Stronger Together! Mechanistic Investigation into Synergistic Effects during Homogeneous Carbon Dioxide Hydrogenation...

A series of group 6 heterobimetallic complexes [M⁰;Ir^III] (M = Cr, Mo, W) were synthesized and fully characterized, and the catalytic behavior was studied. The heterobimetallic complex [Mo⁰;Ir^III] (C1) was by far the most active and has shown a considerable synergistic effect, with both metals actively participating in homogeneous carbon dioxide hydrogenation, leading to formate salts. Based on theoretical calculations, the synergistic interaction is due to Pauli repulsion, lowering the transition state and thus enabling higher catalytic activity. The mechanism of both the hydrogenation itself and the synergistic interaction was studied by NMR spectroscopy, kinetic measurements, and theoretical calculations. The homogeneous nature of the reaction was proven using in situ high-pressure (HP) NMR experiments. The same experiments also showed that the octahedral Mo(CO)₃P₃ moiety of the complex is stable under the reaction conditions. The hydride complex is the resting state because the hydride transfer is the rate-determining step. This is supported by kinetic measurements, in situ HP NMR experiments, and theoretical calculations and is in contrast to the monometallic Ir^III counterpart of C1.