Density functional study of the migratory insertion 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 migratory insertion process [M(CO)(2)I-3(CH3)](-) --> [M(CO)I-3(COCH3)](-) (M = Rh, Ir), which represents an important step in methanol carbonylation. The calculated free energies of activation (Delta G double dagger) are 27.7 kcal mol(-1) (Ir) and 17.2 kcal mol(-1) (Rh), in good agreement with the experimental estimates at 30.6 +/- 1.0 kcal mol(-1) (Ir) and 19.3 +/- 0.5 kcal mol(-1) (Rh). The higher barrier for M = Ir is attributed to a relativistic stabilization of the Ir-CH3 bond. It is indicated that enthalpic and entropic contributions to Delta G double dagger can vary considerably, depending on reaction conditions, without changing Delta G double dagger considerably. Especially, simulations based on ab initio molecular dynamics (AIMD) underlined that the reaction system might prefer to trade entropy for enthalpy in polar solutions by dissociating an I- ligand for M = Ir. A systematic study was also carried out on the general methyl migration reaction [Ir(CO)(2)I2L(CH3)(n-) --> [Ir(CO)I2L(COCH3)(n-) (n = 0, 1), in which an iodide ligand trans to methyl is replaced by another ligand L (where L = CH3OH, CH3C(O)OH, CO, P(OCH3)(3), SnI3-) or an empty coordination site. The free energy of activation for the methyl migration in [Ir(CO)(2)I2L(CH3)] with L trans to methyl follows the order P(OCH3)(3) > CO > SnI3-, none > I- > CH3OH, CH3C(O)OH with respect to the ligand L. This order is to a first approximation determined by the ability of L to labilize the M-CH3 bond trans to it. The order is further shaped by the ability of the pi-acceptors L = CO, P(OCH3)(3) to stabilize the transition state, and, in the case of L = none, by the relocation of an iodide ligand to the site trans to the migrating methyl group. It is finally discussed how placing L cis to the migrating CH3 group might influence the migratory aptitude of methyl.