2-Methylimidazolium cation

ENIM]C1 AICI31 [BP] Cl AICI3 J [BMIM] = 1-butyl-3-methylimidazolium cation [EMIM] = l-ethyl-3-methylimidazolium cation [BP] = N-l-butylpyridinium cation... 

The l-butyl-3-methylimidazolium cation, bmim (Fig. 1), makes a number of ILs with varying properties, when combined with different anions [4]. BmimCl and bmimBr are crystals at room temperature, while bmimi is a RIL. By cooling down molten bmimCl and bmimBr below the melting points, their... 

The general observations made regarding structural influences on melting points are transferable across cation type, and apply in each case. The primary focus is on l-alkyl-3-methylimidazolium cations, coupled with simple organic and inorganic anions. Complex anions, such as mixed /MX systems, are mentioned, as are other series of cations (including some examples of tetraalkylammonium salts). 

Figure 3.1-2 Thermal decomposition temperature ranges (in °C) for ionic liquids containing l-alkyl-3-methylimidazolium cations. The thermal stability of the ionic liquids depends on the nucleophilicity of the anion.

Similar lamellar structures are formed for l-alkyl-3-methylimidazolium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

Figure 5.1 Scheme illustrating potential interactions between methylimidazolium cation and phenyl-based reversed-phase stationary phase. Arrows indicate hydrogen bonding, hydrophobic, and tt-tt interactions. Anion is not shown for clarity. (Adapted from Stepnowski, R, Nichthauser, J., Mrozik, W., and Buszewski, B., Anal. Bioanal. Chem., 385, 1483-1491, 2006.)... 

Wasserscheid and coworkers were the first to attempt to use ILs for the desulfurization of model solutions (dibenzothiophene [DBT], in n-dodecane) and real diesel fuels [41]. For extraction, the authors used ILs with l-alkyl-3-methylimidazolium cations ([C CiIm], n = 2, 4, 6) and various anions. Also, binary mixtures of l-alkyl-3-methylimidazolium chloride with AICI3 (Lewis-acidic ILs), the equimolar mixture of cyclohexyldiethylammonium and tri-butylammonium mefhanesulfonates (Brnnsted-acidic IL) and the equimolar mixture of cyclohexyldiefhylmethylammonium and tributylmefhylammo-nium methanesulfonafes were tested. 

Figure 12.1 Numbering scheme in the l-hexyl-3-methylimidazolium cation, [CjCjImjL showing the three ring protons H2, H4, and H5.

We have shown [46] that the same situation as for [C4Cilm]+ exisfs for longer alkyl chain systems, at least for the l-hexyl-3-methylimidazolium cation. Raman spectra for [CgCjIm]+ cation systems have bands at 698, 623, and 601 cm (but no distinct band at -498 cm i). A comparison between typical experimental spectra is shown in Figure 12.15. 

The enthalpy difference befween the AA and the GA conformers in the [C4CiIm][BFJ is much smaller than the corresponding enthalpy difference befween the conformers of a free butane chain. This indicates that the 1-butyl-3-methylimidazolium cations most likely form local liquid structures specific fo each rofafional isomers [50]. Coexistence of these local structures, incorporating different rotational isomers, seems to hinder crystallization. This is probably the reason for fhe low melting points of such ILs. 

Ozawa, R., Hayashi, S., Saha, S., Kobayashi, A., and Hamaguchi, H., Rotational isomerism and structure of the l-butyl-3-methylimidazolium cation in the ionic liquid state, Chem. Lett., 32, 948-949, 2003. 

Urahata, S. M., and Ribeiro, M. C. C., Structure of ionic liquids of l-alkyl-3-methylimidazolium cations A systematic computer simulation study, /. Ghent. Phys., 120, 1855-1863, 2004. 

More than 600 ionic liquid systems can, in principle, be generated from around 10 simple anions such as (BF4)" and (PFg) and the l-alkyl-3-methylimidazolium cation substimted with various alkyl groups in the 2-, 4-, or 5-position, or N-alkylpridinium substimted in the 3- or 4-position. When heteropolyanions and tetrafkylammonium and tetraalkylphosphonium cations are added to the hst, a grand total of more than a quarter of a million ionic liquid systems is possible. 

The use of two-phase catalysis in water or high-polar organic solvents is one of best approaches to clean catalytic hydrogenation (Hermann and Kohlpainter, 1993). Ionic solvents with a wide range of liquid phase (down to -81°C) based on l- -butyl-3-methylimidazolium cations can be used in these reactions. 

Hydrogenation of aromatics in the ionic liquid system [emim]Cl-AlCl3 ([emim]+ = l-ethyl-3-methylimidazolium cation) with electropositive metals (Li, Zn, Al) and a proton source can be performed with excellent yields and stereoselectivity 480... 

Note on nomenclature, l-alkyl-3-methylimidazolium cations will be abbreviated throughout this Chapter as [C tnim]+, where n is the lenght of the alkyl chain. 

Wilkes launched the field of air- and moisture-stable ionic liquids by introducing five new materials, each containing the Tethyl-3-methylimidazolium cation [EMIMJ+ with one of five anions nitrate [NC>3], nitrite [NO2]-, sulfate [SC>4]2, methyl carbonate [CH3CO2]- and tetrafluoroborate [BF [47]. Only the last two materials had melting points lower than room temperature, and the reactive nature of the methyl carbonate would make it unsuitable for many applications. This led to the early adoption of [EMIM][BF4] as a favored ionic liquid, which has since been the subject of over 350 scientific publications. One of the first appeared in 1997 [50], reporting the investigation of [EMIM][BF4] as the electrolyte system for a number of processes, including the electrodeposition of lithium (intended for use in lithium ion batteries). 

Ion radius is given in parentheses, a Decomposition temperature, [Nuu]+, tetramethylammonium cation [N2222]+, tetraethylammonium cation [EMIM]+, l-ethyl-3-methylimidazolium cation. 

In the case of the l-butyl-3-methylimidazolium cation a stable radical is obtained... 

Our study of the relation between the solubility temperature and anion structure showed that the l-hexyl-3-methylimidazolium cation coupled with the anions, namely Cl, Br, BF4, and CIO4. Roughly, the ILs having smaller anions showed... 

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