Linker-Length Dependence of Crystal Structures and Thermal Properties of Bis(imidazolium) Salts with Tetrafluoroborate Anion

A series of bis(imidazolium) salts with several n-alkylene linkers (carbon numbers: 4-8) incorporating tetrafluoroborate anions were characterized by thermal analysis and single-crystal X-ray analysis. A correlation between macroscopic thermal properties and microscopic crystal structures was investigated. A unique linker-length dependence of the melting points of bis(imidazolium) salts is attributed to variations in the molecular conformation and the packing structure resulting from a balance among cooperative ionic interaction, van der Waals interaction, and pi-pi interaction in the crystalline state. In addition, an even-odd effect of the n-alkylene linker was observed in the crystal structures.