An Iron Bis(carbene) Catalyst for Low Overpotential CO2 Electroreduction to CO: Mechanistic Insights from Kinetic Zone Diagrams, Spectroscopy, and Theory

A common challenge in molecular electrocatalysis is the relationship between maximum activity and the overpotential required to reach that rate, with faster catalysts incurring higher overpotentials. This work follows a strategy based on independent tuning of ligands in the primary coordination sphere to discover a previously unreported iron catalyst for CO₂ reduction with higher activity than similar complexes while maintaining the same overpotential. Iron complexes bearing a bis-N-heterocyclic carbene ligand (methylenebis(N-methylimidazol-2-ylidene), bis-mim) and a redox active 2,2′:6′,2″-terpyridine (tpy) ligand were synthesized and found to catalyze the selective reduction of CO₂ to CO at low overpotential with water as the proton source. Mechanistic studies based on kinetic zone diagrams, spectroscopy, and computation enable comparisons with a previously studied pyridyl–carbene analogue. Changing the bidentate ligand donor ability accelerates catalysis at the same overpotential and changes the nature of the turnover-limiting step of the reaction.