Ionic haloaluminate-based

The first part of this chapter focuses on the synthesis and properties of the so-called first generation of ionic liquids , the haloaluminate-based ionic liquids and in particular on those of chloroaluminate melts. 

Scheme 2.1 General synthesis route to haloaluminate-based ionic liquids.

Physical Data of Haloaluminate-based Ionic Liquids... 

A selection of physical data of selected haloaluminate-based ionic liquids is given in Table 2.1. 

For forty years following the introduction of haloaluminate-based ionic liquids by Hurley and Wier, [44, 45] the majority of research in this field was carried out on systems which were reactive with air and, more specifically, with water. The difficulty of working with these materials, using elaborate Schlenk-line airless techniques or expensive and difficult-to-maintain controlled-atmosphere glove boxes, had the effect of limiting the research to four American-based research groups, mostly funded by the US Air Force [46]. Well aware of this limitation, John Wilkes and coworkers made the decision to substitute the reactive haloaluminate anion... 

The early history of ionic liquids research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminum [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Until recently, electrochemical studies in the ionic liquids were primarily carried out in the haloaluminate-based systems, and this work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past fifteen years, however, has led to an explosion of research in these systems [10,11]. Much of the initial interest in these new ionic liquids has been in areas other than electrochemistry. However, this initial slight has been largely corrected, as evidenced by the dramatic growth over the past five years in electrochemically related publications involving non-haloaluminate ionic liquids and the appearance of several good reviews on the subject [12-17]. 

Ionic liquids based on metal-containing anions and cations, including low-melting coordination complexes, are an area of active investigation. Such materials include the classic haloaluminate salts and related materials. Discussion of the latter is concentrated in Section... 

The room temperature conductivity data for a wide variety of ionic liquids are listed in Tables 3.6-3, 3.6-4, and 3.6-5. These tables are organized by the general type of ionic liquid. Table 3.6-3 contains data for imidazolium-based non-haloaluminate alkylimidazolium ionic liquids. Table 3.6-4 data for the haloaluminate ionic liquids, and Table 3.6-5 data for other types of ionic liquids. There are multiple listings for several of the ionic liquids in Tables 3.6-3-3.6-5. These represent measurements by different researchers and have been included to help emphasize the significant vari-... 

Table 3.2-1 Specific conductivity data for non-haloaluminate alkylimidazolium-based ionic... 

The synthesis ofhaloaluminate-based ionic liquids from halide salts and aluminum Lewis acids (most commonly AIX3 X=C1, Br) can generally be split into two steps (i) fomation of the desired cation by the reaction of a trialkylamine, trialkylphosphine or dialkylsulfide with a haloalkane, and (ii) formation of the haloaluminate anion by addition of an appropriate aluminum halide to this salt (Scheme 2.1). 

Within the binary haloaluminate ionic liquids, increasing the mole percent of the imidazolium salt decreases the density of the liquid (Table 3.2-2). The bromoalumi-nate ionic liquids are substantially denser than their chloroaluminate counter parts, being between 0.57 and 0.83 g cm denser than the analogous chloroaluminate ionic hquids (Table 3.2-2). Variation of the substituents on the imidazolium cation in the chloroaluminate ionic hquids has been shovm to affect the density based upon the cation size [8]. 

Early quantum chemical calculations on ionic liquids were focused towards the haloaluminate, and related metal- (Au and Fe) containing melts, these are examined in the foUovying subsection. As the field has developed, this focus has shifted towards imidazolium-based ionic liquids because of their lower melting points and more fevorable physical properties. Imidazolium-based ionic liquids are discussed in the third subsection which examines imidazolium cations with small alkyl chains (methyl, ethyl and butyl). The ionic liquids which can be formed from imidazolium cations and small anions such as halides or [PFe]" are then discussed, mention is also made of calculations carried out on a few more diverse systems. The electronic structure of the imidazolium-based ionic liquids is the focus of the fourth and final subsection. 

The archetypal ionic liquid is one based on haloaluminate(in) ions, particularly [A1C14]", investigated by pioneers of this topic [13] during the 1970s, taking inspiration from earlier industrial interest in the electrotechnological applications [693, 694] of melts obtained from mixtures of pyridinium halides and aluminium trichloride, as well as in the processing of cellulose [695]. 

---