Ionic liquid, anions cations

Ionic liquid System Cation Anion(s) Temperature, (X Conduc- tivity (k), mS cm Conduc- tivity method Viscosity (n), cP Viscosity method Density (p), gcm Density method Molar conductivity fAJ, cm iT mor Walden product (An) Ref. 

Ionic liquid system Cation(sj Anion(s) Working electrode" Window (V) Ref. 

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-... 

Of course, these concentration effects will be highly dependent on the nature of the substrate dissolved in the ionic liquid, as well as on the nature of the ionic liquid s cation and anion. Given the enormous opportunity to vary these last two, it becomes clear that a detailed understanding of the role of the ionic liquid in reaction mixtures is far from complete. Clearly, this limited understanding is currently restricting our opportunities to benefit from the full potential of an ionic liquid solvent in a given synthetic application. 

In an ionic liquid, the cations are much larger than in typical ionic substances. As a result, the anions and cations cannot be packed together in an orderly way that balances both the sizes and distances of the ions with the charges between them. As a result, they remain in a loosely-packed, liquid form. Because the charges on the cations have the capacity to bind more anions, an ionic liquid has a net positive charge. Notice the large cation of the ionic liquid molecule shown below. This is what makes ionic liquids suitable solvents. 

Ionic liquid system Cation(s) Anion(s) Working electrode Window Ref. (V) ... 

Fig. 8.4. er-Profiles of typical ionic-liquid anions and cations. 

Danielsson et al. [25] have studied the synthesis of PEDOT in ionic liquids that utilize bulky organic anions, l-butyl-3-methylimidazolium diethylene glycol monomethyl ether sulfate and l-butyl-3-methylimidazolium octyl sulfate, the latter of which is a solid at room temperature and thus requires the addition of either monomer or solvent (in this case water) to form a liquid at room temperature. Polymerization in a water-free ionic liquid was only possible in the octyl sulfate species, but the polymerization of EDOT was successful in aqueous solutions of both the ionic liquids (0.1 M). The ionic liquid anions appear to be mobile within the polymer, exchangeable with chloride ions at a polymer/KCl(aq) interface, but it is interesting that when the PEDOT is in aqueous solutions of the ionic liquid, at higher concentrations (0.01-0.1 M) the imidazolium cation can suppress this anion response. The ion mobility in both the ionic liquid and in the polymer film in contact with the solution is significantly increased by addition of water. 

Chiral ionic liquids could become an important class of solvents,194 100 1011 which may even be relatively cheap when prepared from the natural chiral pool with anions such as lactate. However, as yet, their ability to induce enantiomeric excess in catalysed reactions has been only met with limited success.1102 1041 With increasing evidence to suggest that carbene ligands derived from ionic liquid imidazolium cations are involved in certain catalysed reactions it is likely that chiral ionic liquids that act as both solvent and ligand could prove highly useful in stereoselective reactions. Some examples of chiral cations that may lead to ionic liquids are shown in Figure 2.10. 

The square planar coordination compounds [Cu(acac)(tmen)][C104] and [Cu(acac) (tmen)][BPh4] have been used to probe the nucleophilicity and hence the donor power of ionic liquids. The cationic complex shows a good correlation between the donor number (DN) of a solvent and the absorption maximum of the lowest d-d-transition in the absorption spectrum [104], It has also been used to determine the donor numbers of anions in solutions [126], Studies of the solvent polarity of [C4mim]+, [C8mim]+, [C4mmim]+ and [C8mim]+ ionic liquids with [PF6], [OTf]- and... 

Ionic liquid system Cation Anion(s) Temperature (K) Conductivity (K), mS cm 1 Conduc- tivity method Viscosity (V), CP Viscosity method Density (Pi, g c 3 Density method Molar Conductivity (A), cm2 Q -1mor7 Walden product (M) Ref. 

Liquid membranes are ideally suited for this application since they have the potential for removing toxic suhiLances from wastewater down to vety low levels. Both molecular and ionic species (anions, cations, and complex ions) have bean extracted successfully by property constituted liquid membranes. A perlial list of processes reported prior to 1978 is given in Table 19.4-1. These heve all been reviewed.2 3 Recent improvements in thase separations as well as new methods reportnd recently are summerlzed in this section. 

Ionic liquid system Cation Anion(s) Temp, K Viscosity (t ), cP Density, g cm Reference... 

Ionic liquid system Cation h+ Anion(s) tx- Temperature (K) Methodl l Ref. 

However, the effects mainly depend on the charge delocalization in the radical anion and the nature of the ionic liquid s cation plays a secondary role. When the charge is spread out over the entire molecule, as in the 9-diloroanthracene radical anion, the carbon-halogen bond cleavage accelerates in ionic liquids due... 

Very few examples of imine hydrogenation in ionic liquids have been published so far. Giernoth et al. screened eight different ionic liquids (the cations were [BMIMJ+, l-decyl-3-methylimidazolium ([DMIM]" "), N-butyl-3-methylpyridinium ([BMPy]+) and N-decyl-3-methylpyridinium ([DMPy]" ) combined with the anions [Bp4] and [(CF3 S02)zN] ) and compared them with toluene as solvent in the hydrogenation of trimethylindolenine with the Ir-XYLIPHOS catalyst system (Scheme 5.3-12) [120]. 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

In a general (and generalizing) view on this issue it can be stated that suitably selected ionic liquids are very likely to form catalytically active ionic catalyst solutions with a given transition metal catalyst if the latter is neither extremely electrophilic (acidic) nor extremely nucleophilic (basic). While extremely electrophilic catalyst complexes are likely to coordinate strongly even with those anions of the ionic liquid solvent which are generally regarded as weakly coordinating, extremely nucleophilic catalytic centers are likely to react with the ionic liquid s cation. Carbene complex formation by oxidative addition as well as dealkylation of the cation are possible deactivation pathways of the catalyst in such a case. 

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