Ionic liquids imidazolium

XPS measurements showed clearly the interachons of the IL with the metal surface, that occurs through F (for BF4 and PF i) or O (for CF3SO1) of the anions, demonstrating the formation of an IL protechve layer surrounding the iridium nanoparticles. Additional extended X-ray absorption fine structure (EXAFS) analyses also provided evidence for interaction of the IL liquid with the metal surface. 

However, the DLVO model cannot completely explain the stabiUzation properties of imidazoUum ILs towards the lr(0) nanoparticles as it treats counterions as mono-ionic point charges and was not designed to account for sterically stabiUzed systems. Together with the electrostatic stabilization provided by the intrinsic high charge of the IL, a steric type of stabilization can also be envisaged. This is due to the presence of anionic and cationic supramolecular aggregates of the type [(BMl),t(X),( ] [(BMl),t (X)J T where BMl is the l- -butyl-3-methyUmidazoUum cation and X is the anion. 

The D/H exchange occurred mainly after complete consumption of the alkene, and no D-incorporated alkane was detected, which indicated that the coordinated NHC was easily displaced by the alkene and that these carbenes were less strongly bounded to the metal surface than was seen with mononuclear metal compounds [28]. These results strongly suggested that the imidazolium cations reacted with the nanoparticle surface preferentially as aggregates of the type [(DAI) (X) ] [(DAI) e (X)J (where DAI is the 1,3-dialkylimidazolium cation and X the anion), rather than as isolated imidazolium cations. 

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Regarding the color, we only see a need for colorless ionic liquids in very specific applications (see above). One easy treatment that often reduces coloration quite impressively, especially of imidazolium ionic liquids, is purification by column chromatography/filtration over silica 60. For this purification method, the ionic liquid is dissolved in a volatile solvent such as CFF2C12. Usually, most of the colored impurities stick to the silica, while the ionic liquid is eluted with the solvent. By repetition of the process several times, a seriously colored ionic liquid can be converted into an almost completely colorless material. 

Another common reaction is the chlorination of alkenes to give 1,2-dihaloalka-nes. Patell et al. reported that the addition of chlorine to ethene in acidic chloroalu-minate(III) ionic liquids gave 1,2-dichloroethane [68]. Under these conditions, the imidazole ring of imidazolium ionic liquid is chlorinated. Initially, the chlorination occurs at the 4- and 5-positions of the imidazole ring, and is followed by much slower chlorination at the 2-position. This does not affect the outcome of the alkene chlorination reaction and it was found that the chlorinated imidazolium ionic liquids are excellent catalysts for the reaction (Scheme 5.1-39). 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

Figure 3 Purification protocol of imidazolium ionic liquid.

Scheme 7 Partial hydrogenation of benzene by Ru(0) nanocatalysts in room temperature imidazolium ionic liquid...

Recently several pubhcations have examined replacing aqueous solvents with ionic liquids. Since simple and complex sugars are soluble in many imidazolium hahdes, water is not required as a co-solvent and degradation of HMF is minimal. Lansalot-Matras et al. reported on the dehydration of fmctose in imidazolium ionic liquids using acid catalyst (6). Moreau et al. reported that l-H-3-methylimidazolium chloride has sufficient acidity to operate without added acid (7). And we reported that a 0.5 wt% loading (6 mole% compared to substrate) of many metal halides in 1-ethyl-3-methylimidazohum chloride ([EMIM]C1) result in catalytically active materials particularly useful for dehydration reactions (8). 

Dupont, J. (2004) On the solid, liquid and solution structural organization of imidazolium ionic liquids. Journal of the Brazilian Chemical Society, 15 (3), 341-350. 

Migowski, P., Machado, G., Texeira, S.R., Alves, M.C.M., Morais, J., Traverse, A. and Dupont, J. (2007) Synthesis and characterization of nickel nanopartides dispersed in imidazolium ionic liquids. Physical Chemistry Chemical Physics,... 

Dupont, J., Fonseca, G.S., Umpierre, A.P., Fichtner, P.F.P. and Teixeira, S.R. (2002) Transition-metal nanopartides in imidazolium ionic liquids recycable catalysts for biphasic hydrogenation reactions. Journal of the American Chemical Society, 124 (16), 4228—4229. 

Scheme 15.3 Preparation of soluble iridium nanoparticles from in situ reduction of the organometallic precursor [ir(COD)Cl]2 in imidazolium ionic liquids.

Stepnowski, R, Solid-phase extraction of room temperature imidazolium ionic liquids from aqueous environmental samples. Anal. Bioanal. Chem., 381, 189, 2005. 

Ruiz-Angel, M. J. and Berthod, A., Reversed phase liquid chromatography of alkyl-imidazolium ionic liquids,. Chromatogr. A, 1113,101-108, 2006. 

Markuszewski, M. J., Stepnowski, R, and Marszall, R, Capillary electrophoretic separation of cationic constituents of imidazolium ionic liquids, Electrophoresis, 25, 3450-3454, 2004. 

Arenz, S., Babai, A., Binnemans, K., Driesen, K., Giernoth, R., Mudring, A.-V., Nockemann, R, Intense near-infrared luminescence of anhydrous lanthanide(III) iodides in an imidazolium ionic liquid, Chem. Phys. Let., 402, 75-79, 2005. 

Giernoth, R. and Bankmann, D., Transition-metal free ring deuteration of imidazolium ionic liquid cations. Tetrahedron Lett., 47,4293,2006. 

Lesimple, A. et al. Electrospray mass spectral fragmentation study of N,N -disubstituted imidazolium ionic liquids. /. Am. Soc. Mass Spec., 17, 85, 2006. 

Gozzo, F. G. et al.. Gaseous supramolecules of imidazolium ionic liquids Magic numbers and intrinsic strengths of hydrogen bonds. Chem. Eur. J., 10, 6187,... 

Odourless and non-volatile organosulfur compounds grafted to an imidazolium ionic liquid scaffold has been synthesized. The sulfoxides have been used for an efficient oxidation of primary allylic and benzylic alcohols into aldehydes and secondary alcohols to ketones under Swern oxidation conditions and the corresponding sulfides can be recovered and recycled.110... 

Using electrospray ionization mass spectrometry in both positive and negative ion modes, the on-line scanning of the Morita-Baylis-Hillman reaction in the presence of imidazolium ionic liquids has been investigated. The interception of several supramolecular species indicated that ionic liquids co-catalyse the reactions by activating the aldehyde toward nucleophilic enolate attack and by stabilizing the zwitterionic species that act as the main intermediates.175... 

An ionic liquid (IL) is literally an ionic compound (a salt) that is a liquid. Of most current interest are salts that are liquids at room temperature (RTILs), or at least below 100 °C. There is a range of compounds that form room temperature ionic liquids dating back to ethanolammonium nitrate, (EtNFQ+ (N03) (m.p. 14 °C), synthesised by Walden in 1914. Perhaps the most popular and well-studied are those based on the l-butyT3-methylimidazolium (bmim) cation, such as bmim+ PF6 (13.20) and bmim 1 BI 4 which melts at ca. -80 °C. The imidazolium ionic liquids were initially used as their halogenoaluminate salts but they have a major drawback in that they are highly moisture sensitive. 

Corma A, Garcfa H, Leyva A (2005) Comparison between polyethylenglycol and imidazolium ionic liquids as solvents for developing a homogeneous and reusable palladium catalytic system for the Suzuki and Sonogashira coupling. Tetrahedron 61(41) 9848-9854... 

In the light of these results, it is very important to check catalytic results obtained from imidazolium ionic liquid for a possible influence of in-situ formed carbene species. This can be done especially by testing a given reaction as well in ionic liquids which do not form carbene complexes e.g. in pyridinium based ionic liquids. 

Nockemann, P., Binnemanns, K.,and Driesen, K. (2005) Purification of imidazolium ionic liquids for spectroscopic applications. Chem. Phys. Lett., 415,131-136. 

Farmer V. and Welton, T. (2002) The oxidation of alcohols in substituted imidazolium ionic liquids using ruthenium catalysts. Green Chem., 4, 97-102. 

Ionic liquids have been a popular topic of interest in 2002 and a review of the applications of these solvents in organic synthesis has been published (02ACA75>. New, densely functionalized fluoroalkyl-substituted imidazolium ionic liquids have been reported <02TL9497>. An ultrasound-assisted preparation of a series of ambient-temperature ionic liquids, l-alkyl-3-methylimidazolium halides, which proceeds via efficient reactions of 1-methyl imidazole with alkyl halides/terminal dihalides under solvent-free conditions, has been described <02OL3161>. New hydrophilic poly(ethyleneglycol)-ionic liquids have been synthesized from... 

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