Ionic hexafluorophosphate

Hexafluorophosphate derivatives, such as [M(Me2SO)n][PF6]3 (M = La, Lu, Y), have been synthesized (365), and infrared data show O-bonding of the sulfoxide with ionic hexafluorophosphate groups. Analytical data are incomplete for this series, as decomposition, postulated to be to lanthanide fluorides, occurs. The (CH2)4SO complexes [M( CH2 4SO)t.5][PF6]3 are reported (145), which are 3 1 electrolytes with uncoordinated anions, implying a possible semibridged structure, as previously mentioned. Complexes of other cyclic sulfoxides, including thioxane oxide (146) and trans-1,4-dithiane-1,4-dioxide (147) derivatives of hexafluorophosphate salts have also been prepared. 

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. 

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL will form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

Figure 4.2-1 shows the calculated ab initio molecular structure of the ionic liquid [BMIM][PFg] (l-butyl-3-methylimidazolium hexafluorophosphate). 

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. 

The authors correlate the observed catalytic activity with the solubility of the 1,3-butadiene feedstock in the ionic liquid, which was found to be twice as high in the tetrafluoroborate ionic liquid as in the corresponding hexafluorophosphate system. It is noteworthy that the same reaction in a monophasic systems with toluene as the solvent was found to be significantly less active (TOF = 240 h ... 

Anions are just as varied as the cations, and more than 250 different ionic liquids with different anion/cation combinations are commercially available. Hexafluorophosphate, tetrafluoroborate, alkyl sulfates, trifluoromethane-sulfonates (Inflates), and halides are some anion possibilities. 

Lipases from C. antarctica and P. cepacia showed higher enantioselectivity in the two ionic liquids l-ethyl-3-methylimidazolium tetrafluoroborate and l-butyl-3-methylimidazolium hexafluoroborate than in THE and toluene, in the kinetic resolution of several secondary alcohols [49]. Similarly, with lipases from Pseudomonas species and Alcaligenes species, increased enantioselectivity was observed in the resolution of 1 -phenylethanol in several ionic liquids as compared to methyl tert-butyl ether [50]. Another study has demonstrated that lipase from Candida rugosa is at least 100% more selective in l-butyl-3-methylimidazolium hexafluoroborate and l-octyl-3-nonylimidazolium hexafluorophosphate than in n-hexane, in the resolution of racemic 2-chloro-propanoic acid [51]. 

The anion in an ionic liquid can be varied, too. Many ionic liquids contain relatively simple inorganic anions, such as nitrate (NO3 ), tetrafluoroborate (BF4 ), or hexafluorophosphate (PFg ). Anions that are more exotic also are possible, such as the two shown below. Varying the anion provides another way of tuning the properties of an ionic liquid to match a desired application. 

Coupling one of these cations with one of these anions gives an ionic molecule that has high electron density at the anionic end. The computer image at top right shows this for butyl methyl imidazolium hexafluorophosphate. 

Room-temperature ionic liquids have received much attention as green designer solvents. We first demonstrated that ionic liquids acted as good medium for lipase-catalyzed production of polyesters. The polycondensation of diethyl adipate and 1,4-butanediol using lipase CA as catalyst efficiently proceeded in l-butyl-3-methylimidazolinium tetrafluoroborate or hexafluorophosphate under reduced pressure. The polymerization of diethyl sebacate and 1,4-butanediol in l-butyl-3-methylimidazolinium hexafluorophosphate took place even at room temperature in the presence of lipase BC. ... 

A reference to the versatility of ionic liquids was made in Section 4.2.3. These liquids are a new class of solvents which do not have any problems associated with volatile organic liquids. Rogers et al. (1999) have used butylmethylimidazolium hexafluorophosphate to extract a benzene derivative from water. 

The anodic oxidation of the tetrafluoroborate anion occurs at potentials higher than 2.1 V and the remaining hexafluorophosphate and imide anions are oxidised at potentials higher than 2.0 V. Hence, the stability window of the EMImBF4 and BMImBF4 is 4.2 V. Ionic liquids BMImPF6 and EMImN(Tf)2 shows a similar stability window of ca. 4.1 V. However, the window of the BMPyN(Tf)2, is considerably lower ca. 3.0 V. This is consistent with data (ca. 4.1-4.2 V) found for a series of ionic liquids based on EMIm+ and DMPIm+ (l,2-dimethyl-3-propylimidazolium) cations [12],... 

Using a similar protocol, Loupy and coworkers have reported the synthesis of chiral ionic liquids based on (ll ,2S)-(-)-ephedrinium salts under microwave irradiation conditions (Scheme 4.21a) [75]. Importantly, the authors were also able to demonstrate that the desired hexafluorophosphate salts could be prepared in a one-pot protocol by in situ anion-exchange metathesis (Scheme 4.21b). The synthesis and transformation of so-called task-specific ionic liquids is discussed in more detail in Section 7.4. 

Scheme 6.93) [192]. Using either of the two solvent systems, all studied cycloaddition reactions were completed in less than 1 min upon microwave irradiation at 50 °C employing 3 mol% of the catalyst. An additional advantage of using the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) as solvent is that it facilitates catalyst recycling. 

Inter- and intramolecular hetero-Diels-Alder cycloaddition reactions in a series of functionalized 2-(lH)-pyrazinones have been studied in detail by the groups of Van der Eycken and Kappe (Scheme 6.95) [195-197]. In the intramolecular series, cycloaddition of alkenyl-tethered 2-(lH)-pyrazinones required 1-2 days under conventional thermal conditions involving chlorobenzene as solvent under reflux conditions (132 °C). Switching to 1,2-dichloroethane doped with the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and sealed-vessel microwave technology, the same transformations were completed within 8-18 min at a reaction temperature of 190 °C (Scheme 6.95 a) [195]. Without isolating the primary imidoyl chloride cycloadducts, rapid hydrolysis was achieved by the addition of small amounts of water and subjecting the reaction mixture to further microwave irradia-... 

In 2002, Leadbeater and Torenius reported the base-catalyzed Michael addition of methyl acrylate to imidazole using ionic liquid-doped toluene as a reaction medium (Scheme 6.133 a) [190], A 75% product yield was obtained after 5 min of microwave irradiation at 200 °C employing equimolar amounts of Michael acceptor/donor and triethylamine base. As for the Diels-Alder reaction studied by the same group (see Scheme 6.91), l-(2-propyl)-3-methylimidazolium hexafluorophosphate (pmimPF6) was the ionic liquid utilized (see Table 4.3). Related microwave-promoted Michael additions studied by Jennings and coworkers involving indoles as heterocyclic amines are shown in Schemes 6.133 b [230] and 6.133 c [268], Here, either lithium bis(trimethylsilyl)amide (LiHMDS) or potassium tert-butoxide (KOtBu) was em-... 

The microwave-assisted thionation of amides has been studied by Ley and coworkers using a polymer-supported thionating reagent [115]. This polymer-supported amino thiophosphate serves as a convenient substitute for its homogeneous analogue in the microwave-induced rapid conversion of amides to thioamides. Under microwave conditions, the reaction is complete within 15 min, as opposed to 30 h by conventional reflux in toluene (Scheme 7.95). The reaction has been studied for a range of secondary and tertiary amides and GC-MS monitoring showed that it proceeded almost quantitatively. More importantly, this work was the first incidence of the use of the ionic liquid l-ethyl-3-methylimidazolium hexafluorophosphate... 

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. 

Recently, a novel ionic, phenoxaphosphino-modified xanthene ligand has been prepared by van Leeuwen and co-workers [59]. The ligand, containing two l-methyl-3-pentylimidazolium hexafluorophosphate moieties, was synthesised via a six-step route from 9,9-dimethylxanthene according to Scheme 7.6 [60]. 

Finally, research efforts to replace hexafluorophosphate (and other halogen-containing) ionic liquids by some cheap and halogen-free ionic liquids in the Rh-catalysed hydroformylation should be mentioned. The first attempts in this direction were made by Andersen et al. [10] These authors investigated the hydroformylation of... 

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