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.