Soluble, crystalline, and thermally stable alkali CO(2)(−) and carbonite (CO(2)(2−)) clusters supported by cyclic(alkyl)(amino) carbenes

The mono- and dianions of CO(2) (i.e., CO(2)(−) and CO(2)(2−)) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO(2) reduction and fixation chemistry. However, CO(2) anions are highly unstable and difficult to study. As such, examples of stable compounds containing these ions are extremely limited; the unadulterated alkali salts of CO(2) (i.e., MCO(2), M(2)CO(2), M = alkali metal) decompose rapidly above 15 K, for example. Herein we report the chemical reduction of a cyclic (alkyl)(amino) carbene (CAAC) adduct of CO(2) at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO(2)(−) and CO(2)(2−) clusters. One-electron reduction of CAAC–CO(2) adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC–CO(2))](n) (M = Li, Na, K, 2–4) analogous to the alkali CO(2)(−) radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M(2)(CAAC–CO(2))](n) (5–8) with reduced CO(2) units which are structurally analogous to the carbonite anion CO(2)(2−). It is notable that crystalline clusters of these alkali–CO(2) salts may also be isolated via the “one-pot” reaction of free CO(2) with free CAAC followed by the addition of alkali metals – a process which does not occur in the absence of carbene. Each of the products 2–8 was investigated using a combination of experimental and theoretical methods.