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

Abstract

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.

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

“…A previous version of this manuscript has been deposited on a preprint server. 40 The authors declare no competing financial interest.…”

Mechanistic Comparisons of bpy and phen Ligand Backbones in Cr-Mediated Co-Electrocatalytic CO2 Reduction

“…Finally, we can compare the results described with complex 2 to those obtained with complex 3 (Figure 3C). 8 The reduction potential of 3 is 40 mV closer to Ph2DBTD than 2, but the activity only increases by a factor of 21 from the intrinsic response (ratio of catalytic and cocatalytic TOF values), 9 in comparison to a factor of 26 for 2 (Table 1). Notably, the calculated structures of the 4 0 𝐂𝐫(𝐂𝐎 𝟐 𝐇)(𝐑𝐌) −2 intermediate for 2 and 3 where RM is Ph2DBTD show a clear steric hindrance effect on the binding interaction (Figure 8).…”

“…If earth-abundant transition metals and renewable energy sources, like wind or solar, are used to drive the electrochemical reduction of CO2 to carbon monoxide (CO), the production of chemical feedstocks and fuels could be decoupled from petrochemical sources and feasibly scaled. 2 While earth-abundant transition metal complexes have been studied across a range of metal centers for the reduction of CO2, [3][4][5][6] the use of chromium (Cr)-centered catalysts remains relatively underdeveloped with, to our knowledge, only three known catalysts reported by our group being active for the reduction of CO2 to CO. [7][8][9] There is growing interest in the use of redox mediators (RMs) to improve catalytic small molecule conversion processes. Previous examples of using RMs have increased the activity of the catalytic system and/or shifted reaction selectivity by facilitating electron transfer, avoiding high energy intermediates, or avoiding competitive pathways.…”