A Static and Dynamic Density Functional Theory Study of

Methanol Carbonylation

A Static and Dynamic Density Functional Theory Study of

Methanol Carbonylation

Minserk Cheong,a Rochus Schmid,b and Tom Zieglerc

a Department of Chemistry, Kyung Hee University, Seoul 130-701, Koreab Technische Universitat Munchen, Anorganisch-Chemisches Institut, D-85747

Garching, Germanyc Department of Chemistry, University of Calgary, Alberta, Canada T2N 1N4

2

Abstract

Quantum mechanical calculations based on density functional theory (DFT) were carried out in order to investigate the reaction mechanism for the carbonylation of methanol to acetic acid by [M(CO)2I2]

- (M =

Rh, Ir). The study included the initial oxidative addition of CH3I to

[M(CO)2I2]- : (1) [M(CO)2I2]

- + CH3I [M(CO)2I3(CH3)]-, as well as the

migratory insertion of CO into the M-CH3 bond : (2) [M(CO)2I3(CH3)]-

[M(CO)I3(COCH3)]-. Considerations were also given to migratory

insertion processes where the I--ligand trans to methyl was replaced by another ligand L (where L = MeOH, MeC(O)OH, CO, P(OMe)3 or

SnI3-) or an empty coordination site. The calculated free energies of

activation and heat of reactions are in good agreement with the experimental data. A full analysis is provided of how ligands trans to the migrating methyl group influence the barrier of migratory insertion.

3

M

OC I

OC I

M

I I

OC IC

CH3O

M

OC I

OC I

I

CH3

M

OC I

OC I

HI

H2OC

MeI

CH3O

MeCOI

MeOHI MeCO 2H

CO

1

2

3

4

Catalytic Cycle for Acetic Acid Synthesis

M= Rh +, Ir +

4

Thermodynamics of oxidative addition reactions

Meta l H298(g) S298(g) G298(g) Esolv G298(solv)

Rh 29.5 -13.6 33.6 -22.0 13.6Ir 22.4 -15.1 26.9 -22.5 4.4

kcal/mol

Meta l H298(g) S298(g) G298(g) Esolv G298(solv)

Rh -38.9 -28.4 -30.4 18.6 -11.8Ir -41.4 -27.7 -33.1 14.0 -19.1

CH3I + M(CO)2I2−→ CH3 ( )M CO 2I 2 + I

-

CH3M(CO)2I2 + I- → CH3 ( )M CO 2 I3−

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I

R hO

CI

C

O

I

C

r(R

h -H)

r(C

- I)

r (Rh-C)RC=3

G

O R C = r(M-C)-r(I-C)

Transition S tate Region

O

C

OCRh

C

I

I

I

r(C

-I)

r(R

h-C

)

RC=-1.0

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Comparison of static(ADF) calculations and dynamic(PAW) calculation

Comparison of static(ADF) calculations and dynamic(PAW) calculation

Metal ‡ S‡ G‡

RhPAW

ADF

19.2

13.8

-21.1

-43.9

25.5

26.9

Expt 12.0 -39.4 23.7

Ir PAW 12.1 -23.4 19.1

ADF 6.0 -44.6 19.3

Expt 12.9 -26.8 20.9kcal/mol

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I

O

I

C

Rh

C

I

C

O2 .7 7

2.39

1.91

OO

I

CC

Rh

C

I I

O

I

C

Rh

O

C

C

O

I

C

0 kcal/mol

‡ 18 kcal/mol

S‡ 1.1 cal/mol•K

G‡ 17 kcal/mol - 5.6 kcal/mol

S 2.3 cal/mol•K

G = - 6.2 kcal/mol

Migratory Insertion of [MeRh(CO)2I3]-

Transition State

Reactant

Product

‡expt

= 15 kcal/mol S‡

expt = -14 cal/mol•K

G ‡expt

= 19 kcal/mol

expt = -8.8 kcal/mol

S expt = - 13 cal/mol•K

G expt = -5.0 kcal/mol

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I

O

C

I

C

Ir

C

I

O

2 .77

2.50 1. 88

O

I

C

I

I r

C

C

I

O

O

C

IIr

O

C

C

O

C

I

H‡

= 28 kcal/mol

S‡

= 2.0 cal/mol•K

G‡

= 28 kcal/mol

Transition State

kcal/mol

Reactant

Product

H= 4.0 kcal/mol S = 3.6 cal/mol•K G = 2.9 kcal/mol

Migratory Insertion in [MeIrCO2I3]-

2.90

Hexpt‡ = 37 kcal/mol

Sexpt‡ = 22 cal/mol•K

Gexpt‡ = 31 kcal/mol

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Comparison of static (ADF) calculations and dynamic (PAW) calculations

H ‡expt = 155 ± 4 kJ/mol

S ‡ expt = 91 ± 8 J/mol•K G ‡ expt = 128± 4 kJ/m ol

I

O

C

I

C

Ir

C

I

O

2.60

2.42

1.80

H ‡ = 118 kJ/molS ‡ = 8 J/mol•K G ‡ A DF = 116 kJ/m ol

A DF

A DF

H ‡ = 126 kJ/molS ‡ = 60 J/mol• KG ‡ PAW = 111 kJ/m ol

PAW

PAW

-20

0

20

40

60

80

100

120

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

0.0

00

01

RC = r(C-C)

G k

J/m

ol

Free Energy Reaction Profile

3.53

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H‡expt = 37 ± 1 kcal/mol

S‡ expt = 22 ± 2 cal/mol•K G‡ expt = 31 ± 1 kcal/mol

Reduction of migration barrier by substituting iodine trans to methyl

O

C

I

O

C

Ir

C

II

+L or Act

-I-

O

CO

C

Ir

C

II

LL = CO; MeOH; AcOH; P(OMe)3 ; None. Act = SnI2.

H‡expt = 21 ± 1 kcal/mol

S‡ expt = -9 ± 2 cal/mol•K G‡ expt = 24 ± 1 kcal/mol

L= I- L=CO

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Activation parameters for the CO insertionActivation parameters for the CO insertion

kcal/mol

[Ir(CO)2I2(CH3)L]n- [Ir(CO)I2(COCH3)L]n-

L H‡ S‡ G‡

--- 21 -5.2 23

I- 28 2.0 28

CH3OH 33 -5.7 35

CH3C(O)OH 34 -4.7 36

CO 17 -3.9 18

P(OMe)3 14 1.9 13

SnI3- 22 -3.9 23

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Isomers of [M(CO)2I2(CH3)L]n- and their relative energies

Isomers of [M(CO)2I2(CH3)L]n- and their relative energies

L M fac,cis mer,cis mer,trans

I Rh 0.0a 1.3 0.4

Ir 0.0 4.1 4.1

CO Rh 0.0a -2.2 -2.5

Ir 0.0 -1.8 0.4

a Energies(kcal/mol) relative to fac,cis isomer

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Activation parameters for the different isomers of [Ir(CO)3I2(CH3)]

Activation parameters for the different isomers of [Ir(CO)3I2(CH3)]

Isomer H‡ S‡ G‡

fac,cis 17.3 -3.90 18.5

mer,cis 28.8 -1.19 29.2

24.1ª -4.34 25.4

mer,trans 16.8 -0.46 16.9

expt. 21.3 -8.6 23.8

kcal/mol

ª Methyl group migrating to the CO which is trans to another CO

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ConclusionConclusion• Static and dynamic calculation results for the oxidative addition and

the migratory insertion step in the carbonylation of methanol catalyzed by [M(CO)2I2]- (M=Rh, Ir) are in good agreement with the experimental values.

• The rate-determining step for the Rh catalyst is the oxidative addition of CH3I, whereas for Ir it is the migratory insertion step.

• Enthalpic and entropic contributions to G‡ can vary considerably depending on reaction conditions without changing G‡ considerably.

• Detailed study on the methyl migration of [Ir(CO)2I2(CH3)L]n- (L is trans to I-) shows that free energies of activation is in the order of P(OMe)3 < CO < SnI3

-, none < I- < CH3OH, CH3C(O)OH. • In predicting the reaction rate, the relative stabilities of various

isomers should be considered.