Manganese\nElectrocatalysts with Bulky Bipyridine Ligands:\nUtilizing Lewis Acids To Promote Carbon Dioxide Reduction at Low Overpotentials

Abstract

Earth-abundant\nmanganese bipyridine (bpy) complexes are well-established\nmolecular electrocatalysts for proton-coupled carbon dioxide (CO₂) reduction to carbon monoxide (CO). Recently, a bulky bipyridine\nligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy), was\nutilized to significantly lower the potential necessary to access\nthe doubly reduced states of these manganese catalysts by eliminating\ntheir ability to dimerize after one-electron reduction. Although this\nMn mesbpy catalyst binds CO₂ at very low potentials, reduction\nof a resulting Mn­(I)–COOH complex at significantly more negative\npotentials is required to achieve fast catalytic rates. Without reduction\nof Mn­(I)–COOH, catalysis occurs slowly via a alternate catalytic\npathway–protonation of Mn­(I)–COOH to form a cationic\ntetracarbonyl complex. We report the use of Lewis acids, specifically\nMg²⁺ cations, to significantly increase the rate of catalysis\n(by over 10-fold) at these low overpotentials (i.e., the same potential\nas CO₂ binding). Reduction of CO₂ occurs at\none of the lowest overpotentials ever reported for molecular electrocatalysts\n(η = 0.3–0.45 V). With Mg²⁺, catalysis proceeds\nvia a reductive disproportionation reaction of 2CO₂ + 2e^– → CO and CO₃^2–. Insights into the catalytic mechanism were gained by using variable\nconcentration cyclic voltammetry, infrared spectroelectrochemistry,\nand bulk electrolysis studies. The catalytic Tafel behavior (log turnover\nfrequency vs overpotential relationship) of [Mn­(mesbpy)­(CO)₃(MeCN)]­(OTf) with added Mg²⁺ is compared with those of\nother commonly studied CO₂ reduction catalysts.