Ionic liquids are salts composed of cations such as imidazolium ions, pyridinium ions and anions such as BF4-, PF6- and are liquid at relatively low temperatures. Their characteristic properties include (1) essentially no vapor pressure, (2) non-flammability, (3) high thermal stability, (4) relatively low viscosity, (5) liquid state over extended temperature ranges, (6) high ionic conductivity.
Applications of ionic liquids with these features are being vigorously explored in various fields. When an ionic liquid is used as a reaction solvent, the solute is solvated by ions only. Thus, the reaction proceeds in an environment totally different from that when water or ordinary organic solvents are used and therefore, high selectivity is possible. To date application in the Friedel-Crafts reaction, the Diels-Alder reaction, metal-catalyzed asymmetric synthesis and so forth, have been reported.1) Furthermore, some ionic liquids have low solubility in water and low polar organic solvents. By using this property, ionic liquids can be recovered and reused after reaction product is extracted with organic solvents. Recently, methods are being studied which reduce the load on the environment from the viewpoint of green chemistry. Ionic liquids are receiving much attention as a superb solvent from the point of safety, separation/purification procedures, and recyclability.2)
Ionic liquids are used not only as a reaction solvent but also electrochemically, for example, as electrolytes of rechargeable batteries.3) Watanabe and co-workers have developed a novel solid electrolyte in which an ionic liquid is confined in a network polymer called “ion gel”.4) This ion gel is expected to contribute to solidification of various electrochemical systems and safety improvement as it exhibits exceedingly high ion-conductivity compared with conventional solid polymer electrolytes. Hamaguchi and co-workers reported that 1-butyl-3-methylimidazolium tetrachloroferrate is magnetic ionic liquid.5) The new magnetic ionic liquid overcomes various problems of traditional magnetic fluids, and is expected to be applied to many fields including the use as a sealing agent for the motor axis.