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[Fe^II(NP^iso)(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[Fe^II(NP^iso)(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 (ΔG^‡ = 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.