Soluble, crystalline, and thermally stable alkali CO2 - and carbonite (CO2 2-) clusters supported by cyclic(alkyl)(amino) carbenes.

Abstract

The mono- and dianions of CO2 (i.e., CO2− and CO22−) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO2 reduction and fixation chemistry. However, CO2 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 CO2 (i.e., MCO2, M2CO2, 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 CO2 at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO2− and CO22− clusters. One-electron reduction of CAAC–CO2 adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC–CO2)]n (M = Li, Na, K, 2–4) analogous to the alkali CO2− radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M2(CAAC–CO2)]n (5–8) with reduced CO2 units which are structurally analogous to the carbonite anion CO22−. It is notable that crystalline clusters of these alkali–CO2 salts may also be isolated via the “one-pot” reaction of free CO2 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.