The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

Abstract

© 2016 The Royal Society of Chemistry. Here we describe the synthesis and X-ray crystal structures of a panel of cis-[CoII(N4)Cl2] complexes (N4= tetradentate N atom donor ligand). We also examine the catalytic activities of these complexes in the photochemical and electrochemical reduction of CO2to CO using [Ir(ppy)3] as the photosensitizer. Among the complexes studied, cis-[CoII(PDP)Cl2] (C1) (PDP = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrrolidine) displayed the highest catalytic activity. This Co(ii) complex was able to effectively mediate the reduction of CO2to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO2to CO with up to 96% Faraday efficiency at -1.70 V (vs. SCE, SCE = saturated calomel electrode). A selectivity of up to 95% for CO production was achieved in a photocatalytic CO2reduction system by using C1 as the catalyst, Ir(ppy)3as the photosensitizer and triethylamine as the electron donor. The Co(i) species in situ generated by the one electron reduction of cis-[CoII(PDP)Cl]+is suggested to be directly responsible for CO2activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)3to C1 is a key step in the generation of active Co(i) species. The electronic structure and redox properties of the Co(i) species, [CoI(N4)Cl], as well as its role in the catalytic reaction were investigated by DFT calculations. The presence of one chloride ligand cis to the CO2coordination site neutralizes the positive charge on the Co(i) centre, therefore assisting the bound CO2molecule in attracting protons. The reaction mechanism for CO2reduction to CO catalysed by the recently reported [CoII(TPA)Cl]+(TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4ligand from cis-[CoII(N4)Cl2] were found to have a profound effect on the efficiency of CO2reduction by DFT calculations.