1,2-Bis[N-(N′-alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals

Abstract

A new class of organic ionic plastic crystals was discovered. A series of alkylene 1,2-bis[N-(N′-alkylimidazolium)] salts with Br⁻ and PF₆⁻ anions was prepared and most of the ethylene (C₂) linked compounds undergo multiple solid–solid phase transitions as determined by differential scanning calorimetry (DSC). The salts with longer spacers (C₃ or C₄) between the imidazolium units do not display any solid–solid transitions. The PF₆⁻ salts with terminal C₁₀ and C₁₂n-alkyl moieties are “classical” organic ionic plastic crystals by Timmermans' definition; they have low ΔS_f (11 J K⁻¹ mol⁻¹ for C₁₀ and 12 J K⁻¹ mol⁻¹ for C₁₂). Additionally, the C₂ linked bis-imidazolium salts with n-butyl, n-hexyl, n-heptyl and n-octyl termini all undergo two, three or four solid state transitions and exhibit ΔS_f values in the range of 36–49 J K⁻¹ mol⁻¹, which are similar to those of other known ionic plastic crystalline materials. These materials were additionally characterized via ionic conductivity and solid state NMR. These ionic plastic crystals are presumably single-ion conductors, but the ionic conductivities appear to be too low for practical applications. The activation energy for conduction decreases as these compounds are heated through each solid–solid transition. The lack of any change in solid state ²H NMR spectra with temperature indicates that there is no change in phenyl ring flipping, suggesting no change in the imidazolium local environment. A resolution of this apparent dichotomy is perhaps that the counterions reside with the ethylene spacers between imidazolium moieties.