Lewis acidic/basic ionic liquids

Some ionic liquids have tunable Lewis acidities and basicities. The tuning can be achieved simply by varying the anion fraction in the overall ionic liquid composition. In some cases, Bronsted acidity can also be introduced into stable ionic liquids. Many publications show the broad applicability of acidic or basic ionic liquid media in catalysis replacing corrosive liquids and solid catalysts. 

The heptachloroaluminate ion is a strong Lewis acid, and chloride ion is the conjugate Lewis base. The basic ionic liquids, in which equilibrium (1) dominates, are light green in color, and the acidic ones, in which equilibrium (2) dominates, are darker and brownish in color (23). The acidic ionic liquids in this family are less viscous than the basic ones. 

Ionic liquids can be compared to any other liquid in that the reactivity of a species will depend upon its relative activity in solution. To this end it is important to consider the relative Lewis and Bronsted acids that can interact with the solutes to affect their activity. It is also important to remember that ionic liquids with discrete anions have wider potential windows and what we therefore hope to achieve with them is more susceptible to the presence of reactive species. The influence of impurities on the electrochemical behavior of an ionic liquid will depend upon the relative Lewis acidity/basicity of the liquid and of the redox process in question. Eutectic-based ionic liquids behave very differently from ionic liquids with discrete... 

In the case of basic ionic liquids that contain AICI3 at less than 50 mol%, the metal salt acts as a Lewis acid by accepting the chloride anions in the ionic liquids to form a chlorocomplex anion ... 

Ionic liquids based on chloroaluminates (the most common form of Lewis acidic or basic ionic liquids) are formed by reacting a quaternary ammonium chloride salt [QAm]" with aluminium chloride (AICI3) in various ratios [94]. Common examples are l-ethyl-3-methyl imidazoUum chloride ([EMIm]Cl) and l-(l-butyl)pyridinium chloride ([BuPy]Cl) [95]. A Lewis base, neutral species or acid is formed by varying the ratio of the two components of the ionic liquid. Using the letter N to represent the mole fraction of AICI3 in the melt [96], the following classification is given for these ionic liquids ... 

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]. 

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). 

Chloroaluminate ionic liquids (typically a mixture of a quaternary ammonium salt with aluminum chloride see Table 6.9) exhibit at room temperature variable Lewis acidity and have been successfully used as solvent/catalyst for Diels-Alder reactions [57]. The composition of chloroaluminate ionic liquids can vary from basic ([FMIM]C1 or [BP]C1 in excess) to acidic (AICI3 in excess) and this fact can be used to affect the reactivity and selectivity of the reaction. The reaction of cyclopentadiene with methyl acrylate is an example (Scheme 6.31). 

The modest endo/exo ratio observed when the reaction was carried out in basic chloroaluminate ionic liquids is ascribable to the polarity of the medium, while the high diastereoselectivity found in the acidic mixture is due to the increase of Lewis/Bronsted acidity of the medium. The rates of the reactions performed in basic and acidic chloroaluminates ([EMIMJCl AlCh, [BPJCl AlCh) are seven times slower and ten times faster, respectively, than those observed when the reactions were carried out in water [57]. 

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 Lewis 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 anions, on the other hand, determine to a large extent the chemical properties of the system. For example, the main anions present in chloroaluminate ionic liquid systems such as (emim)Cl-AlCl3 are CT, which is a Lewis base (AICI4) , which is neither acidic nor basic and the Lewis acid (AbCb). The concentration of each anion, and therefore the Lewis acidity of the system, varies depending on the relative amounts of AICI3 and (emim)Cl added to the system. 

If the mole fraction XfAlCh) is less than 0.5 in the final product, the ionic liquids are basic, as chloride ions are present which are not bound to aluminum and which act as Lewis bases. For mole fractions XfAlCh) > 0.5 an excess of Lewis acid AICI3 is present and the melts are acidic. If the mole fraction XfAlCl ) = 0.5 the salts are neutral as all of the chloride ions are bound to aluminum and the only species present is the [AlCb] ion. However, as a consequence of the autosolvolysis of... 

The ZnCl2 system has probably been studied in the most detail. Fast atom bombardment mass spectrometry (FAB MS) has been used to identify the species present. It was found that ZnCl3, Zn2Cl5 and Zn3Cl7 species are all present in the liquids. The relative proportions of anionic species depend on the ionic liquid composition. Lecocq et al. [108] used electrospray ionization to look at the various species present and found that in Lewis basic liquids x(ZnCl2) <0.5 ZnC -whereas the di- and tri-metallate species were more prevalent in Lewis acidic liquids. 

Lecocq et al. [108] studied ionic liquids formed between zinc chloride and TbutyT2,3-dimethylimidazolium chloride [BMMIMJC1 with the amount of Zn(H2 between 0 and 0.75 mol%. Analysis using NMR, and mass spectrometry showed Cl and [ZnCl j] in Lewis basic liquids and [ZnCh] and [Zn>,Cl7] in Lewis acidic liquids. Infrared spectra with pyridine were used to quantify the Lewis acidity and high temperature (110 °C) NMR experiments showed that the structure varies with time from [BMMIM][ZnCl3] to [BMMIM... Cl... ZnCy. 

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