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 say' 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, eta(2)-CO2, and N-2 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.