Tetrafluoroammonium salts

Submitted by KARL O. CHRISTE, WILLIAM W. WILSON, CARL J. SCHACK, and RICHARD D. WILSON  

Since [Np4] is a coordinatively saturated complex fluoro cation, the syntheses of its salts are generally difficult. A limited number of salts can be prepared directly from NFj, and these salts can then be converted by indirect methods into other [NFJ salts that are important for solid propellant NFj-Fj gas generators or reagents for the electrophilic fiuorination of aromatic compounds. 

The two direct methods for the syntheses of [NF4] salts are based on the reaction of NFj with either [KrF] salts  

NF3 + [KrF][AsFe] [NF4][AsFe] + Kr or F2 and a strong Lewis acid in the presence of an activation energy source E.  

For the chemist interested in synthesis, the second method is clearly superior, due to its high yields, relative simplicity, and scalability. 

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Other tetrafluoroammonium salts which have been prepd are Tetrafluoroammonium Penta-fluorogermanate, NF4+GeF5—, decomps 249° TetrafI u oroammo ni u m Hexaf luorophosph ate, NF4+PF6, decomps 245° and Bis (tetrafluoroammonium) Hexaf luorogermanate, (NF4+)2GeF6—, decomps 238° (Ref 9)... 

Tetrafluoroammonium salts hydrolyze in water, regenerating NF3. Nitrogen trifluoride reacts with several metals at high temperatures. When heated with copper, it yields copper(ll) fluoride and dinitrogen tetrafluoride ... 

It should be noted that fluorodiazonium hexafluoroantimonate (4) also reacts with aromatic compounds, forming fluorobenzenes, in low yield.193 A more recent report concerns the electrophilic fluorination of methane to fluoromethane using fluorodiazonium and tetrafluoroammonium salts.198... 

Two independent groups were responsible for the original discovery of tetrafluoroammonium salts, both basing the synthesis on the reaction of NFj with fluorine in the presence of a strong Lewis acid, which acts as a fluoride ion acceptor ... 

Several tetrafluoroammonium salts have been prepared by irradiating a mixture of NF3, F2, and a Lewis acid with ultraviolet light, which... 

The hexafluoroantimonate is chosen for this type of reaction because CsSbF6 is sparingly soluble in AHF (1.8 mg/g of HF at —78QC) and can be filtered off at the reaction temperature in a specially designed apparatus (61). The tetrafluoroammonium salt isolated by evaporating the filtrate can then be further purified by recrystallization from HF or BrF5. 

The same principle was used in preparing the compounds NF4UF7 and NF4WF7 from the hexafluorides UF6 and WF6, both of which are weak Lewis acids with only a moderate tendency to form the MF7 ion. It is for this reason that the two tetrafluoroammonium salts cannot be made by direct synthesis from NF3, F2, and the Lewis acid, or indirectly by a metathetical reaction in AHF solution. Repeated treatment of NH4HF2 with the hexafluoride, accompanied by removal of the AHF set free, as in the synthesis of (NF4)2SiF6, did, however, give the required compounds (297) (where X = U, W) ... 

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