Pendant Amine-Promoted Complete Eight-Electron Photoreduction of CO2 to Methane by a Molecular Nickel Catalyst

Abstract

Solar-driven CO2 reduction offers a sustainable route to carbon neutrality by converting greenhouse gases into value-added fuels. Here, we report a secondary coordination sphere design strategy that directs CO2 photoreduction selectivity toward methane (CH4). A redox-active Ni complex (C1), incorporating two (6-amino-2-(phenylazo)pyridine) ligand with a pendant amine, exhibits markedly enhanced CH4 selectivity compared to control complexes lacking the pendant –NH2 group. Systematic evaluation of sacrificial donors, proton sources, and photosensitizers identified a dimeric Cu(I)-based photosensitizer (Cu-PS-1) as a cost-effective alternative to Ir-systems, achieving TONCO2→CO = 4789, TONCO2→CH4 = 1130, and TONCO→CH4 = 3102, rivalling Ir-PS-1. Operando UV-vis, FTIR, and EPR spectroscopy, together with DFT calculations, revealed a stepwise 8e⁻/8H⁺ reduction pathway in which the pendant –NH2 group stabilizes key COOH and CHx intermediates through hydrogen bonding, lowering activation barriers and steering selectivity toward CH4. These results establish OCS engineering as a powerful design principle for earth-abundant molecular catalysts and highlight new opportunities for selective CO2-to-CH4 conversion in artificial photosynthesis and carbon valorization.