Density Functional Study of the Oxidative Addition Step in the Carbonylation of Methanol Catalyzed by [M(CO)2I2]- (M = Rh, Ir)

Quantum mechanical calculations based on density functional theory (DFT) have been carried out on the oxidative addition process [M(CO)2I2]- + CH3I → [M(CO)2I3(CH3)]- (M = Rh, Ir), which represents an elementary step in methanol carbonylation. The calculated free energies of activation (ΔG ⧧) in methanol are 19.3 (Ir) and 26.9 kcal mol-1 (Rh), respectively, in good agreement with the experimental estimates at 20.9 kcal mol-1 (Ir) in dichloromethane and 22.9 kcal mol-1 (Rh) in methanol, respectively. The higher barrier for M = Rh is attributed to a relativistic stabilization of the Ir−CH3 bond. A thermodynamic calculation was carried out on the formation of the intermediate [M(CO)L2ICH3]( n -1)- in the general oxidative addition reaction [M(CO)L2I] n - + CH3I → [M(CO)L2ICH3]( n -1)- + I- (n = 0 or 1) (where L2 = (CO)I-, (CO)2, or (CO)(MeOH) for M = Ir and L2 = (CO)I-, trans-(PEt3)2, Ph2PCH2CH2PPh2, Ph2PCH2P(S)Ph2, or S,P-SC2B10H10PPh2 - for M = Rh). The thermodynamic energy difference between the intermediate [M(CO)L2ICH3]( n -1)- and the ground state follows the order S,P-SC2B10H10PPh2 - < trans-(PEt3)2 < Ph2PCH2P(S)Ph2 < Ph2PCH2CH2PPh2 < (CO)I- < (CO)2 in solution with respect to the ligand L2. This order is to a first approximation determined by the ability of L to stabilize the M−CH3 bond being formed and the ability of the solvent to stabilize the intermediate [M(CO)L2ICH3]( n -1)- relative to ground state. This order agrees well with the experimental kinetic trend.