriate
aldehyde was introduced with further grinding for a few minutes. Interestingly, all
products formed for the different methods have an E-configuration as a major
product.41-44
1.2.1.2 Reaction of ferrocenecarboxaldehyde with methylene active compounds
Knoevenagel condensation of ferrocenecarboxaldehyde with methylene active
compounds is usually performed under classical homogeneous conditions in
ethanol.36,37 Of the two isomers obtained, the E-isomer was found to possess more
enhanced non-linear optical (NLO) properties than the Z-isomer.35 Incidentally, a
whole range of these kinds of ferrocene derivatives have been studied for their NLO
properties.35,36 They have also been used as catalysts for the combustion of
composite explosives.37 The classical homogeneous methods frequently result in low
yields and some reactions require the use of Schlenk techniques.36
8
Fe
CH
O
Fe
CX-CH2-Y, Al2O3C
X
H
Y
X, Y = electron withdrawing group
solvent-free
58-100%
Scheme 1.5: Solvent-free Knoevenagel condensation reaction36
Cooke and co-workers36 have reported on the solvent-free synthesis of novel
ferrocene derivatives from the Knoevenagel condensation reaction of
ferrocenecarboxaldehyde with methylene active compounds, in the presence of an
inorganic support (Scheme 1.5). The role of the inorganic support, basic alumina in
this case, was to increase the surface area and thus improve the conversion of
reagents into the desired products. The reaction was carried out simply by grinding
or stirring (depending on the nature of the methylene active compound) of the
reaction mixture (at room temperature or heated up to 65ºC) and the consequent
addition of a carefully calculated amount of basic alumina. This was accompanied by
an immediate colour change to purple, indicating formation of the product. Isolation
of the product was achieved simply by extraction with dichloromethane and in some
cases further purification by column chromatography was necessary. Stankovic and
co-workers35 obtained better yields, in some cases, when silica was used as an
inorganic support and they postulated that due to the inherent acidity of the silica, it
acted as a catalyst. It is worth mentioning that the method used by Stankovic and co-
worker differs to the one used by Cooke and coworkers. in that they do not grind the
starting reagents. Instead, a solution of ferrocenecarboxaldehyde in dichloromethane
is added to the inorganic support, followed by evaporation of the solvent. A solution
of methylene active compound in dichloromethane is then added and the solvent
evaporated. At this stage the reaction is left to stand at room temperature. The
method of Stankovic takes longer (1-48 h) than that of Cooke (1-5 h), and yet there
is difference in the yields. Bai et al.37 have reported on three methods for the
Knoevenagel condensation of ferrocenecarboxaldehyde and the methylene active
compound, namely in water, grinding under solvent-free conditions and microwave
induced solvent-free reactions. All the methods are performed in the presence of
9
potassium hydroxide. Most interestingly all the methods had shorter reaction times
than the previous methods, with reaction times in water (45 min), grinding (12-30
min) and microwave irradiation (6-8 min). None of the three methods was superior in
terms of the product yields, although microwave assisted reactions had shorter
reaction times.
1.2.1.3 Reaction of ferrocenecarboxaldehyde with an ylid
Fe
CH
O
Fe
HCRCH2P+Ph3X-
Solvent-free, NaOH
CHR
R = ArX = Cl, Br, I
71-95%
Scheme 1.6: Solvent-free Wittig reaction of ferrocenecarboxaldehyde47
The Wittig reaction is a very popular method for the regio- and stereo-selective
formation of alkenes from carbonyl compounds.45,46 A resonance-stabilized
phosphonium intermediate, called an ‘ylid’, is generated by abstraction of a proton
from a phosphonium salt by a base. The reaction of the ylid with a carbonyl group
results in the formation of an alkene and triphenylphosphine oxide as a by-product.
The Wittig reaction and many other synthetic methods used for the synthesis of
vinylferrocene derivatives, have usually been performed in classical homogeneous
media. The first ever solvent-free Wittig reaction was reported in 2001, by Liu and
co-workers.47 They were successful in preparation of vinylferrocene by grinding
ferrocenecarboxaldehyde and triphenylphosphinylbenzylphosphonium chloride in the
presence of a sodium hydroxide, using a mortar and pestle (Scheme 1.6). The
reactions were performed at room temperature (5 min) and slow reactions were
heated at 65 ºC (40-45 min). The conversions of starting reagent were moderate to
excellent and the major products were E-alkenes.
10
1.2.1.4 Synthesis ferrocenyl-1,5-diketone derivatives
1,5-Diketones are very useful synthetic intermediates and are desirable starting
materials for the preparation of heterocyclic and polyfunctional compounds.48-50
These compounds have potential applications in coordination chemistry, molecular
sensing, catalytic reactions, the chemical modification of electrodes and redox active
self-assembly devices. Various procedures have been reported for the synthesis of
1,5-diketones, but require the use of expensive reagents, are carried out under
refluxing conditions, and have longer synthetic routes.
Fe
R
O
PhCOMe, NaOH
solvent-freeFe
O R
Ph
O
R = Ar 68-92%
Scheme 1.7: Solvent-free synthesis of ferrocenyl 1,5-diketone derivatives51
Liu and co-workers51 have reported on the solvent-free synthesis of 1,5-diketone
derivatives containing ferrocene via the Michael addition reaction (Scheme 1.7). The
reaction was performed by grinding a mixture of an α,β-unsaturated ferrocenyl
ketone (or ferrocenylchalcone) and acetophenone with an agate mortar and pestle,
and the reaction mixture heated at 45 ºC. The pure products were obtained after a
short and simple work-up procedure.
1.2.1.5 Synthesis of ferrocenoate esters
Elago et al.52 have recently reported on the synthesis of ferrocenoate esters, amides
and other ferrocenoyl derivatives in ionic liquids and under solvent-free conditions
(Scheme 1.8). Both methods yielded excellent yields irrespective of the nature of the
substituent X. However, the ionic liquid required deaeration and three cycles of
freeze-thaw degassing. Moreover, the reaction requires 16 hours of stirring. In the
solvent-free method ferrocenoyl fluoride and the appropriate substituted phenol were
thoroughly ground in a mortar, and the mixture subjected to microwave irradiation for
1 minute.
11
Fe
CF
O
DMAP, [bmim][BF4] RT, 16 h
or solvent-free, MW
+ HO
XFe
CO
O
X
X = 4-OCH3 = 4-CH3 = 4-H = 4-Cl = 4-NO2 = 4-Br = 4-CHO
60-100%
Scheme 1.8: Solvent-free synthesis of ferrocenoate esters52
1.2.1.6 Synthesis of ferrocenylimines
Several research groups53-55 have prepared ferrocenylimines by heating a solution of
ferrocencarboxaldehyde with aromatic amines under reflux in anhydrous methanol or
ethanol. Ferrocenylimines have been used for metal coordination and have also
been studied for non-linear optical properties.56 The major setback with the solvent
procedure is the decomposition of the imines during the heating process, leading to
moderate yields.
In highlighting the solvent-free procedure, Nyamori et al.42 have successfully
prepared ferrocenylimines under solvent-free conditions (Scheme 1.9). Grinding of a
mixture of ferrocenecarboxaldehyde and the appropriate substituted aniline with a
glass rod in a glass tube resulted in excellent yields of products. The products were
purified by recrystallization from a minimal amount of cold anhydrous methanol. In
cases where the aniline substituent was an electron-withdrawing group or where the
ferrocenecarboxaldehyde was less reactive, gentle heating (50 ºC) was necessary to
obtain good conversions.
Fe
Y C
O
H+
H2N
X
Grind
Solvent-free Fe
Y C
N
H
X
Y = Ph, Ph-Ph, Ph-O-Ph
87-97%
Scheme 1.9: Solvent-free reactions of ferrocenylaldehydes with aromatic amines42
12
1.3 Solvent-free synthesis of ligand systems
It is generally accepted that a metal complex is a chemical species which contains a
metal atom or ion bonded to a greater number of ions or molecules than would be
expected from simple valency considerations.57 The ions or molecules that are
bonded or coordinated with the metal are termed ligands. A ligand is regarded as an
ion or a molecule that has a pair of electrons that it can easily donate. The actual
atom through which a ligand is bonded to a metal is called the ligand atom.
Ligands have played an extremely important role in human lives due to their use in
various types of functions, such as in the synthesis of biologically active
organometallic compounds58-62 and in catalysis.63-65 However, the synthesis of the
ligands has always been performed in the presence of a solvent medium. For
example, binaphthols have been prepared from 2-naphthol in the presence of a
catalyst, oxygen or air as an oxidant and chlorinated solvents (Scheme 1.10).66,67
R1
R2
OH
CuCl(OH).TMEDA (1mol%)
O2, CH2Cl2
R1
R2
OH
OH
R2
R1
Scheme 1.10: Synthesis of binaphthol66
Many research groups68-71 have reported on the synthesis of several kinds of
phosphorus ligands in solution. Some methods required the use of very toxic
reagents, such as potassium cyanide,70 working at very low temperature71 and many
reaction steps. Therefore, in order to maintain the concept of sustainable chemistry
the design of new synthetic procedures that are benign to the environment became a
necessity. In this case special attention is given to the solvent-free approach to the
synthesis of ligands.
A good example of the solvent-free approach is that described by Kwong et al.72 on
the phosphination reactions (Scheme 1.11). In that work, arylbromides and triflates
were converted into the corresponding arylphosphines by palladium catalyzed
13
phosphination with triphenylphosphine under solvent-free conditions. Altough the
yields were an improvement on those reported previously,73 they were still modest.
X
Y2.3 eq. PPh310 mol% Pd(OAc)2
solvent-free, 115°CPPh2
Y
X = Br, OTf
Y = CN, OMe, CHO, COCH3, CO2Me
Scheme 1.11: Solvent-free palladium-catalyzed phosphination reaction72
Another interesting example from the green chemistry perspective is the oxidation of
thiols to disulfides. Thiols and disulfides play an important role in biological
processes.74 Lenardão et al.75 discovered a solid-supported catalyst (Al2O3/KF) for
the solvent-free oxidation of thiols to disulfides (Scheme 1.12). The reactions were
performed either at room temperature, by gently heating or by microwave irradiation.
Without any exception, the microwave assisted reactions proceeded faster and with
higher yields.
R S HAl2O3/KF (40%)
RT, ∆ or MWR S S R
R = Ar = n-C12H25 = HO(CH2)2
48-96%
Scheme 1.12: Solvent-free oxidation of thiols to disulfides75
Vijaikumar and Pitchumani76 have also demonstrated the benefits of the solvent-free
approach in the synthesis of unsymmetrical sulfides from thiols and alkyl halides
using hydrotalcite clays (HT). The proposed mechanism is shown in Scheme 1.13.
14
HT+ OH-
H2O
HX↑↑↑↑
HT+ X- RS- HT+
RSH
R'SRSR'
Scheme 1.13: A mechanism for the synthesis of unsymmetrical sulfides from thiols
and alkyl halides using hydrotalcite clays76
McCluskey77 developed a method for allylation of carbonyl compounds by
tetraallylstannane in the presence of water as a solvent. This procedure successfully
minimised the environmental impact of tetraallylstannane by allowing the isolation of
inorganic stannane salts and recycling the organic solvents used to extract the
homoallylic alcohol products. A more recent method for allylation of carbonyl
compounds is that reported by Andrews et al.78 This involves the synthesis of
homoallylic alcohols via a metal-mediated reaction of carbonyl compounds with allyl
bromide under solvent-free conditions (Scheme 1.14). In some cases the reactions
are heated or subjected to sonification, and also require quenching of the reaction
with water.
Br +R R'
Oi) M
ii) H2O R'
R OH
M = In, Bi, Zn, CuR, R' = H, Aryl, Alkyl
0-97%
Scheme 1.14: Solvent-free metal mediated synthesis of homoallyl alcohols78
Sabitha et al.79 have also shown that the aminolysis of epoxides with ammonium
acetate can proceed efficiently and regioselectively under solvent-free conditions,
especially when subjected to microwave irradiation (Scheme 1.15). This procedure is
15
faster and results in higher yields than the corresponding solvent-mediated methods.
The regioselectivity stems from the preferential attack of the nucleophile on the less
hindered carbon atom of the epoxide ring.
R
O NH4OAc, MW
40 - 120 s R
OH
NH2+
R
NH2
OH
major minor
Scheme 1.15: Regioselective microwave assisted solvent-free synthesis of β-
aminoalcohols79
A more recent method for the aminolysis of epoxides is that reported by Placzek et
al.80. This involves the synthesis of β-aminoalcohols via a ring opening of epoxides
with amines in the presence of a scandium triflate catalyst under solvent-free
conditions (Scheme 1.16). The aryl oxiranes underwent cleavage by various amines
in a regiospecific fashion with preferential attack at the benzylic carbon. The catalyst
could be recycled and reused several times.
OOH
NSc(OTf)3 (5 mol%)
solvent-free, RT+ HNR
R'
R
R'
Scheme 1.16: Regioselective Sc(OTf)3 catalyzed solvent-free synthesis of β-
aminoalcohols80
The synthesis of substituted imidazoles is of importance due to their biological
properties. Compounds with the imidazole ring system have many pharmacological
properties and play an important role in biochemical processes. One of several ways
of synthesizing substituted imidazoles is the four-component condensation of
arylglyoxals, primary amines, carboxylic acids and cyanides on Wang resin.81
Balalaie et al.82 have described a novel one-pot, three-component condensation of
benzil, benzonitrile derivatives, and primary amines on the surface of silica gel under
solvent-free conditions (Scheme 1.17). The reaction was accelerated by microwave
irradiation and excellent yields were obtained.
16
Ph
Ph
O
O
+ + NH2
silica gel
N
N
Ph
Ph
R
PhAr CN
MWR
R = Ar, Alkyl
58-92%
Scheme 1.17: Solvent-free synthesis of tetrasubstituted imidazoles on silica gel
support82
In a recent report, Karimi et al.83 have described another facile method for the
synthesis of tetrasubstituted imidazoles with a silica gel-supported sodium bisulfate
as a catalyst and without any solvent. The reaction involved a four-component
condensation of benzil or benzoin, an aldehyde, amine and ammonium acetate
under microwave irradiation or heating (Scheme 1.18). The method showed
superiority over conventional methods in that it gave excellent yields. Another
advantage of this method is that the catalyst is inexpensive and it avoids problems
associated with catalyst cost, handling, safety and pollution.
Ph
Ph
O
O
+
R
H O
+ R' NH2
NH4OAc/NaHSO4-SiO2
MW or ∆ N
N
Ph
Ph
R' R
Scheme 1.18: Solvent-free synthesis of tetrasubstituted imidazoles on SiO2/NaHSO4
support83
More recently, Adib et al.84 have developed a novel and facile method for the
preparation of 2,4,6-triarylpyridines by reaction of chalcones with ammonium
acetate, with heating under solvent-free conditions (Scheme 1.19). Excellent yields
of triarylpyridine products were obtained and required simple purification techniques.
17
Ar
O NH4OAc, AcOH (cat.)
100 °C, solvent-free, 4 hAr' N
Ar'
Ar Ar
93-97%
Scheme 1.19: Solvent-free synthesis of 2,4,6-triarylpyridines84
Spurred by the success with the above reactions Adib et al.85 went further to develop
a simple and versatile route for the synthesis of 2,4,6-triarylpyrimidines under
solvent-free conditions and microwave irradiation.
The synthesis of Schiff bases has generally been carried out under reflux in
methanol or ethanol solution. The disadvantage with this method is that more
sensitive Schiff bases tend to undergo some degree of decomposition. Recently,
Naeimi et al.86 developed a mild and convenient route of the reaction of carbonyl
compounds with amines, under solvent-free conditions and in the presence of a
catalyst. They also prepared double Schiff bases and obtained low to excellent
yields, depending on the amine used (Scheme 1.20). The only significant
disadvantage of this method is that the catalyst (P2O5/Al2O3) is moisture sensitive.
R
OH
R'
O+ H2N
Y
NH2
P2O5/Al2O3
Solvent-free
R
OH
R'
N
Y
N
R'
HO
R
R = H, NO2
R' = H, CH3
Y = C6H4-O-C6H4, C6H4-CH2-C6H4, C6H4-SO2-C6H4
Scheme 1.20: Solvent-free synthesis of Schiff bases86
1.4 Nitrogen-donor ligand chemistry
P-donor ligands have received a lot of attention in the fields of organometallic
chemistry and catalysis.87 It was only in the mid 1990s that special attention was
18
drawn towards N-donor ligands.88 N-donor ligands have a larger range than that of
any other atom and their organic chemistry is varied. The best way to classify N-
donor ligands could be based on the hybridisation of the nitrogen atom, i.e., sp3, sp2
and sp. Ligands with sp2-hybridised nitrogen, such as imines and pyridines, have a
significant coordination chemistry.87 Examples of some ligands containing sp2-
hybridised nitrogen atoms are summarized in Figure 1.2.
N NN N N
N NH
N
R N N R N
N N
N
Figure 1.2: Some N-donor ligands with sp2-hybridised nitrogen atoms
The comparison of the well-studied phosphorus ligands and the nitrogen ligands can
be done by noting the differences in coordination behaviours as well as the
properties of the N-donor ligands.87
(i) The coordination bonds formed by N-donor ligands are fairly stronger than
those formed by P-donor ligands and the strength of the bonds depends
largely on their σ covalency with potentially significant contribution from
the ionic character of the bond itself.87
(ii) The strength of the M-N bonds will be affected much more by steric effects
than that of the corresponding M-P bonds.87
(iii) The N-donors are generally not as effective in forming low-spin complexes
and consequently form thermodynamically less stable species but more
kinetically labile than their low-spin P-donor analogues.87
(iv) N-donor ligands generally show only a limited π-back bonding ability,
making these ligands less suitable for the stabilization of low oxidation
state transition metals. However, sp2-hybridised N-donors such as
pyridine, have been known to show some π-back bonding effects between
the nitrogen heterocycle and the metal centre.89,90
19
(v) The N-donor ligands exert a very small trans effect in comparison to other
ligands used in organometallic chemistry, resulting in a fast rate of
substitution reactions.87
(vi) The observed reactivity of nitrogen donor ligands is generally high,
particularly with alkyl complexes of transition metals containing these
ligands.87
As it has been mentioned above that N-donor ligands have a larger range than any
other atom, therefore, a few examples of N-donor ligand catalyzed reactions will be
chosen, particularly those reactions catalyzed by transition metal complexes
containing pyridines and Schiff bases (imines). The examples of reactions that will
be focused on include the allylic alkylation, olefin polymerization, cross coupling,
epoxidation and ring opening metathesis polymerization (ROMP).
1.4.1 Polymerization reactions
Polymerization reactions required that for the catalysts to be more efficient, the
following conditions for the catalysts must apply.
The catalyst must:
(i) have high-olefin insertion ability.91
(ii) have two available cis-located sites for polymerization.91
(iii) Be stable enough under the usual polymerization conditions.91
The polymerization was catalyzed efficiently by titanium complexes containing
bis(alkylphenylaminopyridinato) ligands (Figure 1.3(a)-(e)).92 The ligands were
prepared by the reaction of 2-chloropyridine and the appropriate alkylaniline
hydrochlorides. The catalysts were formed at room temperature on reaction of the
ligands with titanium tetrachloride (TiCl4).
20
N N RTi
N NR
ClCl a) R = 2-Etb) R = 3,5-Mec) R = 4-n-Bud) R = 2-t-Bue) R = Ph
I
Figure 1.3: Bis(alkylphenylaminopyridinato) titanium complexes92
The complex Ie (Figure 1.3) exhibited the highest polymerization activity but
produced the lowest molecular weight of polyethylene compared to complexes Ia-d.
The decrease in polymerization activity for complexes Ia-e was attributed to the
electron donating effects of the alkyl substituents. It was also concluded that as the
distance between the alkyl substituent and the metal centre increased, the more
active was the catalyst.92
A series of new zirconium complexes bearing bis(phenoxyketimine) ligands have
been prepared in low to moderate yields (>70%) (Scheme 1.21).93 The effects of the
substituents on the imine carbon and on the phenyl ring of the aniline moiety on
polymerization activity were investigated. It was found that when R1 is H atom the
molecular weight of polyethylene was low (Mw = 12 000) and the activity 1.6 kg PE
mmol-1 Zr .h-1 at 20 ºC. The best results were obtained when the substituent R1 was
electron donating and substituent R2 was an electron withdrawing group.
NR1
R2
O
ZrCl2
2
But But
NR1
R2
OH
But But
1) n-BuLi
2) 0.5 ZrCl4
Scheme 1.21: Synthesis of bis(phenoxyketimine) zirconium complexes93
Cloete et al.94 have synthesized functionalized pyridinylimine complexes of palladium
as precursor catalysts for ethylene polymerization. The nitrogen atom of the imine
functionality contained various substituents, ranging from alkyl to aromatic and allylic
groups (Figure 1.4, II).
21
N
II
a: R = allylb: R = styrylc: R = phenold: R = phenyle: R = propyl
N
N NAr Ar
R'
RR
R'
a: R = Me, R' = H, Ar = Mesb: R = R' = Me, Ar = MesC: R = CH2Ph, R' = H, Ar = Mesd: R = R' =CH2Ph, Ar = Mese: R = Me, R' = H, Ar = DIPPf: R = R' = Me, Ar = DIPPg: R = R' = H, Ar = Mes
Mes = mesityl (2,4,6-trimethylphenyl)DIPP = 2,6-diisopropylphenyl
NRPd
Cl Cl
Fe
Cl Cl
N
N NFe
R2
R1
X
R1
R2
X
R1, R2 = H, MeX = Br, I
Cln
III
IV
Figure 1.4: Pyridyl-imine complexes of iron (Fe) and palladium (Pd).94,97,98
The palladium complex IIc exhibited the highest activity at Al:Pd of 2000:1 and whilst
complexes IIb reached the optimal activity at Al:Pd of 1500:1. The reason complex
IIc required a higher amount of methylalumoxane (MAO) was due the fact that the
hydroxyl group on the aromatic ring reacted with MAO to form an Al phenoxide
adduct.94
The iron-based bis(imino)pyridine complexes have been prepared95-98 and have
been used as catalysts for olefin polymerization.97,98 Upon treatment with MAO, all
complexes became active in the polymerization of ethylene. Complexes that
contained the 2,4,6-trimethylphenyl (Figure 1.4, IIIa-d and IIIg) attached to the imine
nitrogen atom were more productive than those that contained the 2,6-
diisopropylphenyl (Figure 1.4, IIe and IIf). This effect was attributed to less
congested active sites for 2,4,6-trimethylphenyl derivatives resulting in higher
propagation rates.97 The replacement of the ethyl group in IIe with the isopropyl
group to form IIf led to increased polymer molecular weights [Mw 26 000 (IIe), Mw
263 000 (IIf)].
22
The influence of the para-substituent in bis(arylimino)pyridine iron complexes on the
catalytic oligomerization and polymerization of ethylene was investigated.98 These
complexes contained phenyl rings bearing halogen (Br, I) substituents at para-
positions (Figure 1.4, IV). The complexes where R1 was a methyl group, R2 a
hydrogen and bearing a strongly electron withdrawing group at para-position were
found to have the highest activities in oligomerization. On the other hand, those
complexes where R1 and R2 were methyl groups and bearing a halogen at a para-
position were highly active in polymerization. The extremely high polymerization
activities of 4-halo-2-methyl substituted complexes was due to the electronic
influence of the halogen at the para-position.98 The Fe(III) analogues of these
complexes show more enhanced activities due to increased Lewis acid strength and
therefore the coordination of an ethylene molecule is facilitated.98
1.4.2 Cross-coupling reactions
1.4.2.1 Heck reactions
Two pyridine bridged dicarbene palladium(II) complexes (Figure 1.5, V) that were
efficient for the Heck coupling reaction have been reported by Nielsen and co-
workers.99 These complexes were prepared via a dinuclear silver(I) complex, which
was synthesized from the reaction of the tridentate 2.6-bis[(3-methylimidazolium-1-
yl)methyl] pyridine dibromide and silver(I) oxide in dichloromethane or N,N’-
dimethylsulfoxide. The compound V (X = Cl) exhibited a higher activity towards the
coupling reaction of n-butyl acrylate and 4-bromoacetophenone when treated with
hydrazine hydrate, in the absence of the quaternary ammonium salt.99 The main
product of the coupling reaction was the expected n-butyl-(E)-4-acetylcinnamate,
without any detection of the Z isomer by 1H NMR and gas chromatography.99
23
N
N N
N N
Pd
X
X = Cl, Br
BF4
O
O
O
Si(CH2)3N
NPd
Cl
Cl
N N
Pd
ClCl
R1
R2
N N
Pd
R1
R2
E
E
R1 = H, R2 = i-PrR1 = Me, R2 = i-PrR1 = H, R2 = t-Bu E = CO2Me
VVI
VII VIII
Figure 1.5: Examples of Pd-pyridyl complexes used for the Heck reactions99
Horniakova et al.100 have reported on pyridylimine palladium(II) complex immobilized
on a mesoporous silica (Figure 1.5, VI). This complex is heterogeneous in nature
and could be used as an alternative to the homogeneous catalysts for the Heck
reaction as well as the Suzuki reaction.100 The activity of this complex was compared
with quiniline-imine palladium(II) complex. Both catalysts were found to be highly
active in the Heck reaction with 100% conversions of the arylhalides and high
selectivity towards the E isomer was also observed.
Palladium(II) (Figure 1.5, VII) and palladium(0) (Figure 1.5, VIII) complexes based on
pyridyl-imine ligands have been synthesized and used as catalyst precursors for the
Heck reaction.101 As expected all the ligands were active and promoted the complete
conversion of iodobenzene into trans-methyl cinnamate (Z isomer). For the same
ligand, the palladium(II) complexes were more active than their palladium(0)
analogues.101 Although not mentioned in some articles,99,102 it was apparent that
leaching of palladium was a major problem for the homogeneously-catalyzed Heck
reactions.101,103
24
1.4.2.2 Suzuki cross-coupling reactions
A range of nickel(0) complexes (Figure 1.6) that contain pyridine, 2,2’-bipyridine, 2-
(2-oxazolyl)pyridine and 2,2’-bisoxazole ligands have been prepared and their
catalytic activity towards cross-coupling of aryl chlorides by intramolecularly
stabilized dialkylaluminium reagents has been studied.104
N N
Ni
N N
N N
O
Ni
NN
O
N
O
Ni
O
N
N
O O
N
N N
Ni
N N
IX X
XI XII
Figure 1.6: Examples of Ni(0) complexes used for cross-coupling of arylchlorides104
Complexes IX and X exhibited an increase in activity in the presence of THF as a
solvent, which enhanced the homo-coupling problem.104 It was also observed that IX
and X efficiently catalyzed the cross-coupling of chloroarenes without
hydrodehyalogenation occurring.
Some silica-supported imine palladacycles have been reported by Bedford and co-
workers.105 The objective was to study their catalytic activity as potential
heterogeneous catalysts for the Suzuki cross-coupling reaction, and their
recyclability was also tested. The synthesis of silica-supported imine palladacycles is
illustrated in Scheme 1.22.
25
OHC
Br
H2N(CH2)3Si(OEt)3
BrNR
R = (CH2)3Si(OEt)3
EtOH, molecular sieves
Pd(dba)2, toluene
60°C
RN
R = (CH2)3Si(OEt)3
Pd
Br
2
PPh3, CH2Cl2
RN
R = (CH2)3Si(OEt)3
Pd
Br
PPh3
N Pd
Br
2
silica, toluenereflux temperature
Si
HOOHOH
Si Si Si
N Pd
Br
PPh3
Si
HOOHOH
Si Si Si
PPh3, CH2Cl2
XIIIXIV
XVXVI
Scheme 1.12: Synthesis of silica-supported imine palladacycles.105
The silica-supported palladium complexes were very poor in terms of their catalytic
activity towards the Suzuki coupling reaction.105 In addition, the complexes were
easily recycled but the activity deteriorated in the subsequent reactions. The
deterioration in activity was due to the degradation of the catalysts since deep red-
brown palladium nanoparticles, which subsequently decomposed to palladium black,
were observed.105
26
SN R
SNR
PdCl
Cl
SN R
SNR
PdCl
Cl
R = Ph
XVIIXVIII
R = Mes = n-Pr = (S)-MeCHPh
Figure 1.7: Palladium imine and amine complexes for coupling of aryl bromides.106
Wiedermann et al.106 have synthesized palladium imine and amine complexes
(Figure 1.7) and investigated their activity towards the Suzuki cross-coupling of aryl
bromides with phenylboronic acid (Scheme 1.23).
Br
R
+
B(OH)2
catalyst, 2 eq. Cs2CO3
dioxane R
Scheme 1.23: Suzuki cross-coupling of aryl bromides with phenylboronic acid.106
Complex XVIII (R = Mes) showed a higher catalytic activity than all the other
complexes.106 The bulkier mesityl group was responsible for rendering the catalyst
more active.106
1.4.3: Epoxidation reactions
A 2-(phenylazo)pyridine rhuthenium(II) complex has been synthesized by Barf and
co-workers.107 and their catalytic activity in the epoxidation of stilbene was
investigated. Barf and co-workers were able to isolate three isomers, namely trans-
trans-trans (γ), trans-cis-cis (α) and cis-cis-cis (β) (Figure 1.8).
27
N N
N
a
b
2-(phenylazo)pyridine (pap)
Ru
Nb
Nb
Na
Na
Cl
Cl
Ru
Nb
Nb
Na
Na
Cl
Cl
Ru
Nb
Nb
Na
Na
Cl
Cl
αααα (t c c) ββββ (c c c) γγγγ (t t t)
Figure 1.8: Three isomers of Ru(pap)2Cl2107
The two isomers of Ru(pap)2Cl2, β and γ resulted in 100% conversion of stilbene
with good selectivity.107 The α-isomer which was the most stable gave both lower
conversion and selectivity. This was attributed to the steric hinderance of the α-
isomer as was evidenced by the X-ray crystal structure.107
Another interesting example was the use, by Moutet and Ourari,108 of Mn(III) Schiff-
base complexes for the electrocatalytic epoxidation and oxidation of cyclooctene.
The complexes are shown in Figure 1.9 below. The activity of the catalysts was
dependent on the bridging group Z, i.e., the activity decreased from Salen to Salch to
Sal(Cl)2ophen complexes. The effect was due to the decrease in stability of the
ligand as a result of the increased rigidity.108 The best results for epoxidation of
cyclooctene were obtained when salen complex and 2-methylimidazole were used.
Recently, Bruno et al.109 have reported on dioxomolybdenum(IV) complexes bearing
a bidentate and tetradentate salen-type ligands, that were similar to those prepared
by Barf et al.107
28
NZ
N
O OR1
R2 R2
R1
Mn
Cl
Salen
Z = (CH2)2
Salchx
Z =
Salophen
Z =
Sal(Cl)2ophen
Z =
Cl Cl
R1 = R2 = HR1 =Cl, R = HR1 = R2 = ClR1 = OCH3, R2 = HR1 = NO2, R2 = H
Figure 1.9: Mn(III) Schiff-base complexes for electrocatalytic epoxidation of
olefins108
An octahedral titanium binaphthyl-bridged Schiff-base complex110 has been prepared
and used as a catalyst for the regio- and stereoselective epoxidation of allylic
alcohols, under microwave-mediated solvent-free conditions. The cis-β isomer was
the preferentially formed product according to the X-ray crystal structure (Figure
1.10).110
Ti
N
N O
Cl
Cl
O
But
cis-ββββ isomer
N
N
=N
N
Figure 1.10: The cis-β isomer of the binaphthyl-bridged Schiff base titanium
complex110
The epoxyalcohols could be obtained with very high regio- and chemoselective way
by the microwave irradiation of the mixture under solvent-free conditions.
Furthermore, high diastereoselectivity could be observed for secondary allylic
alcohols.110
29
1.4.4 Asymmetric allylic substitution reactions
Palladium complexes based on the chiral pyridine ligands have been reported111,112
and their catalytic activity in asymmetric allylic alkylation of 1,3-diphenylprop-2-enyl
acetate with dimethyl malonate has been studied. The catalysts were prepared in
situ by a reaction of allylpalladium chloride dimer [Pd(η3-C3H5)Cl]2 with the ligands
2,2’-bipyridines, 2,2’,2”-terpyridines and 1,10-phenanthrolines (Scheme 1.24).111
Ph Ph
OCOCH3 CH2(CO2CH3)2
[Pd(η3-C3H5)Cl]2 / ligandPh Ph
CH(CO2CH3)2
*
Scheme 1.24: Allylic alkylation of 1,3-di[henyl-2-enyl acetate with dimethyl
malonate111,112
Phenanthroline catalysts were the most active and their enanioselectivity depended
on the distance of the chiral substituent from the heterocyclic nitrogen. On the other
hand, the enantioselectivity of bipyridines increased when there was a bulky
substituent at the 6-position.111
A series of chiral bidentate ligands containing thiazolyl and pyridyl donors have been
synthesized and used for in situ preparation of copper(I) complexes.113 The
subsequent copper(I) thiazolyl-pyridine complexes were used for the
enantioselective allylic oxidation of cyclohexene with t-butyl perbenzoate. Examples
of the chiral bidentate thiazolyl-pyridyl ligands are shown in Figure 1.11. Complexes
XX gave the highest yields of the product, but with low enantioselectivity, while
complexes XXI exhibited higher enantioselectivity. The reason for the higher
enantioselectivity of the latter complexes was attributed to steric hindrance at the 8-
position of the tetrahydroquinoline ring and the thiazole ring. Ligands with a
substituent at the 8-position of tetrahydrquinoline led to the (R)-configuration and
others led to (S)-configurations.113
30
N
S
N
R
XIX
R = H, CH3
N
S
N
R
XX
R = H, n-Pr, CH2Ph, CH2SiMe3
N
S
N
R1
XXI
R1 = R2 = HR2
R1 = CH3, R2 = HR1 = i-Pr, R2 = HR1 = H2 = n-PrR1 = R2 = n-BuR1 = R2 = CH2PhR1 = R2 = CH2SiMe3
Figure 1.11: Examples of chiral bidentate thiazolyl-pyridine ligands113
The chiral diimine palladium(II) complexes (Figure 1.12) have been prepared for
asymmetric alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate by
Albano et al.114
N
N
S
S
S
S
R
R
PdBF4
Figure 1.12: Chiral diimine palladium(II) catalyst for asymmetric alkylation114
These complexes were less active than their diamine analogues due to the steric
hindrance that prevents nucleophilic attack and the electronic issues taking part
during the overall oxidative-reductive catalytic cycles.114
1.4.5 Ring-opening metathesis polymerization (ROMP)
Matos et al115 have prepared complexes of the type [RuCl2(PPh3)2L2], where L =
pyridine, 4-methylpyridine (picoline), 4-aminopyridine and isonicotinamide. The
31
complexes were investigated for catalytic activity in ring-opening metathesis
polymerization. The reaction pathway of ROMP is illustrated in Figure 1.13.
M CHR
C C
n
C C
M CHR
C C
C CM M
C CM C C CHR
n
Figure 1.13: The reaction pathway of ROMP116
All catalysts were active only in the presence of ethyldiazoacetate (EDA).
Isonicotinamide and aminopyridine gave unimodal polymers with highest molecular
weights Mw, but with slightly lower polydispersity indices Mw/Mn. Pyridine and picoline
gave bimodal polymers.115
Schiff base substituted first and second generation Grubbs catalysts have described
for the polymerization of cyclooctadiene.117 These catalysts showed greater stability
and activity than their parent first generation Schiff base catalysts at high
temperature but much lower activity than their parent second generation Grubbs
catalyst.117
Wright et al.118 have prepared two catalysts that were derivatives of the first
generation Grubbs catalyst. Preparation of the catalysts is illustrated in Scheme
1.25. Complex XXIII was prepared via in situ formation of the ligand (Scheme 1.25b).
32
Ru
P(i-Pr)3
P(i-Pr)3
Cl
Cl Ph
+NN N
N NAr Ar
toluene, RT
68%N
N
N
N
N
Ru
Cl
Cl Ph
Ar
Ar
(a)
(b)
N NN Ar
NKHMDS
THF N NN Ar
N
+
Ru
P(i-Pr)3
P(i-Pr)3
Cl
Cl Ph
THF
Petrol
N
N
N
N
N
Ru
Cl
Cl
Ar
Ar
Ph
XXII
XXIII
Scheme 1.25: Synthesis of Ru complexes derived from 1st generation Grubbs
catalyst.118
Complex XXII exhibited lower activity than the first and second generation catalyst
due to limtations of C-N-C ligand design, particularly on late 4d and 5d transition
metals.118 The limitations originated from low lability of metal-nitrogen heterocyclic
bonds and the blocking of 3d coordination sites by rigid ligands.118 Catalytic studies
of XXIII were hampered by very low yields, coupled with the difficulties to obtain the
free carbene.
1.5 Ferrocenyl-nitrogen donor ligand chemistry
1.5.1 Ferrocenyl-pyridine ligands
Despite the extensive work on the symmetrical ferrocenylphosphine119 and
unsymmetrical ferrocenyl ligands,120 it was apparent from reviewing the literature that
the chemistry of ferrocenyl-nitogen donor ligands, particularly those containing
pyridine, has not been well developed. One of the first ferrocenylpyridine complexes
was described by Miller et al.121 (Figure1.14a). Electrochemical properties of the
33
complexes were investigated to determine whether the changes in the oxidation
state of redox ferrocenyl ligand could lead to changes in the reactivity of the rhenium
centre.
Fe
Fe
N
N
Re
Cl
CO
CO
CO
Fe
Fe
N
N
Pt
Cl
Cl
(a) (b)
Figure 1.14: Re and Pt complexes with ferrocenylpyridine ligands121,122
A platinum complex has been prepared, that contain 3-ferrocenylpyridine and the
specific role of platinum as a linker between the ferrocenyl groups has been studied
(Figure 1.14b).122 A single redox wave was observed, corresponding to a two
electron oxidation at the platinum electrode indicating little or no communication
between the ferrocene groups.
Sakanishi et al.123 have reported complexes containing ferrocenylpyridine ligands
attached to a tungsten metal centre (Figure 1.15).
Fe
Fe
FeN W(CO)5
R
R = H, Me, Ph
XN W(CO)4
X = CH, N
N
W(CO)5
OFe N W(CO)5
R
R = H, Me2
Figure 1.15: Tungsten complexes with ferrocenylpyridine ligands123
34
The spectroscopic and electrochemical studies showed that these complexes have
architectural and electronic properties necessary to exhibit second-order non-linear
optical behaviour. Some complexes that were characterised by X-ray methods
crystallized in a centrosymmetric space group and thus could not show second-order
non-linear optical properties in the bulk state.123
Braga et al.38,39,124 have described the preparation of supramolecular organometallic
materials and coordination networks based on 1,1’-bis(4-pyridyl)ferrocene. Example
of these ligands are shown in Figure 1.16.
Fe
Fe
Fe
N
N
N
N
N
N
N
N
N
Figure1.16: Examples of ferrocenylpyridyl and pyrimidyl complexes38,39,124
The platinum and palladium complexes of 1,1-bis(2-pyridyl)ferrocene have been
prepared and their catalytic reactivity towards carbonyl insertion reactions has been
investigated (Figure 1.17).125
Fe
N
N
Pd
Me
ClFe
N
N
Pt
Cl
Cl
Pt
Cl
Cl
Figure 1.17: Pd and Pt complexes of 1,1’-bis(2-pyridyl)ferrocene125
A rapid insertion of carbon monoxide into the palladium-methyl bond was observed.
However, detailed kinetic studies on the reaction were impossible because of weak
ligand-metal bonding.
35
Rajput et al.126 have prepared some palladium(II), platinum(II), rhodium(I) and
iridium(I) complexes. Cytotoxicity studies were carried out on selected complexes
and these were also screened for activity against oesophageal and cervical cancer
cell lines. Complexes with a significant activity in an initial screening assay, and
which were soluble in the culture media were further tested to determine their IC50
values. These were compared to IC50 values of cisplatin, determined in the same
way and for the same cell line. Interestingly, some of the complexes displayed
similar growth inhibitory activity to that displayed by cisplatin. In a more recent
publication, Rajput et al 127 have reported on the electrochemical properties of the
rhodium(I) complexes. Results obtained showed some communication between
metal centres or a lack of communication in the complexes.
A series of pyridyl- and quinolyl-N-substituted ferrocenyl and ferrocenediyl ligands
have been synthesized by Gibson and co-workers.128 The activity of the ligands in
olefin polymerization was tested after coordination to palladium and nickel metal
centres. Examples of the palladium and nickel complexes are depicted in Figure
1.18.
Fe
N
N
N
N
X Y
X YFe
N
N N
N
X YX Y
Fe
N N
X YFe
N N
X Y
R
R
R
M = Pd or NiR = H, MeX = Y = Cl, Br
M = Pd or NiX = Y = Cl, Br
M = Pd or NiR = H, MeX = Y = Cl, Br M = Pd or Ni
X = Y = Cl, Br
Figure 1.18: Palladium and nickel complexes128
The palladium complexes were found to be inactive, but with the nickel-based
complexes, although no high molecular weight polyethylene was formed.128 The
nickel complexes were highly selective for the formation of short chain oligomers.
36
1.5.2 Ferrocenyl-Schiff base ligands
Recently, there has been an upsurge of interest in preparing ferrocenyl-nitrogen
donor ligands based on Schiff bases.124 These are very interesting compounds since
the presence of the ferrocenyl moiety imparts special electrochemical
properties129,130 and modes of coordination.130 Some of the ferrocenyl-Schiff base
ligands have been studied for second-order non-linear properties.131 Several
research groups have reported on the preparation of cyclopalladated complexes
based on ferrocenylimines132-138 and these complexes have been known to catalyse
cross-coupling reactions, namely Heck and Suzuki reactions. An example is the
dimeric complex prepared by Wu et al.139. The palladium centre is stabilized by a
five-membered ring that incorporates the C=N bond (Figure 1.19). Triethylamine and
1,4-dioxane were found to be the suitable base and solvent, respectively. The
catalysts were highly efficient for the olefination of iodobenzene and resulted in
highly regioselective products (trans-isomers).
Fe
X = Cl, Br
N CH3
PdX
2
CH3
Figure 1.19: Dimeric cyclopalladated ferrocenylimine complex for catalytic Heck
reaction139
Zhao et al140 have also reported on the catalytic activity of a cyclopalladated
ferrocenylimine complex in the Mizoroki-Heck reaction for the arylation of
iodobenzene (Figure 1.20). In this complex the ring is formed by nucleophilic attack
of the palladium by the carbon on the second Cp ring and does not incorporate the
C=N bond. The complex was highly active and showed high regioselectivity for trans-
coupling.
37
FeN
Pd S
Ph3P Cl
Figure 1.20: Monomeric cyclopalladated ferrocenylimine for Mirozoki-Heck
reaction140
Weng et al.141 have described the preparation of a 1,1’-N-substituted ferrocenediyl
palladium(II) complex for the catalysis of Suzuki cross-coupling reaction of aryl
iodides bromides with aryl boronic acids, under non-homogeneous conditions in
aqueous medium (Figure 1.21). The catalyst exhibited a high activity in the coupling
of 4-bromoacetophenone and phenylboronic acid. The catalyst could be easily
recovered, recycled and could be used over a number of runs without loss of activity.
Fe
N
N
PdCl
Cl
Figure 1.21: 1,1’-N-substituted ferrocenediyl Pd(II) complex for Suzuki cross-
coupling reaction141
Zhang et al142 have reported on the synthesis and catalytic activity of the
cyclopalladated ferrocenylketimine complexes for the Suzuki cross-coupling reaction,
under ultrasonic irradiation in aqueous medium (Figure 1.22). Compared to
conventional heating, the reaction occurred faster when accelerated by ultrasonic
irradiation although the yields were comparable. Gong et al.143,144 have reported on
the catalytic activity of mono-and dimeric cyclopalladated ferrocenylimine complexes
for the Suzuki cross-coupling reaction143 and the Buchwald-Hartwig coupling of
amines with aryl halides or sulfonates.144
38
Fe
N
CH3
R
PdX
2
R = X = ClR = Me, X = I
Figure 1.22: Dimeric cyclopalladated ferrocenylketimine complexes for Suzuki
reaction143,144
Chiral ferrocenylphosphine-imine ligands containing a pyridine unit have been
reported by Hu et al.145 On coordination to a palladium metal centre, highly active
catalysts for the asymmetric allylic alkylations. Of particular interest was the
influence of the position of the pyridine nitrogen atom on the reactivity and
enantioselectivity. The presence of a pyridine unit significantly affected the way a
ligand coordinated to the metal centre, which in turn led to dramatic changes in the
reactivity and enantioselectivity of the catalytic reaction.145 The ligand with a 3-
pyridine nitrogen atom (Figure 1.23) resulted in increased reactivity and
enantioselectivity, while the ligand with a 2-pyridine nitrogen atom gave no alkylation
product.
Fe
PPh2
N N
(S,Sp)
Figure 1.23: Chiral ferrocenylphosphine-imine ligand145
Other examples of allylic alkylations catalysts are those prepared by Platero-Prats et
al.146 These compounds were synthesized by treatment of a ferrocenylimine with an
appropriate chloro-bridged dimeric palladium(II)-allyl complex in dichloromethane
solution at 25 ºC. The complexes were active in the allylic alkylation of (E)-3-phenyl-
2-propenyl (cinnamyl) acetate with sodium diethyl-2-methylmalonate as a
nucleophile.
39
N-ferrocenyl salicylaldimine ligands have been synthesized by Bott et al.147 (Scheme
1.26). After coordination to magnesium, titanium and zirconium metal centres, their
catalytic activity in polymerization of olefins was investigated. It was found that on
activation with methylalumnoxane (MAO), the titanium complex showed moderate
activity for ethylene polymerization, while the zirconium complex was highly active
for ethylene oligomerization.
Fe
N
HO ButFe
NH2
+OHC
OH
But EtOH
-H2O
Scheme 1.26: Synthesis of N-ferrocene salicylaldimine ligand147
Several groups have reported on the synthesis of bisferrocenylimines from
ferrocenecarboxaldehyde148-151 or 1,1’-diformylferrocene149,151,152 a range of amines
or anilines. Lee et al153 have synthesized cationic rhodium(I) and iridium(I)
complexes based on bisferrocenylimine for polymerization of phenylacetylene
(Figure 1.24).
Fe
CH
(CH2)
Fe
CH
N2
N
MClO4
M = Rh, Ir
Figure 1.24: Cationic rhodium(I) and Ir(I) complex for olefin polymerization153
Compared to its iridium(I) analogue, the rhodium(I) complex gave high molecular
weight polymers in excellent yields while the iridium(I) complex gave better
polydispersity indices.
40
1.6 Objectives of the project
The elimination of the hazardous materials in synthetic processes is important
especially in the chemical industry and in academia. The versatility of the solvent-
free approach to organic synthesis has been extensively illustrated with examples in
the previous sections.
This project focuses on developing a synthetic procedure that reduces or eliminates
the use of organic solvents using the green chemistry principles. The main objective
of the project is to prepare nitrogen-donor ligands based on ferrocene that can be
used in catalysis. Our group42 has previously reported on successful preparation of
ferrocenylimines under the solvent-free environment. We intend taking this work a
step further and prepare bisferroceylimines ligands for olefin polymerization. The
ligands will then be coordinated to a rhodium(I) metal centre. Lee et al.153 have
demonstrated that rhodium(I) complexes containing bisferrocenylimines are very
effective catalysts for olefin polymerization (Figure 1.24). Based on the work of Lee
et al.153 we will investigate the effect of increasing the alkyl chain length on the
catalytic activity of the complexes.
Rhodium(I) complexes containing diamine ligands have been found to be effective in
hydroformylation reactions. Kim and Alper154 have demonstrated that these
complexes are effective for hydroformylation reactions (Figure 1.25).
Rh
N N
Rh
Cl Cl
Figure 1.25: Cationic rhodium(I) complexes for hydroformylation reactions.154
We intend to prepare similar complexes with bisferrocenylamine ligands and
investigate their catalytic activity towards hydroformylation reactions. The
bisferrocenylamine ligands will be obtained by simple reduction of
41
bisferrocenylimines, which proceeds much cleaner than the lithium aluminium
hydride route.
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50
CHAPTER 2
RESULTS AND DISCUSSIONS
2.1 Solvent-free synthesis of bisferrocenylimines
2.1.1 Introduction
There has been an upsurge of interest in performing chemical transformations under
solvent-free conditions. In comparison to methods that employ solvents, the solvent-
free approach proceeds more cleanly and provides higher yields. Examples of the
solvent-free method have been highlighted in the previous chapter. In the literature,
Nyamori et al.1 have reported on the solvent-free synthesis of ferrocenylimines by
grinding ferrocenecarboxaldehyde and a substituted aromatic amine. This procedure
provided excellent yields and the reactions occurred readily at room temperature.
However, the reactions in which the aromatic amine contained an electron-
withdrawing substituent were slow and thus required heating at 50 ºC.
Fe
Y C
O
H+
H2N
X
Grind
Solvent-free Fe
Y C
N
H
X
Y = Ph, Ph-Ph, Ph-O-Ph
Scheme 2.1: Solvent-free synthesis of ferrocenylimines
The objective of this research was to extend the work and synthesize
bisferrocenylimines (Figure 2.1), by reacting ferrocenecarboxaldehyde with
diamines.
51
Fe
CH
(CH2)
Fe
CH
Nx
N
Figure 2.1: General structure of bisferrocenylimines to be synthesized
The synthesis of bisferrocenylimines by a reaction of diamines and
ferrocenecarboxaldehyde has been reported by several research groups.2,3 The
imines are useful ligands for metal complexation and many ferrocenylimines have
been investigated for non-linear optical properties.4
2.1.2 Synthesis and characterization of bisferrocenyimines
The reaction of ferrocenecarboxaldehyde and the appropriate diamines gave the
corresponding bisferrocenylimines in excellent yields (Scheme 2.2).
Fe
CHOH2N(CH2)xNH2
Grind Fe
CH
(CH2)
Fe
CH
Nx
N
2
x = 2, 3, 4, 6, 8
Scheme 2.2: Solvent-free synthesis of bisferrocenylimines
The solvent-free reaction involved the mixing and grinding of two mole equivalents of
ferrocenecarboxaldehyde and the appropriate diamine. A pictorial description of the
process is shown by a reaction of ferrocenecarboxaldehyde and 1,8-diaminooctane
in Figure 2.2. On grinding the mixture of reagents, a melt was obtained which
eventually solidified at room temperature, to give the bisimines in excellent isolated
yields (Table 2.1). However, for [2.2] the melt obtained only gradually solidified at
room temperature, after removal of water formed in the condensation reaction under
vacuum.
52
Figure 2.2: The pictorial stages of the solvent-free synthesis of N,N’-
octylenebis(ferrocenylmethylidine)amine
(A) Ferrocenecarboxaldehyde (brown)
1,8-Diaminooctane (white)
(B) Ground mixture of
ferrocenecarboxaldehyde
and 1,8-diaminooctane
MELT
PRODUCT
(C) Solidified N,N’-
octylenebis(ferrocenylmethylidine)-
amine
53
Table 2.1: Yields of bisferrocenylimines from the solvent-free reaction of diamines
and ferrocenecarboxaldehyde
Compound
number
Ferrocene-
carboxaldehyde
(Fc-Y-CHO)
Diamine
H2N(CH2)xNH2
Yield of
Bisferrocenyliminea
(%)
[2.1]
FcCHO
H2N(CH2)2NH2
Fe
CH
(CH2)
Fe
CH
N2
N
(99%)
[2.2]
FcCHO
H2N(CH2)3NH2
Fe
CH
(CH2)
Fe
CH
N3
N
(94%)
[2.3]
FcCHO
H2N(CH2)4NH2
Fe
CH
(CH2)
Fe
CH
N4
N
(92%)
[2.4]
FcCHO
H2N(CH2)6NH2
Fe
CH
(CH2)
Fe
CH
N6
N
(94%)
[2.5]
FcCHO
H2N(CH2)8NH2
Fe
CH
(CH2)
Fe
CH
N8
N
(97%)
[2.6]
Fc CHO
H2N(CH2)2NH2
Fe Fe
CH N N CH
(97%)
a The isolated yields are based on starting materials.
54
The solidified melt was initially analyzed by infrared (IR) spectroscopy using a
potassium bromide (KBr) disc, in order to determine that the reaction occurred under
solvent-free conditions. Infrared analysis showed the disappearance of the carbonyl
(C=O) band at approximately 1700 cm-1 and the appearance of the strong imine
(C=N) band at 1646 cm-1. Figure 2.3 shows the IR spectrum of [2.5] immediately
after the melt had solidified. The absence of the carbonyl band in the IR spectrum
was evidence that the reaction was completed under solvent-free conditions.
Figure 2.3: IR spectrum of [2.5]
The solidified melt was ultimately recrystallized from a minimal amount of cold
anhydrous methanol to provide a pure product. 1H NMR spectra of all the
bisferrocenylimines [2.1]-[2.6] showed proton signals in the region δ 8.1-8.2 ppm
which were indicative of the presence of imine (CH=N) protons. Compounds [2.1]2
and [2.6]5 have been synthesized previously; however, they are reported here for the
first time under solvent-free conditions. 1H NMR spectra of [2.1]-[2.5] were expected
to possess some similarities since the only difference in their molecular structures
was the length of the alkyl chain of the diamines. A singlet was observed at δ 3.78
ppm for [2.1], indicatve of the four protons for the two (CH2) groups. Compound [2.2]
has an additional CH2 group in the β-position and exhibited a triplet at δ 3.56 and a
55
multiplet at δ 2.03 ppm due to the four protons of the two terminal (α) CH2 groups
and two protons of the middle (β) CH2 group, respectively. As expected, [2.3] with
four CH2 groups exhibited a triplet at δ 3.7 ppm due to the protons on the terminal (α)
CH2 groups and a multiplet at δ 1.71 ppm due to the protons on the middle (β) CH2
groups. Three signals were observed for [2.4], a triplet at δ 3.45 ppm for the protons
on the terminal (α) CH2 groups and two multiplets at δ 1.66 and 1.44 ppm for the
protons on the inner (β) CH2 groups and the middle (γ) CH2 groups, respectively. An
interesting scenario was observed with [2.5] where instead of four signals that would
have been expected, only three were observed. A triplet at δ 3.42 ppm due to the
protons on the terminal (α) CH2 groups, a multiplet at δ 1.62 ppm for the inner (β)
CH2 groups and a singlet for the protons on the four middle CH2 groups were
observed (Figure 2.4).
Figure 2.3: 1H NMR spectrum of [2.5]
This suggested that the deshielding effect of the nitrogen groups became less
pronounced as the length of the alkyl chain increased. The substituted
cyclopentadienyl (Cp) ring of the ferrocene moiety for all the compounds was
represented by two triplets and a very intense singlet for the unsubstituted
cyclopentadienyl ring. For all the compounds [2.1]-[2.5] the chemical shifts of the
protons on the carbons directly bonded to the nitrogen groups were expected to be
56
very similar. However, this was not the case and the results are summarized in Table
2.2. This table shows that the proton signals are shifted to lower frequencies (less
deshielding) as the length of the alkyl chains increased. This effect could be
attributed to the increased free rotation and flexibility of the chain as the length
increased. A significant change was observed between [2.1] and [2.2] as well as
[2.2] and [2.3], while there was less change from [2.3] to [2.4] to [2.5]. Additionally,
for [2.2]-[2.5], these protons appeared as triplets, indicating some vicinal proton-
proton coupling. For [2.1] a singlet was observed showing that there was no vicinal
coupling between the neighbouring protons.
Table 2.2: Chemical shifts for protons on the carbon directly bonded to nitrogen
group
Compound Chemical shift δ
(ppm)
[2.1]
3.78
[2.2]
3.56
[2.3]
3.47
[2.4]
3.45
[2.5]
3.42
13C NMR spectra for all compounds showed signals, for the imine carbons, in the
region δ 160-163 ppm. The observed carbon signals for all compounds [2.1]-[2.5]
were as expected. Compound [2.1] exhibited a single signal at δ 62.80 ppm for two
CH2 groups while for [2.2] two signals were observed at δ 59.25 and 32.90 ppm
57
representing the carbons directly bonded to nitrogen groups and the middle carbons,
respectively. Compounds [2.3], [2.4] and [2.5] exhibited two, three and four carbon
signals, respectively, in the expected chemical shift regions for the CH2 groups. All
the compounds are very stable in the solid state at room temperature and in open air
while they showed some degree of instability in solution. This was in accordance
with findings of Benito et al.2 Attempts to perform the same reaction using
acetylferrocene were unsuccessful, even after heating the reaction mixture up to 65
ºC. The inability of acetylferrocene to react was attributed to steric hindrance by the
methyl group.
2.1.3 Solvent-free synthesis of arylbisimines
The synthesis of the arylbisimines in a solvent-free environment has been reported
by Naeimi and co-workers.6 The method included the use of a solid-supported
catalyst P2O5/Al2O3 (Scheme 2.3).
R
OH
R'
O+ H2N
Y
NH2
P2O5/Al2O3
Solvent-free
R
OH
R'
N
Y
N
R'
HO
R
R = H, NO2
R' = H, CH3
Y = C6H4-O-C6H4, C6H4-CH2-C6H4, C6H4-SO2-C6H4
Scheme 2.3: Solvent-free synthesis of arylbisimines in the presence of a catalyst.6
Similar types of arylbisimines have been synthesized in this study, where the
aromatic ring contained methyl substituents at 2,3-, 2,5- and 2,4,6-positions. The Y-
spacer was a two-carbon alkyl chain and no catalyst was required. The reactions
involved mixing and stirring of two mole equivalents of an appropriate substituted
benzaldehyde, and ethylenediamine. Stirring the reagents for a few seconds gave a
white solid, an indication that the reaction had taken place since both starting
reagents are liquids (Scheme 2.4).
58
H
O
+H2N
NH2
stirsolvent-free
R1
NN
R1
[2.7]: R1, R2 = CH3; R3, R4, R5 = H
[2.8]: R1, R3 = CH3; R2, R4, R5 =H
[2.9]: R1, R3, R5 = CH3; R2, R4 =H
R1
R2
R3 R5
R5 R3
12
3
45
6
1'2'
3'
4'5'
6'
R2
R4
R4
R2
R3
R4
R5
Scheme 2.4: Solvent-free synthesis of arylbisimines
Although it was apparent that the reaction took place under these solvent-free
conditions, the reaction mixture was analysed by IR spectroscopy using KBr discs to
confirm that the reaction had indeed occurred under these conditions. IR analysis
showed the disappearance of the carbonyl band at approximately 1700 cm-1 and the
appearance of the imine (C=N) band in the region 1637-1639 cm-1. The white crude
solid was eventually recrystallized from cold anhydrous methanol to obtain the pure
product in excellent yield (Table 2.3). 1H NMR revealed that there was no aldehydic
proton signal present in the region δ 9.7-10 ppm for all the compounds. The major
differences between the compounds were expected to be in aromatic region of the
spectra and the methyl substituents. Compound [2.7] was identified by the presence
of two singlets at δ 2.33 and 2.27 ppm representing the methyl groups at 2, 2’ and 3,
3’ positions of the aromatic rings, respectively. For [2.8] two singlets were also
observed at δ 2.39 and 2.29 ppm for the methyl groups at 2, 2’ and 5, 5’ positions,
respectively. Compound [2.9] contained three methyl substituents at 2, 2’, 4, 4’ and
6, 6’ positions. For the aromatic rings, [2.7] exhibited two doublets at δ 7.69 and 7.20
ppm due to protons at the 6, 6’ and 4, 4’ positions, respectively, and a triplet at δ
7.10 ppm due to protons at 5, 5’ positions.. A singlet at δ 7.66 ppm for [2.8]
represents the protons at the 6, 6’ positions.
59
Table 2.3: Yields of arylbisimines from a reaction of substituted benzaldehyde and
ethylenediamine
Substituted
Benzaldehyde
Ethylenediamine
Yield of
Arylbisiminea
(%)
H
O
H2NNH2
NN
[2.7] (86%)
H
O
H2NNH2
NN
[2.8] (84%)
H
O
H2NNH2
NN
[2.9] (88%) a Yields are based on starting materials
Two doublets were also observed at δ 7.11 and 7.07 ppm for the protons at the 3, 3’
and 4, 4’ positions, respectively. As anticipated, the 1H NMR for [2.9] showed some
chemical shift equivalence of the protons on the aromatic rings as only a singlet at δ
2.37 ppm was observed. 13C NMR spectra of all compounds exhibited signals for the
C=N group in the region δ 161-162 ppm. The 13C NMR spectra of [2.7] and [2.9]
showed six and four signals for the aromatic rings, respectively, while the spectrum
of [2.8] gave rise to five signals. Actually, there were six signals since the signal at δ
60
131.1 ppm (highlighted) represented two overlapping signals, one lower in intensity.
Figure 2.5 illustrates the aromatic region of the 13C NMR spectrum of [2.8].
Figure 2.4: 13C NMR spectrum of [2.8]
According to predicted chemical shifts as implemented in ChemDraw, the signals
were assigned to C3, C3’ and C4, C4’. The signals for the methyl groups were
observed at δ 19.81 and 14.10 ppm for [2.7]. The methyl signals for [2.8] were
observed at δ 20.38 and 18.46 ppm while for [2.9] signals appeared at δ 21.52 and
21.06 ppm. The signals for the CH2 groups were observed at δ 62.17, 62.15 and
63.89 ppm for [2.7], [2.8] and [2.9], respectively. Since the major difference between
the three compounds was the number and the positions of the methyl groups on the
aromatic rings, it seemed as though the more methyl groups there were present, the
greater the downfield shift of the CH2 groups.
2.2 Reduction reaction of bisferrocenylimines and arylbisamines
2.2.1 Reduction of bisferrocenylimines
The reduction of bisferrocenylimines with lithium aluminium hydride has been
reported elsewhere in the literature.2 Our interest was to carry out this transformation
using a much cleaner synthetic route. We therefore opted for the catalytic
61
hydrogenation reaction procedure (Scheme 2.5), which takes place cleanly since the
only product is the corresponding bisferrocenylamine.
X
Fe
CH
(CH2)
Fe
CH
Nx
N
H2, Pd/C
MeOH Fe
CH2
(CH2)
Fe
CH2
HN
x
HN
x = 2, 3, 4, 6, 8
Scheme 2.5: Hydrogenation of ferrocenylbisimines
However, attempts to carry out the reactions yielded no results. We were then
forced to use the lithium aluminium hydride route. The reaction was carried out by
heating a solution of the bisferrocenylimine in diethylether for 30 minutes. The
reaction was then quenched with ice/water slurry and the product extracted with
diethyl lether. The products were obtained in excellent yield as pale to bright yellow
solids (Table 2.4). Similar types of compounds that are C2-symmetric have been
reported by Woltersdorf et al.7 The 1H NMR spectra of all the reduction products
showed the disappearance of the imine proton signal and the appearance of two
new signals, the amine (NH) and the CH2 proton signals. For all the compounds
[2.10]-[2.14] the NH proton signal was observed as a broad singlet in the chemical
shift region δ 1.53-2.27 ppm. The CH2 proton signal on the other hand, was
observed as singlet in the chemical shift region δ 3.52-3.58 ppm. It was also
observed that one of the substituted Cp ring proton signals was shifted to lower
chemical shifts for all the compounds. The CH2 proton signal in the reduced form of
the bisferrocenylimines was observed in the expected chemical shift region. The
terminal CH2 proton signals of the alkyl chains appeared to have been shifted to
lower frequencies compared to those in the original bisferrocenylimines. In the
bisferrocenylimines the terminal CH2 proton signal appeared at higher frequencies
largely due to anisotropic effects of the double bond (CH=N). All the terminal CH2
signals were shifted to lower frequencies by ca. 0.8 ppm, which is in agreement with
the literature value.8 13C NMR spectra showed the disappearance of the CH=N
carbon signal and a new CH2 carbon signal, in the expected region, for all the
compounds [2.10]-[2.14].
62
Table 2.4: Yields of bisferrocenylimines
Compound % Yielda
Fe
CH2
(CH2)
Fe
CH2
HN
2
HN
[2.10]
87%
Fe
CH2
(CH2)
Fe
CH2
HN
3
HN
[2.11]
92%
Fe
CH2
(CH2)
Fe
CH2
HN
4
HN
[2.12]
86%
Fe
CH2
(CH2)
Fe
CH2
HN
6
HN
[2.13]
91%
Fe
CH2
(CH2)
Fe
CH2
HN
8
HN
[2.14]
88%
a Yields are based on starting materials
Infrared spectra of all compounds showed the appearance of several peaks
corresponding to the v(NH) stretching vibration which occurred in the region 3300-
3100 cm-1. Furthermore, the v(CH=N) stretching peak which occurs as a strong
absorption in the region 1640-1645 cm-1 for the bisferrocenylimines was lacking. The
above information was in agreement with the proposed molecular structures of the
compounds.
63
2.2.2 Reduction of arylbisimines
The reduction of arylbisimines was carried out using the same method as for the
reduction of bisferrocenylimines. These compounds were obtained in moderate to
good yield as a white powder, except for [2.15] which was obtained as a colourless
oil. However, it was also found that [2.15] solidified to a white solid after prolonged
drying process under suction. 1H NMR spectra of all compounds [2.15]-[2.17]
showed the disappearance of the CH=N proton signal and the appearance of CH2
and NH protons signals. The peak in the chemical shift region δ 3.94-3.96 ppm
represented the CH2 proton signal for all the compounds. The NH proton signal was
obtained as a broad singlet in the region δ 3.76-3.81 ppm. The signal for the CH2
groups of the ethylene chain in arylbisimines appeared at higher frequencies than
those of the arylbisamines largely due to anisotropic effects of the imine double
bond. The shift to lower resonance frequencies in the CH2 signals in arylbisamines
confirmed the reduction of the CH=N double bond to a CH2-NH single bond. The
formation of the arylbisamines was also confirmed by 13C NMR spectrometry. The
appearance of a new signal in the chemical shift region δ 48.2-51.77 ppm was due to
the presence of the CH2 group. Infrared spectra showed the appearance of several
peaks for the ν(NH) stretching vibration at 3350-3150 cm-1 for all the compounds.
2.3 Electronic Spectroscopy
The UV-visible spectra of bisferrocenylimines and bisferrocenylamines prepared
were obtained in dichloromethane solution. Spectral comparisons with unsubstituted
ferrocene as a reference were also made. Ferrocene exhibited two bands at
wavelengths of λmax 326 and 442 nm, which have been assigned to 1A2g→1E2g and
1A1g→1E1g ligand field d-d transitions.9 The UV-vis spectrum of unsubstituted
ferrocene is illustrated in Figure 2.6.
64
Figure 2.6: UV-vis spectrum of unsubstitued ferrocene in dichloromethane
The ferrocenyl bands in [2.1]-[2.5] were observed at longer wavelengths λmax (Figure
2.7, Table 2.5) largely due to conjugation with the CH=N bond. Bathochromic shifts
(shifts to longer wavelengths) are anticipated where conjugation increases in length.
Some absorption bands due to π-π* and n-π* transitions of the imine groups CH=N
in [2.1]-[2.5] were also observed at wavelengths lower than 300 nm. The ferrocenyl
bands in [2.10]-[2.15] were observed at slightly shorter wavelengths λmax (Figure 2.8,
Table 2.5) largely due to the reduction of the CH=N bond resulting to decrease in
conjugation. Shifts to lower wavelengths are termed hypsochromic shifts and are
expected where there is a decrease in conjugation length. The extinction coefficients
of [2.1]-[2.5] were higher for the band at lower wavelengths and lower for the band
at higher wavelengths than that of ferrocene (Table 2.5). On the other hand, for
[2.10]-[2.14], the situation was restored to that of the unsubstituted ferrocene.
65
Figure 2.7: UV-vis spectra of bisferrocenylimines in dichloromethane
Figure 2.8: UV-vis spectra of bisferrocenylamines in dichloromethane
66
Table 2.5: UV-vis data for ferrocene, [2.1]-[2.5] and [2.10]-[2.14]
Compound λmax(nm) ε [(L.mol-1.cm-1)]
Ferrocene 326 [202] 442 [335]
[2.1] 329 [2586] 454 [914]
[2.2] 323 [3638] 448 [1087]
[2.3] 321 [2934] 450 [852]
[2.4] 323 [3180] 448 [910]
[2.5] 321 [2836] 445 [803]
[2.10] 322 [140] 438 [177]
[2.11] 323 [204] 469 [219]
[2.12] 324 [137] 437 [208]
[2.13] 323 [160] 438 [230]
[2.14] 324 [182] 439 [245]
ε = molar extinction coefficient
2.4 Cyclic Voltammetry
The redox behaviour of bisferrocenylimines and bisferrocenylamines was studied by
cyclic voltammetry. The observed redox behaviour of the compounds was compared
with that of ferrocene as a standard reference. Ferrocene exhibited a one-electron
reversible wave with an E1/2 at 90.5 mV (Figure 2.9). The cyclic voltammograms
were recorded in acetonitrile with tetrabutylammonium perchlorate (0.1 M) as a
supporting electrolyte, in an inert environment. The three-electrode system was a
platinum disk working electrode, a platinum wire auxiliary electrode and Ag/AgNO3
reference electrode.
Some typical cyclic voltammograms of selected compounds are shown in Figure
2.10, and show one-electron reversible redox waves similar to that of ferrocene. The
bisferrocenylimines exhibited a positive shift in potential indicating that these
compounds became more difficult to oxidise (Figure 2.10, Table 2.6). These positive
shifts can be attributed to the presence of the CH=N bond in close proximity to the
ferrocene group, resulting in the reduced electron density at the metal centre.
67
Figure 2.9: Cyclic voltammogram of ferrocene in acetonitrile
As expected the bisferrocenylamines exhibited a negative shift since the CH2-NH
bond had no effect on the metal centre.
Figure 2.10: Cyclic voltammograms of [2.2], [2.5], 2.12] and [2.14]
68
Table 2.6: Half-wave potentials of [2.1]-[2.5] and [2.12]-[2.14]a
Compound Epa (mV) Epc (mV) E1/2 (mV)
Ferrocene 140 41 90.5
[2.1] 243 124 133.5
[2.2] 246 167 206.5
[2.3] 243 171 207
[2.4] 238 168 203
[2.5] 242 163 202.5
[2.12] 101 23 62
[2.13] 111 35 73
[2.14] 112 25 68.5 a Due to solubility problems acceptable voltammograms of compounds [2.6], [2.10] and [2.11] could
not be recorded.
2.5 Experimental
2.5.1 Purification procedures
All reagents and solvents were purified using standard purification and drying
methods.10
Table 2.7: General drying agents for solvents
Solvent Drying Agent
Diethyl ether Na wire
Methanol Mg turnings, I2
Ferrocenecarboxaldehyde, 2,3- and 2,5-dimethylbenzaldehyde and 2,4,6-
trimethylbenzaldehyde were purchased from the Sigma Aldrich Chemical Company.
All other common laboratory chemicals were obtained locally and were used without
further purification.
69
2.5.2 Instrumentation
Melting points were determined on an Electrothermal IA 900 series digital melting
point apparatus and were uncorrected.
NMR spectra were recorded on a Bruker DPX (300 MHz) spectrometer at ambient
temperatures. 1H NMR spectra were referenced against the deuterated solvent
(CDCl3: δ 7.28) and the values reported relative to tetramethylsilane (TMS: δ 0.00). 13C NMR spectra were similarly referenced internally to the solvent resonance
(CDCl3: δ 77.0) with values reported relative to tetramethylsilane (TMS: δ 0.00).
Infrared spectra were recorded on a DigiLab FTS 3100 Excalibur HE series, running
DigiLab Resolution 4.0 software with solid samples prepared as potassium bromide
(KBr) disks. Microanalyses were obtained on a Carlo Erba EA 1108 elemental
analyser at the University of Cape Town. Fast atomic bombardment (FAB) and high
resolution (EI) mass spectra were recorded on a micromass auto-Tof mass
spectrometer at the Witwatersrand University in South Africa.
Uv-vis spectra were recorded on a Hewlett Packard 8452A diode array spectrometer
in dichloromethane (10-3 M) with a cell width of 1 cm.
Cyclic voltammograms were obtained on a BAS 100B electrochemical analyser with
a three-electrode system using Ag/AgNO3 (0.01 M) as a reference electrode,
platinum wire as the auxiliary electrode and platinum disc as the working electrode.
Samples (10-3 M) were prepared and run under nitrogen at ambient temperatures, in
acetonitrile with tetrabutylammonium perchlorate (0.1 M) as a background
electrolyte. The scan rate used was 100 mV.s-1. Solutions were saturated with
nitrogen by bubbling for 10 minutes prior to each run. The system gave ferrocene
E1/2 = 90.5 mV.
70
2.6 Synthesis of bisferrocenylimines and arylbisimines
2.6.1 General procedure for the synthesis of bisferrocenylimines
Ferrocenecarboxaldehyde (2 mole equivalents) and the diamine (1 mol equivalent)
were added to a pyrex tube fitted with glass ground joint. The two compounds were
ground together at room temperature (ca. 25ºC) using a glass rod The pyrex tube
was then placed under a high vacuum pump overnight. The products were obtained
as yellow to orange solids after recrystallization from cold anhydrous methanol.
2.6.1.1 N,N’-Ethylenebis(ferrocenylmethylidene)imine [2.1]
The general procedure was followed using
ferrocenecarboxaldehyde (360 mg, 1.68
mmol) and ethylenediamine (50 mg, 0.84
mmol). N,N’-Ethylenebis(ferrocenylmethylidine)amine was obtained as a yellow solid
(380 mg, 99%). M.p. 147-150 °C; 1H NMR (CDCl3): 8.18 (2H, s, N=CH), 4.63 (4H, t,
J = 1.8, C5H4), 4.30 (4H, t, J = 1.8, C5H4), 4.16 (10H, s, C5H5), 3.78 (4H, s, 2 x CH2); 13C NMR (CDCl3): 162.74, 80.85, 70.75, 69.52, 68.86, 62.80; IR (KBr): 3113, 3071,
2917, 2897, 2832, 1782, 1705, 1643, 1462, 1412, 1381, 1327, 1281, 1246, 1215,
1211, 1103, 1049, 1011, 961, 891, 822, 768, 644, 594, 517, 486, 475, 436, 401; m/z
(EI): 453 (29%), 452 ([M+], 80%), 321 (13%), 256 (31%), 241 (22%), 227 (19%) 226
(63%), 213 (92%), 199 (27%), 186 (14%), 160 (14%), 121 (63%), 69 (21%), 56
(16%), 43 (11%), 41(11%), 32 (32%), 30 (11%), 28 (100%); Anal. Calc. for
C24H24N2Fe2: MW, 452.15076. Found: MW, 452.063965.
2.6.1.2 N,N’-Propylenebis(ferrocenylmethylidene)imine [2.2]
The general procedure was followed using
ferrocenecarboxaldehyde (200 mg, 0.93
mmol) and 1,3-diaminopropane (41 mg, 0.55
mmol). N,N’-Propylenebis(ferrocenylmethylidine)amine was obtained as an orange
solid (204 mg, 94%). M.p. 127-129 °C; 1H NMR (CDCl3) 8.18 (2H, s, N=CH), 4.67
(4H, t, J = 1.8, C5H4), 4.39 (4H, t, J = 1.8, C5H4), 4.21 (10H, s, C5H5), 3.56 (4H, t, J =
Fe
CH
(CH2)
Fe
CH
N2
N
Fe
CH
(CH2)
Fe
CH
N3
N
71
7.0, 2 x CH2), 2.03 (2H, m, CH2); 13C NMR (CDCl3) 160.57, 81.88, 70.28, 69.25,
68.64, 59.25, 32.90; IR (KBr) 3108, 3066, 2929, 2946, 2866, 2823, 1640, 1470,
1449, 1323, 1243, 1105, 1004, 823, 544; m/z (EI) 467 (24%), 466 ([M+], 100%), 401
(30%), 335 (56%), 255 (55%), 254 (38%), 253 (30%), 241 (24%), 240 (60%), 233
(26%), 227 (62%), 226 (24%), 225 (21%), 212 (36%), 199 (38%), 186 (52%), 129
(25%), 120 (90%), 56 (52%), 39 (28%); Anal. Calc. for C25H26N2Fe2: C, 64.41; H,
5.62; N, 6.01; MW, 466.17734. Found: C, 63.15; H, 5.82; N, 5.75; MW, 466.082825.
2.6.1.3 N,N’-Butylenebis(ferrocenylmethylidene)imine [2.3]
The general procedure was followed using
ferrocenecarboxaldehyde (200 mg, 0.93
mmol) and 1,4-diaminobutane (44 mg, 0.50
mmol). N,N’-Butylenebis(ferrocenylmethylidine)amine was isolated as an orange
solid (207 mg, 92%). M.p. 152-154 °C; 1H NMR (CDCl3) 8.19 (2H, s, CH=N), 4.65
(2H, t, J = 1.8, C5H4), 4.36 (2H, t, J = 1.7, C5H4), 4.19 (10H, s, C5H5), 3.47 (4H, t, J =
7.0, 2 x CH2), 1.71 (4H, m, 2 x CH2); 13C NMR (CDCl3) 160.29, 81.08, 70.69, 69.47,
68.78, 62.07, 29.09; IR(KBr) 3071, 2930, 2860, 2814, 2364, 1644, 1470, 1439, 1407,
1381, 1369, 1324, 1244, 1104, 1041, 1020, 1001, 962, 930, 819; m/z (EI) 481 (5%),
480 ([M+], 13%), 284 (15%), 268 (23%), 267 (100%), 213 (15%), 199 (17%), 121
(50%), 56 (18%), 55 (15%), 44 (25%), 43 (34%), 41 (26%), 39 (22%), 30 (67%), 28
(61%), 27 (20%). Anal. Calc.1 for C26H28N2Fe2: C, 65.03; H, 5.88, N, 5.83; MW,
480.20392. Found: C, 64.51; H, 6.05; N, 5.74; MW, 480.095172.
2.6.1.4 N,N’-Hexylenebis(ferrocenylmethylidene)imine [2.4]
The general procedure was followed using
ferrocenecarboxaldehyde (100 mg, 0.47
mmol) and 1,6-diaminohexane (24 mg, 0.23
mmol). N,N’-Hexylenebis(ferrocenylmethylidine)amine was isolated as a light yellow
solid (109 mg, 94 %). M.p. 109-111 °C; 1H NMR (CDCl3) 8.16 (2H, s, CH=N), 4.65
(4H, t, J = 1.8, C5H4), 4.36 (4H, t, J = 1.8, C5H4), 4.19 (10H, s, C5H5), 3.45 (4H, t, J =
1 The tendency for the C and N content to be too low and H too high could be due to the presence of traces of water. This also applies to compounds [2.4] and [2.5].
Fe
CH
(CH2)
Fe
CH
N4
N
Fe
CH
(CH2)
Fe
CH
N6
N
72
6.2, 2 x CH2), 1.66 (4H, m, 2 x CH2), 1.44 (4H, m, 2 x CH2); 13C NMR (CDCl3)
161.08, 81.96, 70.67, 69.46, 68.78, 62.30, 31.32, 27.61; IR (KBr) 3099, 2935, 2862,
2822, 1646, 1468, 1454, 1409, 1381, 1351, 1326, 1243, 1204, 1164, 1103, 1063,
1051, 1038, 1004, 957, 936, 876, 865, 846, 826, 807; m/z (EI) 509 (15%), 508 ([M+],
41%), 312 (20%), 296 (19%), 295 (71%), 214 (23%), 213 (26%), 199 (27%), 186
(30%), 121 (100%), 56 (34%), 55 (22%), 43 (24%), 41 (39%), 39 (41%), 30 (27%),
28 (49%), 27 (33%). Anal. Calc. for C28H32N2Fe2: C, 66.17; H, 6.35; N, 5.51; MW,
508.25708. Found: C, 64.31, H, 6.46; H, 5.40; MW, 508.126146.
2.6.1.5 N,N’-Octylenebis(ferrocenylmethylidene)imine [2.5]
The general procedure was followed using
ferrocenecarboxaldehyde (200 mg, 0.93
mmol) and 1,8-diaminooctane (239mg, 0.47
mmol). N,N’-Octylenebis(ferrocenylmethylidine)amine was obtained as a yellow solid
(231 mg, 97 %). M.p. 97-100 °C; 1H NMR (CDCl3) 8.15 (2H, s, N=CH), 4.65 (4H, t, J
= 1.8, C5H4), 4.36 (4H, t, J = 1.8, C5H4), 4.19 (10H, s, C5H5) 3.42 (4H, t, J = 6.6, 2 x
CH2), 1.62 (4H, m, 2 x CH2), 1.40 (8H, s, 4 x CH2); 13C NMR (CDCl3) 161.00, 81.13,
70.66, 69.46, 68.78, 62.35, 31.33, 29.84, 27.73; IR (KBr) 3065, 2923, 2848, 2819,
1646, 1612, 1494, 1471, 1371, 1327, 1243, 1106, 1043, 1022, 1001, 950, 824, 768,
725, 545; m/z (EI) 537 (29%), 536 ([M+],100%), 471 (20%), 341 (14%), 340 (79%),
268 (19%), 226 (15%), 213 (19%), 199 (26%), 186 (15%), 121 (49%), 55 (16%), 43
(14%), 30 (43%). Anal.Calc. for C30H36N2Fe2: C, 67.19; H, 6.77; N, 5.22; MW,
536.31024. Found: 65.23; H, 7.19; N, 5.59; MW, 536.157307.
2.6.1.6 N,N’-Ethylenebis(4-phenylferrocenylmethylidene)imine [2.6]
The general procedure was followed using 4-
Ferrocenylbenzaldehyde (75 mg, 0.26 mmol)
and ethylenediamine (10 mg, 0.15 mmol).
N,N’-Ethylenebis(4-phenylferrocenylmethylidene)amine was obtained as an orange
solid (73 mg, 97 %). M.p 208-210 °C; 1H NMR (CDCl3) 8.33 (2H, s, N=CH), 7.63 (4H,
dd, J = 8.3, C6H4), 7.49 (4H, dd, J = 8.3, C6H4), 4.69 (4H, t, J = 1.8, C5H4), 4.38 (4H,
t, J = 1.8, C5H4), 4.06 (10H, s, C5H5), 3.99 (4H, s, 2 x CH2); 13C NMR (CDCl3)
Fe
CH
(CH2)
Fe
CH
N8
N
Fe Fe
CH N N CH
73
162.99, 128.59, 126.41, 85.01, 70.31, 70.12, 69.81, 67.05; IR (KBr) 3098, 2913,
2847, 2230, 1638, 1605, 1566, 1528, 1454, 1420, 1373, 1362, 1308, 1281, 1227,
1181, 1103, 1084, 1018, 995, 887, 864, 822, 648, 513, 509, 451; m/z (EI) 605 (38%),
604 ([M+, 75%), 302 (17%), 287 (14%), 180 (7%), 152 (9%), 139 (7%), 121 (48%),
69 (10%), 63 (22%), 62 (17%), 56 (38%), 43 (18%), 39 (69%), 38 (28%), 37 (15%),
29 (35%), 28 (100%), 27 (66%), 26 (30%); Anal. Calc. for C36H32N2Fe2: MW,
604.34268. Found: MW, 604.134521.
2.6.1.7 N,N’-Bis-(2,3-dimethylbenzylidene)-ethane-1,2-diimine [2.7]
2,3-Dimethylbenzaldehyde (202 mg, 1.5
mmol) and ethylenediamine (45 mg, 0.91
mmol) were added into a 25 cm3 round-
bottomed flask. The two compounds
were stirred using a magnetic stirrer at room temperature (ca. 25ºC). The round
bottomed flask was placed under high vacuum overnight. N, N’-Bis-(2,3-
dimethylbenzylidine)-ethane-1,2-diamine was obtained as a white powder (188 mg,
86 %). M.p. 119-121 °C; 1H NMR (CDCl3) 8.68 (2H, s, CH=N), 7.69 (2H, d, J = 7.6,
Ar-H), 7.20 (2H, d, J = 7.2, Ar-H), 7.10 (2H, t, J = 7.6, Ar-H), 3.95 (4H, s, 2 x CH2),
2.33 (6H, s, 2 x CH3), 2.27 (6H, s, 2 x CH3); 13C NMR (CDCl3) 161.62, 137.42,
136.41, 134.99, 131.76, 125.88, 125.67, 62.17, 19.81, 14.10; IR (KBr) 3063, 3005,
2964, 2972, 2907, 1637, 1591, 1458, 1374, 1281, 1261, 1197, 1183, 1092, 1010,
990, 980, 956, 903, 798, 784, 760, 716, 488, 426; m/z (EI) 293 (10%), 292 ([M+],
38%), 291 (35%), 162 (30%), 161 (100%), 160 (65%), 159 (19%), 158 (28%), 147
(20%), 146 (86%), 144 (26%), 134 (15%), 133 (69%), 132 (90%), 131 (48%), 130
(37%), 119 (70%), 118 (22%), 117 (29%), 116 (23%), 115 (19%), 105 (15%), 103
(16%), 91 (39%), 77 (21%), 69 (19%), 57 (18%), 55 (16%), 43 (16%), 41 (20%), 28
(31%). Anal. Calc. for C20H24N2: MW, 292.41796. Found: MW, 292.193095.
2.6.1.8 N,N’-Bis-(2,5-dimethylbenzylidene)-ethane-1,2-diimine [2.8]
2,5-Dimethylbenzaldehyde (500 mg, 3.7
mmol) and ethylenediamine (112 mg, 1.90
NN
NN
74
mmol) were added into a 25 cm3 round bottomed flask. The procedure for [2.7] was
followed and N,N’-bis-(2,5-dimethylbenzylidine)-ethane-1,2-diamine was obtained as
a white powder (489 mg, 88 %). M.p. 109-110 °C; 1H NMR (CDCl3) 8.58 (2H, s,
CH=N), 7.66 (2H, s, Ar-H), 7.11 (2H, d, J = 7.1, Ar-H), 7.07 (2H, d, J = 7.8, Ar-H),
3.94 (4H, s, 2 x CH2), 2.39 (6H, s, 2 x CH3), 2.29 (6H, s, 2 x CH3); 13C NMR (CDCl3)
161.21, 135.49, 134.92, 134.51, 131.09, 128.28, 62.15, 20.38, 18.46; IR (KBr) 3019,
2977, 2909, 2878, 2843, 1639, 1609, 1572, 1496, 1463, 1403, 1387, 1372, 1277,
1246, 1211, 1198, 1164, 1117, 1034, 1017, 969, 959, 942, 897, 822, 790, 724, 645,
505, 566, 467, 407; m/z (EI) 293 (20%), 292 ([M+], 47%), 291 (38%), 162 (42%), 161
(100%), 160 (48%), 159 (22%), 158 (39%), 147 (36%), 146 (98%), 145 (24%), 144
(49%), 134 (27%), 133 (85%), 132 (99%), 131 (59%), 130 (51%), 120 (17%), 119
(72%), 118 (26%), 117 (48%), 116 (30%), 115 (33%), 106 (26%), 105 (32%), 104
(23%), 103 (35%), 91 (52%), 79 (22%), 78 (21%), 77 (44%), 65 (19%), 51 (15%), 41
(18%), 39 (17%), 28 (17%). Anal. Calc. for C20H24N2: MW, 292.41796. Found: MW,
292.193616.
2.6.1.9 N,N’-Bis-(2,4,6-trimethylbenzylidene)-ethane-1,2-diimine [2.9]
2,4,6-Trimethylbenzaldehyde (500 mg, 3.4
mmol) and ethylenediamine (101 mg, 1.70
mmol) were added into a 25 cm3 round-
bottomed flask. The same procedure for
[2.7] was used and N,N’-bis-(2,4,6-trimethylbenzylidine)-ethane-1,2-diamine was
obtained as a white crystalline needles (457 mg, 84 %). M.p. 126-127 °C; 1H NMR
(CDCl3) 8.64 (2H, s, CH=N), 6.93 (4H, s, Ar-H), 4.02 (4H, s, 2 x CH2), 2.37 (12H, s, 4
x CH3), 2.28 (6H, s, 2 x CH3); 13C NMR (CDCl3) 162.64, 139.03, 137.89, 131.41,
129.69, 63.89, 21.52, 21.06; IR (KBr) 2957, 2941, 2917, 2881, 2843, 2734, 1637,
1610, 1568, 1481, 1463, 1430, 1392, 1376, 1285, 1223, 1154, 1048, 1032, 1008,
975, 932, 897, 861, 841, 789, 727, 597, 565, 542, 512, 440; m/z (EI) 321 (8%), 320
([M+], 21%), 176 (34%), 174 (94%), 173 (32%), 172 (32%), 161 (27%), 160 (83%),
159 (15%), 158 (32%), 148 (18%), 147 (67%), 146 (100%), 145 (49%), 144 (37%),
133 (39%), 132 (26%), 131 (31%), 130 (32%), 120 (18%), 119 (12%), 117 (18%),
116 (14%), 115 (21%), 105 (27%), 91 (26%), 77 (14%), 41 (15%). Anal. Calc. for
C22H28N2: MW, 320.47112. Found: MW, 320.1650.
NN
75
2.6.2 Reduction of bisferrocenylimines and arybisimines
2.6.2.1 N,N’-Ethylenebis(ferrocenylmethyl)amine [2.10]
To a solution of lithium aluminium hydride (17 mg,
0.44 mmol) in diethyl ether (40 cm3) was added
N,N’-ethylenebis(ferrocenylmethylidene)imine
(100 mg, 0.22 mmol). The resultant mixture was heated under reflux for 1 h, and the
reaction was quenched with ethyl acetate/ice-water slurry. The solution was
extracted with diethyl ether (2 x 30 cm3) and the combined ethereal extracts were
dried over anhydrous sodium sulfate. The solution was filtered and the solvent
removed in vacuo. N,N’-Ethylenebis(ferrocenylmethyl)amine was obtained as a light
yellow powder (88 mg, 87 %). M.p. 82-84 °C; 1H NMR (CDCl3) 4.23 (4H, t, J = 1.8,
C5H4), 4.18 (4H, t, J = 1.9, C5H4), 4.15 (10H, s, C5H5), 3.58 (4H, s, 2 x CH2), 2.84
(4H, s, 2 x CH2), 2.27 (2H, br-s, 2 x NH); 13C NMR (CDCl3) 86.79, 69.18, 68.97,
68.62, 48.72, 30.06; IR (KBr) 3098, 2957, 2925, 2854, 1665, 1626, 1590, 1558,
1472, 1430, 1406, 1364, 1314, 1286, 1263, 1245, 1122, 1105, 1084, 1037, 1025,
1001, 819, 771, 668, 649, 637, 497, 482, 452; m/z (EI) 457 ([M+ +1], 8%), 456 ([M+],
6%), 455 (2%), 289 (9%), 257 (4%), 199 (100%), 154 (48%), 136 (49%); Anal. Calc.
for C24H28N2Fe2: MW, 456.18252. Found: MW, 456.19.
2.6.2.2 N,N’-Propylenebis(ferrocenylmethyl)amine [2.11]
To a solution of lithium aluminium hydride (21 mg,
0.56 mmol) in diethyl ether (40 cm3) was added
N,N’-propylenebis(ferrocenylmethylidene)imine (131
mg, 0.28 mmol). The procedure for [2.10] was
followed and N,N’-propylenebis(ferrocenylmethyl)amine was obtained as a yellow
powder (121 mg, 92 %). M.p. 86-88 °C; 1H NMR (CDCl3) 4.18 (4H, t, J = 1.6, C5H4),
4.16 (10H, s, C5H5), 4.13 (4H, t, J = 1.6, C5H4), 3.53 (4H, s, 2 x CH2), 2.72 (4H, t, J =
6.8, 2 x CH2), 2.02 (2H, br-s, 2 x NH), 1.72 (2H, m, CH2); 13C NMR (CDCl3) 86.78,
68.90, 68.83, 68.25, 49.36, 48.45, 30.08; IR (KBr) 3102, 2928, 2831, 1556, 1468,
Fe
CH2
(CH2)
Fe
CH2
HN
2
HN
Fe
CH2
(CH2)
Fe
CH2
HN
3
HN
76
1435, 1411, 1402, 1390, 1262, 1103, 1062, 1035, 1018, 1001, 929, 825, 807, 776,
668, 646, 499, 490, 470; m/z 471 ([M+ +1], 16%), 470 ([M+], 8%), 307 (8%), 289
(8%), 199 (100%). Anal. Calc. for C25H30N2Fe2: MW, 470.2091. Found: MW,
469.9878.
2.6.2.3 N,N’-Butylenebis(ferrocenylmethyl)amine [2.12]
To a solution of lithium aluminium hydride (32 mg,
0.83 mmol) in diethyl ether (40 cm3) was added N,N’-
butylenebis(ferrocenylmethylidene)imine (200 mg,
0.42 mmol). The procedure for [2.10] was followed and N,N’-
butylenebis(ferrocenylmethyl)amine was obtained as a yellow powder (174 mg, 86
%). M.p.98-99 °C; 1H NMR (CDCl3) 4.19 (4H, t, J = 1.8, C5H4), 4.13 (10H, s, C5H5),
4.12 (4H, t, J = 1.9, C5H4), 3.54 (4H, s, 2 x CH2), 2.65 (4H, t, J = 6.4, 2 x CH2), 2.07
(2H, br-s, 2 x NH), 1.25 (4H, m, 2 x CH2); 13C NMR (CDCl3) 86.59, 68.97, 68.84,
68.31, 49.56, 49.22, 28.26; IR (KBr) 3098, 3056, 2896, 2866, 2801, 1471, 1453,
1442, 1410, 1396, 1383, 1318, 1259, 1227, 1151, 1114, 1105, 1045, 1037, 1028,
998, 968, 923, 900, 878, 857, 848, 834, 804, 774, 737, 724, 623, 517, 498, 487, 463,
415; m/z 485 ([M+ +1], 21%), 484 ([M+], 9%), 483 (4%), 307 (5%), 285 (6%), 199
(100%). Anal. Calc. for C26H32N2Fe2: MW, 484.23568. Found: MW, 484.09989.
2.6.2.4 N,N’-Hexylenebis(ferrocenylmethyl)amine [2.13]
To a solution of lithium aluminium hydride (30 mg,
0.78 mmol) in diethyl ether (40 cm3) was added N,N’-
hexylenebis(ferrocenylmethylidene)imine (200 mg,
0.39 mmol). The procedure for [2.10] was followed
and N,N’-hexylenebis(ferrocenylmethyl)amine was obtained as a yellow powder (183
mg, 91 %). M.p. 102-103 °C; 1H NMR (CDCl3) 4.20 (4H, t, J = 1.8, C5H4), 4.13 (10H,
s, C5H5), 4.12 (4H, t, J = 1.8, C5H4), 3.52 (4H, s, 2 x CH2), 2.63 (4H, t, J = 7.1, 2 x
CH2), 1.53 (2H, br-s, 2 x NH), 1.35 (4H, m, 2 x CH2), 1.77 (4H, m, 2 x CH2); 13C
(CDCl3) 87.16, 68.90, 68.83, 68.25, 49.90, 49.44, 30.34, 27.69; IR (KBr) 3098, 2924,
2849, 2818, ,1478, 1452, 1434, 1309, 1247, 1227, 1210, 1153, 1121, 1105, 1043,
1021, 1000, 960, 923, 873, 822, 768, 731, 692, 665, 615, 520, 489, 481; m/z 513
Fe
CH2
(CH2)
Fe
CH2
HN
4
HN
Fe
CH2
(CH2)
Fe
CH2
HN
6
HN
77
([M+ +1], 9%), 512 (10%), 511 (4%), 313 (7%), 307 (3%), 289 (3%), 199 (100%), 154
(22%). Anal. Calc. for C28H36N2Fe2: MW, 512.28884. Found: MW, 511.7.
2.6.2.5 N,N’-Octylenebis(ferrocenylmethyl)amine [2.14]
To a solution of lithium aluminium hydride (19. mg,
0.52 mmol) in diethyl ether (40 cm3) was added
N,N’-octylenebis(ferrocenylmethylidene)imine (137
mg, 0.26 mmol). The procedure for [2.10] was
followed and N,N’-octylenebis(ferrocenylmethyl)amine was obtained as a yellow
powder (121 mg, 88 %). M.p. 65-66 °C; 1H NMR (CDCl3) 4.20 (4H, t, J = 1.7, C5H4),
4.13 (10H, s, C5H5), 4.13 (4H, C5H4), 3.52 (4H, s, 2 x CH2), 2.62 (4H, t, J = 7.0, 2 x
CH2), 1.67 (2H, br-s, 2 x NH), 1.49 (4H, m, 2 x CH2), 1.31 (8H, s, 4 x CH2); 13C NMR
(CDCl3) 87.28, 68.88, 68.79, 68.15, 50.03, 49.46, 30.39, 29.88, 27.71; IR (KBr)
3095, 2923, 2852, 1473, 1432, 1409, 1326, 1240, 1150, 1120, 1105, 1044, 1018,
999, 961, 923, 884, 864, 848, 819, 772, 723, 668, 614, 523, 501, 484, 456; m/z 541
([M+ +1], 15%), 540 ([M+], 23%), 342 (5%), 341 (15%), 215 (4%), 199 (100%). Anal.
Calc. for C30H40N2Fe2: MW, 540.342. Found: MW, 539.9.
2.6.2.6 N,N’-Bis(2,3-dimethylbenzyl) ethane-1,2-diamine[2.15]
To a solution of lithium aluminium hydride (17 mg,
0.44 mmol) in diethyl ether (40 cm3) was added
N,N’-bis(2,3-dimethylbenzylidene)ethane-1,2-
diimine (65 mg, 0.22 mmol). The procedure for
[2.10] was followed and N,N’-bis(2,3-dimethylbenzyl)ethane-1,2-diamine was
obtained as a colourless oil (37 mg, 56%). M.p.; 1H NMR (CDCl3) 7.18-7.15 (2H, m,
Ar-H), 7.11 (4H, br-s, Ar-H), 3.81 (4H, s, 2 x CH2), 2.86 (4H, s, 2 x CH2), 2.32 (6H, s,
2 x CH3), 2.28 (6H, s, 2 x CH3), 1.78 (2H, s, 2 x NH); 13C NMR (CDCl3) 138.4, 137.7,
137.5, 135.4, 129.2, 127.1, 125.8, 52.7, 49.5, 21.0, 15.2; IR (NaCl) 3308, 3066,
3030, 3013, 2937, 2915, 2821, 2732, 2691, 1588, 1463, 1383, 1353, 1330, 1295,
1248, 1183, 1163, 1115, 1090, 1018, 990, 972, 902, 880, 820, 774, 725, 710, 675,
666; m/z; 297 ([M+ +1], 90%), 296 ([M+], 8%), 295 (25%), 171 (5%), 148 (28%), 119
(100%). Anal. Calc. for C20H28N2: MW, 296.44972. Found: MW, 296.1094.
Fe
CH2
(CH2)
Fe
CH2
HN
8
HN
NH
HN
78
2.6.2.7 N,N’-Bis(2,5-dimethylbenzyl) ethane-1,2-diamine [2.16]
N,N’-Bis(2,5-dimethylbenzylidene)ethane-1,2-
diimine (131 mg, 0.44 mmol) was added to a
solution of lithium aluminium hydride (34 mg,
0.88 mmol) in diethyl ether (40 cm3). The
procedure for [2.10] was followed and N,N’-
bis(2,5-dimethylbenzyl)ethane-1,2-diamine was obtained as a white powder (87 mg,
68%). M.p. 43-46 °C; 1H NMR (CDCl3) 7.13 (2H, s, Ar-H), 7.06 (2H, d, J = 7.6, Ar-H),
6.99 (2H, d, J = 7.6, Ar-H), 3.76 (4H, s, 2 x CH2), 2.86 (4H, s, 2 x CH2), 2.32 (6H, s, 2
x CH3), 2.31 (6H, s, 2 x CH3), 2.00 (2H, s, 2 x NH); 13C NMR (CDCl3) 138.12, 135.77,
133.42, 130.60, 129.64, 128.07, 51.77, 49.24, 21.38, 18.94; IR (KBr) 3265, 3048,
3017, 2971, 2946, 2918, 2894, 2859, 2823, 2758, 2732, 2703, 1610, 1499, 1474,
1457, 1376, 1368, 1303, 1279, 1240, 1229, 1204, 1188, 1156, 1131, 1098, 1051,
1038, 995, 980, 928, 882, 833, 814, 800, 718, 704, 668, 544, 492, 440, 419 ; m/z
297 ([M+ +1], 51%), 296 ([M+], 6%), 295 (18%), 167 (9%), 148 (22%), 119 (100%).
Anal. Calc. for C20H28N2: MW, 296.44972. Found: MW, 296.2178
2.6.2.8 N,N’-Bis(2,4,6-trimethylbenzyl) ethane-1,2-diamine[2.17]
N,N’-Bis(2,4,6-trimethylbenzylidene) ethane-
1,2-diimine (100 mg, 0.31 mmol) was added
to a solution of lithium aluminium hydride
(25 mg, 0.66 mmol) in diethyl ether (40
cm3). The procedure for [2.10] was followed and N,N’-bis(2,4,6-trimethylbenzyl)
ethane-1,2-diamine was obtained as a white powder (79 mg, 79 %). M.p.: 69-72 °C; 1H NMR (CDCl3) 6.85 (4H, s, Ar-H), 3.81 (4H, s, 2 x CH2)), 2.92 (4H, s, 2 x CH2),
2.33 (12H, s, 4 x CH3), 2.26 (6H, s, 2 x CH3), 1.99 (2H, s, 2 x NH); 13C NMR (CDCl3)
137.6, 137.4, 129.6, 129.5, 48.2, 47.2, 21.3, 20.1; IR (KBr) 3202, 3005, 2957, 2926,
2845, 2817, 2789, 1613, 1483, 1461, 1443, 1378, 1355, 1345, 1332, 1262, 1222,
1209, 1129, 1110, 1091, 1029, 1014, 914, 867, 851, 825, 807, 774, 759, 709, 665,
619, 592, 549; m/z 325 ([M+ +1], 37%), 324 ([M+], 4%), 323 (7%), 307 (8%), 297
NH
HN
NH
HN
79
((14%), 154 (19%), 133 (100%), 120 (44%). Anal. Calc. for C22H32N2: MW,
324.50288. Found: MW, 324.2869.
2.7 References
1. C. Imrie, V. O. Nyamori and T. I. A. Gerber, J. Organomet. Chem., 689 (2004)
1617-1622.
2. A. Benito, J. Cano, R. Martínez-Máňez, J. Soto, J. Payá, F. Lioret, M. Julve, J.
Faus and M. Dolores Marcos, Inorg. Chem., 32 (1993) 1197-1203.
3. V. K. Muppidi, T. Htwe, P. S. Zcharias and S. Pal, Inorg. Chem. Commun., 7
(2004) 1045-1048.
4. I. Ratera, D. Ruiz-Molina, C. Sánchez, R. Alcalá, C. Rovira and J. Veciana,
Synth. Met., 121 (2001) 1834.
5. J. Rajput, PhD Thesis: Platinum group metal coordination complexes of
ferrocenyl-N-donor ligands and their potential application in catalysis and
medicinal chemistry, University of Cape Town, 2003, 34.
6. H. Naeimi, F. Salimi and K. Rabiei, J. Mol. Cat. A, 260 (2006) 100-104.
7. M. Woltersdorf, R. Kranich and H.-G. Schmalz, Tetrahedron, 53 (1997) 7219.
8. R. M. Silverstein, F. X. Webster and D. J. Kiemle, Spectroscopic Identification
of Organic Compounds, 7th Ed., John Wiley & Sons, Inc. 2005.
9. R. C. J. Atkinson, V. C. Gibson and N. J. Long, Chem. Soc. Rev., 33 (2004)
313.
10. B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell, Vogel’s
Textbook of Practical Organic Chemistry, Longman Scientific and Technical,
England (5th Ed), 1989.
80
CHAPTER 3
RESULTS AND DISCUSSION
3.1 Synthesis of cationic rhodium(I) complexes
3.1.1 Rhodium(I) complexes containing bisferrocenylimines
The bisferrocenylimines described in the previous chapter were prepared in order for
them to be reacted with a rhodium(I) metal centre, to form cationic rhodium(I)
complexes. Rhodium(I) is understood to form square planar coordination complexes
with π-acceptor ligands and some five-coordinate complexes are known as well.1
Cationic rhodium complexes are known for their application in the field of catalysis.
An appropriate metal precursor for the synthesis of rhodium(I) complexes is the
chloro-bridged rhodium cyclooctadienyl dimer, chloro(1,5-cyclooctadiene)rhodium(I).
The dimer can readily be obtained from the reaction of 1,5-cyclooctadiene with
rhodium trichloride trihydrate under reflux.1 The type of complex formed from the
dimer is dependent on the nature of the ligand and the ratio of the metal to ligand.2
The cationic rhodium(I) complexes were synthesized using a literature procedure as
illustrated in Scheme 3.1.3 A solution of silver perchlorate in acetone was added to a
solution of the rhodium dimer in acetone. On precipitation of silver chloride, a
solvated complex of general formula [Rh(COD)(acetone)2]ClO4 was formed.3,4 The
addition of a bisferrocenylimine ligand resulted in a cationic complex by
displacement of the coordinated solvent from the rhodium coordination sphere. The
complexes [3.1] (x = 2), [3.2] (x = 3) and [3.3] (x = 4) were obtained in low to
excellent yields by redissolving the residue, after concentration of solvent, with
dichloromethane and precipitated by addition of diethyl ether. The complexes were
further purified by recrystallization. Complex [3.2] had the highest yield while [3.1]
was obtained in lowest yield. This effect was attributed to the stability of the six-
membered ring formed by the bidentate ligand [2.2] with the metal centre compared
to five- and seven-membered rings formed by [2.1] and [2.3], respectively. Attempts
to prepare complexes containing [2.4], [2.5] and [2.7]-[2.9] were not successful.
81
+ 2 AgClO4
acetone[Rh(COD)(acetone)2]ClO4 + AgCl(s)Rh
Cl
ClRh
Fe
CH
(CH2)
Fe
CHN
xN
Rh
2L
ClO4
[3.0]
x = 2: [3.1] = 3: [3.2] = 4: [3.3]
Scheme 3.1: Procedure for the synthesis of cationic rhodium(I) complexes.3
1H NMR data for [3.1]-[3.3] is summarized in Table 3.1. The imine (CH=N) chemical
shifts for [3.2] and [3.3] were observed in the expected region while for [3.1] the
peak was shifted remarkably to lower frequencies. This effect has also been
observed by Lee et al.3 and could not be explained (Figure 3.1). The CH=N signal in
[3.1] moved from δ 8.17 ppm in the free ligand to δ 7.42 ppm in the complex. The
CH=N signal moved to higher frequencies for [3.2] and [3.3]. This signal was
observed to have shifted from δ 8.17 ppm in the free ligand [2.2] to δ 8.31 ppm in the
complex. On the other hand, the signal for [3.3] moved from δ 8.18 ppm in the free
ligand [2.3] to δ 8.22 ppm in the complex. In the ferrocene region, a sharp singlet
was observed at δ 4.37 and 4.07 ppm for [3.1] and [3.2] respectively, and it was
assigned to the unsubstituted Cp ring. Two singlets at δ 4.73 and 4.80 ppm for [3.1]
were assigned to the substituted Cp ring. The substituted Cp ring signals for [3.2]
were observed at δ 4.23 and 4.72 ppm. The appearance of additional signals in the
ferrocene region of [3.3] complicated the assignments.
82
Table 3.1: The summarized NMR data for [3.1]-[3.3]
Complex Yield
(%)a
1H (ppm) 13C (ppm)
CH=N COD Fc CH=N COD Fc
[3.1]
32
(47.0)b
7.42 4.16,
2.64,
2.09,
4.80,
4.73,
4.37
168.48 84.49,
30.55
74.70,
73.22,
70.68,
84.65
[3.2]
84
(37.1)b
8.31 5.48,
5.39,
2.58,
1.97
4.72,
4.23,
4.07
170.44 75.61,
30.89
71.73,
70.51,
70.28,
84.89
[3.3]
70
(39.0)b
8.22 6.11,
5.65,
2.62,
1.79
c
170.39 75.63,
31.26
c
a Isolated yields are based on starting reagents.
b Numbers in parentheses are conductivity values in Ohm-1.cm2.mol-1
c The appearance of additional signals in the ferrocene region complicated the assignments.
Two sharp and equally intense singlets were observed at δ 4.26 and 4.14 ppm and
are assigned to the unsubstituted Cp rings. This suggests that the Cp rings could be
chemically inequivalent due to the spatial orientation of the ferrocene, something that
was not observed in [3.1] and [3.2]. Other signals could not be assigned due to the
complexity of the signals in the region. Initially, it was thought that the extra signals
were due to the presence of impurities. However, the signals were persistent even
after recrystallization several times. The COD ligand exhibited the expected patterns
for [3.1] giving rise to a singlet due to CH=CH protons at δ 4.16 ppm, a multiplet δ
2.64 ppm and a doublet δ 2.09 ppm due to CH2 protons. For [3.2] and [3.3] the
CH=CH proton signal was split into two singlets at δ 5.48 and 5.39 ppm and 6.11
83
and 5.65 ppm, respectively. The CH2 signals were
Figure 3.1: 1H NMR spectra of [3.2] (top) and [3.3] (bottom) in CDCl3
observed in the expected region for both [3.2] and [3.3]. 13C NMR showed the
exhibited C=N signals in the expected region, δ 168.48, 170.44 and 170.39 ppm for
[3.1], [3.2] and [3.3], respectively. The ferrocene signals for [3.1] and [3.2] were
observed in the expected region while the same problem as with the 1H NMR was
experienced for [3.3]. The COD signals were also observed in the expected region
84
for all complexes. IR spectra of all the compounds showed that the v(C=N) stretching
signals have moved to lower frequencies. The infrared together with the NMR
information explain the coordination of the ligands to the rhodium metal centre.
3.1.2 X-ray Crystallography
Crystals of [3.2] that were suitable for X-ray crystallographic analysis were obtained
by slow diffusion of diethyl ether into a solution of the complex in dichloromethane.
The complex was observed to crystallize in a triclinic space group P1bar with Z = 4
and contains two molecules (structural isomers) in an asymmetric unit. The structure
was refined successfully with the final R factor of 0.0456.
Table 3.2: Crystal data and structure refinement of [3.2] and [3.3]
Complex [3.2] Complex [3.3]
Empirical formula C33H38ClFe2N2O4Rh C34H40ClFe2N2O4Rh
Formula weight 776.71 790.74
Temperature 113(2) K 173(2) K
Wavelength 0.71073 Å 0.71073
Crystal system Triclinic Monoclinic
Space group P1bar P21/c
Unit cell dimensions
a 12.0890(2) Å 12.983(3) Å
b 16.5931(2) Å 13.536(2) Å
c 17.5512(3) Å 17.729(4) Å
α 67.8240(10)˚ 90˚
β 72.9460(10)˚ 92.580(7)˚
γ 72.3860(10)˚ 90˚
Volume 3042.77(8) A3 3112.6(11) A3
Z 4 4
Calculated density 1.696 Mg/m3 1.687 Mg/m3
Reflections collected 70435 / 11534 48758 / 5716
Unique [R (int) = 0.1029 [R (int) = 0.1694
Goodness-of-fit on F2 1.023 1.034
Final R indices R1 = 0.0456, wR2 = 0.0875 R1 = 0.0611, wR2 =0.0934
R indices (all data) R1 = 0.0758, wR2 = 0.0987 R1 = 0.1324, wR2 = 0.1119
85
All hydrogen atoms were placed geometrically with fixed bond length and refined
with isotropic displacement parameters depending on their carbon atoms. The
parameters for crystal data collection and structure refinements are in Table 3.2.
The ORTEP drawing shown in Figure 3.2 confirms the molecular structure of [3.2].
The bond lengths, angles, torsion angles and other parameters are in Table 3.3. The
rhodium atom is oriented in an essentially square planar geometry defined by two
nitrogen atoms of the bidentate ligand and the two C=C double bonds of the COD
ligand.
Figure 3.2: ORTEP diagram of [3.2]
The six-membered ring formed by the rhodium atom, the two nitrogen atoms of the
ligand and the three carbon atoms of the alkyl chain separating the two nitrogen
atoms is in a chair conformation. The chair conformation is the lowest energy state
that a six-membered ring can be found in, which explains the reason for [3.2] being
obtained in excellent yields. The Rh(1A)-N(1A) and Rh(1A)-N(2A) bond distances for
molecule A are 2.077(4) and 2.095(4) Å, respectively. For molecule B, the Rh(1B)-
86
N(2B) and Rh(1B)-N(1B) the bond distances are 2.076(4) and 2.088(4) Å,
respectively. The bond distances Rh(1A)-C(2A), Rh(1A)-C(6A), Rh(1A)-C(1A) and
Rh(1A)-C(5A) for molecule A, are 2.144(4), 2.150(5), 2.152(4) and 2.158(4) Å,
respectively. On the other hand, for molecule B the bond distances Rh(1B)-C(2B),
Rh(1B)-C(5B), Rh(1B)-C(6B) and Rh(1B)-C(1B) are 2.130(4), 2.143(4), 2.144(4) and
2.171(4) Å, respectively.
Table 3.3: Selected bond distances, bond angles and torsion angles of [3.2]
Molecule A Molecule B
Rh(1A)-N(1A) 2.077(4) Rh(1B)-N(2B) 2.076(4)
Rh(1A)-N(2A) 2.095(4) Rh(1B)-N(1B) 2.088(4)
Rh(1A)-C(2A) 2.144(4) Rh(1B)-C(2B) 2.130(4)
Rh(1A)-C(6A) 2.150(5) Rh(1B)-C(5B) 2.143(4)
Rh(1A)-C(5A) 2.152(4) Rh(1B)-C(6B) 2.144(4)
Rh(1A)-C(2B) 2.158(4) Rh(1B)-C(1B) 2.171(4)
N(1A)-C(9A) 1.294(6) N(1B)-C(9B) 1.289(6)
N(2A)-C(13A) 1.277(5) N(2B)-C(13B) 1.286(6)
N(1A)-(Rh(1A)-N(2A) 85.92(14) N(2B)-(Rh(1B)-N(1B) 85.06(14)
N(1A)-(Rh(1A)-C(2A) 92.34(16) N(2B)-(Rh(1B)-C(2B) 162.39(16)
N(2A)-(Rh(1A)-C(2A) 171.25(16) N(1B)-(Rh(1B)-C(2B) 95.29(16)
N(1A)-(Rh(1A)-C(6A) 154.73(17) N(2B)-(Rh(1B)-C(5A) 95.04(16)
N(2A)-(Rh(1A)-N(6A) 89.01(16) N(1B)-(Rh(1B)-C(5A) 174.44(16)
C(2A)-(Rh(1A)-N(CA) 96.11(18) C(2B)-(Rh(1B)-C(5A) 82.94(18)
N(2A)-Rh(1A)-N(2A)-C(9A) 112.4(4) N(2B)-Rh(1B)-N(1B)-C(9B) 112.8(4)
C(2A)-Rh(1A)-N(1A)-C(9A) -59.0(4) C(2B)-Rh(1B)-N(1B)-C(9B) -49.6(4)
C(6A)-Rh(1A)-N(1A)-C(9A) -168.7(4) C(5B)-Rh(1B)-N(1B)-C(9B) 21.5(19)
C(1A)-Rh(1A)-N(1A)-C(9A) -96.7(4) C(6B)-Rh(1B)-N(1B)-C(9B) -163.7(4)
C(5A)-Rh(1A)-N(1A)-C(9A) 3.1(10) C(1B)-Rh(1B)-N(1B)-C(9B) -86.9(4)
N(2A)-Rh(1A)-N(1A)-C(10A) -65.1(3) N(2B)-Rh(1B)-N(1B)-C(10B) -65.5(3)
The bond distances that have just been mentioned were found to be comparable to
with literature-cited bond distances for similar complexes.5 The bite angles N(1A)-
Rh(1A)-N(2A) and N(2B)-Rh(1B)-N(1B) are 85.92(14) and 85.06(14) for molecule A
and molecule B, respectively. The deviation from the ideal 90° bond angles (square
planar) around the Rh-atom was due to the steric bulk of the COD ligand.5 The bond
distances N(1A)-C(9A) and N(2A)-C(13A) are 1.294(6) and 1.277(5) and are typical
87
bond distances for the C=N bond. The crystal packing in the unit cell of [3.2] is
shown in Figure 3.3 and no intermolecular interactions were exhibited.
Figure 3.3: Crystal packing of [3.2], projection viewed along [100]
Crystals of [3.3] that were suitable for X-ray crystallographic analysis were obtained
by slow diffusion of diethyl ether into a solution of the complex in dichloromethane.
The complex was observed to crystallize in a monoclinic space group P21/c with Z =
4. The structure was refined successfully with the final R factor of 0.0611. All
hydrogen atoms were fixed in geometrically calculated positions with Uiso set at 1.2
or 1.5 those of the parent atoms. The parameters for crystal data collection and
structure refinements are in Table 3.2.
The ORTEP drawing shown in Figure 3.4 confirms the molecular structure of [3.3].
The bond lengths, angles, torsion angles and other parameters are in Table 3.4. The
rhodium atom is oriented in an essentially square planar geometry defined by two
nitrogen atoms of the bidentate ligand and the two C=C double bonds of the COD
ligand.
88
Figure 3.4: ORTEP drawing of [3.3]
Figure 3.5: Crystal packing of [3.3], projection viewed along [100]
The Rh(1)-N(1) and Rh(1)-N(2) bond distances are 2.089(5) and 2.105(5) Å and are
slightly longer than those for [3.2]. The Rh(1)-C(6), Rh(1)-C(2), Rh(1)-C(5) and
Rh(1)-C(1) are 2.135(6), 2.146(6), 2.150(60) and 2.165(6) Å, respectively. The bite
angle N(1)-Rh(1)-N(2) is 89.04(18)° and is much closer to the ideal 90° for square
planar complex. Figure 3.5 shows the crystal packing in the unit cell of [3.3] and no
89
intermolecular interactions are present. The crystal packing of [3.2] completely
differs from that of [3.3].
Table 3.4: Selected bond distances, bond angles and torsion angles of [3.3]
Bond distances Bond angles Torsion angles
Rh(1)-N(1) 2.089(5) N(1)-Rh(1)-N(2) 89.04(18) N(2)-Rh(1)-N(1)-C(9) 107.9(5)
Rh(1)-N(1) 2.105(5) N(1)-Rh(1)-C(6) 158.7(2) C(6)-Rh(1)-N(1)-C(9) 12.3(9)
Rh(1)-C(6) 2.135(6) N(2)-Rh(1)-C(6) 92.9(2) C(2)-Rh(1)-N(1)-C(9) -98.8(5)
Rh(1)-C(2) 2.146(6) N(1)-Rh(1)-C(2) 89.7(2) C(1)-Rh(1)-N(1)-C(9) -61.4(5)
Rh(1)-C(5) 2.150(6) N(2)-Rh(1)-C(2) 153.2(2) N(2)-Rh(1)-N(1)-C(10) -76.6(4)
Rh(1)-C(1) 2.165(6) C(6)-Rh(1)-C(2) 97.8(2) C(2)-Rh(1)-N(1)-C(10) .76.7(4)
N(1)-C(9) 1.283(7) N(1)-Rh(1)-C(5) 163.6(2) C(1)-Rh(1)-N(2)-C(14) 7.4(14)
N(1)-C(10) 1.478(7) N(2)-Rh(1)-C(5) 91.8(2)
N(2)-C(14) 1.289(7) C(6)-Rh(1)-C(5) 37.6(2)
N(2)-C(13) 1.473(7) C(2)-Rh(1)-C(5) 82.2(2)
C(9)-C(21) 1.450(8) C(10)-(11)-C(12) 116.1(5)
C(10)-C(11) 1.512(8) C(13)-C(12)-C(11) 114.7(5)
C(11)-C(12) 1.522(8) C(25)-C(21)-C(22) 106.6(6)
C(12)-C(13) 1.522(8) C(25)-C(21)-C(9) 130.7(6)
C(14)-C(41) 1.470(8) C(22)-C(21)-C(9) 122.4(6)
3.1.3 Rhodium(I) complexes containing bisferrocenylamines
Initially, the objective was to synthesize rhodium complexes similar to those reported
by Kim and Alper (Figure 3.6)6 using bisferrocenylamines as ligands. These
complexes are said to be highly effective in hydroformylation reactions.
Rh
N N
Rh
Cl Cl+ -
Figure 3.6: Cationic rhodium(I) diamine complexes with the [Rh(COD)Cl2]- anion.6
90
The complexes were prepared by stirring equimolar amounts (1 mole) of
[Rh(COD)Cl]2 [3.0] with bisferrocenylamines [2.10]-[2.12], using the Schlenk
technique, at room temperature in an argon environment, for 12 h.6 Yellow
precipitates were formed, almost immediately, on addition of the ligand to a solution
of [Rh(COD)Cl]2 in toluene. However, these complexes were highly insoluble in most
organic solvents and therefore could not be characterized. The insolubility of the
complexes was thought to be due to very high lattice energies in the molecules. It
was then decided that a smaller counterion should be used instead of the bulky
anionic complex. Tetrafluoroborate ion, BF4- was chosen as the anion to be used and
all the complexes were soluble in most organic solvents, making characterization
possible.
The complexes were prepared by a slightly modified literature method.4 Silver
tetrafluoroborate (AgBF4) (2 mol) in acetone was added to [Rh(COD)Cl]2 (1 mol) in
acetone using the Schlenk technique in an argon atmosphere. After filtration of the
precipitated AgCl, the yellow filtrate was treated with the appropriate ligand (2 mol) in
acetone. The mixture was stirred at room temperature for 24 h. Addition of n-pentane
caused the precipitation of a yellow solid after the volume of acetone was reduced to
approximately 5 cm3. The complexes (Figure 3.7) were further purified by
recrystallization and were isolated in moderate to excellent yields (Table 3.5).
Fe
CH2
(CH2)
Fe
CH2
HN
x
HN
RhBF4
X = 2 [3.4] = 3 [3.5] = 4 [3.6]
Figure 3.7: Cationic rhodium(I) complexes
91
Surprisingly, 1H NMR data was not helpful in terms of the characterization of all
complexes since their spectra showed only broad signals. 13 C NMR gave no signals
for complexes even after running the experiment for 24 h. IR spectra of complexes
[3.4], [3.5] and [3.6] showed absorptions in the region 3180-3280 cm-1 representing
the v(NH) stretching frequencies. The frequencies were similar to those reported by
Garrald et al2 and Beller et al.7 for ν(NH) of similar complexes. For ferrocene, three
bands were observed in all complexes in the regions 3090-3100 cm-1, 1405-1415
cm-1 and 1104-1115 cm-1, which were assigned to v(CH) stretching, v(C-C)
stretching and ring breathing, respectively.1
Figure 3.8: IR spectrum of [3.5]
Complexes [3.4], [3.5] and [3.6] each exhibited a band at 1642, 1634 and 1638,
respectively, which was attributed to the v(C=C) stretching frequency of the COD
ligand. Another sharp band was observed in the region 480-490 cm-1 and it was
assigned to the Rh-N stretching frequency. A signal at around 1000 cm-1 was
observed in all complexes which, according to Beller et al.,8 could be assigned to the
BF4- ion, indicative of a cationic species. The infrared spectrum of [3.5] is shown in
Figure 3.8.
92
Table 3.5: Table of yields and conductivity measurements
Number Complex Yield (%) Λm
(Ohm-1.cm2.mol-1)
[3.4] Fe
CH2
(CH2)
Fe
CH2
HN
2
HN
RhBF4
66
37.6
[3.5] Fe
CH2
(CH2)
Fe
CH2
HN
3
HN
RhBF4
82
40.4
[3.6] Fe
CH2
(CH2)
Fe
CH2
HN
4
HN
RhBF4
72
33.6
[3.7] CH2
(CH2)
CH2
HN
2
HN
RhBF4
60
32.0
[3.8] CH2
(CH2)
CH2
HN
2
HN
RhBF4
68
42.8
The aromatic v(C=C) stretching frequencies were observed at 1506 cm-1 for [3.7]
and 1509 cm-1 for [3.8]. A Rh-N stretching band was also observed for [3.7] and
[3.8] in exactly the same region as in the previous complexes.
A sharp band that was assigned to the BF4- ion was observed at 1032 and 1036 cm-1
for both complexes. This was an indication of the cationic nature of the complexes.
The conductivity values of the complexes (Table 3.5) were comparable to the
conductivity values obtained by Denise and Pannetier9 for similar types of
complexes.
93
Figure 3.9: IR spectrum of [3.8]
3.2 Electronic spectroscopy Uv-vis spectra of all complexes were obtained in a dichloromethane solution (10-4
M). In comparison with the free ligands [2.1], [2.2] and [2.3], spectra of complexes
[3.1], [3.2] and [3.3] exhibited an extra band at λmax 350, 382 and 383 nm,
respectively (Figure 3.12, Table 3.6). Moreover, the bands that were observed in the
ligands [2.1]-[2.3] appeared to have shifted to higher wavelengths λmax. This was
clearly indicative of some coordination to the rhodium(I) ion. Both bands at lower
wavelengths, for the complexes, appeared as shoulders. UV-vis spectra of
complexes [3.4], [3.5] and [3.6] exhibited a shoulder at λmax 462, 466 and 468 nm,
respectively, (Figure 3.11, Table 3.6). Bands at λmax 385, 388 and 387 nm in [3.4],
[3.5] and [3.6], respectively, were thought to be due to coordination to the rhodium
metal.
This band appeared in complexes [3.1]-[3.3] as well as in complexes [3.7] and [3.8]
as will been seen later. One band that was observed in the corresponding ligands
[2.10], [2.11] and [2.12] was not observed in the spectra of the complexes. It was
postulated that these bands could have been masked by the bands at λmax 388, 387
and 382 nm.
94
Figure 3.10: UV-vis spectra of [3.1], [3.2] and [3.3]
Figure 3.11: UV-Vis spectra of [3.4], [3.5] and [3.6]
UV-vis spectra of complexes [3.7] and [3.8] exhibited only one band at λmax 382 and
381 nm respectively (Figure 3.10, Table 3.5). These spectra clearly showed ligand
coordination to the metal since [2.16] and [2.17] were inactive in the UV-vis region
(800-200 nm).
95
Figure 3.12: UV-vis spectra of [3.7] and [3.8]
Table 3.6: UV-vis data for complexes [3.1]-[3.8]
Complex λmax (nm) ε (L.mol-1.cm-1)
[3.1] 303 (1198) 350 (670) 469 (219)
[3.2] 347 (593) 382 (392) 469 (469)
[3.3] 348 (711) 383 (456) 467 (282)
[3.4] 385 (174) 462 (34)
[3.5] 388 (345) 466 (50)
[3.6] 387 (219) 468 (32)
[3.7] 382 (228)
[3.8] 381 (141)
ε = molar exctinction coefficient.
3.3 Cyclic Voltammetry
Some typical voltammograms of selected rhodium(I) complexes are illustrated in
Figure 3.13, and depict one-electron reversible redox waves.
96
Figure 3.13: Cyclic voltammograms of [3.2], [3.4], [3.5] and [3.6]
Rhodium(I) complexes containing bisferrocenylimines exhibited positive shifts in
potential meaning that the complexes became more resistant to oxidation than their
corresponding free ligands (Figure 2.11, Table 3.6). Similarly, rhodium(I) complexes
containing bisferrocenylamines exhibited positive shifts in potentials compared to
their corresponding free ligands and thus became more difficult to oxidise (Figure
3.13, Table 3.6). The positive shift in potentials exhibited by the compounds was
evidence that coordination to the rhodium centre had occurred. Rhodium(I)
complexes containing arylbisamines [3.7] and [3.8] were electrochemically inactive
as in the case of their corresponding free ligands.
Table 3.7: Half-wave potentials of [3.1]-[3.6]
Compound Epa (mV) Epc (mV) E1/2 (mV)
[3.1] 413 342 327.5
[3.2] 388 300 344
[3.3] 372 261 316.5
[3.4] 246 105 175.5
[3.5] 222 104 163
[3.6] 157 72 114.5
97
3.4 Experimental
3.4.1 Purification procedures
All reagents and solvents were purified using standard purification and drying
methods.8 Silver tetrafluoroborate and the chloro-(1,5-cyclooctadiene)rhodium(I)
dimer were obtained from Sigma Aldrich Chemical Company, Milwaukee, USA.
Table 3.7: General drying agents for solvents.
Solvent Drying Agent
Dichloromethane CaH2
Hexane Na wire
Toluene Na wire
3.4.2 Instrumentation
Unless otherwise mentioned, all reactions were carried out using standard Schlenk
techniques in an argon gas environment. All the other instruments employed for
characterizations were the same as stated in Chapter 2.
X-ray crystal intensity data were collected on a Nonius Kappa-CCD diffractometer
using graphite monchromated MoKα radiation at the University of Cape Town.
Temperature was controlled by an Oxford Cryostream cooling system (Oxford
Cryostat). The strategy for the data collections was evaluated using the Bruker
Nonius “Collect” program.9 Data were scaled and reduced using DENZO-SMN
software (Ontwinowski & Minor, 1977). An empirical absorption correction utlilized
the program SADABS (Sheldrick, 1996). The structure was solved by direct methods
and refined by full-matrix least-squares with the program SHELXL-97 (Sheldrick,
1997), refining on F2.10,11 Packing diagrams were produced using the program
PovRay and graphic interface X-seed (Barbour, 2001).12 All the non-H atoms were
refined anisotropically.
98
3.5 Synthesis of rhodium(I) complexes
3.5.1 Rhodium(I) complexes containing bisferrocenylimines
3.5.1.1 General procedure3
Silver perchlorate [3.0] (0.23 mmol) (Scheme 3.1) in acetone (2 cm3) was added to a
solution of the chloro-(1,5-cyclooctadiene)rhodium dimer (0.11 mmol) in acetone (30
cm3). After removal of the precipitated AgCl, the reaction mixture was then heated
under reflux for 30 minutes. The reaction mixture was treated with bisferrocenylimine
(0.23 mmol) in toluene (20 cm3) and the resultant dark red solution was left to stir at
room temperature (ca. 25 °C) for 3h. Solvents were removed in vacuo and the dark
red solid was recrystallized from a dichloromethane/hexane mixture. The products
were obtained as dark red to orange solids.
3.5.1.2 Complex [3.1]3
The general procedure in 3.5.1.1 was followed using
N,N’–ethylenebis(ferrocenylmethylidene)imine (104
mg, 0.23 mmol) in toluene (20 cm3). Complex [3.1]
was obtained as a dark red solid (27 mg, 32 %). M.p.
215 °C (decomp) (lit. 192 °C, decomp.); 1H NMR (CDCl3) 7.43 (2H, s, N=CH), 4.80
(4H, t, J = 1.7, C5H4), 4.73 (4H, t, J = 1.8, C5H4), 4.37 (10H, s, C5H5), 4.16 (4H, s,
COD-CH), 3.85 (4H, s, CH2), 2.64 (4H, br-s, COD-CH2), 2.09 (4H, d, J = 7.5, COD-
CH2); 13C NMR (CDCl3) 168.48, 84.65, 84.49, 74.71, 73.23, 70.68, 57.01, 30.55; IR
(KBr); 3249, 2951, 2886, 2836, 1612, 1447, 1412, 1377, 1259, 1219, 1098, 1102,
954, 825, 733, 622, 479; m/z (FAB) 665 ([M++2], 3%), 663 ([M+], 29%), 553 (3%),
489 (2%), 453 (9%), 333 (3%), 308 (18%), 289 (16%), 233 (4%), 154 (100).
Anal.Calc. for C32H36N2Fe2Rh: MW, 663.06324. Found: MW, 662.7.
Fe
CH
(CH2)
Fe
CH
N2
N
ClO4
Rh
99
3.5.1.3 Complex [3.2]
The general procedure in 3.5.1.1 was followed using
N,N’-propylenebis(ferrocenylmethylidene)imine (107
mg, 0.23 mmol) in toluene (20 cm3). Complex [3.2]
was obtained as a reddish orange solid (72 mg, 84
%). M.p. 220 °C (decomp.); 1H NMR 8.31 (2H, s, N=CH), 5.49 (2H, s, COD-CH),
5.39 (2H, s, COD-CH), 4.72 (4H, s, C5H4), 4.68 (4H, s, 2 x CH2), 4.23 (4H, s, C5H4),
4.17 (2H, m, CH2), 4.07 (10H, s, C5H5), 2.59 (4H, br-s, COD-CH2), 1.97 (4H, d, J =
7.7, COD-CH2) 13C NMR (CDCl3) 170.44, 77.62, 75.61, 73.41, 70.51, 70.28, 65.04,
30.89, 30.55; IR (KBr) 3106, 2933, 2852, 1624, 1457, 1411, 1373, 1331, 1253, 1093,
1052, 998, 897, 829, 622; m/z (FAB) 677 ([M+], 11%), 675 (2%), 567 (2%), 467 (5%),
424 (3%), 347 (2%), 307 (18%), 289 (17%), 242 (2%), 154 (100%). Anal. Calc.2 for
C33H38N2Fe2Rh; C, 51.03; H, 4.93; N, 3.61; MW, 677.26372. Found: C, 50.27; H,
4.59; N, 3.48; MW, 676.7.
3.5.1.4 Complex [3.3]
The general procedure in 3.5.1.1 was followed
using N,N’-butylenebis(ferrocenlmethylidene)imine
(111 mg, 0.23 mmol). Complex [3.3] was obtained
as an orange solid (68.3 mg, 70%). M.p. 228 °C
(decomp.); 1H NMR (CDCl3) 8.22 (2H, s, CH=N), 6.11 (2H, s, COD-CH), 5.65 (2H, s,
COD-CH), 4.14 (5H, s, C5H5), 4.12 (5H, s, C5H5), 2.62 (4H, m, COD-CH2), 1.79 (4H,
s, COD-CH2), other signals could not be assigned properly; 13C NMR (CDCl3)
170.41, 84.58, 75.63, 73.74, 73.90, 73.33, 70.04, 69.96, 69.78, 66.46, 58.02, 29.29,
28.73; IR (KBr) 3101, 3013, 2926, 2882, 2838, 1620, 1454, 1436, 1414, 1375, 1331,
1256, 1146, 1103, 1046, 1002, 967, 830, 624, 506, 483; m/z (FAB) 693 ([M+ +2],
9%) 691 ([M+], 65%), 581 (7%), 495 (4%), 481 (6%), 396 (5%), 345 (3%), 307 (21%),
289 (20%), 233 (7%), 154 (100%). Anal. Calc. for C34H40N2Fe2Rh: C, 51.64; H, 5.10;
N, 3.54; MW, 691.09454. Found: C, 51.46, H, 5.43; N, 3.31; MW, 690.7.
2 The tendency for the C and N content to be too low and H too high could be due to the presence of traces of water. This also applies to compounds [3.3]-[3.8].
Fe
CH
(CH2)
Fe
CH
N3
N
ClO4
Rh
Fe
CH
(CH2)
Fe
CH
N4
N
RhClO4
100
3.5.2 Rhodium(I) complexes containing bisferrocenylamines3
3.5.2.1 Complex [3.4]
Silver tetrafluoroborate [3.0] (33.3 mg, 0.171
mmol) in acetone 2 cm3 was added to a solution
of [Rh(COD)Cl]2 (42.1 mg, 0.086 mmol) in
acetone (25 cm3) and the reaction mixture stirred
vigorously at room temperature for a few minutes. The precipitated AgCl was filtered
off and the yellow filtrate was stirred with N,N-ethylenebis(ferrocenylmethyl)amine
(78.1 mg, 0.171 mmol) in acetone (20 cm3). The reaction mixture was left to stir at
room temperature for 24 h. Reduction of the solvent volume and addition of hexane
caused the precipitation of complex [3.4] as a yellow solid. After filtration, the solid
was washed with diethyl ether and recrystallized from a dichloromethane/hexane
mixture (85.4 mg, 66.2%). M.p. 182 °C (decomp.); IR (KBr) 3268, 3101, 2934, 2881,
2829, 1638, 1454, 1410, 1383, 1335, 1304, 1234, 1107, 1085, 1028, 1002, 918, 822,
484; Anal. Calc. for C32H40N2Fe2Rh: C, 50.97, H; 5.35; N, 3.71. Found: C, 49.60, H,
6.01; N, 3.24.
3.5.2.2 Complex [3.5]
The procedure as for [3.4] was followed using
silver tetrafluoroborate (23.2 mg, 0.122 mmol),
[Rh(COD)Cl]2 (30.1 mg, 0.061 mmol) and N,N-
propylenebis(ferrocenylmethyl)amine (55.8 mg,
0.122 mmol). Complex [3.5] was obtained as a yellow solid (94.3 mg, 81.8%). M.p.
134 °C (decomp.); IR (KBr) 3271, 3092, 2937, 2888, 2831, 1633, 1454, 1381, 1332,
1283, 1238, 1124, 1108, 1084, 1039, 1002, 974, 913, 819, 481; Anal. Calc. for
C33H42N2Fe2Rh: 51.60, H, 5.51, N, 3.65. Found: C, 50.40; H, 5.66; N, 3.30.
3 No MS data was obtained for compounds [3.4]-[3.8] owing to a spectrometer breakdown.
Fe
CH2
(CH2)
Fe
CH2
HN
2
HN
RhBF4
Fe
CH2
(CH2)
Fe
CH2
HN
3
HN
RhBF4
101
3.5.2.3 Complex [3.6]
The procedure as for [3.4] was followed using
silver tetrafluoroborate (36.1 mg, 0.185 mmol),
[Rh(COD)Cl]2 [3.0] (45.6 mg, 0.093 mmol) and
N,N-butylenebis(ferrocenylmethyl)amine (89.6
mg, 0.185 mmol). Complex [3.6] was obtained as a yellow solid (84.8 mg, 72.3 %).
M.p. 204 °C (decomp.); IR (KBr) 3285, 3259, 3180, 3092, 2996, 2926, 2882, 2829,
1642, 1476, 1432, 1410, 1379, 1331, 1230, 1125, 1107, 1085, 1037, 1028, 997, 953,
923, 817, 488; Anal. Calc. for C34H44N2Fe2Rh: C, 52.21; H, 5.67; N, 3.58. Found: C,
51.38; H, 6.33; N, 2.57.
3.5.2.4 Complex [3.7]
The procedure as for [3.4] was followed using
silver tetrafluoroborate (29.2 mg, 0.150 mmol),
[Rh(COD)Cl]2 [3.0] (37.0 mg, 0.075 mmol) and
N,N-bis(2,5-dimethylbenzyl)ethane-1,2-
diamine (44.5 mg, 0.150 mmol). Complex [3.7]
was obtained as a light yellow solid (53.8 mg, 60.4 %). M.p. 110 °C (decomp.); IR
(KBr) 3250, 2964, 2926, 2882, 2838, 1629, 1506, 1458, 1388, 1339, 1304. 1164,
1125, 1085, 1059, 1037, 958, 817, 765, 528, 483; m/z (FAB); Anal. Calc. for
C28H40N2Rh: C, 56.58; H, 6.78; N, 4.71. Found: C, 55.52; H, 7.50; N, 3.87.
3.5.2.5 Complex [3.8]
The procedure as for [3.4] was followed using
silver tetrafluoroborate (27.2 mg, 0.139 mmol),
[Rh(COD)Cl]2 (34.3 mg, 0.070 mmol) and N,N-
bis(2,4,6-trimethylbenzyl)ethane-1,2-diamine
(45.1 mg, 0.139 mmol). Complex [3.8] was
obtained as a light yellow solid (63.6 mg, 68.1%). M.p. 170 °C (decomp.); IR (KBr)
3259, 2961, 2917, 2873, 2829, 1638, 1616, 1581, 1458, 1432, 1379, 1340, 1304,
Fe
CH2
(CH2)
Fe
CH2
HN
4
HN
RhBF4
CH2
(CH2)
CH2
HN
2
HN
Rh BF4
CH2
(CH2)
CH2
HN
2
HN
RhBF4
102
1120, 1085, 1063, 1032, 896, 852, 769, 716, 633, 611, 519, 484; m/z (FAB); Anal.
Calc. for C30H44N2Rh: C, 57.89; H, 7.13; N, 4.50. Found: C, 53.71; H, 6.89; N, 3.56.
3.6 References
1. W. P. Griffith, The Chemistry of the Rarer Platinum Metals, John Wiley &
Sons, London 1967.
2. M. A. Garralda and L. Ibarlucea, J. Organomet. Chem., 311 (1986) 225.
3. S. –I. Lee, S. –C. Shim and T. –J. Kim, J. Polym. Sci., Part A: Polymer Chem.,
34 (1996) 2377.
4. P. Pertici, F. D’ Arata and C. Rosini, J. Organomet. Chem., 515 (1996) 163.
5. G. R. Julius and S. Cronje, Helvetica Chim. Acta, 85 (2002) 3737.
6. J. J. Kim and H. Alper, Chem. Commun., (2005) 3059.
7. M. Beller, H. Trauthwein, M. Eichberger, C. Breindl, T. E. Müller and A. Zapf,
J. Organomet. Chem., 566 (1998) 277.
8. B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell, Vogel’s
Textbook of Practical Organic Chemistry, Longmann Scientific and Technical,
England (5th Ed.) 1989.
9. Z. Otwinowski and W. Minor in C. W. Carter, J. Sweet and R. M. Sweet (Eds),
Macromolecular Crystallography Part A, Academic Press, New York, 276
(1997) 307.
10. G. M. Sheldrick, SHELX97, Programme for Solving Crystal Structures,
University of Göttingen, Germany, 1997.
11. G. M. Sheldrick, SHELX97, Programme for the Refinement of Crystal
Structures, University of Göttingen, Germany, 1997.
12. L. J. Barbour, X-Seed, University of Missouri-Columbia, USA, 1999.
103
CHAPTER 4
RESULTS AND DISCUSSION
4.1 Polymerization of phenylacetylene
4.1.1 Introduction
Polyphenylacetylene (PPA) has been found to be a very interesting polymer because
of its photoconductivity,1 photoluminescence,2 non-linear optical3 and membrane
properties.4 Cametti et al.5 have also investigated iodine-doped polyphenylacetylene
for potential application in technology. Polymerization of phenylacetylene has been
carried out in various conditions, including cationic, radical and coordination
mechanisms.6 Transition metal complexes have been used since the early
pioneering work during 1930 to the 1950’s. Masuda et al.7 first polymerized
phenylacetylene in 1974 using WCl6 and MoCl5 as catalysts, to give high molecular
weight polymers. Since then, other transition metal complexes (palladium, rhodium,
iridium, etc) have been investigated for catalytic activity towards polymerization of
polyphenylacetylene.
The zwitterion complex Rh+(COD)BPh4- has been found to produce stereoregular
cis-PPA with molecular weights up to 35 000, under hydrosilation conditions.8 The
[Rh(norbonadiene)Cl]2 complex has, so far, been reported to yield the highest
molecular weight of approximately 4.3 x 106.9 The catalytic activity of Rh(I), Ir(I) and
Ru(IV) complexes containing 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene and
1,3-di(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene ligands has been reported by
Zhang et al.10 Molecular weights ranging between 55 000 and 200 000 have been
obtained in ionic liquids using Rh(I) complexes as catalysts.11
104
4.1.2 Polymer characterization
Four possible stereoisomers can be formed in the catalytic polymerization of
phenylacetylene (Figure 4.1).10 The stereochemistry of PPA can be generated from
the configuration of the C=C bond and the conformation of C-C single bond of the
polymer main chain. The stereoisomers can be easily distinguished by their
physicochemical and spectroscopic properties.6
Ph H
Ph
HPh
H
cis-cisoidal
PhH H Ph
Ph H Ph H
trans-cisoidal
Ph PhH H
Ph H Ph H
cis-transoidal
Ph Ph Ph Ph
trans-transoidal
Figure 4.1: Stereoisomers of polyphenylacetylene
The solubility of the isomers can also be utilized as an aid to distinguish between
them. For example, cis-transoidal and trans-cisoidal isomers are both highly soluble
in benzene, while the cis-cisoidal isomer is insoluble. NMR and IR spectroscopy
show that the cis-cisoidal and cis-transoidal isomers have similar spectroscopic
behaviour, which differentiates them from the trans-cisoidal isomer.
The most significant difference in the IR spectra of the three isomers is the presence
of a band at 740 cm-1 in the cis-cisoidal and cis-transoidal isomers. This band
represents the out-of-plane stretching of the C-H bonds and can also be associated
with the cis content in the polymer. The trans-cisoidal isomer exhibits a band at 1265
cm-1 due to the out-of-plane deformation vibrations of the trans-C-H bonds. The band
at 740 cm-1, present in the cis-cisoidal and cis-transoidal isomers, is lacking in the
trans-cisoidal isomer. On the other hand, the band at 1265 cm-1, present in the trans-
105
cisoidal isomer, is lacking in the cis-cisoidal and cis-transoidal isomers. Hence, the
band at 740 cm-1 can be used as a function of the cis content in the polymer and the
band at 1265 cm-1 as a function of the trans content.
The ratio between the bands at 1500 and 1450 cm-1 provides information about the
stereochemistry of the polymer. A polymer with a cis content can be identified by a
ratio of 1 or smaller, together with a strong band at 740 cm-1. A ratio greater than 1
and a band at 1265 cm-1 can be related to a polymer with a trans content. Some
polymers with cis conformation can isomerise to trans conformation, resulting to
ratios greater than 1 and weak bands at 740 cm-1.
1H NMR spectra of the polymer have been used to differentiate between the
respective isomers by looking at differences in the chemical shifts of the aromatic
protons. Apart from these differences, a signal at approximately 5.82 ppm represents
the olefinic proton in the cis-cisoidal and cis-transoidal isomers. The trans-cisoidal
isomer exhibits a smaller or no signal at 5.82 ppm and also displays a broad weak
signal in the region 3-4 ppm due to the aliphatic protons. The area of the signal at
5.82 ppm can be correlared with the intensity of the band at 740 cm-1 to determine
the cis content in the polymer.
4.2 Catalytic polymerization studies
The catalytic polymerization of phenylacetylene was studied with the complexes
[3.1], [3.2] and [3.3] (Table 4.1).
Ph HCatalyst, MeOH
RT, 24 h Ph H
n
Scheme 4.1: Polymerization of phenylacetylene with Rh(I) catalysts
106
Table 4.1: Polymerization of phenylacetylene with Rh(I) complexes.a
Catalyst Mn Mw Mw/Mn cis-Content (%)b
[3.1] 8129 21203 2.6 98.7
[3.2] 8223 21105 2.6 100
[3.3] 8749 22330 2.5 99.4 a Reaction conditions: 0.3 mol % catalyst in MeOH (15 cm3); at RT for 24 h b Calculated according to reference 12 and 13
The aim was to determine the effect, if any, of increasing the length of the alkylene
chain of the bidentate nitrogen-donor ligands on the catalytic activity of the
complexes.
The number average molecular weight Mn, weighted average molecular weight Mw,
polydispersity index (Mw/Mn) and the cis-content of the polymer samples were
determined (Table 4.1). The molecular weights Mw of the polyphenylacetylene
obtained for all the catalyst were similar, but they were approximately 4 times smaller
than the literature values for similar types of complexes (Table 4.1). For example,
[3.1] has been investigated for its catalytic activity in the polymerization of
phenylacetylene and was found to have the highest catalytic activity with a molecular
weight of approximately 86 000.6 However, according to the results in Table 4.1,
[3.3] produced the highest molecular weight polymer. Moreover, since there was not
much of a difference between the molecular weights and the fact that the molecular
weight decreased from [3.1] to [3.2], it was very difficult to conclude on the influence
of increasing the length of the alkylene chain. The lower polydispersity values (~2.5)
that were obtained imply a more uniform distribution of the polymers.6 All the
catalysts that were investigated resulted in polymers with high a cis content (Table
4.1). Complex [3.2] produced a polymer with the highest cis content.
4.2.1 Spectroscopic properties of polymers
As already mentioned, the stereochemistry of the isomers can be deduced from the
physicochemical and spectroscopic properties. All the catalysts that were
investigated produced polymers with similar 1H NMR and IR spectra. All spectra
107
exhibited a signal at δ 5.86 ppm, due to olefinic protons, which was indicative of cis-
isomer polymers (Figure 4.2). Two further signals at δ 6.96 and 6.65 ppm were due
to the aromatic protons. According to the literature, the pattern of the signals
indicates a stereoregular polyphenylacetylene with a predominantly cis-transoidal
structure.14 The insolubility of the polymer in methanol made its separation very
easy.
Figure 4.2: 1H NMR spectrum of PPA, catalyzed by [3.2]
Infrared spectra of the polymers that were prepared exhibited a strong absorption
band at λmax 737 cm-1 and lacked the absorption band at λmax 1265 cm-1 (Figure 4.3).
This information confirmed a cis-isomer polymer and that the polymers produced
were linear.14 The ratio of the infrared absorption band at λmax 1500 and 1450 cm-1
can provide information about the stereochemistry of the polymer. A ratio of 1 or
smaller, together with a band at λmax 740 cm-1 indicates the cis content, while a ratio
greater than 1 and a band at λmax 1265 cm-1 indicates the existence of trans content
of the polymer.
108
Figure 4.3: IR spectrum of PPA prepared using [3.2]
As shown in Figure 4.3, all polymers produced absorption bands at λmax 1488 and
1443 cm-1 (Figure 4.3). Ratios of approximately 1.00 were calculated for these bands
and all were consistent with the cis content of the polymer (Table 4.2).
Table 4.2: Determination of cis-content of polymers
Catalyst Band Ratio cis-Content (%) Polymer
(1488 vs 1443) Colour
[3.1] 1.05 98.7 Yellow
[3.2] 1.03 100 Yellow
[3.3] 1.02 99.4 Yellow
4.2.2 Thermal analysis
The TGA curve showed that the polymer was stable up to 260°C in a nitrogen
atmosphere (Figure 4.4). Decomposition of the polymer continued slowly as the
temperature increased until a residue of 7% remained, at 475°C.
wavenumber (cm-1
)
109
Figure 4.4: TGA and DSC curves of PPA obtained with [3.1]
The DSC thermogram displayed peaks associated with the cis content of the
polymer and no glass transition was observed. Two exothermic peaks at 172 and
250°C were observed. The two peaks corresponded to the cis-trans isomerization
and the crystallization phenomena, respectively.15 An endothermic peak occurred at
300°C which corresponded to thermal decomposition of the polymer.
4.2.3 Mechanistic pathways for polymerization of phenylacetylene
The transition metal catalyzed polymerization of phenylacetylene is widely known to
occur via two main mechanisms: the four-centre acetylene insertion mechanism
(insertion mechanism)16 and metallacycle (or metathesis) mechanism.17 The first
step of the insertion mechanism is the displacement of the cyclooctadiene in the
catalyst precursor by the solvent, leading to the formation of the species
[Rh(NN)(solvent)2]+ in solution. The formation of a hydridoacetylenic species follows
via the oxidative addition of phenylacetylene to the rhodium(I) metal centre. The
coordination is followed by a migratory insertion, resulting in the formation of a vinylic
rhodium intermediate species, consequently resulting in the formation of the polymer
(Scheme 4.1).
110
C C
Ph H
M P
HC CH** C C
Ph H
M P
C
C
H
H*
*
CC
H
M
P
H
CC
H
H**
CC
C
H
M
P
H
CC
H
H
**
CH
H
P = polymer chain
M = Rh
Scheme 4.2: Insertion mechanism for polymerization of phenylacetylene16,18
The metallacycle occurs via the formation of a metal-carbene complex with a vinylic
metallacycle intermediate. A key step is the rearrangement of a monomer unit
formed by the stepwise addition of a second metal-carbene complex. The polymer is
formed by the repetition of the above steps (Scheme 4.2).
C
M
HP
P = polymer chain
M = Rh
HC CH**
C C
M CH
H
*
*P
H
C
M
H
P
C
C
H**
H
HC CH
C C
C C
C
P
H HH
H H
M*
*
Scheme 4.3: Metallacyclic (metathesis) mechanism for the polymerization of
phenylacetylene19
111
An apparent distinction between the two mechanisms is that the insertion
mechanism predicts that two carbons of a monomer unit become doubly bonded to
each other, while the metallacycle mechanism predicts that two carbons of a
monomer unit end up singly bonded to one another in the resulting polymer.20
However, it is generally believed that the polymerization of acetylene catalyzed by
rhodium complexes proceeds via the insertion mechanism, resulting in stereoregular
polymers.
4.3 Hydroformylation of styrene
4.3.1 Introduction
The hydroformylation or “oxo” reaction was discovered by Otto Roelen in 1938,
through modification of the Fischer-Tropsch synthesis to produce aldehydes and
ketones rather than hydrocarbons as the main products.21-23 He observed that
ethylene, H2 and CO were converted into propanal and diethyl ketone (high
pressures) in the presence of Co2(CO)8 as a catalyst. However, aldehydes are the
primary products of the hydroformylation of olefins or alkenes (Scheme 4.3).
RH2, CO
CatalystR
CHO+ R
CHO
linear branched
*
Scheme 4.4: Hydroformylation of olefins
Hydroformylation reactions are very important industrial processes in that most
aldehydes (linear) produced can be reduced to alcohols or oxidized to carboxylic
acids.23 Alcohols are used for the synthesis of phthalate plasticizers by esterification
reaction with phthalic anhydride. In turn, the phthalate plasticizers are used primarily
for polyvinyl chloride plastics. The aldehydes are also used for the production of
detergents, surfactants, solvents, lubricants, cosmetics and other widespread
chemicals.23,24 Branched aldehydes are also very useful for stereoselective and
asymmetric synthesis.24 Thus, hydroformylation reactions have attracted more
attention from both industrial and academic research groups.
112
Until the early 1970s, cobalt carbonyl complexes were the most employed
homogeneous catalysts in the hydroformylation of alkenes.21,23 The dimeric
dicobaltoctacarbonyl complex Co2(CO)8 was believed to be rapidly converted into a
cobalt hydrocarbonyl species HCo(CO)4, by H2/CO under pressure and was the
active species in the hydroformylation of alkenes. However, the regioselectivity of the
HCo(CO)4 towards linear aldehydes depends on the reaction conditions and the
alkene substrate used. Modification of the HCo(CO)4 catalyst by replacing one
carbonyl ligand with a trialkylphosphine PR3 to produce HCo(CO)3(PR3), led to an
improvement in the rate of reaction and regioselectivity.25 The replacement of the CO
ligand with trialkylphosphine causes stronger Co-CO bonding and consequently
decreases the CO partial pressure, thereby stabilizing the catalyst. It also prevents
the formation of Co metal.
The success of rhodium catalysts led to a decrease in the use of cobalt carbonyl
catalysts. The reason was mainly because rhodium catalysts were more catalytically
active than cobalt catalysts. The use of rhodium carbonyl complexes favoured
formation of a higher proportion of linear aldehydes at comparable temperatures.
Hydroformylation of alkenes26 and alkynes27 using
tris(triphenylphosphine)chlororhodium as a catalyst have been achieved by Osborn
et al.26,27 Union Carbide developed a ligand BIPHEPHOS, which in conjunction with
Rh(CO)2(acac), resulted in regioselective formation of linear aldehydes from various
functionalized terminal alkenes under mild conditions (Scheme 4.4). The aldehyde
was then converted to the indolizidine alkaloid. Many rhodium complexes have been
employed as catalysts for hydroformylation reactions and examples can be obtained
in the review article of Clarke.24
113
N
BOC
+ CO + H2
Rh(CO)2(acac)
BIPHEPHOS
60 °C, 5 atm, 83%
N
BOC
CHO
BIPHEPHOS =
MeO OMe
tButBu
O
PO O
P
O
OO
Scheme 4.5: Synthesis of precursor to the indolizidine alkaloid
4.3.2 Catalytic hydroformylation studies The catalytic activity of the rhodium(I) complexes was investigated for the
hydroformylation of styrene. Scheme 4.5 illustrates the possible aldehydes expected
from the hydroformylation of styrene, that is, 2-phenylpropanal (branched) and 3-
phenylpropanal (linear).
+ CO + H2
catalyst
800 psi, 20 h
CHO
CHO
+
2-phenylpropanal 3-phenylpropanal
Scheme 4.6: Hydroformylation of styrene catalyzed by [3.4]-[3.8]
The two products and unreacted styrene were identified and distinguished by 1H
NMR spectroscopy. 2-Phenylpropanal was identified by a high intensity doublet at δ
9.72 ppm due to the CH=O group, a a doublet of quartets at δ 3.67 ppm due to the
α-proton and a high intensity doublet at δ 1.48 ppm due to the methyl group (Figure
4.5). 3-Phenylpropanal exhibited very low intensity signals, a triplet at δ 9.84 ppm
due to the CH=O group and two triplets at δ 2.99 and 2.81 ppm due to the β- and α-
CH2 groups, respectively. Unreacted styrene was identified by a doublet of doublets
at δ 6.76 ppm due to the vinylic CH group and two doublets at δ 5.80 and 5.31 ppm
due to the cis- and trans-protons of the vinylic CH2 group.
114
Figure 4.5: 1H NMR spectrum of the products of hydroformylation of styrene
catalyzed by [3.6]
All complexes [3.4]-[3.8] exhibited excellent catalytic activity and selectivity towards
the hydroformylation of styrene (Table 4.3). Conversions of styrene were
comparable; however, [3.8] resulted in the highest conversion of styrene. All
complexes selectively favoured the formation of the branched aldehyde, 2-
phenylpropanal. It has been shown that the bite angle of chelating ligands have a
direct effect on the regioselectivity of hydroformylation reactions.28 Chelating ligands
with bite angles greater than 90° favour linear over branched aldehydes. Complexes
[3.4]-[3.8] are expected to be in a square planar configuration, and thus would have
bite angles of approximately 90°, which explains why all compounds favoured
branched over linear aldehydes. Complexes [3.7] and [3.8] provided excellent yields
with [3.8] recording the highest yield of all the complexes. Complexes [3.4]-[3.6]
provided excellent conversion of styrene and selectivity towards branched
aldehydes, however, the yields were moderate. The reason was thought to probably
be that the aldehyde formed was further hydrogenated to an alcohol or that the
styrene was hydrogenated to ethylbenzene. In addition, the GC chromatograms of
[3.5] and [3.6] showed an extra peak before 13 minutes and after 17 minutes,
115
respectively. These peaks were not observed in any of the other chromatograms.
However, the side products were not isolated from the reaction mixture.
Table 4.3: Hydroformylation of styrene catalyzed by rhodium(I) complexes
Entry Catalyst Conversionb
(%)
Selectivity
B/L Ratioc
Yield (%)d
2-PP 3-PP
1 [3.4] 90 27 74 2.4
2 [3.5] 92 25 70 2.7
3 [3.6] 89 26 69 1.9
4 [3.7] 95 27 91 1.8
5 [3.8] 99 25 95 1.8 aReaction conditions: cat. (10 mg), toluene (150 cm3), CO/H2 (400/400 psi), alkene/cat. ratio (1000),
RT, 20h. bDetermined by GC. cDetermined by 1H NMR and GC. dDetermined by GC.
Kim and Alper have reported that sterically hindered rhodium catalysts resulted in
low conversions of styrene.29 This effect was due to the bulkiness of the chelating
nitrogen donor ligands in the rhodium catalysts. The presence of the bulky ferrocenyl
substituents on the chelating ligands in [3.4]-[3.8] explains the reason for the lower
conversions, compared to the smaller benzene substituents on the ligands in [3.7]
and [3.8]. In conclusion, [3.7] and [3.8] gave the best results since both recorded
high conversions of styrene and excellent aldehyde yields.
4.3.3 Mechanism for hydroformylation of styrene
The mechanism for the hydroformylation of styrene is proposed here although it was
not investigated in this project. It is well documented that the COD ligand is readily
replaced by the carbonyl ligand even at room temperature.29,30 Therefore, it has
been suggested that in the presence of CO and H2, the COD in the rhodium catalyst
is replaced by CO, resulting in the pentacoordinated rhodium species
RhH(NN)(CO)2. The RhH(NN)(CO)2 is believed to be active in the hydroformylation
reaction. Scheme 4.6 illustrates the possible reaction pathways for the
hydroformylation of styrene.
116
N
N
Rh CO
H2
N
N
Rh
CO
CO
H
Ph
N
N
Rh
CO
CO
H PhN
N
Rh
CO
CO
Ph
N
N
Rh
CO
CO
Ph
less favoured
highly favoured
N
N
Rh
CO
Ph
C
O
H2
N
N
Rh
CO
C
O
HH
Ph
+
Ph
CHO
N
N
Rh
CO
H
CO
Scheme 4.7: Possible mechanism for hydroformylation of styrene catalyzed by [3.4]-
[3.8]
The mechanism is also thought to occur in an analogous manner as the one
proposed by Wilkinson.30 The initial step involves the addition of styrene to the
RhH(NN)(CO)2 species, followed by the insertion of styrene resulting in the rhodium
alkyl complex that undergoes migratory insertion of CO to form the rhodium acyl
complex. This step is followed by the oxidative addition of H2 to give the dihydrido
acyl rhodium complex. This step is rate determining and is the only one that results
in the change in oxidation state of the rhodium. The final step involves the reductive
elimination of the product and the reformation of the active rhodium hydride species.
117
4.4 Experimental
4.4.1 Purification procedures
All solvents used were purified and dried as in the previous Chapters.
Phenylacetylene and styrene were obtained from the Sigma Aldrich Chemical
Company, Milwaukee, USA. The synthetic gases H2 and CO were obtained locally
from Afrox.
4.4.2 Instumentation
Gel Permeation Chromatography was conducted at the Stellenbosch University with
a Waters 717 autosampler, a Waters 619 flow unit, a Waters 410 difractometer, a
Waters 600E system controller, a Waters 515 HPLC pump, a Valveco 8-port switch
for high and low temperature HPLC applications and a Wyatt technology laser
photometer. The programme was controlled using a MilleniumTM software.
Gas Chromatography was conducted on a Focus GC Thermo Finnigan instrument
(Model no: AI 3000) equipped with a flame ionization detector (FID) and a DB 1701
column (film thickness 0.25 µm, internal diameter 0.25 mm, length 30 m). The Delta
Chromatography software was used for recording and integration of chromatograms.
4.4.3 Polymerization of phenylacetylene6
Polymerization of phenylacetylene was carried out in a 2-necked round-bottomed
flask (25 cm3) under an argon atmosphere using Schlenk techniques. The catalyst
(0.02 mmol) was added to a solution of phenylacetylene (0.55 g, 5.44 mmol) in
methanol (15 cm3). The reaction mixture was stirred at RT for 24 h. The yellow
precipitates which formed were filtered off, washed with methanol and then dried
under the vacuum. The polymers were analysed by NMR and IR spectroscopy,
thermal analysis and gel permeation chromatography.
118
4.4.4 Hydroformylation of styrene29
All hydroformylation reactions were conducted in a 2 L Parr stirred reactor. The
reactor was charged with an appropriate amount of styrene (1000 equiv.), catalyst
(10 mg) and toluene (150 ml). The reactor was flushed three times with CO and
pressurized to 400 psi. After attaching and purging of the H2 line, the reactor was
pressurized to 800 psi. After stirring the reaction for 16 h, the excess CO and H2
were released. The reaction mixture was analysed with 1H NMR and GC. Similar
conditions were used for hydroformylation reactions 4.4.4.1-4.4.4.5.
4.4.4.1 Hydroformylation of styrene (1381 mg, 13.26 mmol) with [3.4] (10 mg,
0.01326 mmol). Conversion: 90%.
4.4.4.2 Hydroformylation of styrene (1356 mg, 13.02 mmol) with [3.5] (10 mg,
0.01302 mmol). Conversion: 92%.
4.4.4.3 Hydroformylation of styrene (1332 mg, 12.79 mmol), with [3.6] (10 mg,
0.01279 mmol). Conversion: 89%.
4.4.4.4 Hydroformylation of styrene (1753 mg, 16.83 mmol) with [3.7] (10 mg,
0.01683 mmol). Conversion: 92%.
4.4.4.5 Hydroformylation of styrene (1674 mg, 16.07 mmol) with [3.8] (10 mg,
0.01607 mmol). Conversion: 95%.
4.5 References
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(2000) 4437.
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119
6. S. –I. Lee, S. –C. Shim and T. –J. Kim, J. Polym. Sci. A: Polymer Chemistry,
34 (1996) 2377.
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8. Y. Goldberg and H. Alper, Chem. Commun., (1994) 1209.
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690 (2005) 5728.
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17. T. Masuda and T. Higashimura, Adv. Polym. Sci., 81 (1986) 121.
18. G. Wegner, Angew. Chem. Intl. Ed. Engl., 20 (1981) 361.
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20. C. S. Yannoni and R. D. Kendrick, J. Chem. Phys., 74 (1981) 747.
21. M. M. T. Khan and A. E. Martell, Homogeneous Catalysis by Metal
Complexes, Academic Press, Inc., New York, 1974.
22. J. Tsuji, Transition Metal Reagents and Catalysis: Innovations in Organic
Synthesis, John Wiley & Sons, LTD, New York, 2000.
23. http:/ chemistry.Isu.edu/Stanley/webpub/4571-Notes/Chap16-
hydroformylation.doc.
24. M. L. Clarke, Current Organic Chemistry, 9 (2005) 701.
25. L. H. Slaugh and D. Mullineaux, J. Organomet. Chem., 13 (1968) 469.
26. J. A. Osborn, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chem. Soc. A.
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120
28. C. P. Casey, G. T. Whiteker, M. G. Melville, L. M. Petrovich, J. A. Gavney, Jr.
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121
CHAPTER 5
CONCLUSION
5.1 Conclusion
Bisferrocenylimines and arylbisimines were successfully prepared under solvent-free
conditions and were fully characterized. The solvent-free reactions carried more
advantages than the solvent reactions, and these included shorter reaction times,
better yields and required no heat. The reduction of the above compounds was
achieved by lithium aluminium hydride (LAH).
Cationic rhodium(I) complexes containing bisferrocenylimines were successfully
prepared and fully characterized. Suitable single crystals grown allowed the
determination of the structures of [3.2] and [3.3] using X-ray crystallography. The
successful synthesis of cationic rhodium(I) complexes containing
bisferrocenylamines and arylbisamines was also achieved. Characterization was
only possible with IR and UV-vis spectroscopy, cyclic voltammetry and
conductometry. For example, UV-vis spectra of [3.1], [3.2] and [3.3] showed an
extra band at λmax 350, 382 and 383 nm, respectively compared to the corresponding
free ligands [2.1], 2.2], and [2.3]. The bands that also appeared in the free ligands
were shifted to higher wavelengths. Similarly, [3.4], [3.5], [3.6], [3.7] and [3.8]
exhibited bands at λmax 385, 388, 387, 382 and 381 nm. All these bands were
attributed to coordination of the ligands to the rhodium(I) ion.
Complexes [3.1], [3.2] and [3.3] were catalytically active in the polymerization of
phenylacetylene. The molecular weights of the polymers were very low, with Mw =
22330 being the highest molecular weight recorded for [3.3]. All polymers produced
with the catalysts were in the cis-transoidal configuration. Complexes [3.4]-[3.8]
were catalytically active in the hydroformylation of styrene. All complexes favoured
the formation of the branched (iso) aldehyde 2-phenylpropanal. Complexes [3.7] and
[3.8] were most active catalysts with the highest conversions of styrene.