Abstract:
Salts of the formula NF 4   +  MF 7   -  are produced by the  fowing reaction 
     
       NF.sub.4 HF.sub.2 nHF+MF.sub.6 →NF.sub.4 MF.sub.7 +(n+1)HF 
     
     wherein M is uranium (U) or tungsten (W).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to energetic inorganic salts and more particularly to salts containing the NF 4   +  cation. 
     2. Description of the Prior Art 
     NF 4   +  salts are key ingredients for solid propellant NF 3  -F 2  gas generators, as shown by D. Pilipocich in U.S. Pat. No. 3,963,542, and for high detonation pressure explosives, as shown by K. O. Christe in U.S. Pat. No. 4,207,124. The synthesis of NF 4   +  salts is unusually difficult because the parent molecule NF 5  does not exist and the salts must be prepared from NF 3  which amounts formally to a transfer of F +  to NF 3  accordingly to: 
     
         NF.sub.3 +F.sup.+ →NF.sub.4.sup.+ 
    
     Since fluorine is the most electronegative of all elements, F +  cannot be generated by chemical means. This difficult synthetic problem was overcome by K. O. Christe and co-workers, as shown in U.S. Pat. No. 3,503,719. By the use of an activation energy source and a strong volatile Lewis acid, such as AsF 5 , the conversion of NF 3  and F 2  to an NF 4   +  salt became possible: ##STR1## However, only few Lewis acids are known which possess sufficient strength and acidity to be effective in this reaction. Therefore, other indirect methods were needed which allowed conversion of the readily accessible NF 4   +  salts into other new salts. Two such methods are presently known. The first one involves the displacement of a weaker Lewis acid by a stronger Lewis acid, as shown by K. O. Christe and C. J. Schack in U.S. Pat. No. 4,172,881 for the system: 
     
         NF.sub.4 BF.sub.4 +PF.sub.5 →NF.sub.4 PF.sub.6 +BF.sub.3 
    
     but obviously is again limited to strong Lewis acids. The second method is based on metathesis, i.e., taking advantages of the different solubilities of NF 4   +  salts in solvents such as HF or BrF 5 . For example, NF 4  SbF 6  can be converted to NF 4  BF 4  according to: ##STR2## This method has successfully been applied by K. O. Christe and coworkers, as shown in U.S. Pat. Nos. 4,108,965; 4,152,406; and 4,172,884, to the syntheses of several new salts. However, this method is limited to salts which have the necessary solubilities and are stable in the required solvent. The limitations of the above two methods are quite obvious and preempted the syntheses of NF 4   +  salts of anions which are either insoluble in those solvents or are derived from a Lewis acid weaker than the solvent itself and therefore are displaced from their salts by the solvent. 
     SUMMARY OF THE INVENTION 
     Accordingly an object of this invention is to provide methods which permit the syntheses of new NF 4   +  salts containing anions derived from very weak Lewis acids. 
     Another object of this invention is to provide new energetic NF 4   +  compositions which are useful in explosives and solid propellants. 
     A further object of this invention is to provide NF 4   +  compositions for solid propellant NF 3  -F 2  gas generators for chemical HF-DF lasers which deliver a maximum of NF 3  and F 2  while not producing any gases which deactivate the chemical laser. 
     Yet another object of this invention is to provide NF 4  fluorotungstates which on burning with tungsten powder can produce hot WF 6  gas in high yield. 
     These and other objects of this invention are achieved by providing: 
     Salts of the formula NF 4   +  MF 7   -  by the following reaction 
     
         NF.sub.4 HF.sub.2.nHF+MF.sub.6 ⃡NF.sub.4 MF.sub.7 +(n+1)HF 
    
     wherein M is uranium (U) or tungsten (W). These salts are useful as ingredients in solid propellants and in high detonation pressure explosives. 
     A method of generating hot WF 6  gas by burning a mixture of NF 4  WF 7  and tungsten metal. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Surprisingly, it has now been found that the salts NF 4  UF 7  and NF 4  WF 7  can be prepared from the very weak and volatile Lewis acids UF 6  and WF 6 . The salts are prepared by the following methods. 
     First, readily available NF 4  SbF 6  salt is converted by metathesis into NF 4  HF 2  according to the reaction ##STR3## The details of this procedure are disclosed by K. O. Christe, W. W. Wilson, and R. D. Wilson in Inorg. Chem., 19, pp. 1494+(1980), herein incorporated by reference. A method of preparing NF 4  SbF 6  is disclosed by K. O. Christe, C. J. Schack, and R. D. Wilson, J. Fluorine Chem., 8, pp. 541+(1976), herein incorporated by reference. 
     The NF 4  HF 2  produced by the above procedure will be complexed with HF and can be represented by the formula NF 4  HF 2 .nHF. Because HF is a stronger Lewis acid than either UF 6  or WF 6 , as much HF as possible has to be removed from the NF 4  HF 2  without decomposing the NF 4  HF 2 . This can be achieved by judicious pumping at about 0° C. This is continued until a solid having the composition NF 4  HF 2 .nHF wherein n is from about 0.5 to about 10.0 is obtained. 
     NF 4  UF 7  and NF 4  WF 7  are produced by the following reactions: 
     
         NF.sub.4 HF.sub.2.nHF+UF.sub.6 ⃡NF.sub.4 UF.sub.7 +(n+1)HF (2) 
    
     
         NF.sub.4 HF.sub.2.nHF+WF.sub.6 ⃡NF.sub.4 WF.sub.7 +(n+1)HF. (3) 
    
     These reactions may be run at ambient (25° C.) temperature. Repeated treatments of NF 4  HF 2 .nHF with a large excess of UF 6  or WF 6 , followed by the removal of the volatile products at ambient temperatures, surprisingly shifted the equilibrium in reaction (2) and the equilibrium in reaction (3) quanitatively to the right. This is probably due to the thermal stability of NF 4  UF 7  and of NF 4  WF 7  being significantly higher than that of NF 4  HF 2 . 
     The addition of UF 6  or WF 6  and subsequent evacuation of volatile reaction products is continued until the conversion of NF 4  HF 2 .nHF to NF 4  UF 7  or NF 4  WF 7  is substantially completed. This will be the point at which no significant amount of UF 6  or WF 6  is taken up and no significant amount of volatile reaction products (e.g., HF gas) is generated. Thus, by monitoring the gases evacuated from the reaction chamber, the progress of the reaction may be monitored. 
     Examples 1 and 2 further illustrate these procedures. 
     NF 4  UF 7  and NF 4  WF 7  are useful as key ingredients for solid propellant NF 3  -F 2  gas generators and for high detonation pressure explosives. 
     NF 4  WF 7  is of particular interest as an ingredient for hot WF 6  gas generators. Hot WF 6  is an excellent electron capturing agent and therefore useful for reducing radar signatures. For example, formulations based on 
     
         6NF.sub.4 WF.sub.7 +5W→11WF.sub.6 +3N.sub.2         ( 4) 
    
     can theoretically produce up to 97 weight percent of WF 6  with flame temperatures in excess of 2000° C. A pyrotechnic mixture of finely powdered NF 4  WF 7  and tungsten in approxiately a 6:5 molar ratio may be used. 
    
    
     The general nature of the invention having been set forth, the following examples are presented as specific illustrations thereof. It will be understood that the invention is not limited to these examples but is susceptible to various modifications that will be recognized by one of ordinary skill in the art. 
     EXAMPLE 1 
     Preparation of NF 4  WF 7   
     Dry CsF (15.0 mmol) and NF 4  SbF 6  (15.0 mmol) were loaded in the drybox into one half of a prepassivated Teflon double U-metathesis apparatus. Dry HF (15 ml liquid) was added on the vacuum line and the mixture was stirred with a Teflon coated magnetic stirring bar for 15 minutes at 25° C. After cooling the apparatus to -78° C., it was inverted and the NF 4  HF 2  solution was filtered into the other half of the apparatus. Tungsten hexafluoride (22.5 mmol) was condensed at -196° C. onto the NF 4  HF 2 . The mixture was warmed to ambient temperature, and two immiscible liquid phases were observed. After vigorous stirring for 30 minutes at 25° C., the lower WF 6  layer dissolved in the upper HF phase. Most of the volatile products were pumped off at ambient temperature until the onset of NF 4  HF 2  decomposition became noticeable (NF 3  evolution). An additional 8.0 mmol of WF 6  was added at -196° C. to the residue. When the mixture was warmed to ambient temperature, a white solid product appeared in the form of a slurry. All material volatile at -31° C. was pumped off for 1 hour and consisted of HF and some NF 3 . An additional 14.5 mmol of WF 6  was added to the residue and the resulting mixture was kept at 25° C. for 14 hours. All material volatile at -13° C. was pumped off for 2 hours and consisted of HF and WF 6 . The residue was kept at 22° C. for 2.5 days and pumping was resumed at -13° C. for 2.5 hours and at 22° C. for 4 hours. The volatiles collected at -210° C., consisted of some HF and small amounts of NF 3  and WF 6 . The white solid residue (5.138 g, 84% yield) was shown by vibrational and  19  F NMR spectroscopy to consist mainly of NF 4  WF 7  with small amounts of SbF 6   -  as the only detectable impurity. Based on its elemental analysis, the product had the following composition (weight %): 
     NF 4  WF 7 , 98.39; CsSbF 6 , 1.61. Anal. Calcd: NF 3 , 17.17; W, 44.46; Cs, 0.58; Sb, 0.53. Found. NF 3 , 17.13: W, 44.49; Cs, 0.54; Sb, 0.55. 
     EXAMPLE 2 
     Preparation of NF 4  UF 7   
     A solution of NF 4  HF 2  in anhydrous HF was prepared from CsF (14.12 mmol) and NF 4  SbF 6  (14.19 mmol) in the same manner as described for example 1 (NF 4  WF 7 ). Most of the HF solvent was pumped off on warm up from -78° C. towards ambient temperature, until the onset of NF 4  HF 2  decomposition became noticeable. Uranium hexafluoride (14.59 mmol) was condensed at -196° C. into the reactor, and the mixture was stirred at 25° C. for 20 hours. The material volatile at 25° was briefly pumped off and separated by fractional condensation through traps kept at -78°, -126° and -210° C. It consisted of HF (6.3 mmol), UF 6  (9.58 mmol) and a trace of NF 3 . Since the NF 4  HF 2  solution had taken up only about one third of the stoichiometric amount of UF 6 , the recovered UF 6  was condensed back into the reactor. The mixture was stirred at 25° C. for 12 hours and the volatile material was pumped off again and separated. It consisted of HF (12.8 mmol), UF 6  (1.7 mmol) and a trace of NF 3 . Continued pumping resulted in the evolution of only a small amount of UF 6 , but no NF 3  or HF, thus indicating the absence of any unreacted NF 4  HF 2 . The pale yellow solid residue (5.711 g, 88% yield) was shown by vibrational and  19  F NMR spectroscopy and elemental analysis to have the following composition (weight %): NF 4  UF 7 , 97.47; NF 4  SbF 6 , 1.50; CsSbF 6 , 1.03. Anal. Calcd: NF 3 , 15.34; U, 50.32; Sb, 0.90; Cs, 0.37. Found: NF 3 , 15.31 U, 50.2; Sb, 0.90; Cs, 0.37. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.