Neutral ionic liquids

The first electrodeposition of lithium from an ionic liquid was reported in 1985 by Lipsztajn and Osteryoung [2], They were able to deposit lithium from a 1-ethyl-3-methyl-imidazolium chloride/aluminum trichloride ionic liquid. They noticed that a neutral ionic liquid, a neutral basic ionic liquid (neutral + small excess of RC1) and a neutral acidic ionic liquid (neutral + small excess of AICI3) each have unique features. Both the basic and the neutral acidic ionic liquids show an extension of 1.5 V of the electrochemical window, making them interesting for electrochemical applications. 

Eor ionic liquids that do not mix completely with water (and which display sufficient hydrolysis stability), there is an easy test for acidic impurities. The ionic liquid is added to water and a pEf test of the aqueous phase is carried out. If the aqueous phase is acidic, the ionic liquid should be washed with water to the point where the washing water becomes neutral. Eor ionic liquids that mix completely with water we recommend a standardized, highly proton-sensitive test reaction to check for protic impurities. 

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 behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

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). 

Diels-Alder reactions Neutral ionic liquids have been found to be excellent solvents for the Diels-Alder reaction. The first example of a Diels-Alder reaction in an ionic liquid was the reaction of methyl acrylate with cyclopentadiene in [EtNH3][N03] [40], in which significant rate enhancement was observed. Howarth et al. investigated the role of chiral imidazolium chloride and trifluoroacetate salts (dissolved in dichloromethane) in the Diels-Alder reactions between cyclopentadiene and either crotonaldehyde or methacroline [41]. It should be noted that this paper describes one of the first examples of a chiral cationic ionic liquid being used in synthesis (Scheme 5.1-17). The enantioselectivity was found to be < 5 % in this reaction for both the endo (10 %) and the exo (90 %) isomers. 

A similar study performed by Welton and co-workers studied the rate and selec-tivities of the Diels-Alder reaction between cyclopentadiene and methyl acrylate in a number of neutral ionic liquids [44]. It was found that endo. exo ratios decreased slightly as the reaction proceeded, and were dependent on reagent concentration and ionic liquid type. Subsequently, they went on to demonstrate that the ionic liquids controlled the endo. exo ratios through a hydrogen bond (Lewis acid) interaction with the electron-withdrawing group of the dienophile. 

Miscellaneous reactions in neutral ionic liquids Kitazume et al. have also investigated the use of [EDBU][OTf as a medium in the formation of heterocyclic compounds [58]. Compounds such as 2-hydroxymethylaniline readily condense with... 

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). 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

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 chloroaluminate(III) ionic liquids - [EMIM][C1-A1C13], for example (where EMIM is l-ethyl-3-methylimidazolium) - are liquid over a wide range of AICI3 concentrations [24]. The quantity of AICI3 present in the ionic liquid determines the physical and chemical properties of the liquid. When the mole fraction, X(A1C13), is below 0.5, the liquids are referred to as basic. When X(A1C13) is above 0.5, the liquids are referred to as acidic, and at an X(A1C13) of exactly 0.5 they are referred to as neutral. 

Giirtler and Jautelat of Bayer AG have protected methods that use chloroalumi-nate(III) ionic liquids as solvents for both cyclization and polymerization reactions of acyclic dienes [52]. They employed the neutral ionic liquid [EMIM][G1-A1G13]... 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts, while different levels of proton impurities in chloroalumi-... 

According to the complexing ability of their anions, ionic liquids are classified as basic, neutral and acidic [50]. Some examples of neutral ionic liquids are reported in Table 6.9. 

Earle and coworkers [54] have performed Diels-Alder reactions in neutral ionic liquids. The results of reactions of cyclopentadiene with dimethyl maleate, ethyl acrylate and acrylonitrile are reported in Table 6.10. The cycloadditions proceeded at room temperature in all of the ionic liquids tested, except [BMIMJPF4, and gave almost quantitative yields after 18-24h. The endo/exo selectivity depends on dienophile. No enantioselectivity was observed in the [BMIM] lactate reaction. 

Table 6.10 Diels-Alder reactions of cyclopentadiene with dimethyl maleate, ethylacrylate and acrylonitrile in neutral ionic liquids...

Ionic LCs are interesting systems because they combine the properties of LCs with those of ionic liquids. Although alkali metal soaps were among the first thermotropic LCs to be systematically studied, ionic liquid crystalline derivatives have been reported less frequently than those based on neutral molecular and macromolecular species [39]. When the halide of [AuX(CNR)] complexes is substituted by a second isocyanide, ionic complexes [Au(CNR)2][Y] [R = C6H40C H2 + i (27a),... 

However, if we consider the alternative nucleophilic displacement, it is known that nucleophilic processes are accelerated by ionic liquids, but more pertinent is the fact that the Sn2 displacement of iodide from alkyl iodide (Mel) by Rh(CO)2l2 is slightly accelerated by ionic liquids (7). Unfortunately, ionic liquids would also be expected to accelerate the nucleophilic displacement of iodide from ethyl iodide by propionic acid to form ethyl propionate (Reaction 8). In fact, as an Sn2 Type II displacement (the interaction of two neutral species), the ester formation from propionic acid and ethyl iodide would be expected to be significantly increased compared to the reaction of Rh(CO)2l2 with EtI. Therefore, by operating in iodide containing ionic liquids, we had set up a situation in which we suppressed the normally predominant hydride mechanism, slightly accelerated the alternative nucleophilic mechanism, but dramatically increased the ethyl propionate by-product forming pathway. 

Thiolates, generated in situ by the action of ammonium tetra-thiomolybdate on alkyl halides, thiocyanates, and disulfides, undergo conjugate addition to a, (1-unsaturatcd esters, nitriles, and ketones in water under neutral conditions (Eq. 10. II).29 Conjugate addition of thiols was also carried out in a hydrophobic ionic liquid [bmim]PF6/water-solvent system (2 1) in the absence of any acid catalyst to afford the corresponding Michael adducts in high to quantitative yields with excellent 1,4-selectivity under mild and neutral conditions (Eq. 10.12). The use of ionic liquids helps to avoid the use of either acid or base catalysts... 

In practical cases, it is the solute charges that are modeled explicitly, and treated as permanent source charges. In contrast, the whole solvent medium is usually treated as a continuum, without any explicit, permanent, source charges. (This is reasonable for a solvent made of small, neutral molecules ionic liquids would obviously need a different treatment.) Since there are no permanent charges in the solvent,... 

Table 3 shows conductivity of 2mol/dm3 solutions of EMImBF4 and EMImPF6 in a number of molecular solvents. A high increase of conductivity, in comparison to neat ionic liquids, can be observed after dilution with electrically neutral molecular liquids. However, solutions of ionic liquids in molecular liquids are simply conventional solutions of organic salts in nonaqueous solvents, and no distinction can be seen between them and commonly employed solutions of (C2H5)4NBF4. 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

Scheme 7.1. Activation of a neutral catalyst precursor by chloroaluminate ionic liquids...

The first investigations of the rhodium-catalysed hydroformylation in room temperature ionic liquids were published by Chauvin et al. in 1995 [44], The hydroformylation of 1-pentene with the neutral catalysts [Rh(CO)2(acac)]/triarylphosphine was carried out in a biphasic reaction using [BMIM][PF6] as the ionic liquid (see Scheme 7.2). 

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