lig and substitution

8/14/2019 Lig and Substitution

1/22

11/5/2013 Ligand Substitution and Activation 1

Ligand substitution reactions:

dissociativeExample:

Factors influencing ease of dissociation:

1erow < 2erow > 3erow

d8-ML5> d10-ML4> d

6-ML6

stable ligands (CO, olefins, Cl-) dissociate easily

(as opposed to e.g. CH3, Cp).

LnM CO L

nM L

nM L'+ CO

L'

18 e 16 e 18 e


2/22


Dissociative substitution in ML6

16-eML5complexes are usually fluxional;

the reaction proceeds with partial inversion,

partial retention of stereochemistry.

18-e

oct

16-e

SP distortedTBP

or


3/22


Example:

Sometimes the solvent is involved:


associative

16 e 18 e

L'Ln-1M L'LnM LnM L'

- L

16 e

L2MCl2 L2M(Cl)(Br)

L2M(Cl)(H2O)+

Br-

- Cl-

H2O - Cl- Br-

- H2O (cis-platin !)


4/22


Ligand rearrangement

Several ligands can switch between n-eand

(n-2)-esituations, thus enabling associative

reactions:

M N

O

M N O

3-e 1-e

M M

3-e5-eM

CO

RM

O

R

(1+2)-e 1-e


5/22



redox-induced

For 17-e19-eusually fast equilibrium:

Reduction promotes dissociative substitution.

Oxidation promotes associative substitution.

In favourable cases, the product

oxidizes/reduces the starting material

redox catalysis.

LnM L'L

nM + L'


6/22


Initiation by added reductant.

Sometimes, radical abstraction produces a 17-e species

(see C103).

CO

L

Fe(CO)4L

Fe(CO)4L

Fe(CO)4

Fe(CO)5

Fe(CO)5


7/22



photochemicalVisible light can excite an electron from an M-L

bonding orbital to an M-L antibonding orbital (LF

transition). This results in fast ligand dissociation.

Requirement: the complex must absorb,

so it must have a colour.

d

d

h

M(CO)6


8/22


Some ligands have a low-lying * orbital and

undergo MLCT excitation. This leads to easyassociative substitution.

The excited state is formally (n-1)-e !

M-M bonds dissociate easily on irradiation

(n-1)-eassociative substitution

d

d

h

M(CO)4(phen)

*


9/22


Ligand activation:

electrophilic and nucleophilic attack Electron-rich metal fragment:

ligands activated for electrophilic attack.

N4: strong -donor

NN

N

RhS

N

++

S

H+N

N

N

Rh

N

+


10/22


Ligand activation:

electrophilic and nucleophilic attack Electron-poor metal fragment:

ligands activated for nucleophilic attack.

-

Cr

OCOC

CO

Bu

H

Li+

Cr

OCOC

CO

BuLi

CO: strong -acceptor


11/22


Electrophilic attack on ligand

Hapticity may increase or decrease.

Formal oxidation state of metal may

increase.

H+MI

+

MI

H+M(0) MII

+


12/22


Electrophilic addition

Electrophilic abstraction

Alkyl exchange also starts with electrophilic attack.

O

Fe(CO)3

OEt+

Fe(CO)3

Et3O+

Cp2Zr

Me

Me

B(C6F5)3Cp2Zr

Me

+

MeB(C6F5)3-

16 e 14 e

+


13/22


Competition: attack of electrophile on metal

(may be followed by shift to ligand)

Fe

+

Fe H

Ni

H+

H+

Ni

+


14/22


Electrophilic attack on metal

Can be the start of oxidative addition

Key reaction in the Monsanto acetic acid

process:

I2(CO)

2Rh Me I I

2(CO)

2RhMe + I- I

3(CO)

2RhMe

MeOH + CO MeCOOHHI"Rh"


15/22


MeOH

MeI

HI

H2O

MeCOOH

MeCOI

Rh(CO)2I2

Rh(CO)2I3Me Rh(CO)2I3(COMe)

CO

oxidative

addition

reductive

elimination

insertion


16/22


Nucleophilic attack on ligand

Nucleophile "substitutes" metal

hapticity usually decreases

Oxidation state mostly unchanged

Competition: nucleophilic attack on metal

usually leads to substitution


17/22


Nucleophilic abstraction

NaH

CrOCOC

CO

Na+

-

CrOCOC

CO

Cp2WH2BuLi Cp2WH Li


18/22


Nucleophilic addition

Key reaction of the Wacker process:

(H2O)Cl2Pd + OH- (H2O)Cl2Pd

OH

-

C2H4 + O2PdCl2, H2O

CH3CHO

CuCl2


19/22


H2O Pd Cl

Cl

H2O PdH

ClOH

H2O PdH

Cl O

Cl Pd ClCl

Cl 2--

Cl Pd Cl

Cl

H2O PdCl

OH

H2O PdCl

OH-H elim

- 2 e(CuCl2CuCl)

C2H4 H2O

H2O PdCl

ClOH

-

ins

Cl-

CH3CHO

H2O Pd ClH

Cl-

Pd(0)+ H++

H2O + 2 Cl-

OH-

- Cl-

-H elim


20/22


How can you distinguish between internaland

externalattack of OH-?

Use trans-CHD=CHD and trap the intermediate

Pd-C-C-OH with CO:

H2O PdCl

Cl

H2O Pd

OH

Cl-

OH- H2O PdCl

Cl

OH-

-

H2O Pd

O

Cl

H

ins

OH- - Cl-??

H2O PdCl

ClOH

-

PdCO

OH

O

O

CO

- Cl-

Pd

O

HO

ins

nuclattack


21/22


OH-

O

O

D

D

Pd

HO

D

D

Pd

HO

D

D

Pd

D

D

O

O

D

D

PdD

HO

D

Pd

OHD

D

CO

CO


22/22


Could acetaldehyde be formed directly as

vinyl alcohol ?Perform reaction in D2O:

diss-H elim OPd

D

ins

CH3

OH

PdOD

D2OCH2D

OH

diss

ODOD

Pd

H

lig and substitution

Documents