N-Methyl-N-alkylpyrrolidinium Hexafluorophosphate Salts: Novel Molten Salts and Plastic Crystal Phases

A new series of salts, based on the N -methyl- N -alkylpyrrolidinium cation and the PF 6 - anion, are reported and their thermal properties described for alkyl = Me, Et, Pr, Bu, Hx, and Hp. X-ray structures of several of the salts are also reported. The N, N -dimethylpyrrolidinium hexafluorophosphate has a melting point greater than 390 °C; however, the N -methyl- N -butylpyrrolidinium derivative melts at 70 °C. Most of the PF 6 - salts were observed to have lower melting points in comparison with the analogous iodide salts. Most of the salts exhibit one or more thermal transitions prior to melting and a final entropy of melting less than 20 J K -1 mol -1, behavior which has previously been associated with the formation of plastic crystal phases. Good crystal structure solutions were obtained at low temperatures in the case of the alkyl = propyl and heptyl derivatives. The loss of diffraction peaks and changes in symmetry at higher temperatures indicated the presence of dynamic rotational disorder, supporting the understanding that the plastic properties arise from rotational motions in the crystal.