Thermal properties of N-alkyl-N-methylpyrrolidinium and N-butylpyridinium fluorometallates and physicochemical properties of their melts

Abstract

A series of N-alkyl-N-methylpyrrolidinium (RMPyr+, where R = E: ethyl, B: butyl, and H: hexyl) and N-butylpyridinium (BPy+) salts based on the fluorocomplex anions, BF₄⁻, PF₆⁻, SbF₆⁻, NbF₆⁻, TaF₆⁻, and WF₇⁻, have been synthesized and their thermal behavior has been investigated. The melting points of the RMPyr⁺ salts are above room temperature with the trend; BMPyrAF₆ < HMPyrAF₆ < EMPyrAF₆ for the hexafluorocomplex salts. Some of the salts containing BMPyr⁺ and HMPyr⁺ exhibit phase transitions in the solid states. Similar melting points of BPy⁺ salts of PF₆⁻, SbF₆⁻, TaF₆⁻, and WF₇⁻ are observed at around 350 K. Ionic conductivity and viscosity for BMPyrNbF₆ (3.0 mS cm⁻¹ and 164 cP at 328 K) are similar to those for BMPyrTaF₆ (3.0 mS cm⁻¹ and 165 cP at 328 K), resulting from the similarity of the anions in size. The activation energies of ionic conductivity for the NbF₆⁻ and TaF₆⁻ salts are 18 and 20 kJ mol⁻¹, and those for viscosity are 23 and 25 kJ mol⁻¹, respectively calculated by Arrhenius equation in the temperature range between 328 and 348 K. Electrochemical windows of BMPyrNbF₆, BMPyrTaF₆, and BMPyrWF₇ are about 4.0, 5.0 and 3.1 V, respectively.