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.