Hexafluorophosphoric acid salts

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. 

A. o-Bromobenzenediazonium hexafluorophosphate. A solution of 95 ml. of 12N hydrocMoric acid in 650 ml. of water is added with stirring to 60 g. of o-bromoaniline (0.35 mole Note 1) in a 2-1. three-necked flask equipped with stirrer and thermometer. Solution is effected by heating the mixture on a steam bath (Note 2). A solution of 29 g. (0.42 mole) of sodium nitrite in 75 ml. of water is added with stirring while the mixture is maintained at — 5° to —10° by means of a bath of ice and salt or of dry ice and acetone. At the end of the addition there is an excess of nitrous acid, which can be detected with starch iodide paper. Seventy-four milliliters (134 g., 0.60 mole) of 65% hexafluorophosphoric acid (Note 3) is added in one portion, with vigorous stirring, to the cold solution of the diazonium salt. Cooling and slow stirring are continued for an additional 30 minutes, and the precipitated diazonium hexafluorophosphate is then collected on a Bilchner funnel. The diazonium salt is washed on the funnel with 300 ml. of cold water and with a solution of 80 ml. of methanol in 320 ml. 

Hexafluorophosphoric acid and its salts can be employed for halogen- and alkoxy-exchange fluorination at germanium and silicon atoms in transition metal complexes3 or chloro(or-gano)silanes,4 to give, for example, 33 and 4.4... 

Reaction of the ketosulfone (ix) with 2-chloro-1,3-bis(dimethylamino)trimethinium salt (xix) in the presence of an equimolar amount of tert-BuOK followed by treatment with acetic acid and TFA and reflux with an excess of ammonium hydroxide. 2-Chloro-1,3-bis(dimethylamino)trimethinium hexafluoro-phosphat (xix) is obtained by reaction of chloroacetic acid with hot dimethylformamide and POCI3. Finally the reaction mixture is treated with NaOH and hexafluorophosphoric acid in water. 

Protonation of dienol complexes (53) with hexafluorophosphoric acid in diethyl ether affords air-stable salts (55) in high yield [Eq. (27)] (206). While the iron derivatives (21) are rapidly hydrolyzed, the cyclopentadienyl rhodium and iridium salts (55) are much less reactive (207). 

The addition is mildly exothermic. Sodium hexafluorophosphate is commercially available although significantly more expensive. The sodium salt may be used in place of the hexafluorophosphoric acid by omitting the initial sodium hydroxide charge. 

The primary advantage in the first step of the method described here (using 1-chlorobutane diluted in MeCN) is that it eliminates long reaction periods and allows the use of secondary alkyl halides without competitive elimination reactions. For example, the reaction of sec-butyl bromide with N-methylimidazole using the classical method (in neat alkyl halide) produces, along with the desired product, 20-30% of butenes and 1-methylimidazole hydrobromide. In the second step, the use of water as solvent allows the anion metathesis reaction to be quantitative in a very short time and allows the easy purification of the ionic liquids. Moreover, employing the potassium salt avoids the use of corrosive and difficult to handle hexafluorophosphoric acid and the expensive silver tetrafluoro borate. ... 

The arylsulfonium metal halide complex salts are not readily available. Triphenylsulfoniurn hexafluorophosphate can be prepared from diphenyliodonium hexafluorophosphate by heating with phenyl sulfide, as described by Knapczyk and McEwen ( ), or by reaction of phenyl magnesium bromide with diphenyl sulfoxide followed by reaction with hexafluorophosphoric acid according to the procedure of Wildi, Taylor and Potratz (60). 

The hexafluorophosphate ion has proved to be very useful for studies with complexes, since the [PF6] anion has a very weak coordinating ability.1 Lan-thanoid hexafluorophosphates can be obtained in very concentrated aqueous solutions by reaction of a freshly prepared solutions of hexafluorophosphoric acid and a hydrated lanthanoid basic carbonate.2 The resulting solution, after filtration, is evaporated to near dryness. Attempts to isolate the hydrated salts are unsuccessful because of decomposition accompanied by hydrogen fluoride evolution. Nevertheless, the complexes containing diphenylphosphinic amide are isolable and are quite stable.3... 

This compound may be used for the preparation of many salts of hexafluorophosphoric acid. 

Ref. 58) Solid [bmim][Cl] (200g, 1.15mol) was melted in a lOOOmL flask by heating. After cooling at room temperature, it was immediately diluted with water (200 mL) to reduce the viscosity. Hexafluorophosphoric acid (60% in water, 200 mL) was added dropwise over 2h. The mixture was stirred for 1 h, and the ionic liquid phase (bottom layer) was separated and washed with water (3 x 100 mL). After drying, 260 g (0.90 mol, 80%) of the [bmim][PF6] salt were obtained as a very pale yellow liquid of low viscosity. 

Barlow et al. reported the synthesis of cationic (T) -cyclopentadienyl)(r -fulvene)ruthenium(ll) salts from ruthenocenylmethanol (57) either by treatment with trimethylsilyl triflate to achieve 58 in 79% yield. Alternatively, the reaction of 57 with tetrafluoroboric acid or with hexafluorophosphoric acid gave the tetrafluoroborate or hexafluorophosphate salts in 59 and 60 in 92% and 52% yield, respectively. Treatment of 59 with sodium tetrakis[3,5-bis(trifluoromethyl)]borate afforded the tetraarylborate salt 61 in 69% yield. 

Perchloric acid, a superacid, is one of the strongest Bronsted-Lowiy acids. Its pKa is -10. It provides strong acidity without interference from potential nucleophiles such as sulfate or chloride that complicate the use of sulfuric and hydrochloric acids. Other acids of noncoordinating anions, such as fiuoroboric acid and hexafluorophosphoric acid are susceptible to hydrolysis, whereas perchloric acid is not. Despite hazards associated with the explosiveness of its salts, the acid is often preferred in certain syntheses. For similar reasons, it is a useful eluent in ion-exchange chromatography. 

Suitable salts were those of perchloric acid or, preferably, of hexafluorophosphoric acid." " Experiments carried out with bromine-terminated polystyrene or polybutadiene gave a rapid precipitation of silver bromide even at low temperatures, due to the resonance stabilization of the benzylic or the allylic carbenium ions thus generated. The efficiency of block copolymer formation was found to be determined by factors such as the structure of the carbenium ion, the nature of the gegenion, the experimental conditions and, of course, the nature of the second monomer. Under carefully chosen conditions, THF could be polymerized to form living cationic block copolymers... 

Bis-(N-methyl-N-nitroso)terephthalamide and N,N -dimethyl-N,N -dinitrosooxamide are convenient precursors of diazomethane. From N,N -dinitrosooxamides, other diazoalkanes have been prepared as well . Improved yields in the Schiemann reaction have been obtained with hexafluorophosphoric acid . Reaction of dia-zonium salts with nickel carbonyl and acetic acid may provide a single-step alternative to the Sandmeyer reaction, especially where the nitriles from the latter contain other groups susceptible to hydrolysis . The highly reactive 1,2,4-thiadiazolediazonium salts have permitted an extension of diazonium salt chemistry by new reactions . 

Although tetrafluoroboric and hexafluorophosphoric adds do not exist as such, but only in the form of oxonium salts, for example, H502BF4, in nonaqueous solvents such as carboxylic anhydrides or ethers there can be strong acid behavior. Tetrafluoroboric acid in diethyl ether is a useful strong add, [Et2OH]+BF4. 

The use of dry aryldiazonium salts of naphthalene-1-sulfonic [70], naphthalene-1,5-disulfonic [70], ZnCl2 complex [70], hexafluorophosphoric or tetrafluoroboric acid [61] in non-aqueous medium under as mild as possible reaction conditions is substantial to reach higher yields of biaryls. An alternative method for non-aqueous GBH reaction is the aprotic diazotation of aromatic amines with alkyl nitrites such as butyl or pentyl nitrite with subsequent arylation of aromatic compound, as demonstrated by Cadogan [71,72]. This method is realized by simple heating the mixture of aromatic amine, alkyl nitrite and liquid arene at an elevated temperature. When a mixture of 3-aminopyridine (46), benzene and pentyl nitrite is heated at reflux, 3-phenylpyridine (47) is obtained with a 55% yield [71], Scheme 16. 

Other commonly used anions for ionic liquids, such as tetrafluoroborate and hexafluorophosphate, also imdergo hydrolysis at sufficiently rapid rates that would cause difficulties for their use with either aqueous solvents or hydroxylic cations. While the hexafluorophosphate anion imdergoes hydrolysis most rapidly only under acidic conditions (Freire et al. 2010), the preparation of the ionic liquids using hexafluorophosphoric add can lead to significant formation of the (hydrolyzed) phosphate salt (Lall et al. 2000). Similarly, the alternative approach for the prepwation of the phosphate salts (liquid ionic phosphates, LIPs) involving the use of 98% phosphoric acid (Lall et al. 2002) similarly experiences difficulty when cations are used that bear hydroxylic sites. Presumably the "anion exchange" method (Lall et al. 2002) would not experience this difficulty, but this process is extremely ineffident for the preparation of any sizable quantity of material. Similarly, the tetrafluoroborate anion imdergoes hydrolysis sufficient rapidly (Freire, et al. 2010) and can be anticipated to read with free hydroxyhc functions such that it would not be a useful anion for ionic hquids based on carbohydrate and carbohydrate-related materials. 

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