Cyclic(Alkyl)(Amino) Carbene-Assisted Reduction of Carbon Dioxide: Isolation of Crystalline Alkali Metal Clusters of Supported CO2 Radical Anions and Dianions

The reduction of the relatively inert carbon–oxygen bonds of CO2to access useful CO2-derived organic products is one of the most important fundamental challenges in synthetic chemistry. Achieving this reduction using earth-abundant main group elements (MGEs) is especially arduous because of the difficulty in achieving strong inner-sphere reactions and bond activation events between CO2and the MGE. Herein we report the first successful chemical reduction of a zwitterionic carbene-CO2adduct by either one or two equivalents of light alkali metals to form isolable, room-temperature-stable crystalline clusters exhibiting remarkably diverse electronic and structural characteristics. The reduction of a CAAC-CO2adduct [CAAC–CO2, 1, CAAC = cyclic (alkyl)(amino) carbene] with one equivalent of lithium, sodium or potassium metal yields the monoanionic radicals (THF)3Li2(CAAC–CO2)2(2), (THF)4Na4(CAAC–CO2)4(3), or (THF)4K4(CAAC–CO2)4(4). The reduction of 1by two or more equivalents of lithium, sodium, or potassium yields the open-shell, dianionic clusters (THF)2Li6(CAAC–CO2)3(5), Li12(CAAC–CO2)6(6), Na12(CAAC–CO2)6(7), and K10(CAAC–CO2)5(8). Each of the clusters was studied by a combination of X-ray crystallography, FTIR, UV-Vis, EPR and NMR spectroscopies, and theoretical calculations. The synthetic transformation described in this report results in the facile net reduction of CO2at room temperature by lithium, sodium, and potassium metal without the need for additional metallic promoters, catalysts, or reagents – a process which does not occur in the absence of carbene.