Cation-Anion Interactions in 1-Ethyl-3-methylimidazolium-Based Ionic Liquids with Aprotic Heterocyclic Anions (AHAs)

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for CO₂ capture but have been considered for other applications as well. Previously, we have shown that AHA ILs, where the only site for reaction with CO₂ is the anion, the CO₂ capacity correlates with anion basicity. Moreover, we have shown that 1-ethyl-3-methylimidazolium ([Emim]⁺)AHA ILs can react with CO₂ in two ways. The first is with the anion to form a carbamate. The second is by reaction of the deprotonated C2 of the imidazolium cation to form a carboxylate. Here we show that the amount of carboxylate formed when [Emim]⁺ AHA ILs are exposed to CO₂ is not proportional to the anion basicity, contrary to expectations. Rather, it is roughly the same for all AHA ILs investigated, as long as the anion is able to readily deprotonate the C2. Moreover, the strength of the hydrogen bond between C2-H and the anion is not proportional to the anion basicity, once again contrary to expectations. In addition, we discovered that deuterium exchange occurs not only with the acidic proton on the C2 of the cation but also with the less acidic C4 and C5 protons of the imidazolium cation, when the anions are sufficiently basic. These conclusions were reached based on quantification of cation-CO₂ and anion-CO₂ complex formation, IR spectra, ¹H NMR chemical shifts, and deuterium exchange equilibrium and kinetics.