Photocatalytic CO2-to-Formate Conversion by a Dinuclear Bismuth-bis(dithiolene) Complex

Abstract

Reducing and utilizing atmospheric carbon dioxide (CO2) is a globally significant challenge. Among the promising approaches to address this issue, photocatalytic CO2 reduction has attracted considerable attention. This approach enables the direct conversion of CO2 into reduced carbon products using sunlight. However, achieving high product selectivity in photocatalytic CO2 reduction requires the rational design of suitable catalysts. Formate dehydrogenase (FDH) in nature is known to reversibly convert CO2 to formate, and in metal complexes that mimic this enzyme, high⁻valent W or Mo centers with strong Lewis acidity have been reported to react with CO2 to form carbonate intermediates. Elucidating the role of intermediates formed through interactions between Lewis acidic metal centers and CO2 in the CO2 reduction process is a critical task for catalyst design and mechanistic understanding aimed at controlling product selectivity. In our prior work, a bismuth oxycarbonate active species in a heterogeneous system selectively converted CO2 to formate. The central metal, bismuth (Bi)—a heavy element analogous to Mo and W—exhibits Lewis acidity in the Bi(III) oxidation state and reacts with bicarbonate; the resulting intermediate has been reported to function as the active species in catalysis. However, heterogeneous systems inherently limit precise mechanistic elucidation. In this study, we synthesized a homogeneous dinuclear bismuth–bis(dithiolene) complex, elucidated its reactivity toward bicarbonate, and investigated photocatalytic CO2 reduction under homogeneous conditions. As a result, CO2 was selectively converted to formate, and the intermediate formed upon reaction of the complex with bicarbonate was observed spectroscopically. Based on spectroscopic analysis of this intermediate, we elucidated the associated photocatalytic reaction mechanism.