ReviewRoom temperature ionic liquids of alkylimidazolium cations and fluoroanions
Introduction
A room temperature ionic liquid, or molten salt, is defined as a material containing only ionic species without any neutral molecules and having a melting point of lower than 298 K. A dramatic increase in activity in this area was afforded by the discovery of 1-ethyl-3-methylimidazolium chloroaluminates [1]. This system has a wide liquid range and electrochemical window of more than 3 V, and has been of great interest for both electrochemical and synthetic purposes. However, the applications have been limited due to the chemical instability, mainly against moisture. Since the discovery of water stable 1-ethyl-3-methylimidazolium tetrafluoroborate [2], the compound 2 in Table 1, Table 2, Table 3, several combinations of alkylimidazolium cations and inorganic and organic anions have been investigated to date. A survey of these salts including fluoroanions will be given in this short review. In this area, studies on the compound 2 have been reported the most to date. Recently, reports are increasing concerning the salts of bis((trifluoromethyl)sulfonyl)amide anion (h). All the salts found in the present literature survey were those containing one singly charged cation and one singly charged anion (single, 1-1 salt), except the salt 3 in which the existence of anions of more than one kind are suggested [14].
Section snippets
Preparation
The literature method for preparation of these salts is the reaction of N-alkylimidazole, RIm, with alkyl salts, R′X in appropriate organic solvents [3];where X− is the anion of interest. The anion exchange using N-alkylimidazolium halide is an alternative way for preparation [3]. In the first step, alkylimidazolium halides are synthesized;where X′ = Cl, Br, I. Some of the bromides and iodides are ionic liquids by themselves [3]. Anion exchanges are made by
Structures
No literatures are so far available for structural works on room temperature ionic liquids of imidazolium salts including fluoroanions. However, the single crystal structure at 296 K has been reported for 1-ethyl-3-methylimidazolium hexafluorophosphate [5]. Although the melting point of this compound is 331–333 K, higher than room temperature (therefore, it is not listed in Table 1, Table 2, Table 3), some of the structural features would be helpful to explain the physical and chemical properties
Melting points
The melting points of these liquids reported in the literatures are summarized in Table 5. There seems to be no clear correlation between the melting points and their chemical structures. It has been pointed out that the compounds containing symmetric cations such as A and C exhibit higher melting points than those containing asymmetric cations [3]. It seems that the compounds containing anion j exhibit relatively low melting temperatures. Some of the salts do not crystallize but exhibit
Stability
The ionic liquids discussed here are moisture stable. As mentioned in Section 1, some of them are prepared from aqueous solutions. The compounds containing anion h is immiscible with water reflecting the lack of hydrogen bonding ability of it [3].
Thermal stability has been reported for several liquids containing cation B. The thermogravimetric study revealed that the compounds 4 and 5 are stable up to 673 K while the compound 7 is stable up to 423 K [3]. The compound 2 starts to lose its weight
Applications
The liquids reviewed here are now being studied in various fields. The electrolyte performances of liquid 2 have been studied in battery systems using alkaline metals and lithium-aluminum alloy as anode materials [6], [7]. The interfacial stability has been reported on the liquids 27 and 28 with lithium metal [26]. The electrochemical charge and discharge characteristics have also been reported using intercalation and de-intercalation reactions at both the graphite cathode and anode electrodes
Conclusions
The room temperature ionic liquids of alkylimidazolium salts possess many excellent characteristics as electrolyte and reaction solvents. It has been demonstrated that the combinations of these cations with some inorganic and organic fluoroanions control their physical and chemical properties. The salts with melting points of more than 298 K were not included in Table 1, Table 2, Table 3. However, there are several interesting salts with low melting points not much higher than room temperature.
Acknowledgements
The authors thank the Ministry of Education, Science, Sports and Culture for financial support by a Grant-in-Aid for Scientific Research.
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