Anisotropic, Organic Ionic Plastic Crystal Mesophases from Persubstituted Imidazolium Pentacyanocyclopentadienide Salts

Abstract

We describe the synthesis, supramolecular organization, and thermal characteristics of an unprecedented family of symmetric 1,2,3,4,5-pentaalkylimidazolium ([(Cn)5im]+) salts equipped with halide, nitrate, or pentacyanocyclopentadienide ([Cp(CN)5]−) counterions. Salts containing relatively small anions were obtained as low-melting solids, whereas those with [Cp(CN)5]− anions were found to be ionic liquids even below room temperature. A permethylated derivative, [(C1)5im][Cp(CN)5], proved to be exceptional. Upon heating, the salt self-organized into a new type of organic ionic plastic crystal (OIPC) mesophase, which was termed Mhex and whose anisotropic structure featured hexagonally ordered, rotating anionic stacks positioned within a continuum composed of disordered cations. The structure of the mesophase resembles that of classical columnar liquid-crystalline phases, despite the absence of long, flexible chains. In the Mhex phase, the cations surrounding the anionic columns effectively fulfill the role of “softening” structural constituents, much in the same way as flexible chains. The discovery of the novel mesophase, which displays a two-dimensional, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies around a columnar axis, represents a new paradigm in the field of OIPCs. Relatively high ionic conductivities were measured in the Mhex phase, particularly after doping with the corresponding sodium salt, Na[Cp(CN)5], demonstrating the materials’ potential for use in electrochemical applications such as sodium-ion batteries.