Chemistry in heterocyclic ammonium fluorohydrogenate room-temperature ionic liquid

Investigation on alkali fluoride-HF system has been initiated in the 19th century. The technique is currently utilized in fluorine-chemical industry. But, the problem is that this system readily releases hazardous HF. Although organic base, e.g., amine, with HF, which is mainly applied to fluorination treatment for organic compound, reduces the HF release, the solution still requires careful handling because of limited amount of free HF. Recently family of fluorohydrogenate room-temperature ionic liquid, XF(HF)_2.3, that consists of heterocyclic ammonium cation (X⁺), F(HF)₂⁻, and F(HF)₃⁻, has gotten a lot of attentions due to the interesting physicochemical properties such as negligible vapor pressure (<7.5 × 10⁻³ Torr (=1 Pa) at 298 K), high conductivity, and low corrosiveness. This novel solvent will greatly contribute to development of fluorine chemistry. In this article, fundamental techniques and physicochemical data on the fluorohydrogenate RTIL are summarized, and molecular science in the dialkylimidazolium fluorohydrogenates leading to the understanding of the unusual properties is reviewed based on recent experimental and theoretical considerations.