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

Abstract

<i>fac</i>-[Mn^I(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 (TON_HCOO-) exceeded 1700 when <i>fac</i>-[Mn^I(bpy)(CO)₃((OC(O)OC₂H₅N(C₂H₅OH)₂) (<b>Mn-CO</b>_<b>2</b><b>-TEOA</b>) as the catalyst, [Os^II(4,4'-dimethyl-bpy)(5,5'-dimethyl-bpy)₂]²⁺ (<b>Os</b>) as the photosensitizer, and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (<b>BIH</b>) as the reductant were used in conjunction with irradiation at λ_ex ≥ 620 nm. In contrast, for the same photocatalytic system, irradiation at λ_ex ≥ 480 nm lowered the TON_HCOO- 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] (<b>Dim-Mn</b>), an intermediate produced during the photocatalytic reaction, on the wavelength of the irradiated light for its photoreactivity. That is, the irradiation of <b>Dim-Mn</b> at λ_ex ≥ 620 nm selectively induces splitting of the Mn-Mn bond to produce [Mn⁰(bpy)(CO)₃] (<b>Mn</b>^•) and, contrary to this, splitting of the Mn(0)-CO bonds and further decomposition processes are induced by irradiation at λ_ex ≥ 480 nm.