Cation in ionic liquids

The cations in ionic liquids are generally bulky monovalent organics. The typical cations of ionic liquids, not including the familiar alkylammonium and alkylphosphonium ions, are shown in Fig. 2. It is primarily the cations, which account for the low melting points of ionic liquids. The dialkylimidazolium ions, such as 1-butyl-3-methyl imidazolium [BMIM], have been widely investigated because low-melting ionic liquids can be made readily from such cations and because of their thermal and chemical stability. 

Dimethylimidazolium cation, in ionic liquids, 1, 855 Dimethyl iridium complexes, preparation, 7, 306 l-A -Dimethylnaphthylamines, in cobalt(III) complexes,... 

The size and shape of cations in ionic liquids are important in controlling the melting points of the salts. The formation oflow melting ionic liquids results from reduction in the magnitude of the Coulombic attraction term, and disruption of packing efficiency. On a simple basis, large ions tend to lead to reductions in the melting... 

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.)...

Fig. 7.19 A list of representative anion and cations in ionic liquid solvents used in MD simulations [75-78]...

Fig. 1. Important types of cations in ionic liquids. R and R" are lineal chains ethyl, propyl, butyl, pentyl, hexyl.

Scheme 2.14 Important types of cations in ionic liquids...

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. 

The viscosities of non-haloaluminate ionic liquids are also affected by the identity of the organic cation. For ionic liquids with the same anion, the trend is that larger allcyl substituents on the imidazolium cation give rise to more viscous fluids. For instance, the non-haloaluminate ionic liquids composed of substituted imidazolium cations and the bis-trifyl imide anion exhibit increasing viscosity from [EMIM], [EEIM], [EMM(5)IM], [BEIM], [BMIM], [PMMIM], to [EMMIM] (Table 3.2-1). Were the size of the cations the sole criteria, the [BEIM] and [BMIM] cations from this series would appear to be transposed and the [EMMIM] would be expected much earlier in the series. Given the limited data set, potential problems with impurities, and experimental differences between laboratories, we are unable to propose an explanation for the observed disparities. 

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. 

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. 

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]. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

The controlled synthesis of polymers, as opposed to their undesired formation, is an area that has not received much academic interest. Most interest to date has been commercial, and focused on a narrow area the use ofchloroaluminate(III) ionic liquids for cationic polymerization reactions. The lack of publications in the area, together with the lack of detailed and useful synthetic information in the patent literature, places hurdles in front of those with limited loiowledge of ionic liquid technology who wish to employ it for polymerization studies. The expanding interest in ionic liquids as solvents for synthesis, most notably for the synthesis of discrete organic molecules, should stimulate interest in their use for polymer science. 

Fig. 2. Typical cations used in ionic liquids, excluding alkylammonium and alkylphosphonium ions.

The stability of dialkylimidazolium cation-containing ionic liquids can be a problem even at moderate temperatures in the presence of some reagents or catalysts. For example, when CsF and KF were used in the ionic liquid [BMIM]PFg to perform a halogen exchange reaction in an attempt to replace Br from bromo-carbons with F , it was found that alkyl elimination from the [BMIM] cation took place, forming methyl imidazole, 1-butene, 1-fluorobutane, and other unidentified products at 150°C overnight 69). The fluoride ion acted as a base that promotes elimination or substitution processes. 

The strong acidity of the proton at the C2 position of a [AMIM] ion has been well recognized 183). This cation can react with palladium complexes to form inactive l,3-dialkylimidazol-2-ylidene palladium complexes 200), as confirmed in a study of the conventional Pd(OAc)2/PPh3/base catalyst in ionic liquids for the telomerization of butadiene with methanol at 85°C 201). 

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