Room Temperature Alkali Metal Reduction of Carbon Dioxide

The reduction of the relatively inert carbon–oxygen bonds of CO₂ to access useful CO₂-derived organic products is one of the most important fundamental challenges in synthetic chemistry. Facilitating this bond-cleavage using earth-abundant, non-toxic main group elements (MGEs) is especially arduous because of the difficulty in achieving strong inner-sphere interactions between CO₂ and the MGE. Herein we report the first successful chemical reduction of CO₂ at room temperature by alkali metals, promoted by a cyclic(alkyl)(amino) carbene (CAAC). One-electron reduction of CAAC-CO₂ adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals clusters [M(CAAC–CO₂)]_n(M = Li, Na, K, 2-4) and two-electron alkali metal reduction affords open-shell, dianionic clusters of the general formula [M₂(CAAC–CO₂)]_n (5-8). It is notable that these crystalline clusters of reduced CO₂ may also be isolated via the “one-pot” reaction of free CO₂ with free CAAC followed by the addition of alkali metals – a reductive process which does not occur in the absence of carbene. Each of the products 2-8 were investigated using a combination of experimental and theoretical methods.