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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-Re4, 4'- (MeO) 2bpy (CO) 3P (OEt) 3+, for which the optimized quantum yield for CO formation was 0. 59.
Takeda et al. (Sat,) studied this question.