Ionic liquids cations

With respect to the ionic liquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, an imidazolium moiety as an ionic liquid cation can act as a ligand precursor for the dissolved transition metal. Its transformation into a lig-... 

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

A common observation with all the solutes studied is that, no matter how well-defined are the solute-cation and solute-anion interactions, the ionic liquid cation-anion structure is relatively unaffected—cations are still surrounded by a bulk-like arrangement of anions, and vice versa. 

Stepnowski, P. and Mrozik, W., Analysis of selected ionic liquid cations by ion exchange chromatography and reversed-phase high performance liquid chromatography, /. Sep. Sci, 28,149-154, 2005. 

Stepnowski, R, Nichthauser, Mrozik, W., and Buszewski, B., Usefulness of jt- -n aromatic interactions in the selective separation and analysis of imidazolium and pyridinium ionic liquid cations. Anal. Bioanal. Chem., 385, 1483-1491,2006. 

Figure 10.1 Comparison of the structures of an ionic liquid and an organic solvent. In the ionic liquid, cations are solvated by anions and vice versa. ...

Figure 13.1 Ionic liquid cations and anions discussed in this chapter.

Mele, A., Romano, G., Giannone, M. et al.. The local structure of ionic liquids Cation-cation NOE interactions and internuclear distances in neat [bmim][BF4] and [bdmim][ BFJ, Angew. Chem. Int. Ed., 45,1123, 2006. 

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

Fig. 8.3. (7-Surfaces of typical ionic liquid cations (upper row l-butyl-2,3-methylimidazolium, l-butyl-3-methylimidazolium, and 4-methyl-N-bu-tylpyridinium) and anions (tetrafluoroborate, perchlorate, hexafluoro-phosphate, and bis-(trifluoromethylsulfonyl)imide).

Table 2.1 Standard ion transfer potentials of different A/yV-alkylimidazolium and l-butyl-4-methylpyridinium ionic liquid cations [29]...

G-h Tao, He L, Sun N et al (2005) New generation ionic liquids cations derived from amino acids. Chem Commun 28 3562-3564... 

Poly(pyrrole) films grown by constant potential in the ionic liquid tri(hexyl) (tetradecyl) phosphoniumbis(trifluoromethanesulfonyl)amide, [P6,6,6,u][NTf2], and subsequently cycled 100 times and reduced in the monomer-free ionic liquid, showed the presence of both ionic liquid cations and anions within the film. Interestingly, poly(pyrrole) films grown by constant potential in the ionic liquid but with no subsequent electrochemical cycling also contained both ionic liquid cation and anion within the film (Figure 7.19). 

The crucial parameter for the anodic decomposition of halides is the anodic potential. This is simply due to the dilemma that a minimum potential for decomposition is needed but degradation of the ionic liquid cation is enhanced with increasing potential. It was found for the [BMIM] cation that at a voltage about 20% above the decomposition value the appearance turned gradually from colorless to light yellow. 

Ionic liquids are immiscible with many organic solvents and compounds, which lends themselves to biphasic or multiphasic catalytic reactions. Most are also immiscible with fluorous phases and some are immiscible with water. In the ideal biphasic process involving ionic liquids, a soluble polar substrate is converted to a less polar - and thus insoluble -product, which will then form a separate phase. Attaching fluorous groups to the ionic liquid cation can reverse the solubility properties. 

Figure 2.10 Examples of chiral ionic liquid cations...

The effect of nitrile-functionalised ionic liquids on the recycling potential was investigated in the coupling between iodobenzene and tributylphenyltin.[15] Several, closely related palladium-precursors, based on the ionic liquid cation [C3CNpy]+ and PdCP (Figure 6.5), were tested in the reaction, but the nature of the pre-catalyst did not have a significant influence on the rate. Independent from the ionic liquid used, rather low catalytic activity was observed and with 5 mol% catalyst, yields ranged between 43-65% after 12 hours at 80°C. 

Finally, though not strictly a hydroamination reaction, the asymmetric addition of alkynes to imines with a copper-bis(oxazoline) complex is worth briefly mentioning.[144] The nature of the ionic liquid cation has a strong effect on the enantioselectivity of the reaction and it appears that a good balance between hydrophobicity and acidity play an important role with best results obtained with [C4Ciim][Tf2N]. 

Ionic liquids differ from classical molten salts by being liquids at room temperature, or their melting points are below 100 °C. They have an ionic structure and usually consist of an organic cation and an inorganic or organic anion. The most common cations found in ionic liquids are the tetraalkylammonium, tetraalkyl-phosphonium, N-alkylpyridinium, 1,3-dialkylimidazolium, and trialkylsulfonium moieties [11]. Some common ionic liquid cations and anions are presented in Fig. 7.1. 

Fig. 3.6 -2 Structures forthe ionic liquid cations listed in Tables 3.6-1-3.6-10.

We note that there has been very recent work devoted to the development of force fields for other types of ionic liquid cations including pyridinium [123] and triazolium [124]. Moreover, an electronically polarizable model has been developed and applied to the simulation of [EMIM][N03] [125]. Results from these studies will be discussed in the next section. 

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