Ligand Fluxionality in a Coordinatively Saturated Iron Complex Enables Selective Electrocatalytic Reduction of CO2 to Formate

Transition metal complexes for small molecule activation are typically designed with an open coordination site at the metal center for substrate binding and activation. Herein we show that a coordinatively saturated Fe(II) tris(bipyridine) complex, [L3Fe]2+ (L=2,2’-bipyridine, abbr. bpy), catalyzes the selective and facile electroreduction of CO2 to formate via electron transfer-induced ligand fluxionality of the bpy scaffold. In a coordinating solvent, electrochemical reduction induces partial dissociation of a bipyridine ligand, creating transient binding sites that enable CO2-to-formate conversion with <1% H2 at low overpotentials. Cyclic voltammetry, finite-element simulations and DFT calculations reveal that the timescale of ligand dissociation and coordinating properties of the dielectric medium are critical for the observed activity. We further show that the extent of bpy dissociation triggered by electron transfer can be controlled by switching the supporting electrolyte cation from TBA+ to Li+, which we find, leads to the complete suppression of formate production and results in H2 evolution instead. This ligand “dissociation-rebound” mechanism, coupled to environmental control, offers an alternate blueprint for designing earth-abundant, first-row transition metal based molecular electrocatalysts.