Thermal phase behaviour of N-alkyl-N-methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide salts

The phases, ion crystal packing and thermal properties of the N-alkyl-N-methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl) imide (PYR_1RTFSI and PIP_1RTFSI (subscript R ≤ 1 for methyl and = 2 for ethyl), respectively) salts are compared using powder and single-crystal x-ray diffraction (XRD) and differential scanning calorimetry (DSC). The crystal structure of PIP₁₂TFSI has been determined at 123 K. The salt crystallizes in the triclinic space group with Z ≤ 8. Structural data are also reported for PYR₁₁TFSI at 153 K and PIP ₁₂TFSI at 223 K. PIP₁₁TFSI has identical ion crystal packing to the analogous pyrrolidinium salt PYR₁₁TFSI. Since increasing the cation alkyl chain length to propyl or butyl (R ≤ 3 or 4) reduces the melting point of the salts below room temperature, this study may provide valuable insight into why these pyrrolidinium and piperidinium salts form room-temperature ionic liquids.