Synthesis and applications of ionic liquids as pharmaceutical materials

Ionic liquids (ILs) consist of ions and remain liquid at temperatures below 100°C. The so-called room temperature ionic liquids (RTILs) are most suitable as solvents for chemical reactions and other applications. Comparing with conventional organic solvents, ILs have many favorable properties, such as very low vapor pressure, a wide liquid range, low flammability, high ionic conductivity, high thermal conductivity, high dissolution power toward many substrates, high thermal and chemical stability, and a wide electrochemical potential window. Due to these unique properties, ILs have been widely recognized as solvents or reagents in a variety of applications, including organic catalysis, inorganic synthesis, biocatalysis, polymerization, and engineering fluids. Typical IL cations are nitrogen containing (such as alkylammonium, N, N′-dialkylimidazolium, N-alkylpyridinium, and pyrrolidinium) or phosphorous containing (such as alkylphosphonium). The common choices of anions include halides, BF₄, PF₆ ^–, CH₃CO₂ ^–, CF₃CO₂ ^–, NO₃ ^–, Tf₂N^– (i.e., (CF₃SO₂)₂N^–), [RSO₄]^–, [R₂PO₄]^–, etc. Some typical cations and anions are illustrated in Scheme 5.1a.