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 phi(CO) = 0.60 to 0.74 using fac-[Re-I(bpy)(CO)(3)(CH3CN)](+), Phi(HCOOH) = 0.58 using trans(Cl)-Ru-II(dtbb)(CO)(2)Cl-2 and Phi(HCOOH) = 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.