Robust and SelectiveCobalt Catalysts Bearing Redox-ActiveBipyridyl‑ N ‑heterocyclic Carbene Frameworksfor Electrochemical CO 2 Reduction in Aqueous Solutions

An original series of cobalt complexes bearing redox-active ligands based on bipyridyl-N-heterocyclic carbene donors has been developed for electrocatalytic CO2 reduction in acetonitrile and aqueous solutions. The mechanism was examined by electrochemical methods and electronic structure calculations. From controlled potential electrolysis (CPE) in CH3CN/2% H2O solutions, 1-Co supported by a non-macrocyclic ligand gives a Faradaic efficiency (FE) for CO2-to-CO conversion of 78%, while 2-Co and 3-Co supported by tunable macrocycles afford higher selectivities for CO evolution with FEs of 91% and 98%, respectively, with the balance of charge going to H-2 production in each case. Electrochemical experiments show that the turnover frequencies across the catalyst series increase systematically from 66 s(-1) (1-Co) to 570 s(-1) (3-Co). These results demonstrate that increasing the rigidity of the ligand framework enhances catalytic activity and selectivity for CO2 reduction over the competing H-2 evolution reaction. Indeed, catalysis was extended to water, where the same trend was observed, and CPE in CO2-saturated aqueous solutions using a mercury electrode revealed that 3-Co catalyzes CO2 reduction at an overpotential of 420 mV in 0.1 M NaHCO3 buffer with a Faradaic yield for CO of 93%. Homogeneous behavior was observed in acetonitrile solutions at carbon-based electrodes, while the catalysts were found to adsorb to mercury and exhibit heterogeneous behavior in aqueous solutions. Density functional theory (DFT) calculations and localized orbital bonding analysis indicate that the first reduction of each cobalt(II) catalyst is ligand-based to form [Co-II(L center dot-)](+) species. The second reductions are computed to transition from being ligand-localized (1-Co) to mainly metal-centered (3-Co) across the series as the ligand is more constrained, which yields a more nucleophilic cobalt center in 3-Co that enhances CO2 reduction, kinetically and thermodynamically, with respect to 1-Co and 2-Co.