Controlling the Interface between Salts, Solvates, Co-crystals, and Ionic Liquids with Non-stoichiometric Protic Azolium Azolates

A non-stoichiometric approach to control the solid-state behavior of protic ionic liquids (PILs) was demonstrated by direct mixing of 4,5-dicyanoimidazole (HDCNim) with either 1-ethylimidazole (C(2)im) or 1-butylimidazole (C(4)im) in different mole fractions. Isolation and characterization of three crystalline materials (all having melting points < 100 degrees C, thus fitting the PIL definition) revealed three different but closely related systems. The structure of [HC(2)im][DCNim] consists of a confused proton system, which is so named for the fast proton exchange between basic and acidic fragments and the formation of hydrogenbonded anionic oligomers. The compound [HC(2)im][DCNim]center dot HDCNim consists of an oligomeric delocalized anion (a confused proton located between two azole fragments, in which one acts as a neutral moiety while the other as an ionic fragment, DCNim-H-DCNim) and [HC(4)im][DCNim]center dot HDCNim can be classified as a salt co-crystal. We believe that these observations will allow a deeper understanding of the behavior of confused protons and provide additional strategies for controlling the properties of important classes of materials such as active pharmaceutical ingredients.