Chloroaluminate systems

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],... 

Closely related catalytic systems have also been used for the selective dimerization of ethene to butenes [99]. Dupont et al. dissolved [Ni(MeCN)<3][BF4]2 in the slightly acidic [BMIM]Cl/AlCl3/AlEtCl2 chloroaluminate system (ratio = 1 1.2 0.25) and obtained 100 % butenes at -10 °C and 18 bar ethylene pressure (TOF = 1731 h Y Unfortunately, the more valuable 1-butene was not produced selectively, with a mixture of all linear butene isomers (i.e., 1-butene, cis-2-butene, trans-2-butene) being obtained. 

Biphasic oligomerization with ionic liquids is not restricted to chloroaluminate systems. Especially in those cases where the - at least - latent acidity or basicity of the chloroaluminate causes problems, neutral ionic liquids with wealdy coordinating anions can be used with great success. 

The electrodeposition of Ag has also been intensively investigated [41 3]. In the chloroaluminates - as in the case of Cu - it is only deposited from acidic solutions. The deposition occurs in one step from Ag(I). On glassy carbon and tungsten, three-dimensional nucleation was reported [41]. Quite recently it was reported that Ag can also be deposited in a one-electron step from tetrafluoroborate ionic liquids [43]. However, the charge-transfer reaction seems to play an important role in this medium and the deposition is not as reversible as in the chloroaluminate systems. 

In many ways, chloroaluminate molten salts are ideal solvents for the electrodeposition of transition metal-aluminum alloys because they constitute a reservoir of reducible aluminum-containing species, they are excellent solvents for many transition metal ions, and they exhibit good intrinsic ionic conductivity. In fact, the first organic salt-based chloroaluminate melt, a mixture of aluminum chloride and 1-ethylpyridinium bromide (EtPyBr), was formulated as a solvent for electroplating aluminum [55, 56] and subsequently used as a bath to electroform aluminum waveguides [57], Since these early articles, numerous reports have been published that describe the electrodeposition of aluminum from this and related chloroaluminate systems for examples, see Liao et al. [58] and articles cited therein. 

The binary mixture of A1C13 and NaCl is the alkali metal chloroaluminate most commonly used as an electrochemical solvent. The preparation, purification, and general properties of this and several related inorganic chloroaluminate systems have... 

Seddon and co-workers described the Friedel-Crafts acylation reaction of benzene with ace-tylchloride using acidic chloroferrate ionic liquids as catalysts [38], In contrast to the same reaction in presence of acidic chloroaluminate systems the ketone product could be separated from the ionic liquid by solvent extraction, provided that the molar ratio of FeCl3 is in the range 0.51-0.55 in the applied ionic liquid catalyst (Scheme 1). 

The recognised definition of an ionic liquid is an ionic material that is liquid below 100 °C but leaves the significant question as to what constitutes an ionic material. Some authors limit the definition to cations with discrete anions e.g. BF4-, NO3. This definition excludes the original work on chloroaluminate systems and the considerable work on other eutectic systems and is therefore unsatisfactory. Systems with anionic species formed by complex equilibria are difficult to categorise as the relative amounts of ionic species depend strongly on the composition of the different components. 

The electrodeposition of silver from chloroaluminate ionic liquids has been studied by several authors [45-47], Katayama et al. [48] reported that the room-temperature ionic liquid l-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) is applicable as an alternative electroplating bath for silver. The ionic liquid [EMIM]BF4 is superior to the chloroaluminate systems since the electrodeposition of silver can be performed without contamination of aluminum. Electrodeposition of silver in the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and l-butyl-3-methylimidazoliumhexafluorophosphate was also reported [49], Recently we showed that isolated silver nanoparticles can be deposited on the surface of the ionic liquid Tbutyl-3-methylimidazolium trifluoromethylsulfonate ([BMIMJTfO) by electrochemical reduction with free electrons from low-temperature plasma [50] (see Chapter 10). This unusual reaction represents a novel electrochemical process, leading to the reproducible growth of nanoscale materials. In our experience silver is quite easy to deposit in many air- and water-stable ionic liquids. 

In acidic melts, the A12C17 ions are predominant. In basic melts, per is constant, in the presence of solid NaCl [401], and it is equal to 1.1 at 175°C. Likewise, in the molten chloroaluminate systems the terms acid and base denote a chloride ion acceptor and a chloride ion donor, respectively. The pCL may be measured with either an aluminum or a chlorine electrode immersed in the melt. 

Chloroaluminate Systems. In case of chloroaluminate ionic liquids, the potential of an aluminum electrode immersed in the acidic ionic liquids which contain AICI3 at more than 50 mol% is usually assumed as the potential standard. 

Room-temperature ionic liquids (denoted RTILs) have been studied as novel electrolytes for a half-century since the discovery of the chloroaluminate systems. Recently another system consisting of fluoroanions such as BF4 and PFg , which have good stability in air, has also been extensively investigated. In both systems the nonvolatile, noncombustible, and heat resistance nature of RTILs, which cannot be obtained with conventional solvents, is observed for possible applications in lithium batteries, capacitors, solar cells, and fuel cells. The nonvolatility should contribute to the long-term durability of these devices. The noncombustibility of a safe electrolyte is especially desired for the lithium battery [1]. RTILs have been also studied as an electrodeposition bath [2]. 

Chloroaluminate systems have been intensively studied over the past few decades mainly in electrochemical research fields. Many of the electrochemical properties of RTILs have been sorted out using the chloroaluminate systems [50]. Chloroaluminate anions form RTILs with various organics, including not only EMI and BP but also triazolium [51] and aliphatic onium cations [7,52-54]. Other unique RTILs similar to the chloroaluminate systems such as the chlorogallate (GaCLj ) [55], chloroborate (BCL ) [56], and bromoaluminate (AIBr4 ) systems [57] have been reported. 

Figure 4.6 shows the EWs of chloroaluminate systems with the correction of the potential differences derived from the melt composition and the kind of cation using the redox potential of ferrocene. This figure clearly indicates the effect of using the ferrocene internal reference. The cathodic limiting or anodic limiting potential is almost the same when the same cation is contained in RTILs, and is independent of the measurement conditions such as the difference of melt composition, not considered in Figure 4.5. The potential variation seen in Figure 4.5 even for the same RTILs is almost eliminated.

The nonchloroaluminate system has been intensively investigated, since fluoro-anions are also attractive candidates for RTILs [29, 60, 61]. The significant difference from the chloroaluminate system is its stability in air. The number of anions, which form RTILs with various cations, demonstrate an upward trend [40-45, 47, 62, 63]. 

As was stated above, ferrocene is considered to be a potential internal reference for chloroaluminate systems. There are only a few studies that mention the redox potential of ferrocene in nonchloroaluminate systems. Table 4.5 gives the potential data corrected with the redox potential of ferrocene in each RTIL, if available. Figure 4.7 shows the EW of the RTILs. Shown in the figure are the EWs of both chloroaluminate (solid line) and nonchloroaluminate (dotted line) systems. It is interesting to note that both the cathodic and anodic limiting potentials of the RTILs based on the same cation are the same whether the system is chloroaluminate... 

Figure 4.5 Electrochemical window of chloroaluminate system. The asterisk indicates the composition of the melt in RE of N — 0.60. Another case is U = 0.67. The potential difference between N = 0.60 and 0.67 (130 180 mV) is not considered. Working electrodes GC (glassy carbon) W (tungsten) Pt (platinum).

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