A Static and Dynamic Density Functional Theory Study of Methanol Carbonylation Minserk Cheong, a Rochus Schmid, b and Tom Ziegler c a Department of Chemistry, Kyung Hee University, Seoul , Korea b Technische Universitat Munchen, Anorganisch-Chemisches Institut, D Garching, Germany c 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) 2 I 2 ] - (M = Rh, Ir). The study included the initial oxidative addition of CH 3 I to [M(CO) 2 I 2 ] - : (1) [M(CO) 2 I 2 ] - + CH 3 I  [M(CO) 2 I 3 (CH 3 )] -, as well as the migratory insertion of CO into the M-CH 3 bond : (2) [M(CO) 2 I 3 (CH 3 )] -  [M(CO)I 3 (COCH 3 )] -. 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 SnI 3 - ) 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 OCI I M II I C CH 3 O M OCI I I CH 3 M OCI I HI H 2 O C MeI CH 3 O MeCOI MeOH I MeCO 2 H CO Catalytic Cycle for Acetic Acid Synthesis M= Rh +, Ir +

4 Thermodynamics of oxidative addition reactions Metal  H 298(g)  S (g)  G (g)  E solv  G 298(solv) Rh Ir kcal/mol Metal  H 298(g)  S (g)  G (g)  E solv  G 298(solv) Rh Ir

5 RC=3  G O RC = r(M-C)-r(I-C) Transition State Region O C O C Rh C I I I r ( C - I ) r ( R h - C ) RC=-1.0

6 Comparison of static(ADF) calculations and dynamic(PAW) calculation Metal  ‡ S‡S‡ G‡G‡ Rh PAW ADF Expt IrPAW ADF Expt kcal/mol

7 I O I C Rh C I C O O O I C C Rh C I I O I C Rh O C C O I C 0 kcal/mol  ‡  18 kcal/mol  S ‡  1.1 cal/molK  G ‡  17 kcal/mol    kcal/mol  S  2.3 cal/molK  G =   kcal/mol Migratory Insertion of [MeRh(CO) 2 I 3 ] - Transition State Reactant Product  ‡ expt = 15 kcal/mol  S ‡ expt = -14 cal/molK GG ‡ expt = 19 kcal/mol  expt = -8.8 kcal/mol  S expt = - 13 cal/molK GG expt = -5.0 kcal/mol

8 O I C I Ir C C I O  H ‡ = 28 kcal/mol  S ‡ = 2.0 cal/molK  G ‡ = 28 kcal/mol Transition State  kcal/mol Reactant Product  H= 4.0 kcal/mol  S= 3.6 cal/molK  G= 2.9 kcal/mol Migratory Insertion in [MeIrCO 2 I 3 ]  H expt ‡ = 37 kcal/mol  S expt ‡ = 22 cal/molK  G expt ‡ = 31 kcal/mol

9 of static (ADF) calculations and dynamic (PAW) calculations  H ‡ expt = 155 ± 4 kJ/mol  S ‡ ex = 91 ± 8 J/molKlK  G ‡ ex = 128± 4 kJ/mol C I O  H ‡ = 118kJ/mol  S ‡ = 8 J/molKlK  G ‡ ADF = 116 kJ/mol ADF ADF    H ‡ = 126 kJ/mol S ‡ = 60 J/mol K G ‡ PAW = 111 kJ/mol PAW PAW RC = r(C-C)  G k J / m o l Free Energy Reaction Profile

10  H ‡ expt = 37 ± 1 kcal/mol  S ‡ expt = 22 ± 2 cal/molK  G ‡ expt = 31± 1 kcal/mol Reduction of migration barrier by substituting iodine trans to methyl O C I O C Ir C I I +L or Act -I - O C O C Ir C I I L L = CO; MeOH; AcOH; P(OMe)3)3 ; None. Act = SnI2.I2.  H ‡ expt = 21 ± 1 kcal/mol  S ‡ expt = -9 ± 2 cal/molK  G ‡ expt = 24 ± 1 kcal/mol L= I - L=CO

11 Activation parameters for the CO insertion kcal/mol [Ir(CO) 2 I 2 (CH 3 )L] n-  [Ir(CO)I 2 (COCH 3 )L] n- L H‡H‡ S‡S‡ G‡G‡ I-I CH 3 OH CH 3 C(O)OH COCO P(OMe) SnI

12 Isomers of [M(CO) 2 I 2 (CH 3 )L] n- and their relative energies LMfac,cismer,cismer,trans IRh 0.0 a Ir COCORh0.0 a Ir a Energies(kcal/mol) relative to fac,cis isomer

13 Activation parameters for the different isomers of [Ir(CO) 3 I 2 (CH 3 )] Isomer H‡H‡ S‡S‡ G‡G‡ fac,cis mer,cis ª mer,trans expt kcal/mol ª Methyl group migrating to the CO which is trans to another CO

14 Conclusion Static and dynamic calculation results for the oxidative addition and the migratory insertion step in the carbonylation of methanol catalyzed by [M(CO) 2 I 2 ] - (M=Rh, Ir) are in good agreement with the experimental values. The rate-determining step for the Rh catalyst is the oxidative addition of CH 3 I, 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) 2 I 2 (CH 3 )L] n- (L is trans to I - ) shows that free energies of activation is in the order of P(OMe) 3 < CO < SnI 3 -, none < I - < CH 3 OH, CH 3 C(O)OH. In predicting the reaction rate, the relative stabilities of various isomers should be considered.