Geometric control of Fe(i) intermediates in CO2 photoreduction by tetrahedral tripodal phosphine complexes

The development of homogeneous CO2 photoreduction catalysts based on Earth-abundant metals remains limited due to an insufficient mechanistic understanding of multielectron activation pathways. Here we show that a pseudotetrahedral Fe(II) complex supported by a tripodal tetradentate phosphine ligand, 5[FeII(NPiso)(Cl)](BPh4), functions as an efficient and selective molecular catalyst for visible-light-driven CO2-to-CO conversion. Under the optimized conditions in acetonitrile, 5[FeII(NPiso)(Cl)](BPh4) achieves turnover numbers exceeding 1300, turnover frequencies of up to 445 h−1, and quantum yields of up to 0.64%, placing it among the most active Fe-based molecular catalysts for CO2 photoreduction. Electrochemical, spectroelectrochemical, fluorescence quenching, and high-resolution ESI-MS measurements, supported by computational studies, reveal that catalysis proceeds via a one-electron-reduced Fe(I) acetonitrile adduct formed by ligand substitution of the Fe(II) precursor. This Fe(I) species promotes CO2 binding and proton-coupled reduction through well-defined Fe(I/II) intermediates, culminating in CO release and regeneration of the active complex. The CO-release step is found to be the rate-determining step (deltaGx = 12.9 kcal mol-1) with the generation of a Fe(II) complex displaying a coordination vacancy. The addition of a new acetonitrile molecule in tandem with one electron reduction regenerates the catalytically active species. These results demonstrate that pseudotetrahedral P3N coordination environments stabilize reactive Fe(I) intermediates essential for CO2 activation, offering mechanistic design principles towards next-generation iron catalysts.