Photocatalyses of Ru(II)-Re(I) binuclear complexes connected through two ethylene chains for CO2 reduction

•New Ru(II)Re(I) photocatalysts were synthesized with a cyclic bridging ligand.•These complexes photocatalytically reduced CO2 to CO.•The bridging ligand strongly affected the photocatalysis.•Detailed mechanistic studies on the photocatalyses are reported.

We synthesized two novel supramolecular photocatalysts for CO2 reduction [Ru(BL1)Re(CO)3Cl]2+ (12+) and [Ru(BL1)Re(CO)2{P(p-F-C6H4)3}2]3+ (33+), where BL1 is two 2,2′-bipyridine moieties connected through two ethylene chains. Compared to the corresponding supramolecular photocatalysts (22+, 43+) with only one ethylene chain in the bridging ligand (BL2), these complexes had stronger electronic interactions between the Ru and Re units through the two ethylene chains, which strengthened the oxidation power of the excited Ru unit. Although BL1 compounds functioned as photocatalysts for CO2 reduction by selectively providing CO, their photocatalyses were very different, especially, when compared with BL2 compounds. In the case of 12+, the quantum yield of CO formation (ΦCO) was larger than that of the corresponding BL2 compound. Conversely, ΦCO of the system using 33+ was approximately 40% less than the corresponding BL2 compound. The reasons for these differences were successfully clarified through mechanistic studies of these photocatalytic CO2 reductions.

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