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

Earth-abundant manganese bipyridine (bpy) complexes are well-established molecular electrocatalysts for proton-coupled carbon dioxide (CO2) 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 CO2, 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 Mg2+ cations, to significantly increase the rate of catalysis (by over 10-fold) at these low overpotentials (i.e., the same potential as CO2, binding): Reduction of CO2, occurs at one of the lowest overpotentials ever reported for molecular electrocatalysts (eta = 0.3-0.45 V). With Mg2+, catalysis proceeds via a reductive disproportionation reaction of 2CO(2) + 2e(-) -> CO and CO32-. 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 Mg2+ is compared with those of other commonly studied CO2, reduction catalysts.