Imidazolium halide

Imidazolium halides pyrolysis, 5, 449 Imidazolium ions acylation, 5, 402 H NMR, 5, 352 hydrogen exchange, 5, 417 nucleophilic attack, 5, 375 reactivity, 5, 375 ring opening, S, 375 Imidazolium oxides in pyrrole synthesis, 4, 344 Imidazolium perchlorate, 1,3-diphenyl-acylation, 5, 402 Imidazolium salts 1-acetyl-... 

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. 

The transition states (27) calculated at the B3LYP/6-31+-l-G and B3LYP/6-31G levels of theory for the alkylation of 1-methylimidazole (26) by alkyl bromides and chlorides, forming imidazolium halide ionic liquids (28),110 had a hydrogen bond between the incipient halide ion and the hydrogen on C(2) of the substrate. 

As a new class of materials, ionic liquids require special analytical methods. In the case of imidazolium halides and similar compounds the most common impurities are amines, alkyl halides and of course water. Seddon et al. described a method for the detection of residual amines using the strong UV absorbance of copper tetramine complexes. These complexes are readily formed by the addition of Cu2+ ions [24]. The detection of both amines and alkyl halides is possible by NMR spectroscopy but with limited resolution [25]. By far the most powerful analytical method is liquid chromatography combined with UV detection. This sensitive method allows the detection of traces of amines and halides [26]. Unreacted amines can be also detected by ion chromatography combined with a suppressor module. In this case detection is achieved using a continuous flow conductivity cell since amines are protonated and thus detectable. For traces of other ionic impurities ion chromatography is also the most powerful analytical tool [27]. Finally, residual water can be quantified using Karl Fischer titration or coulometry [28]. 

Numerous methods for the generation of imidazole carbenes have been reported. For example, starting from an imidazolium halide, the use of systems such as sodium hydride in ammonia or dimethylsulphoxide (DMSO), sodium in ammonia, alkali metals in tetrahydrofuran (THF), metal ferf-butoxides in THF or DMSO, etc. Recently, Seddon and Earle reported a simple procedure for the generation of the imidazolium carbene 2 in 90-95% yield from an imidazolium chloride 1 which does not require solvents, filtrations, or produce noxious waste products (Scheme 4) [40],... 

Ionic liquids with anions containing transition metal complexes were among the earliest developed room temperature ionic liquids [60], Transition metal based ionic liquids have been synthesized either by reaction of phosphonium or imidazolium halides with the corresponding metal halides, or by metathesis with alkali salts of the metal-based anions. Among the metal containing ionic liquids, ionic liquid-crystals are excluded in this section as they were reviewed thoroughly in 2005 [61], Synthesis of metal based salts can be divided in to three groups (1) transition metal salts, (2) p-block metal salts and (3)/-block metal salts. 

Halide-based ionic liquids are simple to prepare. Conventionally, they are synthesized by reacting heterocyclic starting materials, for example N-methylimidazole, pyridine, or thiazole, with an appropriate alkyl halide in a nonpolar solvent, for example toluene (Scheme 7.1). The reactions can be also performed using a large excess of the alkyl halide as reagent and solvent. The reactions usually take between 3 and 72 h to reach completion. Symmetrical N,N -disubstituted imidazolium halide... 

At 100 °C no product was ever observed. At 150 °C the imidazolium-based ionic liquids could be prepared. For ionic liquids bearing pyridinium cations a reaction temperature of 200 °C was required. The reactions could be scaled up to 50 g and no large excess of alkyl halide was needed. Deetlefs and Seddon studied the synthesis of pyrazolium, thiazolium, imidazolium, and pyridine-based ionic liquids [21]. Selected results are summarized in Table 7.1. The authors reported that the reactions are up to 72 times faster than when using conventional heating. They also found that if the microwave irradiation is prolonged, decomposition of the ionic liquid occurs. Imidazolium halide based ionic liquids could be prepared in 150,... 

Although the first ionic liquid expressly categorized as being task-spedfic featured the incorporation of function within the cation core, subsequent research has focused on the incorporation of functionality into a branch appended to the cation [11]. In this fashion, a great number of the task-specific ionic liquids have been prepared that have been built-up from 1-methyl and 1-butylimidazole. By far the most of these ionic liquids have been prepared by quaternization of the aforementioned imidazoles with a functionalized alkyl halide to afford the corresponding functionalized imidazolium halides in usually good yield (Scheme 2.3-1). 

It is noteworthy that deprotonation of imidazole with NaH or KH, followed by addition of two equivalents of a functionalized alkyl halide, as well as reaction of 1-trimethylsilylimidazole with two equivalents of a functionalized alkyl halide, also gives access to 1,3-bis-fimctionalized imidazolium halides [21]. The latter can be further converted by anion exchange. 

In the first step (Step A, Scheme 1), an imidazolium halide is formed by the reaction of an alkyl halide R X with an N-alkylimidazole (a variety of N-alkyl-imidazoles are commercially available, while others can be obtained by reacting potassium imidazolide with 1 equiv. of an alkyl halide). The conditions of the reactions mainly depend on the nature of the alkyl halide chloride reactants often require several hours at reflux, while it is necessary to manage the exothermicity of the reaction when bromide and iodide compounds are used. 

The potential detrimental effect of halide has been described previously. A first alternative to the two-step synthesis is the direct alkylation of N-alkylimidazole. H owever, the range of available alkylating reagents is limited and they are sometimes expensive. Two-step syntheses of ILs which do not use imidazolium halide as an intermediate were then developed. For example, Seddon described the reaction of N-alkylimidazole with ethyl trifluoroacetate, which leads to imidazolium trifluoro-... 

Another method has been described for the synthesis of halide-free ILs which uses an imidazolylidene carbene as an intermediate. The starting material is an imidazolium halide which is treated with sodium tert-butoxide to form an imidazolylidene carbene. The latter is distilled in a Kugelrohr apparatus, then reacted with a Br0nsted acid. In this way, it is possible to obtain an IL free from halide contamination, but traces of N-alkylimidazole or acid may be present. 

A microwave-assisted preparation of a series of l-alkyl-3-methylimidazolium halide ILs has been described [17]. The reaction is run in solvent-free conditions with a near-stoichiometric amount of reactants, and the imidazolium halides are obtained in high yield. It is also possible to perform the subsequent metathesis reaction with sodium hexafluorophosphate by means of microwave radiation and then to form the final product in a one-pot reaction [18]. Due to the fact that ILs absorb microwave energy in a very efficient way, they are believed to be well suited for large-scale microwave-assisted synthesis (that is, for reaction mixtures of more than 100 L). 

Ionic liquids used as solvents have to be isolated with high chemical purity. Their non-volatility is a disadvantage in terms of their preparation because, unlike classical solvents, they cannot be purified by distillation. The starting materials are therefore purified. Typically, for fhe preparation of imidazolium halides, 1 -methylimidazolium must be distilled over NaOH and the haloalkane should be washed with concentrated sulfuric acid (to remove coloration), neutralized with an NaHCOs solution, washed with water, dried and distilled before use. 

Matsumi et al. (2009) published novel ILs bearing an ortfto-carborane anion with different 1,3-dialkylated imidazolium cations (l-ethyl-3-methyl, l-butyl-3-methyl and l-butyl-3-ethyl residues) (Figure 29.13). They are prepared easily from the carbene obtained from imidazolium halides. The ionic conductivity of the [EM1M]+ salt, 2.9 x 10" S cm at 51°C, is quite low compared to other ILs. The melting point of this ortfto-carborane salt (30°C) is lower than the melting points for [EMIM]+ salts of the carboranes [1-C3H7CB H ] (45°C) and [1-C4H9CB H ] (40°C) described by Larsen et al. (2000). 

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