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

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• 作者：Matthew D. Sampson ; Clifford P. Kubiak
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：February 3, 2016
• 年：2016
• 卷：138
• 期：4
• 页码：1386-1393
• 全文大小：503K
• ISSN：1520-5126

文摘

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 (η = 0.3–0.45 V). With Mg²⁺, catalysis proceeds via a reductive disproportionation reaction of 2CO₂ + 2e^– → CO and CO₃^2–. 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 Mg²⁺ is compared with those of other commonly studied CO₂ reduction catalysts.