Robust and Selective Cobalt Catalysts Bearing Redox-Active Bipyridyl-NHC Frameworks for Electrochemical CO2 Reduction in Aqueous Solutions

An original series of cobalt complexes bearing redox-active bipyridyl-N-heterocyclic carbene-based ligands 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 electrolyses (CPEs) 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 macrocycles afford higher selectivities for CO evolution with FEs of 91% and 97%, respectively, with the balance of charge going to H2 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 H2 evolution reaction. Indeed, catalysis...