Effect of counter ions on physical properties of imidazole-based proton conductors

Abstract

Heterocycles are an interesting class of compounds for crystal engineering due to their ability to form moderate and weak hydrogen bonds. The physical properties of the newly synthesized proton conducting materials: imidazolium phthalate (salt 1) and imidazolium terephthalate (salt 2) were investigated. These salts differ in the acidic moiety: for salt 1 the carboxylic groups are in ortho position and for salt 2 in the para position of the phenyl ring. The structures of salts 1 and 2 were studied by NMR and X-ray diffraction combined with DFT calculations. The study of critical points and structure of these crystals shows that there are two types of intermolecular hydrogen bonding: medium strength hydrogen bonds (NH⋯O) and weak hydrogen bonds (CH⋯O). These intermolecular hydrogen bonding system types allow us to describe conductive properties using the Grotthuss mechanism. In the electrical conduction mechanism acid molecules, as well as imidazole ions are involved. In salt 1 the oscillating properties as a function of temperature were examined and show an additional band above the temperature of 373 K which is related to the presence of a phthalic acid anhydride. Salt 1 with ortho substitution has good conductive properties (σmax ∼10−1 S/m), while the salt 2 with para substitution has good thermal stability (melting point 464 K).