Effect of Substituent Groups in Rhenium Bipyridine Complexes on Photocatalytic CO2 Reduction

We synthesized four different rhenium complexes, Re(bpy-R)(CO)3Cl (bpy = 2,2-bipyridine and R = H, CH3, COOH, or CN), as photocatalysts that selectively reduce CO2 to CO and investigated the effect of substituent groups (Rs) on the absorption and photocatalystic properties for CO2 reduction under 365-nm light irradiation. The Re(bpy-R)(CO)3Cl (R = H or CH3 or COOH) reduced CO2 to CO in CO2-saturated DMF-triethanolamine solution, which was irradiated with 365-nm light. The amount of CO produced by CO2 reduction differed, depending on the introduced Rs in the bipyridine moiety. We found that the ability of Re(bpy-R)(CO)3Cl (R = H or CH3 or COOH) to produce CO has a linear relationship to molar absorption coefficients of rhenium complexes at the irradiated light wavelength. Introduction of the COOH group, which has the highest molar absorption coefficient among four rhenium complexes, enhanced CO2-to-CO reduction capacity (6.59 mol/cat-mol2h) five times that of Re(bpy-H)(CO)3Cl with no R.

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