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

Abstract

Here we describe the synthesis and X-ray crystal structures of a panel of cis-[Co^II(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 CO₂ to CO using [Ir(ppy)₃] as the photosensitizer. Among the complexes studied, cis-[Co^II(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 CO₂ to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO₂ to 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 CO₂ reduction system by using C1 as the catalyst, Ir(ppy)₃ as the photosensitizer and triethylamine as the electron donor. The Co(I) species in situ generated by the one electron reduction of cis-[Co^II(PDP)Cl]⁺ is suggested to be directly responsible for CO₂ activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)₃ to C1 is a key step in the generation of active Co(I) species. The electronic structure and redox properties of the Co(I) species, [Co^I(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 CO₂ coordination site neutralizes the positive charge on the Co(I) centre, therefore assisting the bound CO₂ molecule in attracting protons. The reaction mechanism for CO₂ reduction to CO catalysed by the recently reported [Co^II(TPA)Cl]⁺ (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N₄ ligand from cis-[Co^II(N₄)Cl₂] were found to have a profound effect on the efficiency of CO₂ reduction by DFT calculations.