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.