Hexafluorophosphate ionic liquid

The future price of ionic liquids will also reflect intellectual property considerations. While the currently most frequently requested ionic liquids, the tetrafluoroborate and hexafluorophosphate ionic liquids, are all patent-free, many recently developed, new ionic liquid systems are protected by state of matter patents. Table 2.2-2 gives an overview of some examples published after 1999. 

Without a doubt, tetrafluoroborate and hexafluorophosphate ionic liquids have shortcomings for larger-scale technical application. The relatively high cost of their anions, their insufficient stability to hydrolysis for long-term application in contact with water (formation of corrosive and toxic HF during hydrolysis ), and problems related to their disposal have to be mentioned here. New families of ionic liquid that should meet industrial requirements in a much better way are therefore being developed. FFowever, these new systems will probably be protected by state of matter patents. 

However, a number of limitations are still evident when tetrafluorohorate and hexafluorophosphate ionic liquids are used in homogeneous catalysis. The major aspect is that these anions are still relatively sensitive to hydrolysis. The tendency to anion hydrolysis is of course much less pronounced than that of the chloroalu-minate melts, hut it still occurs and this has major consequences for their use in transition metal catalysis. For example, the [PF ] anion of l-hutyl-3-methylimida-2olium ([BMIM]) hexafluorophosphate was found (in the author s laboratories) to hydrolyze completely after addition of excess water when the sample was kept for 8 h at 100 °C. Gaseous HF and phosphoric acid were formed. Under the same conditions, only small amounts of the tetrafluorohorate ion of [BMlMjjBFJ was converted into HF and boric acid [10]. The hydrolytic formation of HF from the anion of the ionic liquid under the reaction conditions causes the following problems with... 

In the author s group, much lower-melting benzenesulfonate, tosylate, or octyl-sulfate ionic liquids have recently been obtained in combination with imidazolium ions. These systems have been successfully applied as catalyst media for the biphasic, Rh-catalyzed hydroformylation of 1-octene [14]. The catalyst activities obtained with these systems were in all cases equal to or even higher than those found with the commonly used [BMIM][PF6]. Taking into account the much lower costs of the ionic medium, the better hydrolysis stability, and the wider disposal options relating to, for example, an octylsulfate ionic liquid in comparison to [BMIM][PF6], there is no real reason to center future hydroformylation research around hexafluorophosphate ionic liquids. 

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. 

Taking into account the much lower costs of the ionic medium, the better stability against hydrolysis and the wider disposal options related to, for example, an octylsulfate or a tosylate ionic liquid in comparison to BMIM PF6], there is no real reason to centre future hydroformylation research around hexafluorophosphate ionic liquids. 

Carda-Broch, S., Berthod, A., and Armstrong, D. W., Solvent properties of the l-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. Anal. Bioanal. Chem., 375,191-199, 2003. 

The first publication describing the synthesis of tetrafluoroborate and hexafluorophosphate ionic liquids by metathesis reaction from the corresponding alkali salts [13] opened up the way towards a commercial ionic liquid production. Nowadays, a number of commercial suppliers offer ionic liquids even in large quantities [14]. Moreover, the availability of many ionic liquids on a rapid delivery basis has been established through internationally operating distributors [15]. 

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... 

The common route to bis(indoyl)methanes is via condensation of indoles with aldehydes or ketones in the presence of either protic or Lewis acids. The reaction has been evaluated in tetrafluoroborate and hexafluorophosphate ionic liquids and of the metal salts tested best results were obtained with In(OTf)3 and FeCl3-6H20. Although In(OTf)3 is somewhat more active, its higher price makes the use of iron(III)chloride more attractive. Furthermore, whereas the activity of In(OTf)3 decreases quickly upon recycling, ionic liquid solutions of FeCl3 remain reasonably active for at least four runs.[62] It was found that in hydrophilic ionic liquids the reaction did not proceed at all, whereas fast conversion was observed with the [PF6] -anion, see Scheme 9.17. As water is produced in the course of the reaction it is possible that elimination of the water from the reaction medium helps to protect the catalyst, however, it cannot be excluded that at least some of the catalytic activity is due to the formation of HF. While the... 

Yu B, Zhou F, Pang C J, et al. Tribological evaluation of a, w-diimi-dazoliumalkylene hexafluorophosphate ionic liquid and benzotriazole as additive. Tribology International. 2008. 41, 797-801. 

Schrekker H S, Stracke M P, Schrekker C M L, et al. Ether-functionalized imidazolium hexafluorophosphate ionic liquids for improved water miscibilities. Ind. Eng. Chem. Res. 2007. 46, 7389-7392. 

From that moment ionic liquids started to be regarded as fluids formed by a polar domain with the structure of a tridimensional network of ionic channels (the polar network), and nonpolar domain(s) arranged as a dispersed nanophase in the case of ionic liquids with relatively short alkyl-side chains and as a continuous one for longer side-chains (Fig. 6). In the systems first analyzed (1-alkyl-methylimidazolium hexafluorophosphate ionic liquids), the butyl side-chain marked the onset of the transition from dispersed nonpolar nanodomains to a bicontinuous nanosegregated system. The nanosegregation is also observed in other families of ionic liquids, the presence of nonpolar side chains both in the cation or in the anion being determinant in the structure of the ionic liquid (Fig. 7). 

For similar reasons organic solvents used during the last synthesis step such as methanol, dichloromethane and water have to be removed. Water can not only enter the ligand sphere of metal cations as a strong ligand in ionic liquids but also lead to degradation of the ionic liquid itself by hydrolysis. It is well known that water leads to the decomposition of tetrafluoroborate and hexafluorophosphate ionic liquids fluoride or hydrogen fluoride is released in this reaction. 

Yao MH, Fan MJ, Liang YM et al (2010) Imidazolium hexafluorophosphate ionic liquids as high temperature lubricants for steel-steel contacts. Wear 268 67-71... 

Swatloski RP, Visser AE, Reichert WM, Broker GA, Farina LM, Holbrey ID, Rogers RD (2002) On the solubilization of water with ethanol in hydrophobic hexafluorophosphate ionic liquids. Green Chem 4 81-87... 

Nara, S.J., Harjani, J.R., Salunkhe, M.M., Mane, A.T., and Wadgaonkarb, P.P. (2003) Lipase-catalysed polyester synthesis in l-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. Tetrahedron Lett., 44 (7), 1371-1373. 

Wasserscheid et al. demonstrated that the use of hexafluorophosphate ionic liquids allows, for the first time, a selective, biphasic oligomerization of ethylene to 1-oleflns with the cationic Ni-complex [(mall)Ni(dppmo)][SbF6] (Scheme 5.3-32) [44,249]. 

Most tetrafluoroborate and hexafluorophosphate ionic liquids are patent free however, owing to their cost, their insufficient stability toward hydrolysis and issues concerning their disposal, such ionic liquids are not useful for industrial applications. These problems prompted researchers to develop new types of ionic liquids with different anions, many of them being protected by state-of-the-matter patents, for example, HSOJ (Ref. 184), PF3(C2F5)J (Ref 185) and borate derivatives. To solve the problem of the high cost of imidazolium-based ionic liquids, one patent claims broad coverage of cheap phosphonium ionic liquids. ... 

Schroder, U., Wadhawan, J. D., Compton, R. G., Marken, F., Suarez, P. A. Z., Consorti, C. S., de Souza, R. F., and Dupont, J. [2000]. Water-induced accelerated ion diffusion voltammetric studies in l-methyl-3-[2,6-[S]-dimethylocten-2-yl]imidazolium tetrafluoroborate, l-butyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids. New J. Chem., 24, pp. 1009-1015. 

Giridhar, P., Venkatesan, K., Srinivasan, T., Rao, P. V. (2004). Effect of alkyl group in l-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids on the extraction of uranium by tri-n-butylphosphate diluted with ionic liquids. Journal of Nuclear Radiochemical Science, 5, 21-26. 

Zhang DP, Liu QH, Shi XS, Li YD. Tetrabutylammonium hexafluorophosphate and l-ethyl-3-methyl imidazohum hexafluorophosphate ionic liquids as supporting electrolytes for non-aqueous vanadium redox flow batteries. J Power Sources 2012 203 201-5. 

Keglevich G, Grun A, Hermecz I, Odinets L (2011) Quaternary phosphonium salt and 1, 3-dialkylimidazolium hexafluorophosphate ionic liquids as green chemical tools in organic syntheses. CurrOrg Chem 15 3824-3848... 

Neutron scattering data for the series of l-allq l-3-methyliniidazolium hexafluorophosphate ionic liquids, [C mim][PFg] (n = 4, 6, or 8), collected over a wide Q range (0.05-50 A" ) in... 

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