Ligand Substituent Effects on Rhenium Tricarbonyl Catalysts in CO2 Reduction

The Re (I) complexes originally reported by Lehn et al. is one of the most important catalysts used for photocatalytical reduction of CO2 in homogeneous system. The mechanism for the photocatalytic reduction of CO2 to CO with Re (I) complexes has been thoroughly investigated recently. In this study, a series of rhenium tricarbonyl catalysts (Re-Me, Re-Ac, Re-Qa and Re-Im) with different substituents on 2,2-bipyridine ligand were synthesized and characterized. These catalysts were successfully applied to a light induced CO2 reduction system with triethanolamine (TEOA) as sacrificial reagent, exhibiting different turnover numbers for different catalysts. The highest turnover number was achieved for the catalyst of Re-Qa, and Re-Me and Re-Ac exhibit similar activity, while Re-Im exhibits almost no activity in the photocatalytic conversion of CO2 to CO. UV-vis spectra show that the rate of deactivation is linked to the decomposition of the catalysts in the photocatalytic system. No decomposition was observed in the absence of TEOA, suggesting that the deactivation occurs via the intermediate of one-electron-reduced (OER) species. The transient absorption spectra conformed the formation of OER in the catalytic system. The reasons for the highest turnover number of Re-Qa may be attributed to the quaternary ammonium salt group, which can serve as a mediator to facilitate the reduction process. While in the case of Re-Im, the imidazolium group might accelerate the deactivation of OER species by an intramolecular interaction. Further experiments on this effect are the subject of ongoing investigations.