Anion in ionic liquids

The relative basicity of anions in ionic liquids is directly related to the question of their acid-base properties which cannot be determined by pH measurements due to the non-aqueous nature of these media. However, MacFarlane and Forsyth argue that an acid dissolved in an ionic liquid based on anions which are typically more basic than water, such as acetate, must be more dissociated than when dissolved in water [122],... 

Vidyaa P, Chadhaa A (2009) The role of different anions in ionic liquids on Pseudomonas cepacia lipase catalyzed transesterification and hydrolysis. J Mol Catal B Enzym 57 145-148... 

Finally, the data related to the application of ionic liquids for the cleavage of ethers are also notable. Chloroaluminates [44], anhydrous hydrobromic acid in 1-methylimidazole [45] and concentrated hydrobromic acid in [BMIM][BF4] or p-TsOH/[BMIM][Br] in [BMIM][BF4] [46] have been used to regenerate phenols from the corresponding aryl alkyl ethers. In ionic liquids, in the presence of an efficient proton donor (HBr or TsOH), the bromide anion behaves as a strong nucleophile able to cleave ethers. These latter reactions seem, therefore, to indicate that, as opposed to the situation in the absence of adds, the nudeophihcity of the bromide anion in ionic liquids in the presence of proton donors is higher than in molecular solvents (both pro tic and aprotic). This behavior may be a consequence of a... 

Figure 1.5 (A) Examples of cations in ionic liquids. (B) Examples of anions in ionic liquids. (Reproduced from Ref. [24] with permission of the author.)...

Zhang J, Bond AM, MacEarlane DR, Eorsyth SA, Pringle JM, Mariotti AWA, Glowinski AE, Wedd AG (2005) Voltammetric studies on the reduction of polyoxometalate anions in ionic liquids. Inorg Chem 44(14) 5123-5132. doi 10.1021/ic050032t... 

Table 18.2 Diffusion coefficient of superoxide anion in ionic liquid...

Despite the advantage of sulphate-based anions in ionic liquids over fluorinated or halide-containing anions in terms of greenness , the former ([bmim][0S04] and [moim][MDEGS04]) have in general not proven to be effective in the separation of aromatic-aliphatic mixtures, with the exception of [emim][ES04]. This indicates that a small sulphate-based anion is more effective for the separation of the separation problems presented in... 

Fig. 6.3 Organic cations and inorganic or organic anions in ionic liquids...

Thus we turn our attention to the design of cationic (and particularly polycationic) derivatives related to glycosides, simple carbohydrates, and the reduced polyols. It is necessary that such parent structures bear functionahties that may be used readily for the incorporation of cationic sites. Most prominently, in all of these types of compounds, the presence of primary hydroxyl group(s) readily jjermits such cationic site incorporation. We will contemplate the reaction systems allowing such transformations in a later section of this treatment. Initially, however, we should note the hmitations on the nature of the anion in ionic liquids generated from such structures if one is to allow facile production of the ionic liquid without side reactions that could modify the inherent organic (carbohydrate) structure. 

Mutual Lewis-Acid-Base Interaction of Cations and Anions in Ionic Liquids , Markus Holzweber, Ralf Lungwitz, Denise Dorfler, Stefan Spange, Mihkel Koel, Herbert Hutter and Wolfgang Linert,... 

Dahl K, Sando GM, Fox DM, Sutto TE, Owrutsky JC (2005) Vibrational spectroscopy and dynamics of small anions in ionic liquid solutions. J Chem Phys 123(8), 084504. doi 10.1063/1.2000229... 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

Transport numbers are intended to measure the fraction of the total ionic current carried by an ion in an electrolyte as it migrates under the influence of an applied electric field. In essence, transport numbers are an indication of the relative ability of an ion to carry charge. The classical way to measure transport numbers is to pass a current between two electrodes contained in separate compartments of a two-compartment cell These two compartments are separated by a barrier that only allows the passage of ions. After a known amount of charge has passed, the composition and/or mass of the electrolytes in the two compartments are analyzed. Erom these data the fraction of the charge transported by the cation and the anion can be calculated. Transport numbers obtained by this method are measured with respect to an external reference point (i.e., the separator), and, therefore, are often referred to as external transport numbers. Two variations of the above method, the Moving Boundary method [66] and the Eiittorff method [66-69], have been used to measure cation (tR+) and anion (tx ) transport numbers in ionic liquids, and these data are listed in Table 3.6-7. 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

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 successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Rh(nbd) (PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions. 

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

An example of a biphasic, Ni-catalyzed co-dimerization in ionic liquids with weakly coordinating anions has been described by the author s group in collaboration with Leitner et al. [12]. The hydrovinylation of styrene in the biphasic ionic liq-uid/compressed CO2 system with a chiral Ni-catalyst was investigated. Since it was found that this reaction benefits particularly from this unusual biphasic solvent system, more details about this specific application are given in Section 5.4. 

Interestingly, the specific environment of the ionic solvent system appears to activate the chiral Ni-catalyst beyond a simple anion-exchange reaction. This becomes obvious from the fact that even the addition of a 100-fold excess of Fi[(CF3S02)2N] or Na[BF4] in pure, compressed CO2 produced an at best moderate activation of Wilke s complex in comparison to the reaction in ionic liquids with the corresponding counter-ion (e.g., 24.4 % styrene conversion with 100-fold excess of Fi[(CF3S02)2N], in comparison to 69.9 % conversion in [EMIM][(CF3S02)2N] under otherwise identical conditions). 

In 1992, the ionic liquid methodology received a substantial boost when Wilkes and Zaworotko described the synthesis of non-chloroaluminate, room temperature liquid melts (e. g. low melting tetrafluoroborate melts) which may be regarded as second generation ionic liquids [6]. Nowadays, tetrafluoroborate and (the slightly later published [7]) hexafluorophosphate ionic liquids are still widely used in ionic liquid research. However, their use in many technical applications will be clearly limited by their relatively high sensitivity towards hydrolysis. Of course, the tendency of their anions to hydrolyse is much less pronounced than for the chloroaluminate melts but it still clearly exists. Consequently, the technical application of tetrafluoroborate and hexafluorophosphate ionic liquids will be effectively restricted to those applications where water-free conditions can be realised at acceptable costs. 

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