Exchange Coupling Determines Metal-Dependent Efficiency for Iron-and Cobalt-Catalyzed Photochemical CO₂ Reduction

Catalysts promoting multielectron charge delocalization offer selectivity for the CO₂reduction reaction (CO₂RR) over the competing hydrogen evolution reaction. Here, we show metal-ligand exchange coupling as an example of charge delocalization that can determine the efficiency for photocatalytic CO₂RR. A comparative evaluation of iron and cobalt complexes supported by the redox-Active ligand tpyPY2Me establishes that the two-electron reduction of [Co(tpyPY2Me)]²⁺([Co]²⁺) occurs at potentials 770 mV more negative than the [Fe(tpyPY2Me)]²⁺([Fe]²⁺) analogue by maximizing the exchange coupling in the latter compound. The positive shift in the reduction potential promoted by metal-ligand exchange coupling drives [Fe]²⁺to be among the most active and selective molecular catalysts for photochemical CO₂RR reported to date, maintaining up to 99% CO product selectivity with total turnover numbers (TONs) and initial turnover frequencies exceeding 30,000 and 900 min^-1, respectively. In contrast, [Co]²⁺shows much lower CO₂RR activity, reaching only ca. 600 TON at 83% CO product selectivity under similar conditions accompanied by rapid catalyst decomposition. The spin density plots of the two-electron reduced [Co]⁰complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe]⁰, rationalizing the observed negative shift in two-electron reduction potentials from the [M]²⁺species and lowered CO₂RR efficiency for the cobalt complex relative to its iron congener. This work emphasizes the contributions of multielectron metal-ligand exchange coupling in promoting effective CO₂RR and provides a starting point for the broader incorporation of this strategy in catalyst design.