Rhenium(i) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction.

We developed new cyclic Re(i)-based trinuclear redox photosensitizers with both high oxidation power in the excited state and strong reduction power in the reduced form. These excellent properties were achieved by introducing electron-donating groups on the diimine ligand of the Re(i) metal centre and by connecting each Re(i) unit with polyphenyl-bisphosphine bridging ligands. These Re-rings were applied to homogenous visible light-driven photocatalytic CO2 reduction in conjunction with various mononuclear catalysts, such as Re(i), Ru(ii) and Mn(i) metal complexes, employing a relatively weak sacrificial electron donor, triethanolamine. Each system showed good product selectivity (CO or HCOOH) and an excellent quantum yield of product formation ΦCO = 0.60 to 0.74 using fac-[Re(I)(bpy)(CO)3(CH3CN)](+), ΦHCOOH = 0.58 using trans(Cl)-Ru(II)(dtbb)(CO)2Cl2 and ΦHCOOH = 0.48 using a fac-[Mn(I)(dtbb)(CO)3(CH3CN)](+) catalyst. The high photocatalytic efficiencies for CO2 reduction are attributed to efficient reductive quenching of the Re-ring by triethanolamine and fast electron transfer from the generated one-electron-reduced species of the ring to the catalyst.