Manganese Electrocatalysts with Bulky Bipyridine Ligands: Utilizing Lewis Acids To Promote Carbon Dioxide Reduction at Low Overpotentials

摘要

Earth-abundant manganese bipyridine (bpy) complexes are well-established molecular electrocatalysts for proton-coupled carbon dioxide (CO_2) reduction to carbon monoxide (CO). Recently, a bulky bipyridine ligand, 6,6'-dimesityl-2,2'-bipyridine (mesbpy), was utilized to significantly lower the potential necessary to access the doubly reduced states of these manganese catalysts by eliminating their ability to dimerize after one-electron reduction. Although this Mn mesbpy catalyst binds CO_2 at very low potentials, reduction of a resulting Mn(Ⅰ)-COOH complex at significantly more negative potentials is required to achieve fast catalytic rates. Without reduction of Mn(Ⅰ)-COOH, catalysis occurs slowly via a alternate catalytic pathway-protonation of Mn(Ⅰ)-COOH to form a cationic tetracarbonyl complex We report the use of Lewis acids, specifically Mg~(2+) cations, to significantly increase the rate of catalysis (by over 10-fold) at these low overpotentials (i.e., the same potential as CO_2 binding). Reduction of CO_2 occurs at one of the lowest overpotentials ever reported for molecular electrocatalysts (η = 0.3-0.45 V). With Mg~(2+), catalysis proceeds via a reductive disproportionation reaction of 2CO_2 + 2e~-→CO and CO_3~(2-). Insights into the catalytic mechanism were gained by using variable concentration cyclic voltammetry, infrared spectroelectrochemistry, and bulk electrolysis studies. The catalytic Tafel behavior (log turnover frequency vs overpotential relationship) of [Mn(mesbpy)(CO)_3(MeCN)](OTf) with added Mg~(2+) is compared with those of other commonly studied CO_2 reduction catalysts.