Photocatalytic CO2 reduction catalysed by 3d transition metal complexes bearing an S2N2 ancillary ligand equipped with pyridine pendants as binding sites for Lewis acids

Abstract

To solve the current environmental and energy problems, the development of an efficient photocatalytic system for CO₂ reduction, producing useful chemical resources such as CO, is a promising approach. Herein, we have synthesized 3d-transition metal complexes (Mn, Fe, and Co) using an S₂N₂-type ligand (1), inspired by the structure of a natural CO₂-fixing enzyme, [NiFe]CODH, as CO₂-reducing catalysts. When a Zn²⁺ salt was added as a Lewis acid to a solution of one of the complexes, containing [Ru(bpy)₃]²⁺ as a photosensitizer and BIH (= 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole) as a sacrificial reductant in dimethylacetamide/H₂O (9 : 1, v/v), the amount of CO evolved under photoirradiation at 450 nm was increased in comparison with that without the Zn²⁺ salt. Notably, the selectivity of CO formation by 1-Co was enhanced from 73% to 98% under photoirradiation at 450 nm, and the TON for CO formation by 1-Mn increased to 875 after irradiation for 4 h.