Chloroaluminate

U.S. Air Force Academy in 1961. He was an early researcher in the development of low-temperature molten salts as battery electrolytes. At that time low temperature meant close to 100 °C, compared to many hundreds of degrees for conventional molten salts. His work led directly to the chloroaluminate ionic liquids. 

The pyridinium- and the imidazolium-based chloroaluminate ionic liquids share the disadvantage of being reactive with water. In 1990, Mike Zaworotko (Eigure 1.4) took a sabbatical leave at the Air Eorce Academy, where he introduced a new dimension to the growing field of ionic liquid solvents and electrolytes. 

When [EMIMJCl is present in a molar excess over AICI3, only equilibrium (2.1-1) need be considered, and the ionic liquid is basic. When a molar excess of AICI3 over [EMIMJCl is present on the other hand, an acidic ionic liquid is formed, and equilibria (2.1-2) and (2.1-3) predominate, further details of the anion species present may be found elsewhere [23]. The chloroaluminates are not the only ionic liquids prepared in this manner. Other Eewis acids employed have included AlEtCl2 [24], BCI3 [25], CuCl [26], and SnCl2 [27]. In general, the preparative methods employed for all of these salts are similar to those indicated for AlCl3-based ionic liquids as outlined below. 

The following subsections attempt to comment upon common impurities in commercial ionic liquid products and their significance for known ionic liquid applications. The aim is to help the reader to understand the significance of different impurities for their application. Since chloroaluminate ionic liquids are not produced or distributed commercially, we do not deal with them here. 

Without special drying procedures and completely inert handling, water is omnipresent in ionic liquids. Even the apparently hydrophobic ionic liquid [BMIM][(CF3S02)2N] saturates with about 1.4 mass% of water [15], a significant molar amount. For more hydrophilic ionic liquids, water uptake from air can be much greater. Imidazolium halide salts in particular are laiown to be extremely hygroscopic, one of the reasons why it is so difficult to make completely proton-free chloroaluminate ionic liquids. 

Immobilized ionic liquids Chloroaluminate ionic liquids on inorganic supports IGI, UK 2001 27... 

Despite the utility of chloroaluminate systems as combinations of solvent and catalysts in electrophilic reactions, subsequent research on the development of newer ionic liquid compositions focused largely on the creation of liquid salts that were water-stable [4], To this end, new ionic liquids that incorporated tetrafiuoroborate, hexafiuorophosphate, and bis (trifiuoromethyl) sulfonamide anions were introduced. While these new anions generally imparted a high degree of water-stability to the ionic liquid, the functional capacity inherent in the IL due to the chloroaluminate anion was lost. Nevertheless, it is these water-stable ionic liquids that have become the de rigueur choices as solvents for contemporary studies of reactions and processes in these media [5],... 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

The size of the cation in the chloroaluminate ionic liquids also appears to have an impact on the viscosity. For ionic liquids with the same anion(s) and compositions, the trend is for greater viscosity with larger cation size (Table 3.2-2). An additional contributing factor to the effect of the cation on viscosity is the asymmetry of the alkyl substitution. Highly asymmetric substitution has been identified as important for obtaining low viscosities [17]. 

In the binary haloaluminate ionic liquids, an increase in the mole percent of the imidazolium salt decreases the density of the liquid (see Table 3.2-2). The bromo-aluminate ionic liquids are substantially denser than their chloroaluminate counterparts, being between 0.57 g cm and 0.83 g cm denser than the analogous chloroaluminate ionic liquids (see Table 3.2-2). Variation of the substituents on the imidazolium cation in the chloroaluminate ionic liquids has been shown to affect the density on the basis of the cation size [17]. 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

The electrochemical windows exhibited by the chloroaluminates tend to fall into three ranges that correspond to the types of chloroaluminate ionic liquids basic. 

Table 3.6-2 The room-temperature electrochemical potential windows for binary and ternary chloroaluminate and related ionic liquids.

The exact nature of the anions is unknown anions listed are those that would be expected if the system behaved similarly to the chloroaluminates. 

Species dissolved in ionic liquids A number of systems have been investigated in both chloroaluminate and second generation ionic liquids... 

Figure 4.1-11 The EXAFS data and pseudo-radial distribution functions of Co(ll) in (a) basic and (b) acidic chloroaluminate ionic liquid. Reproduced from reference 46 with permission.

Dent et al. [47] also investigated the V K-edge EXAFS for the dissolution of [EMIM][VOCl4] and [NEt4][V02Cl2] in basic [EMIM]C1/A1C13 and compared the data with those of solid samples. In both cases the dissolved and the solid samples showed similar EXAFS and no coordination of the chloroaluminate species to, for example, the vanadyl oxygen was found. 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

The field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

Reactions in chloroaluminate(III) salts and other related binary salts often proceed smoothly to give products. However, it should be noted that these salts are water-sensitive and must be handled under dry conditions. They react with water to give hydrated aluminium(III) ionic species and HCl. When a reactant or product contains a heteroatomic functional group, such as a ketone, a strong ketone/alumini-um(III) chloride adduct is formed. In these cases, this adduct can be difficult to separate from the ionic liquid at the end of a reaction. The isolation of the product often... 

Chloroaluminate(III) salts are described in more detail in Chapter 2. The composition of a tetrachloroaluminate(III) ionic liquid is best described in this chapter by the apparent mole fraction of AICI3 X(A1C13) present. Ionic liquids with X(AlCl3) <0.5 contain an excess of CF ions over [AFCly] ions, and are... 

Polycyclic aromatic hydrocarbons dissolve in chloroaluminate(III) ionic liquids to give brightly colored solutions (due to the protonated aromatic compound [31]). The... 

Singer and co-workers have shown that benzoyl chloride reacts with ethers to give alkyl benzoates [33] in chloroaluminate(III) ionic liquids. This reaction results in... 

Scheme 5.1-16 The Diels-Alder reaction in a chloroaluminate(lll) ionic liquid.

Eee has used chloroaluminate(III) ionic liquids in the Diels-Alder reaction [36]. The endo. exo ratio rose from 5.25 to 19 on changing the composition of the ionic liquid from X(A1C13) = 0.48 to X(A1C13) = 0.51 (Scheme 5.1-16). The reaction works well, giving up to 95 % yield, but the moisture-sensitivity of these systems is a major disadvantage, the products being recovered by quenching the ionic liquid in water. 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) > 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the... 

Scheme 5.1-38 The chlorination of benzene in acidic and basic chloroaluminate ionic liquids.

In an attempt to study the behavior and chemistry of coal in ionic liquids, 1,2-diphenylethane was chosen as a model compound and its reaction in acidic pyri-dinium chloroaluminate(III) melts ([PyHjCl/AlCb was investigated [69]. At 40 °C, 1,2-diphenylethane undergoes a series of alkylation and dealkylation reactions to give a mixture of products. Some of the products are shown in Scheme 5.1-40. Newman also investigated the reactions of 1,2-diphenylethane with acylating agents such as acetyl chloride or acetic anhydride in the pyridinium ionic liquid [70] and with alcohols such as isopropanol [71]. 

The chemical behavior of Franklin acidic chloroaluminate(III) ionic liquids (where X(A1C13) > 0.50) [6] is that of a powerful Lewis acid. As might be expected, it catalyzes reactions that are conventionally catalyzed by aluminium(III) chloride, without suffering the disadvantage of the low solubility of aluminium(III) chloride in many solvents. 

Details of two related patents for the alkylation of aromatic compounds with chloroaluminate(III) ionic or chlorogallate(III) ionic liquid catalysts have become available. The first, by Seddon and co-workers [81], describes the reaction between ethene and benzene to give ethylbenzene (Scheme 5.1-51). This is carried out in an... 

Scheme 5.1-51 The alkylation of aromatic compounds in chloroaluminate(lll) or chlorogallate(lll) ionic liquids.

The production of linear alkyl benzenes (LABs) is carried out on a large scale for the production of surfactants. The reaction involves the reaction between benzene and a long-chain alkene such as dodec-l-ene and often gives a mixture of isomers. Greco et al. have used a chloroaluminate(III) ionic liquid as a catalyst in the preparation of LABs [83] (Scheme 5.1-53). 

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