Pre-equilibrium reactions involving pendent relays improve CO2 reduction mediated by molecular Cr-based electrocatalysts

Abstract

Homogeneous earth abundant transition-metal electrocatalysts capable of carbon dioxide (CO₂) reduction to generate value-added chemical products are a possible strategy to minimize rising anthropogenic CO₂ emissions. Previously, it was determined that Cr-centered bipyridine-based N₂O₂ complexes for CO₂ reduction are kinetically limited by a proton-transfer step during C–OH bond cleavage. Therefore, it was hypothesized that the inclusion of pendent relay groups in the secondary coordination sphere of these molecular catalysts could increase their catalytic activity. Here, it is shown that the introduction of a pendent methoxy group favorably impacts a pre-equilibrium protonation prior to the catalytic resting state, resulting in a significant increase in catalytic activity without a loss of product selectivity for generating carbon monoxide (CO) from CO₂. Interestingly, combining the pendent methoxy group with a cationic acid causes a positive shift of the catalytic reduction potential of the system, while maintaining increased activity and quantitative selectivity. This work suggests that tuning the secondary coordination sphere with respect to cationic proton sources can result in activity improvements by modifying the kinetic and thermodynamic aspects of proton transfer in the catalytic cycle.