Carbon Dioxide Reduction with Dihydrogen and Silanes at Low-Valent Molybdenum Terphenyl Diphosphine Complexes: Reductant Identity Dictates Mechanism

The reaction chemistry of both silanes and hydrogen at para-terphenyl diphosphine-supported molybdenum complexes was explored within the context of carbon dioxide (CO₂) reduction. CO₂ hydrosilylation commonly affords reduction products via silyl acetals. However, while silyl hydride complexes were characterized in the present system, synthetic, spectroscopic, and kinetic studies suggest C–O cleavage of CO₂ occurs independently of silanes. In their presence, a putative molybdenum oxo intermediate is hypothesized to undergo O-atom transfer, yielding silanol. In contrast, hydrogenation chemistry does occur through an intermediate molybdenum dihydride capable of inserting CO₂ to yield a formate hydride complex. This process is reversible; slow deinsertion under dinitrogen affords a mixture of molybdenum dihydride, η²-CO₂, and N₂ complexes. The molybdenum hydride formate species is a competent precatalyst for both CO₂ hydrogenation to formate (in the presence of lithium cations and base) and formic acid dehydrogenation to CO₂ and hydrogen (in the presence of base). Mechanistic studies of both catalytic processes are presented.