Protic amidinium-based ionic liquids: superior properties relative to imidazolium analogs?

Ionic liquids are versatile materials with great potential to improve existing technologies and enable new applications across a wide range of fields. Among this diverse material class, cations based on the imidazolium moiety are most frequently used, as they typically exhibit low melting points, high stability, and fast dynamics. However, imidazolium ionic liquids suffer from drawbacks in terms of toxicity, limited biodegradability, and complex synthesis. Here, we introduce novel protic formamidinium-based ionic liquids with two frequently used imide-type anions and compare their properties with structurally similar imidazolium counterparts. We investigate their thermal behavior, transport properties (viscosity, conductivity, and self-diffusion coefficients), and internal rotational dynamics. The previously unreported amidinium ionic liquids exhibit a generally favorable combination of physicochemical properties and outperform their imidazolium counterparts in most respects. These amidinium systems have lower melting points, arising from a lower enthalpy of melting that more than compensates for the lower entropy of melting, presumably due to altered hydrogen-bonding interactions. Moreover, the amidinium ionic liquids display significantly faster dynamics, especially at lower temperatures. Variable-temperature nuclear magnetic resonance measurements reveal that the rotation about the amidinium C-N partial double bond is strongly affected by the anion. These insights into the structure-property relationships of a novel cation class expand the chemical space of ionic liquids and enable the tailored design of next-generation ionic liquids to overcome current limitations across numerous applications.