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-[Mn^I(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.