Halide impurities

All the halide exchange reactions mentioned above proceed more or less quantitatively, causing greater or lesser quantities of halide impurities in the final product. The choice of the best procedure to obtain complete exchange depends mainly on the nature of the ionic liquid that is being produced. Unfortunately, there is no general method to obtain a halide-free ionic liquid that can be used for all types of ionic liquid. This is explained in a little more detail for two defined examples the synthesis of [BMIM][(CF3S02)2N] and the synthesis of [EMIM][BF4]. 

---

Halide impurities may have a negative effect on the rate of a hydrogenation reaction, as was observed by Cobley et al. These authors studied the asymmetric hydrogenation of 2-methylenesuccinamic acid using [(S,S)-(Et-DuPHOS)Rh-(COD)]BF4 as catalyst [76]. They were able to obtain a 30-fold acceleration upon removal of a chloride impurity from the substrate (Scheme 44.9). 

H4Ru4(ri -C6H6)](BF4)2, that was not expected to be sensitive to poisoning by halide impurities showed a significant increase in the turnover rate when the cluster was immobilized in the halide-free [BMIM]BF4. 

In certain cases, the halide impurities could be countered by using additives that serve to bind the halides, but this is not always a desired solution. In the rhodium-catalyzed polymerization of phenylacetylene (777), it was shown that the deleterious effect of halide impurities can be nullified by the addition of NEt3 co-catalyst to a Rh(diene)(acac)-containing catalyst. 

It is well established that small levels of halide impurities in ILs can limit their applications in various catalytic processes [1,30,31]. The minimization of halide and water content in the IL constitute two vital purification steps... 

Berthier, D., Varenne, A., Gareil, R, Digne, M., Lienemann, C.-P, Magnac, L., and Olivier-Bourbigouc, H., Capillary electrophoresis monitoring of halide impurities in ionic liquids. Analyst, 129,1257-1261, 2004. 

Minimal metallic or halide impurities in the final product... 

The products from ATRP will most likely contain metallic and halide impurities. The sulfonyl halide initiators for ATRP have considerable advantages over alkyl halides in cost and in operating with any monomer capable of undergoing ATRP. 

The above characterizations primarily concern the interactions between molecular solutes and ILs. However, ILs are also good solvents for ionic compounds, and have been studied extensively as media for transition metal catalysis [4, 38, 219] and for the extraction of heavy metals [23]. ILs are capable of solvating even simple salts, such as NaCl, to some degree [219], and in fact the removal of halide impurities resulting from synthesis can be a considerable challenge [68]. However, ionic complexes are generally far more soluble than simple salts [220], and we focus our attention on these systems as they have received greater study and are more relevant to the processes noted above. 

Figure 9.2 shows a typical cyclic voltammogram of ultrapure 1-butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Pyi,4] TFSA) on Au(lll) [5, 6]. Ultrapurity means that the supplier (here Merck KGaA/EMD) guarantees that water and halide impurities are below the 10 ppm level. Routinely the liquids are dried to water contents below 3 ppm prior to use in our laboratory. 

A further error in IL synthesis can originate from purification processes. In order to remove the often yellowish color of ionic liquids after synthesis they are commonly purified over silica or alumina powder (see above). Once we obtained a liquid where the supplier invested a lot of effort to deliver Endres-quality . [EMIMJTFSA was made with the best available educts in the add-base routine from diluted aqueous [EMIMJOH and H-TFSA. This approach exdudes metal and halide impurities. The supplier removed the slight yellowish color by purification over silica. For this purpose the supplier used quite a fresh silica, which had not been used in any purification process before. One has to bear in mind that the dominant impurities, even in hiqh quality silica, are aluminum species. Figure 11.26 shows the 1st, the 7th and the 15th cydes of this liquid on Au(l 11). Apparently... 

Metal ion and halide impurities are an issue in ionic liquids with discrete anions. As we have demonstrated in Chapter 11.5 Li+ (and K+) are common cationic impurities, especially in the bis(trifluoromethylsulfonyl)amides which typically contain 100 ppm of these ions from the metathesis reaction. Although Li and K are only electrodeposited in the bulk phase at electrode potentials close to the decomposition potential of the pyrrolidinium ions, there is evidence for the underpotential deposition of Li and K on gold and on other rather noble metals. For a technical process to deposit nickel or cobalt from ionic liquids the codeposition of Li and/or K, even in the underpotential deposition regime, has to be expected. 

Halide impurities can alter the complex chemistry in ionic liquids and can lead to unexpected oxidation reactions at the counter electrode. Furthermore even low amounts of e.g. chlorine can be formed, leading to some side reactions. 

Both the melting point and viscosity of an ionic liquid are highly dependent on their purity and values in the literature differ accordingly. In the 1-alkyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids, which are the ones most widely used in catalysis thus far, halide impurities are usually present to varying degrees, due to the metathesis route commonly used for their preparation (see Section 2.3). The effect of different chloride concentrations present on the viscosity of [C4Ciim][BF4] is shown in Figure 2.5.1161... 

Those catalysts that are employed in oxidation reactions are often relatively tolerant with respect to the presence of halide impurities. This is underlined from the fact that NaOCl or [C4Ciim]Br are used as oxidant or solvent, respectively. However, some catalysts are able to oxidise halides to... 

Methyltrioxorhenium (MTO) is an extensively used catalyst for olefin epoxidation1241 and a number of reviews are available on its application.125"281 The complex reacts with hydrogen peroxide to form the mono r 2-peroxo species mpMTO and the diperoxide dpMTO. Both complexes are capable of transferring an oxygen atom to olefins or other substrates, as shown in Scheme 5.1, but the diperoxide species is about five times more reactive than the monoperoxo complex.1171 Care has to be taken that halide impurities are absent from the solvent as these are oxidised much faster than olefins.1171... 

One characteristic of Friedel-Crafts reactions in ionic liquids is their pronounced solvent dependence in that different anion-cation combinations can determine complete and fast conversion and total inactivity of a given catalyst. It is somewhat striking that the catalytic activity is often lowest in hydrophilic ionic liquids from which halide impurities are harder to remove. In any case, screening of a selection of cations and anions appears to be necessary in order to evaluate the suitability of any potential catalyst. 

Syntheses that exploit the solubility of the alkaline-earth metals in liquid ammonia have proven practical for alkoxide work, as they generate high yields, reaction rates, and purity (Table 8, Equation (3)). In a refinement of this approach, Caulton and co-workers have used dissolved ammonia in an ethereal solvent, usually THF, to effect the production of a number of alkoxides of barium, and this method has also been examined with calcium and strontium (Table 8, Equations (4a) to (4c)). Displacement reactions using alkali metal alkoxides and alkaline-earth dihalides (Table 8, Equation (5)), and between alkaline-earth hydrides or amides and alcohols (Table 8, Equations (6) and (7)), have been examined, but alkali-metal halide impurities, incomplete reactions, and unexpected equilibria and byproducts can affect the usefulness of these approaches. 

Halide impurities can be assessed using many techniques with varying degrees of accuracy. Techniques include ... 

Ge et al. [122] also investigated the impact of both water and halide impurities where it was found that, in the case of small quantities, i.e., up to a mass fraction of 0.01 for water and 0.05 for halide (chloride), no significant effect on thermal conductivity was observed. Above this the thermal conductivity of the mixture could be modeled using the Jamieson correlation ... 

---