Efficient and Selective Photocatalytic Conversion of Low-Concentration CO2 to CO Using Mn-Complex Catalysts.

For the practical application of photocatalytic CO2 reduction systems, it should be important that the catalyst directly reduces low concentrations of CO2 in photocatalytic systems consisting of only Earth-abundant elements. Despite the development of numerous photocatalytic CO2 reduction systems, CO2 utilization at low CO2 concentrations has not been thoroughly investigated. Although fac-[MnI(diimine)(CO)3L]n+-type complexes are among the most abundant transition-metal complexes and have been studied as CO2 reduction catalysts in electrocatalytic systems, their application in photocatalytic reactions has been limited by the formation of photochemically unstable Mn(0) dimers. Thus, the photocatalytic activities of systems using Mn complexes as catalysts have not been evaluated at low CO2 concentrations. In this work, we synthesized a novel Mn(I) complex by introducing one sterically bulky mesityl group at the 6-position of the 4,4'-dimethyl-2,2'-bipyridine (dmb) ligand and used it as a catalyst in photocatalytic reactions. In the presence of trifluoroethanol (TFE) and diisopropylethylamine, the Mn complex captures CO2 to form the corresponding carbonate ester complex (MnMes-CO2TFE), and the addition of an organic photosensitizer (4DPAIPN) enables the selective reduction of CO2 to CO. MnMes-CO2TFE demonstrated excellent catalytic durability, owing to the complete suppression of Mn dimer formation in the photocatalytic reactions. The turnover number (TON) of CO formation reached a maximum of 8770 based on the MnMes-CO2TFE used, and the quantum yield of CO formation reached 40%. Furthermore, MnMes-CO2TFE exhibited high selectivity and catalytic rates for CO production, even at low CO2 concentrations (1-10%), attributed to the efficient CO2 capture reaction.