Key Process of the Photocatalytic Reduction of CO2 Using [Re(4,4‘-X2-bipyridine)(CO)3PR3]+ (X = CH3, H, CF3; PR3 = Phosphorus Ligands): Dark Reaction of the One-Electron-Reduced Complexes with CO2

Reduction of CO 2 to CO was efficiently photocatalyzed by [Re(4,4‘-X 2 bpy)(CO) 3 PR 3 ] + (X = H, Me; PR 3 = P(OEt) 3, P(O- i -Pr) 3 ) in quantum yields of 0.16−0.20. Complexes with CF 3 as X or trialkylphosphine as PR 3 have much lower photocatalytic ability. One-electron-reduced species of the complexes, which were produced by the photoinduced electron-transfer reaction with triethanolamine, reacted with CO 2 in the dark with rate constants of 3.5 × 10 - 4 −1.9 × 10 - 2 M - 1 s - 1 . The faster the rate of this process, the higher the quantum yield of CO formation. The calculated amount of CO formation, based on the assumption that the process gives CO quantitatively, was similar to the actually observed amount under various conditions. This is consistent with the thermal process being one of the rate-limiting steps in the photocatalyzed reduction of CO 2 by the rhenium complexes.