Development of an Efficient Photocatalytic System for CO2 Reduction Using Rhenium(I) Complexes Based on Mechanistic Studies

Abstract

The reaction mechanism of photocatalytic CO₂ reduction using rhenium(I) complexes has been investigated by means of a detailed comparison of the photocatalyses of three rhenium(I) complexes, fac-[Re(bpy)(CO)₃L] (L = SCN^- (1-NCS), Cl^- (1-Cl), and CN^- (1-CN)). The corresponding one-electron-reduced species (OER) of the complexes play two important roles in the reaction: (a) capturing CO₂ after loss of the monodentate ligand (L) and (b) donation of the second electron to CO₂ by another OER without loss of L. In the case of 1-NCS, the corresponding OER has both of the capabilities in the photocatalytic reaction, resulting in more efficient CO formation (with a quantum yield of 0.30) than that of 1-Cl (quantum yield of 0.16), for which OER species have too short a lifetime to accumulate during the photocatalytic reaction. On the other hand, 1-CN showed no photocatalytic ability, because the corresponding OER species does not dissociate the CN^- ligand. Based on this mechanistic information, the most efficient photocatalytic system was successfully developed using a mixed system with fac-[Re(bpy)(CO)₃(CH₃CN)]⁺ and fac-[Re{4,4‘-(MeO)₂bpy}(CO)₃{P(OEt)₃}]⁺, for which the optimized quantum yield for CO formation was 0.59.