11 - transitionmetal

Transition-metal Organometallics

Peter H.M. Budzelaar

Transition-metal Organometallics2

Transition metals are never on time

Early

Middle

Late


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


"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


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


Front- and Back-benchers

1 rowst

2 rownd

3 rowrd


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


M-H and M-C σ-bonds

M H

M C

M CC

M C C

M

Hydride

Alkyl

Vinyl (alkenyl)

Acetylide (alkynyl)

Aryl

⇐

⇐

⇐


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


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


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


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

?


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

???


Reactions of metal alkyls

• Insertion, of both polar and non-polar C=X bonds:– olefins, acetylenes, allenes, dienes– (ketones etc)– CO, isocyanides

• Reductive elimination


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


Structure of WH6(PiPr2Ph)3


Synthesis of metal hydrides

• Hydrogenolysis

H2N

N

N

Co

Ar

HN

N

N

Co

Ar

Et


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!)


Metal aryls

Usually much more stable than alkyls

Synthesis:• Metathesis• Oxidative addition

Reaction/decomposition:• Reductive elimination

M

HH

M +???


The other ligands...

Common ligands for transition metals:π ligands, CO, phosphines

General characteristic: σ-donating, π-accepting, "soft"

CO

PR3


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 !


Donor and acceptor orbitals

σ-donor π-acceptor

phosphine

CO

alkeneπ π

LP

LP

*

π*

σ*


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


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



Variation:reductive synthesis

NiSO4S2O4

2-

CONi(CO)4

CrCl3Al/AlCl3C6H6

Cr

+

Cr + e-



Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl)

FeCl2CpNa

Fe

Fe(CO)5

IFe

OCOC COI



Mn(CO)5-

ClMn(CO)5

Mn(CO)4Δ

or hν


Modification of π ligandsby H+/H- addition/abstraction

M

M

H+

- H+

- H-

H-

M

+

M

-

acidbase

H- donor: NaBH4, LiBHEt3H- abstractor: Ph3C+ BF4

-


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-


Reactivity of π-ligand complexes

Change of hapticity

ring slippageM

σ/π allylMM

M


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


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


σ complex ?

2.10 1.92N

RhN

H

H


σ complex ?

LPt

L

H

+


σ complexes

• C-Si bonds

3.22

2.63

103°

La

Me3SiMe3Si

H

Si

SiMe3H

Me MeMe

THF


σ complexes

• Si-H bonds

• etc: B-H, Sn-Cl, P-H, ..

Mn

OCOC SiHPh2

H??


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

11 - transitionmetal

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