Visible-Light-Mediated CO2 Reduction to Formate in Water Catalyzed by a Molecular Fe Cluster

Abstract Photocatalytic CO2 reduction in aqueous conditions offers a promising approach for conversion of CO2 into formate. Aqueous conditions are desirable for fuel formation since they provide a proton source for the formation of hydride intermediates and are compatible with very large-scale installations. However, several challenges remain in formate formation from CO2, including the selective generation of a single product along with the requirement that the catalyst is stable in water. Herein, we report CO2-to-formate reduction using [Fe4N(CO)12]– as a hydride-transfer electrocatalyst, Ru(bpy)3Cl2·6H2O (bpy = 2,2'-bipyridine) as the visible-light photosensitizer, and sodium ascorbate as the sacrificial reductant in phosphate-buffered water at pH 6.3. Under visible-light excitation, we observe a turnover number of 420 for formate production with > 99% product selectivity. The key reaction condition for selectively generating formate is the choice of pH; we see a maximum TON in a phosphate buffer relative to the more acidic acetate buffer. Stern–Volmer analyses of the reaction mechanism indicate that a reductive quenching mechanism is operable such that [Ru(bpy)3]+ plays a role as the reductant in the catalytic cycle.