Ligand Non-Innocence and Proton Channel Promote Cobalt-Catalyzed Electrochemical CO2 Reduction with Predominant CO Selectivity

Enzymes and heterogenous catalysts for CO2 reduction reactions (CO2RR) use secondary interactions between metal sites and protein-derived coordination spheres to control the precise transfer of protons and electrons to minimize overpotential and maximize selectivity over the competitive hydrogen evolution reaction. We now report a molecular cobalt (II) complex [1-Co]2+ that uses a similar strategy under homogenous condition through the use of a redox non-innocent ligand, Hbbpya, containing two 2,2′-bipyridine chelating groups linked by a -NH moiety. By acting as a structural anchor to form a hydrogen-bonded network of four phenol groups, the -NH group enables efficient binding and protonation of CO2 at a cobalt center to form CO under electrocatalytic conditions at a moderate overpotential and with high selectivity. Methylation of the -NH group in [2-Co]2+ results in a loss of CO2RR selectivity and increased production of hydrogen. The complexes [1-Co]2+ and [2-Co]2+, and their one and two electron-reduced counterparts are extensively characterized by X-ray diffraction, cyclic voltammetry, electron paramagnetic resonance and density functional theoretical calculations. The electronic structure of the catalytically active doubly-reduced [1-Co]0 and [2-Co]0 can be best described as containing a cobalt(I) center and a mono reduced ligand system. Most importantly, in stoichiometric reactions, due to the presence of an efficient proton relay, [1-Co]0 performs fast two-electron reduction of CO2 to form [1-Co]2+ and CO, thereby, avoiding the formation of the high-energy CO2 radical anion, reminiscent of the CO2RR mechanism proposed in NiFe-carbon monoxide dehydrogenase. In contrast, a one-electron chemistry prevails in reactions of [2-Co]0 and CO2.