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)(2)I-2](-) + (CHI)-I-3 -> [M(CO)(2)I-3(CH3)](-) (M = Rh, Ir), which represents an elementary step in methanol carbonylation. The calculated free energies of activation (Delta G(double dagger)) 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 L-2 = (CO)I-, (CO)(2), or (CO)(MeOH) for M = Ir and L-2 = (CO)I-, trans-(PEt3)(2), Ph2PCH2CH2PPh2, Ph2PCH2P(S)Ph-2, or SP-SC2B10H10PPh2- for M = Rh). The thermodynamic energy difference between the intermediate [M(CO)L2ICH3]((n-1))- and the ground state follows the order SP-SC2B10H10PPh2- < trans-(PEt3)(2) < Ph2PCH2P(S)Ph-2 < 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.