Crystal Growth, Structure, and Polymorphic Behavior of an Ionic Liquid: Phthalate Derivative of <i>N</i>‑Butyl,<i>N</i>‑methylimidazolium Hexafluorophosphate

After the multistep synthesis of an original imidazolium hexafluorophosphate ionic liquid, [pbmim]­[PF₆], two polymorphic forms were isolated from methanolic solution and characterized by XRPD, DSC, and Raman spectroscopy. Stable Form A (mp 90.3 °C) was obtained by conventional crystallization at a moderate cooling rate (<10 K/min), whereas metastable Form B (mp 83.2 °C) was spontaneously produced from a liquid–liquid demixing (oiling out) when a faster cooling rate (>10 K/min) was applied. Structural analyses carried out by using single-crystal (Form A) and powder (Form B) X-ray diffraction revealed a rotational disorder of anionic octahedrons and, more interestingly, large conformational differences between cationic moieties caused by their molecular flexibility. Crystal growth of [pbmim]­[PF₆] (Form A) in methanol often leads to numerous crystal defects and revealed that most of them consist of liquid inclusions. The supersaturation ratio (β) appeared to be the predominant factor influencing the crystal growth behavior under isothermal and stagnant conditions. At low β values, a morphological transition from rod-shaped crystals to platelets was observed, presumably caused by a change in the growth mechanism of specific faces. Using high β values promotes either the formation of microscopic (