3 - overview
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Organometallic Chemistry
an overview reactions
Peter H.M. Budzelaar
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Overview of Organometallic Chemistry2
Between organic and inorganic...
Organic chemistry:
more or less covalent C-X bonds
rigid element environments
fixed oxidation states (better: valencies)
??Organometallic chemistry??
Inorganic chemistry:
primarily ionic M-X bonds, dative M-L bonds variable and often fluxional environments
variable oxidation states
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Overview of Organometallic Chemistry3
Organometallic reactivity
Since most organometallics are intermediates, the focus in
organometallic chemistry is usually on understanding and tuning
reactivity
This starts with analyzing reaction mechanisms in terms ofelementary steps
The number of possible elementary steps is larger than in
"pure organic" chemistry, but the ideas are similar
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Overview of Organometallic Chemistry4
Organometallic structures
Knowledge of inorganic and coordination chemistry is useful to
understand geometries, electron counts and oxidation states of
organometallic compounds
Organometallics are more covalent and often less symmetricthan coordination compounds, so orbital symmetry arguments
are not as important
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Overview of Organometallic Chemistry5
Trends in organometallic chemistry
Organometallic chemistry is concerned with allmetals, in
combination with all"organic" elements.
there are manymetals !
Generalization is importantthe chemistry of e.g. Fe is not much more complicated
than that of C, but after that there are 80 more metals...
we divide reactions in broader categories than organic chemists do
We concentrate on the M side of the M-C bond,and on how to tune its reactivity
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Overview of Organometallic Chemistry6
Elements of interest
Organic elements
Main group metals
Transition metals
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Organic vs organometallic reactivity
Organic chemistry:
C-C / C-H : nearly covalent
Cd+-Xd- : polar (partly ionic)
reactivity dominated by nucleophilic attack at C
SN2 and SN1 like reactivity
Organometallic chemistry:
C is the negative end of the M-C bond ("umpolung")
reactivity dominated by electrophilic attack at C
or nucleophilic attack at M
associative and dissociative substitution at M
X
SN2 TS (10-e ?)
Y
- X Y
- XY
X
Y
SN1 int (6-e)
(C2H4)2PdCl2 (C2H4)(CO)PdCl2
(C2H4)PdCl2
(C2H4)2(CO)PdCl2
?
CO- C2H4
- C2H4CO
dissociative
associative
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Main-group organometallics
s andp orbitals.
8-e rule, usually.
with a lotof exceptions
More electropositive and larger:
higher coordination numbers,
regardless of the number of electrons.
Early" groups and not very electropositive:
lower coordination numbers.
AlMe
Al
MeMe
MeMe
Me
N
Cl
Sn
O
MeCl Me
Me Zn Me
8-e
10-e
4-e
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Main-group organometallics
Metal is the "d+" side of the M-C bond.
Chemistry dominated by nucleophilic attack of Cd- at electrophiles.
this is also the main application in organic synthesis
MeMgBr + Me2CO Me3COMgBr
note: this is a simplifiedpicture
of the Grignard reaction
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Main-group organometallics
M-M multiple bonds are fairly weak and rather reactive
they are a curiosity and relatively unimportant,
certainly compared to C-C multiple bonds
Bond strengths in kcal/mol:
C-C 85 C=C 150
N-N 40 N=N 100
P-P 50 P=P 75
Multiple-bonded compounds often have unusual geometries
Me4C2
"Me4
Sn2
"
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Transition-metal organometallics
s,p and dorbitals
18-e rule, sometimes 16-eother counts relatively rare
CO
Cr
CO
OC CO
COOCFe
Ni
18-e
18-e
18-e
PtEt3P Me
ClEt3P
16-e
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Transition-metal organometallics
Lower electron counts if metals are sterically saturated:
nor
Co
nornor
nor
nor =
13-e
12-e
TiMe
Me
16-e
Me
WMe
Me
MeMeMe
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Overview of Organometallic Chemistry13
Transition-metal organometallics
Often ligands capable of donating 2-8 electrons
Preference forp-system ligands (good overlap with d-orbitals)
Bonding to neutral ligands (olefin/diene/CO/phosphine)
relatively weak
Important for catalysis!
allyl
3 e
cyclopentadieny
5 e
O
C
2 e
C
carbene
2 e
olefin
2 e
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Overview of Organometallic Chemistry14
Transition-metal organometallics
An olefin complex: (Acac)Ir(NCMe)(C2H4)
donation
backdonation
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Overview of Organometallic Chemistry15
Transition-metal organometallics
Forbidden reactions ?
+?
Symmetry-forbiddenDoes not happen
Mo Mo+
No symmetryVery fast
a
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Reactivity of the M-C bond
Polar reactive towards e.g.
Water:
Me3Al explodes with water; Me4Sn does not react.
Oxygen:
Me2Zn inflames in air; Me4Ge does not react.
Carbonyl groups:
MeLi adds at -80C, Me3Sb not even at +50C.
M-OH + H-C
M-O-O-C
M-O-C C
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Overview of Organometallic Chemistry17
Reactivity of the M-C bond
Oxidation and hydrolysis: large driving force
Bond strengths in kcal/mol:
Al-C 65 As-C 55 Si-C 74
Al-O 119 As-O 72 Si-O 108Al-Cl 100 Si-Cl 91
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Organometallic reaction steps
Ligand dissociation / coordination
Me3Al + NMe3 Me3Al-NMe3
6e 8e
Mo(CO)6 Mo(CO)5 + CO
18e 16e
note: free Me3Al
dimerizes to Me6Al2
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Overview of Organometallic Chemistry19
Organometallic reaction steps
Insertion and b-elimination
MeMgBr + MeC N Me2C=NMgBr
Fe
N
NN Me Fe
N
NN Me Fe
N
NN
Me
N
N NFeAr Ar
+
Ar =
FeN
NN Me =
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Organometallic reaction steps
Insertion and b-eliminationH
PdP
PPdP
P
H
Ph2P PPh2=PP
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Organometallic reaction steps
Oxidative addition / Reductive elimination
PRh
P
Cl
P
PRh
P
H
P
H
Cl
H2"P" = PPh3
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Organometallic reaction steps
Oxidative addition / Reductive elimination
Rh
L
O
O
L
RhL
O
O
L
Me
I
L = P(OPh)3
RhMe
O
O
I
L
L
L = PPh3
MeI
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Organometallic reaction steps
s-bond metathesis
14 e
Lu CH3 Lu *CH3*CH4
Lu
C
*CH
H3
H3
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Organometallic reaction steps
Redox reactions
Homolysis
FeFe- e-
Et Hg Et Et Hg Et+
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Organometallic reaction steps
Reactivity of coordinated ligands
F t i t t d ti it
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Factors governing structure and reactivityof organometallic compounds
M-C, M-X bond strengths
Electronegativity of M (polarity of M-C etc bonds)
Number of (d) electrons
Coordination number
Steric hindrance
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Trends in the periodic table
Main group metals:
left and down: more electropositive
down: higher oxidation states less stable
Transition metals: middle: strongest preference for 18-e
2nd and 3rd row: strong preference for
paired electrons (low-spin states)
down: higher oxidation states more stable
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Working with organometallics
Synthesis and reactivity studies (inert atmosphere!):
Glove box
Schlenk line, specialized glassware
Characterization: Xray diffraction structure bonding
NMR structure en dynamic behaviour
(calculations)
IR
MS
EPR
Not:
GC
LC
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