Metal chloroaluminate

The ionic conductivity of alkali-metal chloroaluminates was also investigated by Weppner and Huggins [37] but also only in the temperature range between room temperature and just above the melting point. At room temperature the ionic conductivity... 

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... 

The useful potential window of the equimolar AlCl3-NaCl melt extends from about 2.2 to 0 V vs. the A1(III)/A1 couple in NaCl(satd) melt. Plambeck [30] summarizes the physical properties of the equimolar (mp = 151 °C) and 63-37 mol% eutectic melts. Pyrex cells are satisfactory for use with the AlCl3-NaCl melt. Teflon has also been used in these melts, but it slowly decomposes. Experimentation with the alkali metal chloroaluminates requires the use of an inert-atmosphere glove box or a gas-tight cell. 

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 first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

This is surprising in view of the fact that a great deal of effort was made to study transition metal complexes in chloroaluminate ionic liquids in the 1980s and early 1990s (see Section 6.1 for some examples). The investigations at this time generally started with electrochemical studies [41], but also included spectroscopic and complex chemistry experiments [42]. 

As early as 1990, Chauvin and his co-workers from IFP published their first results on the biphasic, Ni-catalyzed dimerization of propene in ionic liquids of the [BMIM]Cl/AlCl3/AlEtCl2 type [4]. In the following years the nickel-catalyzed oligomerization of short-chain alkenes in chloroaluminate melts became one of the most intensively investigated applications of transition metal catalysts in ionic liquids to date. 

The only reports of directed synthesis of coordination complexes in ionic liquids are from oxo-exchange chemistry. Exposure of chloroaluminate ionic liquids to water results in the formation of a variety of aluminium oxo- and hydroxo-contain-ing species [4]. Dissolution of metals more oxophilic than aluminium will generate metal oxohalide species. FFussey et al. have used phosgene (COCI2) to deoxochlori-nate [NbOa5] - (Scheme 6.1-1) [5]. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

Zinc and tin The electrodeposition of Zn [52] has been investigated in acidic chloroaluminate liquids on gold, platinum, tungsten, and glassy carbon. On glassy carbon only three-dimensional bulk deposition was observed, due to the metal s underpotential deposition behavior. At higher overvoltages, codeposition with A1... 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate(III) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [L7, 20, 27, 28]. These... 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

Use of low-temperature molten systems for electrolytic processes related with tantalum and niobium and other rare refractory metals seems to hold a promise for future industrial use, and is currently of great concern to researchers. The electrochemical behavior of tantalum, niobium and titanium in low-temperature carbamide-hilide melts has been investigated by Tumanova et al. [572]. Electrodeposition of tantalum and niobium from room/ambient temperature chloroaluminate molten systems has been studied by Cheek et al. [573],... 

Electrodeposition of Transition Metal-Aluminum Alloys from Chloroaluminate Molten Salts... 

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

The coordination of transition metal ions in acidic chloroaluminate melts has not been firmly established. However, in the case of AICb-EtMelmCI. the E0 values of simple redox systems that are electrochemically accessible in both acidic and basic melt, e.g., Hg(II)/Hg [51], Sb(III)/Sb [52], and Sn(II)/Sn [53] exhibit a large positive potential shift on going from basic melt, where metal ions are known to exist as discrete anionic chloride complexes, to acidic melt. Similar results were observed for Cu(I) in AlCh-NaCl [48]. This dramatic decrease in electrochemical stability isprima facie evidence that metal ions in acidic melt are probably only weakly solvated by anionic species such as AICI4 and AECI-. Additional evidence for this is derived from the results of EXAFS measurements of simple metal ions such Co(II), Mn(II), and Ni(II) in acidic AlCh-EtMelmCl, which indicate that each of these ions is coordinated by three bidentate AICI4 ions to give octahedrally-coordinated species such as [ M (AIC14) 2 ] [54]. Most transition metal chloride compounds are virtually... 

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. 

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