Photocatalytic Reduction of Carbon Dioxide to CO and HCO2H Using fac-Mn(CN)(bpy)(CO)3

Studies are reported regarding the use of Mn(CN)(bpy)(CO)3 (1) as a catalyst for CO2 reduction employing [Ru(dmb)3](2+) as a photosensitizer in mixtures of dry N,N-dimethylformamide-triethanolamine (N,N-DMF-TEOA) or acetonitrile-TEOA (MeCN-TEOA) with 1-benzyl-1,4-dihydronicotinamide as a sacrificial reductant. Irradiation with 470 nm light for up to 15 h yields both CO and HCO2H with maximum turnover numbers (TONs) as high as 21 and 127, respectively, with product preference dependent on the solvent. Further data suggests that upon single electron reduction this catalyst avoids the formation of a Mn-Mn dimer and instead undergoes a disproportionation reaction, which requires 2 equiv of [Mn(CN)(bpy)(CO)3](•-) to generate 1 equiv each of the active catalyst [Mn(bpy)(CO)3](-) and the starting compound 1. Additional characterization by cyclic voltammetry (CV) and infrared spectroelectrochemistry (IR-SEC) indicates that the stability of the singly reduced [Mn(CN)(bpy)(CO)3](•-) differs slightly in the N,N-DMF-TEOA solvent system compared to the MeCN-TEOA system. This contributes to the observed selectivities for HCO2H vs CO production.