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" -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 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 -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 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

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 Organometallics33

complex ?

2.10 1.92

N

Rh

N

H

H

Transition-metal Organometallics34

complex ?

LPt

L

H

+

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??

Transition-metal Organometallics37

complexes

A complex is an (arrested) intermediate for oxidative addition:

Stable complexes are formed when the metal is not-basic enough to enable completion of the addition.

MX

Y

XYM M

X

Y

M(m) M(m+2)M(m)

n-e (n+2)-e (n+2)-e