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

Catalysts promoting multielectron charge delocali-zation offer selectivity for the CO2 reduction 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]degrees complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe]degrees, 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.