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 (CH 3 )] - → [M(CO)I 3 (COCH 3 )] - (M = Rh, Ir), which represents an important step in methanol carbonylation. The calculated free energies of activation (Δ G ⧧ ) 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−CH 3 bond. It is indicated that enthalpic and entropic contributions to Δ G ⧧ can vary considerably, depending on reaction conditions, without changing Δ G ⧧ 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 I 2 L(CH 3 )] n - → [Ir(CO)I 2 L(COCH 3 )] n - ( n = 0, 1), in which an iodide ligand trans to methyl is replaced by another ligand L (where L = CH 3 OH, CH 3 C(O)OH, CO, P(OCH 3 ) 3, SnI 3 - ) or an empty coordination site. The free energy of activation for the methyl migration in [Ir(CO) 2 I 2 L(CH 3 )] with L trans to methyl follows the order P(OCH 3 ) 3 > CO > SnI 3 -, none > I - > CH 3 OH, CH 3 C(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−CH 3 bond trans to it. The order is further shaped by the ability of the π-acceptors L = CO, P(OCH 3 ) 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 CH 3 group might influence the migratory aptitude of methyl.