Homogeneous Electrocatalytic Reduction of CO 2 by a CrN 3 O Complex: Electronic Coupling with a 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 carbon dioxide (CO 2 ) 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 CO 2 to carbon monoxide (CO) in the presence of an added proton donor, which contained a redox-active 2,2'-bipyridine (bpy) fragment, CrN 2 O 2 . 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 the selectivity and activity during electrocatalytic CO 2 reduction. Here, we report a new CrN 3 O catalyst, Cr(tpy tbu pho)Cl 2 ( 1 ), where 2-(2,2':6',2″-terpyridin-6-yl)-4,6-di- tert -butylphenolate = [tpy tbu pho] - , which reduces CO 2 to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr( tbu dhbpy)Cl(H 2 O) 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 the intrinsic ligand charge (L 3 X vs L 2 X 2 ) and an increase in the 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 CO 2 binding becoming the rate-determining step, which is an uncommon mechanism during protic CO 2 reduction.