Synthesis and characterization of diimidazole-based hexafluorophosphate ionic liquids

The hexafluorophosphate type ionic liquids, as one kind of functionalized ionic liquids, have a good application prospect in many fields. In this work, a series of diimidazole hexafluorophosphate ionic liquids with different carbon chain lengths (CnDIMF, n = 1, 2, 3, 4, 5, 6) were synthesized and characterized, so as to find the influence of cation on their structure and properties. The results showed that these products are needlelike or lamellar crystals, and their solubility and thermal stability were similar, but there are significantly different in their melting points and crystal structures. They were insoluble in non-polar or weak polar solvents, while have good solubility in polar aprotic solvents. Surprisingly, they could exhibit the temperature-controlled characteristics in methanol and water. These products have a good thermal stability up to 300 °C, and their decomposition temperature also slightly increases from 360 to 395 °C with the increase in the spacer chain length. The melting points of them are above 120 °C, which increased with the decrease of molecular weight if they belong to the same crystal system. Crystallography data showed that two cationic 1-vinylimidazolium were bound to the both sides of alkyl group, and the two imidazole rings were crystallographically dependant plane, and their structure changed from a semi-capsule with a close angle to a linear with a wide angle between the two imidazole rings. The study and determination of these properties, especially the temperature-controlled performance, is interesting and can afford reliable reference for future applications.