transition-metal organometallics peter h.m. budzelaar
TRANSCRIPT
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" -donors (N/O/F)– weak complexation of -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 -donor/-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 -donor/weak -acceptor ligands (phosphines)• Typical catalysis: Hydroformylation
HM M
HM
COCO
M
M
O
MH2
O
M
H
O
H
H
O
H2
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 -bonds
M H
M C
M C
C
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
Ar
EtN
N
N
Co
Ar
Ar
H
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: -hydrogen elimination
Alternatives:• homolysis• //-eliminations• reductive elimination (especially with H or another alkyl)• ligand metallation
MH
MH
MH
MH
Transition-metal Organometallics12
How to prevent -hydrogen elimination ?
• No -hydrogenCH3, CH2CMe3, CH2SiMe3, CH2Ph
• No empty site cis to alkyl
• Product of elimination unstable
OH2
Co
ON
NOH
NN
OH
Et
?
Transition-metal Organometallics13
How to prevent -hydrogen elimination ?
• Planar transition state inaccessible
even for 5-membered metallacycles -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
• -elimination
• Protonation / oxidative addition
WCl6 + LiBEt3H + PR3 WH6(PR3)3
MH
MH
MH
LnMHX
LnMH
X
X-+LnM H
LnMH2
LnMH
HLnM H2
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
H
H
M +
???
Transition-metal Organometallics20
The other ligands...
Common ligands for transition metals: ligands, CO, phosphines
General characteristic: -donating, -accepting, "soft"
CO
PR3
Transition-metal Organometallics21
ligands, CO, phosphines
-donor character:phosphines > alkenes, CO
-acceptor character:CO > alkenes > phosphines
Depends strongly on the substituents on P, C=C !
Transition-metal Organometallics22
Donor and acceptor orbitals
-donor -acceptor
phosphine
CO
alkene *
LP*
LP *
Transition-metal Organometallics23
ligands, CO, phosphines
• CO is one of the best -acceptors (-acids)– isocyanides are stronger donors, weaker acceptors
• PMe3 very weak -acceptor, good -donor
• PF3 nearly as strongly -acidic as CO
• C2H4 weak -acceptor
• C2(CN)4 very strong -acceptor– even forms stable radical anions
Transition-metal Organometallics24
Synthesis of CO and -ligand complexes
Stable, neutral ligands:generate empty site(s) in presence of free ligand
Cr(CO)6reflux in
C6H6
Cr(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 -ligand complexes
Variation:reductive synthesis
NiSO4S2O4
2-
CONi(CO)4
CrCl3Al/AlCl3C6H6
Cr
+
Cr+ e-
Transition-metal Organometallics26
Synthesis of CO and -ligand complexes
Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl)
FeCl2CpNa
Fe
Fe(CO)5
I
Fe
OCOC COI
Transition-metal Organometallics27
Synthesis of CO and -ligand complexes
Mn(CO)5-
ClMn(CO)5
Mn(CO)4
or h
Transition-metal Organometallics28
Modification of 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 -ligand complexes
Ligand activated for nucleophilic attackboth internal and external
MMX-
M C O+ -
C O- + X-
-
M
X
O
M X-
Transition-metal Organometallics30
Reactivity of -ligand complexes
Change of hapticity
ring slippageM
/ allylMM
M
Transition-metal Organometallics31
More than 18 e ?
1.22
1.99
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
complexes
A 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
HhH2
N
Rh
N
H H
LPt
L H
+
CO
Cr
CO
OC CH4
COOCIn "matrix"Characterized by IR
Transition-metal Organometallics35
complexes
• C-Si bonds
3.22
2.63
103°
La
Me3Si
Me3SiH
Si
SiMe3H
Me MeMe
THF
Transition-metal Organometallics36
complexes
• Si-H bonds
• etc: B-H, Sn-Cl, P-H, ..
Mn
OCOC SiHPh2
H??