Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H2 and the hydride transfer. The heterolytic cleavage of H2 is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H2 heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H2. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO2. This ligand assisted pathway is more favorable than the direct attack of hydride to CO2. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide.