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Transition Metal Organometallics

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Transition-metal
Organometallics
Peter H.M. Budzelaar
Transition metals are never on time
Early
Late
Middle
2
Transition-metal Organometallics
Early Transition Metals
Groups 3,4
•
•
•
•
Strongly electrophilic and oxophilic
Few redox reactions (exception: Ti)
Nearly always < 18e
Polar and very reactive M-C bonds
(to alkyl and aryl)
• Few d-electrons:
– preference for "hard" s-donors (N/O/F)
– weak complexation of p-acceptors (olefins, phosphines)
• Typical catalysis: Polymerisation
Me
M
Me
M
M
etc
Me
3
Transition-metal Organometallics
"Middle" Transition Metals
Groups 5-7
•
•
•
•
•
•
4
Many accessible oxidation states
Mostly 18e
Ligands strongly bound
Strong, not very reactive M-C bonds
Preference for s-donor/p-acceptor combinations (CO!)
Typical catalysis: Alkene and alkyne metathesis
CH2
CH2
M
CH2
CH2
M
CH2
CH2
M
CH2
CH2
CH2
Transition-metal Organometallics
Late Transition Metals
Groups 8-10 (and 11)
• Many accessible oxidation states
• Mostly 18e or 16e
16e common for square-planar complexes
•
•
•
•
•
Easy ligand association/dissociation
Weak, not very reactive M-C bonds
Even weaker, reactive M-O/M-N bonds
Preference for s-donor/weak p-acceptor ligands (phosphines)
Typical catalysis: Hydroformylation
H
H
M
M
M
O
CO
CO
M
O
O
O
H
M
H
5
H2
M
H
M
H2
Transition-metal Organometallics
Front- and Back-benchers
1st row
2nd row
3rd row
6
Transition-metal Organometallics
Going down...
1st row:
• often unpaired electrons
• different spin states (HS/LS) accessible
• "highest possible" oxidation states not very stable
– MnO4- is a strong oxidant
2nd/3rd row:
• nearly always "closed shell"
• virtually same atomic radii (except Y/La)
• highest oxidation states fairly stable
– ReO4- is hardly oxidizing
• 2nd row often more reactive than 3rd
7
Transition-metal Organometallics
M-H and M-C s-bonds
M
H
H ydride
пѓњ
M
C
A lkyl
пѓњ
C
M
C
M
C
M
8
V inyl (alkenyl)
C
A cetylid e (alkyn yl)
A ryl
пѓњ
Transition-metal Organometallics
Synthesis of metal alkyls
• Metathesis
TiC l 4 + 4 B zM gC l
TiB z 4 + 4 M gC l 2
(B z = benzyl, C 6 H 5 C H 2 )
• Electrophilic attack on metal
M n (C O ) 5
M eI
• Insertion
M e M n (C O ) 5
Ar
Ar
N
N
N
Co H
N
N
Co Et
N
Ar
9
C 2H 4
Ar
Transition-metal Organometallics
Synthesis of metal alkyls
• Oxidative addition
– often starts with electrophilic attack
Me
O
O
O
O
L
Rh
L
Rh
L = P (O P h ) 3
L
I
M eI
L
L
O
O
I
Rh
L = PPh3
Me
L
I
10
Transition-metal Organometallics
Decomposition of metal alkyls
Dominant: b-hydrogen elimination
M
M
H
M
H
H
M
H
Alternatives:
• homolysis
• a/g/d-eliminations
• reductive elimination (especially with H or another alkyl)
• ligand metallation
11
Transition-metal Organometallics
How to prevent b-hydrogen elimination ?
• No b-hydrogen
CH3, CH2CMe3, CH2SiMe3, CH2Ph
• No empty site cis to alkyl
Et
H
O
N
N
Co
N
N
O
H
O
O H2
• Product of elimination unstable
?
12
Transition-metal Organometallics
How to prevent b-hydrogen elimination ?
• Planar transition state inaccessible
H
H
L 2P t
L 2P t H
L 2P t
???
even for 5-membered metallacycles b-elimination is difficult !
(basis of selective ethene trimerization)
13
Transition-metal Organometallics
Reactions of metal alkyls
• Insertion, of both polar and non-polar C=X bonds:
– olefins, acetylenes, allenes, dienes
– (ketones etc)
– CO, isocyanides
• Reductive elimination
14
Transition-metal Organometallics
Synthesis of metal hydrides
• Metathesis
W C l 6 + LiBEt 3 H + PR 3
•
W H 6 (PR 3 ) 3
b-elimination
M
M
M
H
H
H
• Protonation / oxidative addition
H
L nM
L nM
HX
X
L nM
15
L nM
+
H
X-
H2
H
L nM
L nM
H2
H
Transition-metal Organometallics
Structure of WH6(PiPr2Ph)3
16
Transition-metal Organometallics
Synthesis of metal hydrides
• Hydrogenolysis
Ar
Ar
N
N
H2
N
Co Et
N
17
N
Co H
N
Transition-metal Organometallics
Reactivity of metal hydrides
• "Hydride is the smallest alkyl"
• Can react as H+ or H– HCo(CO)4 nearly as acidic as H2SO4
– Cp2ScH gives H2 with acids, alcohols, ...
• Insertion reactions
– CO insertion rare (endothermic!)
18
Transition-metal Organometallics
Metal aryls
Usually much more stable than alkyls
H
H
M
M
+
???
Synthesis:
• Metathesis
• Oxidative addition
Reaction/decomposition:
• Reductive elimination
19
Transition-metal Organometallics
The other ligands...
Common ligands for transition metals:
p ligands, CO, phosphines
CO
PR3
General characteristic: s-donating, p-accepting, "soft"
20
Transition-metal Organometallics
p ligands, CO, phosphines
s-donor character:
phosphines > alkenes, CO
p-acceptor character:
CO > alkenes > phosphines
Depends strongly on the substituents on P, C=C !
21
Transition-metal Organometallics
Donor and acceptor orbitals
s-donor
alkene
p-acceptor
p
p*
CO
LP
p*
phosphine
LP
22
s*
Transition-metal Organometallics
p ligands, CO, phosphines
• CO is one of the best p-acceptors (p-acids)
– isocyanides are stronger donors, weaker acceptors
• PMe3 very weak p-acceptor, good s-donor
• PF3 nearly as strongly p-acidic as CO
• C2H4 weak p-acceptor
• C2(CN)4 very strong p-acceptor
– even forms stable radical anions
23
Transition-metal Organometallics
Synthesis of CO and p-ligand complexes
Stable, neutral ligands:
generate empty site(s) in presence of free ligand
C r(C O ) 6
re flu x in
C 6H 6
C r(C O ) 3 (C 6 H 6 )
1.13
1.37
1.417
1.410
2.223
1.841
1.913
1.141
24
2.141
1.157
Transition-metal Organometallics
Synthesis of CO and p-ligand complexes
Variation:
reductive synthesis
+
C rC l 3
N iS O 4
25
A l/A lC l 3
Cr
+ e-
Cr
C 6H 6
S 2 O 4 2CO
N i(C O ) 4
Transition-metal Organometallics
Synthesis of CO and p-ligand complexes
Anionic ligands:
introduce via metathesis (Cp-), substitution
or oxidative addition (allyl)
F eC l 2
C pN a
Fe
I
F e(C O ) 5
Fe
OC
OC
26
I
CO
Transition-metal Organometallics
Synthesis of CO and p-ligand complexes
Cl
M n (C O ) 5
-
M n (C O ) 5
пЃ„
or hпЃ®
27
M n (C O ) 4
Transition-metal Organometallics
Modification of p ligands
by H+/H- addition/abstraction
+
M
H+
- H+
acid
b ase
M
-
M
28
H- H-
M
H - d o n o r: N aB H 4 , L iB H E t 3
H - ab stracto r: P h 3 C + B F 4 -
Transition-metal Organometallics
Reactivity of p-ligand complexes
Ligand activated for nucleophilic attack
both internal and external
M
X-
M
X
M
d-
C
d+
O
d+
M
C
d-
O
X
X
-
M
O
29
Transition-metal Organometallics
Reactivity of p-ligand complexes
Change of hapticity
30
M
M
s /p allyl
M
M
ring slippage
Transition-metal Organometallics
More than 18 e ?
1.47
1.40
1.51
2.33
2.45
1.95
1.15
1.22
1.99
2.97
2.30-2.32
2.46
1.40
2.35
2.32
1.41
1.41-1.44
1.40
31
Transition-metal Organometallics
s complexes
A s bond as 2-electron donor for a metal.
• H2 complexes (non-classical hydrides)
CO
OC
CO
Cr
OC
CO
hпЃ®
H2
Usually intramolecular
Cr
H
CO
OC
CO
+
L
N
Rh
Pt
L
H
OC
CO
• C-H bonds
CO
H
N
H
Sometimes intermolecular
H
CO
OC
Cr
OC
CH4
CO
In "matrix"
Characterized by IR
CO
32
Transition-metal Organometallics
s complex ?
2.10
N
1.92
N
Rh
H
H
33
Transition-metal Organometallics
s complex ?
+
L
L
Pt
H
34
Transition-metal Organometallics
s complexes
• C-Si bonds
H
La
THF
H
M e 3S i
M e 3S i M e
Si
S iM e 3
Me
2.63
3.22
Me
103В°
35
Transition-metal Organometallics
s complexes
• Si-H bonds
Mn
OC
OC
S iH P h 2
??
H
• etc: B-H, Sn-Cl, P-H, ..
36
Transition-metal Organometallics
s complexes
A s complex is an (arrested) intermediate for oxidative addition:
M
XY
M
M
Y
M (m )
n-e
X
X
Y
M (m )
M (m + 2)
(n+ 2)-e
(n+ 2)-e
Stable s complexes are formed when the metal is not
p-basic enough to enable completion of the addition.
37
Transition-metal Organometallics
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