Substituents and Cocatalyst Effect on the Catalytic Response and Overpotential of Re(I) Catalysts for CO2 Reduction

To design an efficient electrocatalyst for CO2 reduction, a comprehensive understanding of the catalytic architecture and the reaction mechanism is required. Herein, we synthesized and analyzed a series of fac-[Re(bpy-X) (CO)3Cl] (abbreviated as ReI(bpy-X); bpy-X = 4,4′-disubstituted-2,2′-bipyridine; X = tBu, CH3, mesityl, H, phenyl, and methyl ester) catalysts for electrocatalytic CO2 reduction with and without adding the cocatalyst [Zn(cyclam)]2+ (cyclam = 1,4,6,11-tetraazacyclotetradecane). Both computational and experimental results show that in the experiments bearing the [Zn(cyclam)]2+ cocatalyst, the Re(I) catalysts require less energy input to reduce the CO2, indicating that the CO2-to-CO conversion occurs at a lower overpotential. Interestingly, in the Re(I) electrocatalyst bearing a strong electron-withdrawing group (methyl ester), [Zn(cyclam)]2+ does not improve the CO2 reduction activity. These results emphasize the importance of how the electronic distribution throughout the molecular architecture can enhance or suppress the nucleophilicity of ReI(bpy-X) electrocatalysts, even in the presence of a cocatalyst.