Development of a Highly Durable Photocatalytic CO2 Reduction Using a Mn-Complex Catalyst: Application of Selective Photosplitting of a Mn(0)–Mn(0) Bond

fac-[MnI(diimine)(CO)3(L)]0/+ has attracted significant attention as a catalyst for the photocatalytic reduction of CO2. However, in such photocatalytic systems, the photoexcitation of Mn complexes and reaction intermediates induces their decomposition, which lowers the durability of these systems. In this study, we clarified the primary process whereby the Mn complex catalyst decomposes during the photocatalytic reaction. Based thereupon, we successfully constructed a highly durable photocatalytic system, of which the turnover number of formate (TONHCOO–) exceeded 1700 when fac-[MnI(bpy)(CO)3((OC(O)OC2H5N(C2H5OH)2) (Mn-CO2-TEOA) as the catalyst, [OsII(4,4′-dimethyl-bpy)(5,5′-dimethyl-bpy)2]2+ (Os) as the photosensitizer, and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the reductant were used in conjunction with irradiation at λex ≥ 620 nm. In contrast, for the same photocatalytic system, irradiation at λex ≥ 480 nm lowered the TONHCOO– to less than 60. The significant difference in the durability of the photocatalytic system arises from the dependence of the Mn(0)–Mn(0) dimer [Mn02(bpy)2(CO)6] (Dim-Mn), an intermediate produced during the photocatalytic reaction, on the wavelength of the irradiated light for its photoreactivity. That is, the irradiation of Dim-Mn at λex ≥ 620 nm selectively induces splitting of the Mn–Mn bond to produce [Mn0(bpy)(CO)3] (Mn•) and, contrary to this, splitting of the Mn(0)–CO bonds and further decomposition processes are induced by irradiation at λex ≥ 480 nm.