The main types of ionic liquids

Room temperature ionic liquid refers to a salt composed entirely of cations and anions that exists in a liquid state at or near room temperature, also known as low-temperature molten salt. The main reason for the low melting point of room temperature ionic liquids as ionic compounds is due to the asymmetry of certain substituents in their structure, which prevents ions from regularly stacking into crystals. It is generally composed of organic cations and inorganic or organic anions. Common cations include quaternary ammonium salt ions, quaternary phosphonium salt ions, imidazolium salt ions, and pyrrole salt ions (as shown in Figure 1), while anions include halogen ions, tetrafluoroborate ions, hexafluorophosphate ions, etc.
In the room temperature ionic liquids studied, the cations are mainly imidazole cations, and the anions are mainly halide ions and other inorganic acid ions (such as tetrafluoroborate ions). But in recent years, a series of new types of ionic liquids have been synthesized. For example, in terms of cations, the research team led by Shreeve has synthesized some new cationic ionic liquids as follows:
In terms of anions, Yoshida's research team has also synthesized some new types of anionic ionic liquids, as shown in Figure 2:
Due to the many advantages that room temperature ionic liquids have over traditional solvents and their application as green solvents in the synthesis of organic and polymer substances, they have attracted increasing attention from chemists.