Structures and phase transitions in molecular complexes containing tetrafluoroboric acid and tetramethylpyrazine

Abstract

Crystals of tetrafluoroboric acid with tetramethylpyrazine, TMP + HBF₄ + H₂O (1 : 1 : 2, A), TMP + HBF₄ (1 : 1, B) and TMP + HBF₄ (1 : 2, C) were synthesized and characterized by physicochemical experimental methods. The crystal structures for A and B were determined at 100 K. Since C undergoes a phase transition at 217/223 K (cooling/heating) the structures were measured at 100 and 250 K. Infinite chains of molecules linked via the hydrogen bonds are present in the crystals A and B. A BF₄⁻⋯TMPH₂²⁺⋯BF₄⁻ assembly was found in the crystal structure of C. The dielectric responses have been measured for all three crystals, A, B and C, in a wide temperature (160–300 K) and frequency range (100 Hz–2 MHz). The mechanism of the phase transition at 217/223 K found in C as well as the origin of the ac conductivity was determined to be related to the dynamics of protons in the hydrogen bonds.