Transition metal complexes in organic synthesis

Hydrogenation

Methanol to acetic acid process

Olefin polymerization and oligomerization

Organometallic catalysts in industrial synthesis : Three Nobel Prizes 2000, 2005 and 2010

RHC CH2 + H2 RCH2CH3

CH3OH + CO CH3COOH

**

n **

n **

n

Isotactic polypropylene Syndiotactic polypropylene Atactic polypropylene

nC4-C8 40%C10- C18 40 %C20 & > 20 %

Organometallic complexes

Organometaliic complexes allow impossible reactions to occur.

Have direct metal to carbon bond.

The bond may be metal-carbon sigma bond to give metal alkyls such as R4Ti or by the interaction of metals with pi-system of unsaturated organic systems.

Metals whose energetically accessible d-orbitals allow efficient overlap with pi-orbitals of the carbon-carbon and carbon-heteroatom multiple bonds to give pi-complexes.

The moiety containing pi-bond may be neutral (R-CH=CH2), cation (R-CH=CH2+) or anion (R-CH=CH2-).

Organometallic ChemistryM-C bond can be a type or type bond

Organo transition metal compounds can be divided into two broad classes:1. Sigma bonded organometallics:In which a single carbon atom of the ligand is attached directly to the metal through a sigma

bond (two electron two centre bond). Ligands of this type increases electron density on the centralmetal atom. Examples are: R-Cu, R-TiCl3, R2TiCl2 etc.

2. Pi-bonded organometallicsIn which a metal atom (ion or complex species) is bonded to ligands such as CO, RNC, C=C andother unsaturated molecules. These ligands in addition to the presence of lone pair of electrons havein common the presence of low lying vacant orbitals of correct symmetry to form pi bonds byaccepting electrons from transition metal d-orbitals (back bonding). This decreases the electrondensity on metal atom. These can be further subdivided as; a) in which the ligand is bonded throughentire unsaturated hydrocarbon molecule and b) in which only one carbon of the ligand is within thebonding distance of the metal, examples CO, carbene, benzyne etc.

C

C

C

CM M

donation back donationfrom to *

*

M-C bond can be a type or type bond. The metal therefore acts as both Lewis acid (electron acceptor) and a Lewis base (electron donor). Back donation into pi-antibonding orbitals leads to weakening of

carbon-carbon double bond.

18 electron rule

The rule states that thermodynamically stable transition metal organometallic compounds are formedwhen the sum of the metal d electrons and the electrons conventionally considered as being supplied bythe surrounding ligands equals 18.

When this happens the metal acquires the electronic configuaration of next higher noble gas. In general, the conditions favouring adherence to the 18 electron rule are, an electron rich metal (one

that is in a low oxidation state) and ligands that are good -acceptors.

The requirement for 18 electrons comes from the need to fill one s, three p and five d orbitals

There are exceptions to 18 electron rule including complexes of Ni, Pt, Pd, Zr and Ti which all form stable 16-electron complexes.

The 16 electron complexes are very important in catalytic processes, which have one high energy vacant d-orbital, which act as a site for other ligands in chemical reactions.

The symbol indicates bridging normally we have 2 and rarely 3 bridgingExamples: 2-CO, 3-CO, 2-CH3, 2-H, 2-Cl, , 3-Cl, 2-OR, 2-PR2, 2-NR2

Examples:1-R, 1-Ar 2-C2R4 1-allyl, 3-allyl, 4- Cb, 5-Cp, 6-C6H6 8-C8H8 2-C60, 5-R5C60

Hapto ligands and Sandwich compounds

The hapto symbol, (eta) with a numerical superscript, provides a topological description by indicating the number ofcarbon atoms at a bonding distance to the metal. For example, if all the five carbon atoms of a cyclopentadienylmoiety are equidistant from a metal atom, we term it as 5-cyclopentadienyl.

Sandwich

Bent Sandwich Half Sandwich Triple decker & polycyclic

(5-C5H5)2Fe (6-C6H6)2Cr

Unique reactions in organometallic chemistry

• Ligand exchange

• Oxidative Addition

• Reductive Elimination

• Migratory Insertion

• - Hydrogen Elimination

1. Ligand exchange: A transition metal complex can lose a ligand by dissociation and combine with anotherligand by association process. For example, the rhodium complex can react with an alkene as follows:

It takes place in two steps; in the first step, one of the ligands dissociates. This leads to a complex in whichrhodium has only 16 valence electrons and is therefore coordinatively unsaturated.

In the second step, rhodium associates with other ligand to become coordinatively saturated again.

Kr 4d8 5s1

When addition of ligands is accompanied by oxidation of the metal, it is called an oxidative addition reaction

L n M + X Y L n (X ) (Y )M

d n d n -2

• availability of nonbonded electron density on the metal,

• two vacant coordination sites on the reacting complex (LnM), that is, the complex must be coordinatively unsaturated,

• a metal with stable oxidation states separated by two units; the higher oxidation state must be energetically accessible and stable.

• ligands can be H-H, H-X, R-X, RCO-H, R-CO-X etc.

Requirements for oxidative addition

2. Oxidative addition

OX state of metal increases by 2 units

Coordination number increases by 2 units

2 new anionic ligands are added to themetal

RhPh3P

Ph3P PPh3

ClRh

Ph3P

H PPh3

Cl

H

PPh3

H2 oxidative addition

Rh+1 Rh+3

IrPh3P

Cl PPh3

CO

IrPh3P

Cl PPh3

CO

Cl

Cl

IrPh3P

O PPh3

CO

O

Cl

IrPh3P

I PPh3

CO

Me

Cl

IrPh3P

Cl PPh3

CO

Me

I

Cl2

O2

MeI

16E

18E

Examples of Oxidative addition : Cis or trans ?

Homonuclear systems (H2, Cl2, O2, C2H2) Cis

Heteronuclear systems (MeI) Cis or trans

PtPPh2

Ph2P CH3

CH3

CH3

CH3

reductive elimination PtPPh2

Ph2P CH3

CH3

+ H3C CH3165 °C, days

3. Reductive elimination

• a high formal positive charge on the metal,• the presence of bulky groups on the metal, and • an electronically stable organic product.

Cis orientation of the groups taking part in reductive elimination is a MUST

Almost the exact reverse of Oxidative Addition

Factors which facilitate reductive elimination

Oxidation state of metal decreases by 2 units

Coordination number decreases by 2 units

2 cis oriented anionic ligands form a stable bond and leave the metal

Pt4+Pt2+

Final step in many catalytic cycles

Hydroformylation ( conversion of an alkene to an aldehyde)

Sonogashira Coupling (coupling of a terminal alkyne to an aryl group

Cativa Process (Methanol to Acetic acid)

4. Migratory Insertion

YM

X

[M-Y-X] + LYM

L

X

d n d n

MnOC

OC CO

COOC

CH3

MnOC

OC C

COOC

Ph3P

+ PPh3

O

CH3

No change in the formal oxidation state of the metal

A vacant coordination site is generated during a migratory insertion (which gets occupied by the incoming ligand)

The groups undergoing migratory insertion must be cis to one another

These reactions are enthalpy driven and although the reaction is entropy prohibited the large enthalpy term dominates

1, 2-migratory insertion

Rh

CO

Ph3P

H

Ph3P

R

RhOC

Ph3P CH2CH2R

PPh3

RhOC

Ph3P

CO

PPh3

CH2CH2R

RhOC

Ph3P CCH2CH2R

PPh3

O1, 1-migratory insertion

M A

X

B M A

B

X

M

XA

BM A

B

X

1, 1 - migratory insertion

1, 2 - migratory insertion

Types of Migratory Insertion

5. -Hydride elimination

C

C

H

H

H

MH

HC

C

H

H

H

MH

H C

C

H

H

H

M

H

H

Beta-hydride elimination is a reaction in which an alkyl group having a hydrogen, bonded to a metal centre isconverted into the corresponding metal-bonded hydride and a bonded alkene. The alkyl must have hydrogens on thebeta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex musthave an empty (or vacant) site cis to the alkyl group for this reaction to occur.

mechanism

Can either be a vital step in a reaction or an unwanted side reaction

No change in the formal oxidation state of the metal

-hydrogen elimination does not happen when

• the alkyl has no -hydrogen (as in PhCH2, Me3CCH2, Me3SiCH2)

• (ii) the -hydrogen on the alkyl is unable to approach the metal (as in C≡CH)

• the M–C–C–H unit cannot become coplanar

PtEt3P

Et3P C

C

C

C

H

HB

PtPh3P

Ph3P

C

PtPh3P

Ph3P

D

PtPh3P

Ph3P

A

SiMe3SiMe3

Select the most unstable platinum complex from the given list. Justify your answer

No -H -H unable to MCCH unit will not be approach M coplanar