11 - transitionmetal
DESCRIPTION
TransitionMetalTRANSCRIPT
Transition-metal Organometallics
Peter H.M. Budzelaar
Transition-metal Organometallics2
Transition metals are never on time
Early
Middle
Late
Transition-metal Organometallics3
Early Transition MetalsGroups 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" σ-donors (N/O/F)– weak complexation of π-acceptors (olefins, phosphines)
• Typical catalysis: Polymerisation
MeM M
Me
MeM etc
Transition-metal Organometallics4
"Middle" Transition MetalsGroups 5-7
• Many accessible oxidation states• Mostly 18e• Ligands strongly bound• Strong, not very reactive M-C bonds• Preference for σ-donor/π-acceptor combinations (CO!)• Typical catalysis: Alkene and alkyne metathesis
M CH2
CH2 CH2
MCH2
CH2
CH2
CH2 CH2
M CH2
Transition-metal Organometallics5
Late Transition MetalsGroups 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 σ-donor/weak π-acceptor ligands (phosphines)• Typical catalysis: Hydroformylation
HM M
HM CO
COM
M
O
MH2
O
MH
O
HH
OH2
Transition-metal Organometallics6
Front- and Back-benchers
1 rowst
2 rownd
3 rowrd
Transition-metal Organometallics7
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
Transition-metal Organometallics8
M-H and M-C σ-bonds
M H
M C
M CC
M C C
M
Hydride
Alkyl
Vinyl (alkenyl)
Acetylide (alkynyl)
Aryl
⇐
⇐
⇐
Transition-metal Organometallics9
Synthesis of metal alkyls
• Metathesis
• Electrophilic attack on metal
• Insertion
TiCl4 + 4 BzMgCl TiBz4 + 4 MgCl2(Bz = benzyl, C6H5CH2)
Mn(CO)5MeI
MeMn(CO)5
C2H4N
N
N
Co
Ar
Et
Ar
N
N
N
Co
Ar
H
Ar
Transition-metal Organometallics10
Synthesis of metal alkyls
• Oxidative addition– often starts with electrophilic attack
L
O
OL
RhL
O
OL
Me
I
L = P(OPh)3
RhMe
O
OI
L
L
L = PPh3
MeIRh
I
Transition-metal Organometallics11
Decomposition of metal alkyls
Dominant: β-hydrogen elimination
Alternatives:• homolysis• α/γ/δ-eliminations• reductive elimination (especially with H or another alkyl)• ligand metallation
MH
MH
MH
MH
Transition-metal Organometallics12
How to prevent β-hydrogen elimination ?
• No β-hydrogenCH3, CH2CMe3, CH2SiMe3, CH2Ph
• No empty site cis to alkyl
• Product of elimination unstable
OH2
Co
ON N
OH
N NO
HEt
?
Transition-metal Organometallics13
How to prevent β-hydrogen elimination ?
• Planar transition state inaccessible
even for 5-membered metallacycles β-elimination is difficult !(basis of selective ethene trimerization)
L2PtH
L2Pt H L2PtH
???
Transition-metal Organometallics14
Reactions of metal alkyls
• Insertion, of both polar and non-polar C=X bonds:– olefins, acetylenes, allenes, dienes– (ketones etc)– CO, isocyanides
• Reductive elimination
Transition-metal Organometallics15
Synthesis of metal hydrides
• Metathesis
• β-elimination
• Protonation / oxidative addition
WCl6 + LiBEt3H + PR3 WH6(PR3)3
MH
MH
MH
LnM HXLnM
H
X
X-+LnM H
LnMH2 LnM
H
HLnM H2
Transition-metal Organometallics16
Structure of WH6(PiPr2Ph)3
Transition-metal Organometallics17
Synthesis of metal hydrides
• Hydrogenolysis
H2N
N
N
Co
Ar
HN
N
N
Co
Ar
Et
Transition-metal Organometallics18
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!)
Transition-metal Organometallics19
Metal aryls
Usually much more stable than alkyls
Synthesis:• Metathesis• Oxidative addition
Reaction/decomposition:• Reductive elimination
M
HH
M +???
Transition-metal Organometallics20
The other ligands...
Common ligands for transition metals:π ligands, CO, phosphines
General characteristic: σ-donating, π-accepting, "soft"
CO
PR3
Transition-metal Organometallics21
π ligands, CO, phosphines
σ-donor character:phosphines > alkenes, CO
π-acceptor character:CO > alkenes > phosphines
Depends strongly on the substituents on P, C=C !
Transition-metal Organometallics22
Donor and acceptor orbitals
σ-donor π-acceptor
phosphine
CO
alkeneπ π
LP
LP
*
π*
σ*
Transition-metal Organometallics23
π ligands, CO, phosphines
• CO is one of the best π-acceptors (π-acids)– isocyanides are stronger donors, weaker acceptors
• PMe3 very weak π-acceptor, good σ-donor• PF3 nearly as strongly π-acidic as CO
• C2H4 weak π-acceptor• C2(CN)4 very strong π-acceptor
– even forms stable radical anions
Transition-metal Organometallics24
Synthesis of CO and π-ligand complexes
Stable, neutral ligands:generate empty site(s) in presence of free ligand
Cr(CO)6reflux in
C6H6Cr(CO)3(C6H6)
1.417
2.141
1.410
2.223
1.841
1.1571.141
1.913
1.37 1.13
Transition-metal Organometallics25
Synthesis of CO and π-ligand complexes
Variation:reductive synthesis
NiSO4S2O4
2-
CONi(CO)4
CrCl3Al/AlCl3C6H6
Cr
+
Cr + e-
Transition-metal Organometallics26
Synthesis of CO and π-ligand complexes
Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl)
FeCl2CpNa
Fe
Fe(CO)5
IFe
OCOC COI
Transition-metal Organometallics27
Synthesis of CO and π-ligand complexes
Mn(CO)5-
ClMn(CO)5
Mn(CO)4Δ
or hν
Transition-metal Organometallics28
Modification of π ligandsby H+/H- addition/abstraction
M
M
H+
- H+
- H-
H-
M
+
M
-
acidbase
H- donor: NaBH4, LiBHEt3H- abstractor: Ph3C+ BF4
-
Transition-metal Organometallics29
Reactivity of π-ligand complexes
Ligand activated for nucleophilic attackboth internal and external
MM X-
M C Oδ+ δ-
C Oδ- δ+ X-
-
MX
O
M X-
Transition-metal Organometallics30
Reactivity of π-ligand complexes
Change of hapticity
ring slippageM
σ/π allylMM
M
Transition-metal Organometallics31
More than 18 e ?
1.221.99
2.332.45
2.97
1.401.51
1.47
1.40
1.41
1.402.32
2.46
2.35
1.41-1.44
1.151.95
2.30-2.32
Transition-metal Organometallics32
σ complexes
A σ bond as 2-electron donor for a metal.• H2 complexes (non-classical hydrides)
• C-H bondsUsually intramolecular
Sometimes intermolecular
CO
Cr
CO
OC CO
COOC
CO
Cr
CO
OC
COOC
H
HhνH2
NRh
N
H H
LPt
L H
+
CO
Cr
CO
OC CH4
COOCIn "matrix"Characterized by IR
Transition-metal Organometallics33
σ complex ?
2.10 1.92N
RhN
H
H
Transition-metal Organometallics34
σ complex ?
LPt
L
H
+
Transition-metal Organometallics35
σ complexes
• C-Si bonds
3.22
2.63
103°
La
Me3SiMe3Si
H
Si
SiMe3H
Me MeMe
THF
Transition-metal Organometallics36
σ complexes
• Si-H bonds
• etc: B-H, Sn-Cl, P-H, ..
Mn
OCOC SiHPh2
H??
Transition-metal Organometallics37
σ complexes
A σ complex is an (arrested) intermediate for oxidative addition:
Stable σ complexes are formed when the metal is notπ-basic enough to enable completion of the addition.
MX
Y
XYM MX
Y
M(m) M(m+2)M(m)
n-e (n+2)-e (n+2)-e