Homogeneous Electrocatalytic Reduction of CO2 by a CrN3O Complex: Electronic Coupling with Redox-Active Terpyridine Fragment Favors Selectivity for CO

Electrocatalyst design and optimization strategies continue to be an active area of research interest for the applied use of renewable energy resources. The electrocatalytic conversion of CO2 is an attractive approach in this context, because of the added potential benefit of addressing its rising atmospheric concentrations. In previous experimental and computational studies, we have described the mechanism of the first molecular Cr complex capable of electrocatalytically reducing CO2 to CO in the presence of an added proton donor, which contained a redox-active 2,2'-bipyridine (bpy) fragment, CrN2O2. The high selectivity for CO in the bpy-based system was dependent on a delocalized Cr(II)(bpy•−) active state. Subsequently, we became interested in exploring how expanding the polypyridyl ligand core would impact selectivity and activity during electrocatalytic CO2 reduction. Here, we report a new CrN3O catalyst, Cr(tpytbupho)Cl2 1, where 2-([2,2':6',2''-terpyridin]-6-yl)-4,6-di-tert-butylphenolate = [tpytbupho]–, which reduces CO2 to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr(tbudhbpy)Cl(H2O) catalyst. Computational analyses indicate that although the stoichiometry of both reactions is identical, changes in the observed rate law are the combined result of a decrease in intrinsic ligand charge (L3X vs L2X2) and an increase in ligand redox activity, which result in increased electronic coupling between the doubly reduced tpy fragment of the ligand and the Cr(II) center. The strong electronic coupling enhances the rate of protonation and subsequent C–OH bond cleavage, resulting in CO2 binding becoming the rate-determining step, which is an uncommon mechanism during protic CO2 reduction.