Imidazolium chloroaluminate

Scheme 4.1 The synthesis of imidazolium-chloroaluminate ionic liquids (where R is an...

Takahashi, S., Curtiss, L. A., Gosztola, D., Koura, N., and Saboungi, M.-L., Molecular orbital calculations and raman measurements for l-ethyl-3-methyl-imidazolium chloroaluminates, Inorg. Chem., 34, 2990-2993, 1995. 

Acidic chloroaluminate ionic liquids were used as reaction media for Friedel-Crafts reactions as early as 1976 [34], Systematic investigations into Friedel-Crafts alkylations of benzene with the same acidic systems followed in 1986 by Wilkes et al. [35]. The alkylation of benzene with alkenes in acidic imidazolium chloroaluminate melts was disclosed in a patent by BP Chemicals in 1994 [36]. Here, as advantages over the reaction with aluminum trichloride in organic solvents, claims are made regarding the easy isolation of the product, the practically total reusability of the liquid catalyst and the better selectivity to the desired products. 

Yeung, K.-S., Parkas, M. E., Qiu, Z., and Yang, Z. 2002. Friedel-Crafts acylation of indoles in acidic imidazolium chloroaluminate ionic liquid at room temperature. Tetrahedron Lett. 43 5793-5795. 

There followed research into pyridinium and imidazolium chloroaluminate ionic liquids by Hussey, Seddon and Welton (investigation of transition metal complexes... 

Carbonate Catalyzed with Ionic Liquids Imidazolium Chloroaluminate and Chlorostannate Melts." Maerotnoleeular Rapid Commun, 23, 757-760. 

Historically, ionic liquids initial advances in electrochemistry were encouraged by difficulties and safety issues in the aluminum deposition process known as SIGAL (Siemens Galvano-Aluminium). Major concerns were related with the flammability of the aluminum precursors and of the volatile organic solvents used. In the search for low melting, nonvolatile, and nonaqueous electrolytes, pyridinium [25] and imidazolium chloroaluminates (III) were investigated [26]. These ionic liquids are able to dissolve various metal salts. Their biocompatibility is questionable due to their potential toxicity and because they are also corrosive and unstable in air and/or... 

The imidazolium chloroaluminate ionic liquid was also found to be effective as an unconventional reaction media and as Lewis acid catalyst for the Friedel-Crafts sulfonylation reaction of benzene and substituted benzene with p-toluenesulfonyl chloride giving rise to the diarylsulfones in high yield (Scheme 6.14) [16]. 

ILs have also been employed to prepare a protective aluminium thin layer on carbon steel surface by electroreduction and elect rodepwsition of l-butyl-3methyl-imidazolium chloroaluminate (AICI3/ [BMIM]C1) (Caporali et al., 2008, Yue et al., 2009). 

Despite this early discovery, little attention has been paid to the ILs for catalysis. The first example of RTILs for catalysis in open literature should be the Friedel-Crafts reactions catalyzed with dialkyl imidazolium chloroaluminate ILs by Wikes et al. in 1986 [2]. The research motivation was just to know what would occur when Friedel-Crafts reactions were carried out in the RTILs. The chloroaluminate... 

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. 

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. 

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]. 

The synthetic routes used to prepare ionic liquids vary depending upon the ionic liquid being made. Ionic liquids with metal halide anions are, at least in principle, very simple to prepare. Scheme 4.1 illustrates the synthesis of imidazolium-based ionic liquids with a chloroaluminate anion, commencing with methylimidazole [6],... 

Smith, G.R, Dworkin, A.S., Ragni, R.M., Zingg, S.R, Bronsted superacidity of HCl in liquid chloroaluminate, A1C13—l-ethyl-3-methyl-lH-imidazolium chloride, J. Am. Chem. Soc., Ill, 525-529,1989. 

Chloroaluminate ILs, the first-generation ILs, consist of a mixture of an -onium chloride (typically, of course, imidazolium or ammonium) and varying proportions of aluminum frichloride (AICI3). (The sfrucfures of all cations and anions that are discussed in this chapter are depicted in Figure 13.1.)... 

So far the historical development of ionic liquids has mainly been driven by combining imidazolium, pyridinium, ammonium and phosphonium cations with different classes of anions. Chloroaluminate ionic liquids were the first more detailed studied ionic liquids. As early as 1948 they were synthesized by Hurley and Wier at the Rice Institute in Texas as bath solutions for electroplating aluminum [1], Later in the seventies and eighties, these systems were further developed by the groups of Osteryoung [2], Wilkes [3], Hussey [4] and Seddon [4c, 5], Due to their chemical nature, chloroaluminate ionic liquids must be classified as extremely hygroscopic and labile towards hydrolysis. 

In 1992, Wilkes and Zaworotko described the synthesis of the first imidazolium tetra-fluoroborate ionic liquids [6], These systems together with the slightly later published [7] hexafluorophosphate analogues are the working horses of the actual research with ionic liquid. However, their use in many technical applications is still limited by their relatively high sensitivity versus hydrolysis. The tendency of anion hydrolysis is of course much less pronounced than for the chloroaluminate melts but still existent. The [PFe] anion of 1-butyl-3-methylimidazolium ([BMIM]) hexafluorophosphate - for example - has been found in our laboratories to completely hydrolyse after addition of excess water when the sample... 

The last electrolyte system to be mentioned in connection with lithium electrodes is the room temperature chloroaluminate molten salt. (AlCl3 LiCl l-/ -3/ "-imidazolium chloride. R and R" are alkyl groups, usually methyl and ethyl, respectively.) These ionic liquids were examined by Carlin et al. [227-229] as electrolyte systems for Li batteries. They studied the reversibility of Li deposition-dissolution processes. It appears that lithium electrodes may be stable in these systems, depending on their acidity [227], It is suggested that Li stability in these systems relates to passivation phenomena. However, the surface chemistry of lithium in these systems has not yet been studied. 

These values show that lithium and sodium are at the negative potential limit of the electrochemical window (-2 V) (see Figure 49), close to the reduction potential of the imidazolium cation to neutral radical. Therefore, there is a competition between these processes with a resulting decrease in current efficiency. But Reichel and Wilkes [454], Campbell and Johnson [468], Scordilis-Kelley and Carlin [467,469] and Gray et al. [470] showed that an extension of the electrochemical window to -2.4 V is obtained by the addition of HC1 to the AICI3-MEIC neutral melt buffered with NaCl or LiCl. Under these conditions, plating and stripping of sodium and lithium occurs at inert electrodes in room temperature chloroaluminate molten salts. The effect of HC1 addition disappears quickly because of evaporation. 

---