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)2I3(CH3)]- → [M(CO)I3(COCH3)]- (M = Rh, Ir), which represents an important step in methanol carbonylation. The calculated free energies of activation (ΔG ⧧) 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 ΔG ⧧ can vary considerably, depending on reaction conditions, without changing ΔG ⧧ 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)2I2L(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)2I2L(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 π-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.