Hexafluorophosphate hydrolysis

The compound is a convenient starting material for the preparation of hexafluorophosphates (hydrolysis with the respective hydroxides see p. 196 under KPFg) and of PFg (thermal decomposition at 80°C). 

A number of salts of the monofluoro- and hexafluorophosphoric acids are known and some are commercially important. The salts of difluorophosphoric acid are typically less stable toward hydrolysis and are less well characterized. Sodium monofluorophosphate [7631-97-2] the most widely used dentifrice additive for the reduction of tooth decay, is best known (see Dentifrices). Several hexafluorophosphates can be prepared by neutralization of the appropriate base using hexafluorophosphoric acid. The monofluorophosphates are usually prepared by other methods (57) because neutralization of the acid usually results in extensive hydrolysis. 

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. 

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. 

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 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. 

Swatloski, R.P., Holbrey, J.D., and Rogers, R.D., Ionic liquids are not always green hydrolysis of l-butyl-3-methylimidazolium hexafluorophosphate. Green Chem., 5, 361-363, 2003. 

Swatloski, R. P., Holbrey, J. D., Rogers, R. D. (2003), Ionic Liquids are not always Green Hydrolysis of l-butyl-3-Methylimidazolium Hexafluorophosphate, Green Chem. 5, 361-363. 

Sodium diimidotrimetaphosphate, Na3P308(NH)2, formation and basic hydrolysis of, in preparation of pentasodium diimido-triphosphate, 6 105ra., 106 Sodium diimidotriphosphate, Na5P308(NH)2-6H20, 6 104 Sodium diphosphates, 3 98, 99, 100 Sodium dithionate, 2 170 Sodium (ethylenediaminetetra-acetato)cobaltate(III), 5 186 Sodium hexafluorophosphate, 3 111, 115... 

Alkaline hydrolysis of hexafluorophosphate (PFg ) occurs without accumulation of lower fluorophosphates. The rates have been examined between 160 and 190 °C, with lithium, sodium and potassium hydroxides. At unit ionic strength (nitrates) the rates were dependent on the nature of the cation (K" < Na" " ionic strength, behaviour is more predictable and the rate expression is... 

Both examples indicate that the water scavenging ability of some ionic liquids may influence the thermodynamics of a reaction, although in the light of more recent findings concerning the hydrolysis of hexafluorophosphate and tetrafluoro-borate to acidic HF [178], which in turn may act as catalyst, the scope of ionic liquid anions should be extended to verify the results. 

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. 

After hydrolysis of the anion of Tc(C6Hf,)2j[AlCl4], the cation was precipitated as hexafluorophosphate. The yellow-green, diamagnetic complex salt is stable in air, acids and bases [617]. Prior to the preparation of ponderable amounts, the cation was produced by irradiating h/A(benzene)molybdenum with thermal neutrons ... 

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