Imidazolium-based ionic liquid

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. 

When Heck reactions and other C-C coupling reactions are carried out in imidazolium-based ionic liquids, the base can react with the acidic proton on the... 

Ionic liquid crystals are liquid crystals containing ionic moieties. Ionic hquid crystals having ammonium and pyridinium salts were reported to show stable thermotropic liquid crystalline behavior [35 0]. Seddon, Bruce, and coworkers showed that some imidazolium and pyridinium salts exhibit low melting points and liquid crystalline behavior [41]. Ionic liquid crystals with lower temperature mesophase ranges are called hquid crystalline ionic liquids. A large variety of thermotropic liquid crystaUine ionic liquids exhibiting anisotropic fluid states were prepared. In particular, the ionic liquid crystals based on imidazolium salts 1 [42-46], 2... 

Ionic liquids (imidazolium salts) were utilized as green solvents in the Friedlander synthesis. Reactions of o-amino-substituted aromatic carbonyls with a variety of ketones (cyclic, acyclic, aromatic) result in the desired quinolines in uniformly high yield. Conditions are mild and no hazardous acids or bases are used <03JOC9371>. 

Dobbs W, Douce L, AUouche L et al (2006) New ionic liquid crystals based on imidazolium salts. New J Chem 30 528-532... 

The real breakthrough in the use of ionic liquids as solvents came in 1992, when Wilkes and Zaworotko studied the formation of dialkylimidazolium salts associated with water-stable anions. This led to the discovery of the ionic liquids [emim][BF4] 5 (Figure 2.2) and [emim][N03], with melting points of 15°C and 38" C, respectively. These ionic liquids are hygroscopic, but they are not degraded in the presence of moisture. Shortly afterward, the synthesis of the corresponding hexafluorophosphate derivative 6 was described. Today, most ionic liquids are based on imidazolium derivatives associated with non-covalent anions. 

Certain common families of ionic liquids are based on aromatic cations, namely, imidazolium or pyridinium, and their spatial ordering is affected by interactions involving 7t-systems. The structure of solutions of benzene in imidazolium ionic liquids has been studied both experimentally by neutron diffraction and through simulation [47]. Inclusion conpounds have been identified in the solid-liquid phase diagrams [48] and the role of cation-n-interactions confirmed, also in the structure of solutions of toluene in ionic liquids [37]. 

Lin, I. J. B. Vasam, C. S., (2005). Metal-containing Ionic Liquids and Ionic Liquid Crystals Based on Imidazolium Moiety. /, Orgnomet. Chem., 690, 3498-3512. 

Zhang, Q. X. Shan, C. S. Wang, X. D. Chen, L. L. Niu, L. Chen, B. (2008d). New ionic liquid crystals based on azobenzene moiety with two symmetric imidazolium ion group substituents. Liq. Cryst, 35,1299-1305. 

The pyridinium- and the imidazolium-based chloroaluminate ionic liquids share the disadvantage of being reactive with water. In 1990, Mike Zaworotko (Eigure 1.4) took a sabbatical leave at the Air Eorce Academy, where he introduced a new dimension to the growing field of ionic liquid solvents and electrolytes. 

The room temperature conductivity data for a wide variety of ionic liquids are listed in Tables 3.6-3, 3.6-4, and 3.6-5. These tables are organized by the general type of ionic liquid. Table 3.6-3 contains data for imidazolium-based non-haloaluminate alkylimidazolium ionic liquids. Table 3.6-4 data for the haloaluminate ionic liquids, and Table 3.6-5 data for other types of ionic liquids. There are multiple listings for several of the ionic liquids in Tables 3.6-3-3.6-5. These represent measurements by different researchers and have been included to help emphasize the significant vari-... 

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... 

The use of imidazolium-based ionic liquids in Pd-catalyzed Heck reactions always carries with it the possibility of in situ formation of Pd-carbene complexes (for more details see Section 5.2.2.3). The formation of these under the conditions of the Heck reaction was confirmed by investigations by Xiao et al. [30], who described a significantly enhanced reactivity of the Heck reaction in [BMIM]Br in relation to the same reaction in [BMIM][Bp4] and explained this difference by the fact that formation of Pd-carbene complexes was observed only in the bromide melt. 

With the enthusiasm currently being generated by the (so-called) stable carbenes (imidazolylidenes) [7], it is surprising that there are few reports of imidazolium-based ionic liquids being used to prepare metal imidazolylidene complexes. Xiao et al. have prepared bis(imida2olylidene)palladium(II) dibromide in [BMIMJBr [8]. All four possible conformers are formed, as shown in Scheme 6.1-3. 

Olefin dimerisation with Ni-NHC complexes became a topic of interest following reports of Ni(II) phosphine complexes being employed in imidazolium-based ionic liquid solvents [23, 24]. It had previonsly been established that aIkyl-Ni(II) complexes containing NHC ligands can rapidly decompose via imidazolium formation (Scheme 4.1) [5], and it was thus of interest to explore the effect that an excess of the imidazolinm cation would have on this reaction. 

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

Banerjee, T., Singh, M. K., Khanna, A. Prediction of binary VLE for imidazolium based ionic liquid systems using COSMO-RS. Ind. Eng. Chem. Res. 2006, 45, 3207-3219. 

The rhodium catalyzed carbonylation of ethylene and methanol can be conducted in the absence of added alkyl halide if the reactions are conducted in iodide based ionic liquids or molten salts. In the case of ethylene carbonylation, the imidazolium iodides appeared to perform best and operating in the absence of ethyl iodide gave improved selectivities relative to processes using ethyl iodide and ionic hquids. In the case of... 

Costa C, Hihn JH, Rebetez M et al (2008) Transport-limited current and microsonoreactor characterization at 3 low frequencies in the presence of water, acetonitrile and imidazolium-based ionic liquids ([BuMIm] [(CF3S02)2N]). Phys Chem Chem Phys 10 2149-2158... 

Lee, C.M., Jeong, H.J., Lim, S.T., Sohn, M.H. and Kim, D.W. (2010) Synthesis of iron oxide nanopartides with control over shape using imidazolium-based ionic liquids. ACS Applied Materials S, Interfaces, 2 (3), 756-759. 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

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