Product Control in Visible-Light-Driven CO₂ Reduction by Switching Metal Centers of Binuclear Catalysts

Controlled selectivity of products for visible-light-driven photocatalytic reduction of CO₂ in water-containing systems is highly desirable. Here, we report two highly efficient and selective binuclear complex catalysts, [Co₂(^MeL-S)­(OAc)₂]­(OAc) (CoCo) and [Cu₂(^MeL-S)­(H₂O)]­(CF₃SO₃)₂·2H₂O (CuCu), bearing a N₆S-type polypyridine sulfur ligand (^MeL-S^–) in situ formed from 2,6-bis­[(bis­(pyridylmethyl)­amino)­methyl]-4-methylmercaptophenylsulfide (^MeL-S-S-L^Me), which can promote the selective reduction of CO₂ into CO and HCOOH employing [Ru­(phen)₃]­(PF₆)₂ as photosensitizer, respectively, under the irradiation of visible light in CH₃CN/H₂O (4/1 v/v) solution. We found that CoCo can catalyze the conversion of CO₂ to CO with a high selectivity (96%) and a TON value of 6188. However, HCOOH was found for the CuCu case with a high selectivity (98%) and TON value (7540). Experimental results and DFT calculations revealed that the close Cu···Cu distance in CuCu facilitates the hydrogenation process through a 3-center-4-electron (3c-4e^–) bond to give high efficiency and high HCOOH selectivity. However, 3c-4e^– is absent in CoCo due to the well-separated Co centers and high total valence of the Co atoms, which lead to a high CO selectivity.