Photocatalytic reduction of CO2 to CO and formate by a novel Co(ii) catalyst containing a cis-oxygen atom: photocatalysis and DFT calculations

The conversion of carbon dioxide (CO2) to fuels or value-added chemicals by a photocatalytic system has recently been of growing research interest. One of the challenges is the development of new catalysts with high activity and low cost. Cobalt complexes have long been used as catalysts for the reduction of CO2 in either electrochemical or photochemical systems. Recently, a series of cis-CoII complexes of tetradentate pyridine-amine ligands (N4-ligands) exhibited high activity in the reduction of CO2 in homogeneous photocatalytic systems. However, only CO was obtained as the reduction product. In this regard, herein, we report a novel cis-CoII complex C1 supported by an N4 ligand derivatized with TPA (TPA = tris(2-pyridylmethyl)amine). In contrast to the aforementioned CoII catalysts, which contain two halogen atoms at cis-positions, C1 contains one oxygen atom at one cis-coordination site. The structure of C1 was fully characterized by MS, elemental analysis, and single-crystal X-ray diffraction. Experiments on the photocatalytic reduction of CO2 revealed that C1 is able to convert CO2 to not only CO but also formate in a homogeneous system containing C1 as a catalyst, Ir(ppy)3 as a photosensitizer, and triethylamine as an electron donor under visible-light irradiation. The catalytic activity and distribution of reduction products of this system are highly affected by the solvent environment. The presence of water in this system enhances the efficiency of 2H+-to-H2 and CO2-to-formate conversions. Electrochemical and steady-state emission quenching experiments indicate that photoinduced electron transfer from excited Ir(ppy)3 to C1 is thermodynamically feasible. A photogenerated CoI species is suggested to be the active species involved in the reduction of CO2 and protons. DFT calculations were performed to elucidate the catalytic pathways of the formation of CO, formate, and H2 in this system; four pathways, namely, one for the formation of CO, one for the formation of hydrogen, and two for the formation of formate, were suggested. The results revealed that the oxygen atom at the cis-coordination site in C1 plays an important role in stabilizing the transition state during the transformation of CO2 at the cobalt center.