Electrocatalytic Promiscuity of a Coordinatively Saturated Iron Complex via Ligand Fluxionality and Internal Electric Fields

Transition metal complexes for small-molecule activation typically require an open coordination site at the metal centre. Herein we show that a coordinatively saturated Fe(II) tris(bipyridine) complex, [L₃Fe]²⁺ (L = 2,2′-bipyridine), catalyses the selective electroreduction of CO₂ to formate via electron-transfer-induced ligand fluxionality. In a coordinating solvent, single-electron 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 density functional theory reveal that the timescale of ligand dissociation and coordinating properties of the dielectric medium are critical for activity. Near the first reduction potential of [L₃Fe]²⁺, we also observe a ligand-centred CO₂ capture pathway operating through a multi-electron cascade. Further, by changing the supporting electrolyte cation from tetra-n-butylammonium to Li⁺, the same complex can be steered from CO₂-to-formate conversion to selective ligand-centred H₂ evolution, with internal electric fields around the Fe(II) centre modulating product distribution. This ligand “dissociation-rebound” mechanism, coupled to environmental control, offers a new blueprint for designing robust, earth-abundant catalysts with fluxional ligands.