A series of group 6 heterobimetallic complexes [M0;IrIII] (M = Cr, Mo, W) were synthesized and fully
characterized,
and the catalytic behavior was studied. The heterobimetallic complex
[Mo0;IrIII] (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 IrIII counterpart of C1.