Traditionally, ionic liquids are divided into two classes. The first ionic liquid, [C2
] (melting point13–14 ᵒC), was synthesized by Walden via the neutralization of ethylamine with concentrated HNO3
, as reported in 1914.1
Because a proton-transfer reaction between an organic base and an acid is used during synthesis, this first class of ionic liquids is classified as protic ionic liquids. In 1951, Hurley and Wier reported new ionic liquids prepared by mixing alkylpyridinium chlorides with AlCl3
This work marked the genesis of the second class of ionic liquids, i.e., aprotic ionic liquids. The cations of the aprotic ionic liquids are derived from the alkyl–cation-transfer reaction between an organic base and an alkyl halide. Since then, many ionic liquids composed of various organic cations and inorganic or organic anions have been developed.3,4
Because there are large varieties of cations and anions available, their combination amounts to numerous potential ionic liquids. We have recently developed a third class of ionic liquids.5-7
The essence of our strategy is to synthesize the cations of ionic liquids via the metal–ion-transfer reaction between a neutral organic ligand and an inorganic salt. The key feature of this class of ionic liquids is that their cations consist of metal–ion complexes. These metal-containing ionic liquids provide not only ideal ionic environments, but also liquid inorganic precursors for electroplating of metals, gas separation, and synthesis of inorganic materials. The recent advances in this class of ionic liquids will be discussed.
Acknowledgment: This research was sponsored by the SERDP program.
(1) Walden, P. Bull. Acad. Imp. Sci. 1914, 8 405.
(2) Hurley, F. H.; Wier, T. P. Journal of the Electrochemical Society 1951, 98, 203.
(3) Welton, T. Chem. Rev. 1999, 99, 2071.
(4) Wasserscheid, P.; Keim, W. Angew. Chem.-Int. Edit. 2000, 39, 3773.
(5) Huang, J. F.; Luo, H. M.; Dai, S. Journal of the Electrochemical Society 2006, 153, J9.
(6) Zhu, H. G.; Huang, J. F.; Pan, Z. W.; Dai, S. Chem. Mat. 2006, 18, 4473.
(7) Ma, Z.; Yu, J. H.; Dai, S. Adv. Mater. 2010, 22, 261.