Determination of Missing Crystal Structures in the 1-Alkyl-3-methylimidazolium Hexafluorophosphate Series: Implications on Structure-Property Relationships

We have performed single crystal X-ray diffraction analyses of two 1-alkyl-3-methylimidazolium hexafluorophosphate ([C(n)mim]PF6) salts, [C(1)mim]PF6, and [C(3)mim]PF6. The thermo-dynamically metastable crystalline phase (beta phase) of the former was obtained by crystallization from the melt in a capillary for the crystallographic analysis. Both the cation and the anion in the beta phase of [C(1)mim]PF6 show higher point group symmetry than those in the stable alpha phase. This result is shown to be consistent with the fact that the melting point of the beta polymorph (314 K) is significantly lower than that of the alpha polymorph (364 K). The crystal structure of [C(3)mim]PF6 seems to be typical of that characteristic of the crystals of imidazolium-based room temperature ionic liquids. The cation conformation is the one in between those characteristic of the crystal structures of [C(2)mim]PF6 and [C(4)mim]PF6. When taken together, all crystallographic data for the [C(n)mim]PF6 series (n = 1-4) demonstrate the influence of the alkyl chain length dependence on structural packing in these salts. The free volume fractions for these structures are calculated and compared to the lattice energy simply based on formula unit volume. It is shown that the difference in the alkyl chain length of the imidazolium cations does not monotonically change with the lattice energy of the crystal structure, and the latter also depends on the packing efficiency of the constituent ions in the structure.