Stronger Together! Mechanistic Investigation into Synergistic Effects during Homogeneous Carbon Dioxide Hydrogenation Using a Heterobimetallic Catalyst

The catalytic activity of the heterobimetallic complexes[M-0;Ir-III] (M = Cr, Mo, W) in homogeneous carbondioxide (CO2) hydrogenation was studied. [Mo-0;Ir-III] was the most active and showed an increase inreactivity compared to a mixture of its monometallic counterparts.The homogeneous nature of the reaction was confirmed and the mechanismelucidated. The increase in activity is due to Pauli repulsion loweringthe transition state of the hydride transfer to CO2. A series of group 6 heterobimetallic complexes [M-0;Ir-III] (M = Cr, Mo, W) were synthesized and fullycharacterized,and the catalytic behavior was studied. The heterobimetallic complex[Mo-0;Ir-III] (C1) was by far themost active and has shown a considerable synergistic effect, withboth metals actively participating in homogeneous carbon dioxide hydrogenation,leading to formate salts. Based on theoretical calculations, the synergisticinteraction is due to Pauli repulsion, lowering the transition stateand thus enabling higher catalytic activity. The mechanism of boththe hydrogenation itself and the synergistic interaction was studiedby 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 octahedralMo(CO)(3)P-3 moiety of the complex is stable underthe reaction conditions. The hydride complex is the resting statebecause the hydride transfer is the rate-determining step. This issupported by kinetic measurements, in situ HP NMR experiments, andtheoretical calculations and is in contrast to the monometallic Ir-III counterpart of C1.