Abstract:
Simple nitronium salts are formed by adding nitrogen dioxide, a simple nitronium salt former, and fluorine gas to a vessel held at a low temperature, and allowing the vessel to warm until a reaction occurs.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to nitronium salts and more particularly to a new method for the preparation of simple nitronium salts. 
     BACKGROUND OF THE INVENTION 
     The existence and characteristics of simple nitronium salts such as nitronium hexafluoroarsenate, nitronium hexafluorophosphate and nitronium tetrafluoroborate have been known for some time. For example, nitronium hexafluoroarsenate is a colorless, crystalline solid that is stable at room temperature and sensitive to moisture. The salt is soluble in nitromethane and, like other nitronium salts, is an excellent nitrating agent for aromatics and a good oxidizing and intercalating agent for covalent materials such as graphite and polyacetylene. 
     The recent interest in nitronimum hexafluoroarsenate, (NO 2  AsF 6 ) as an oxidizing agent has emphasized the need for a simple, one-step, high yield synthesis of this compound. Previous syntheses have involved the initial preparation of nitryl fluoride (FNO 2 ) (from NO 2  and F 2 ) and subsequent reaction with arsenic pentafluoride (AsF 5 ); the use of hydrogen fluoride (HF) with nitric acid (HNO 3 ), nitryl chloride (ClNO 2 ) or nitrate esters; the reaction of nitrogen dioxide (NO 2 ), bromine trifluoride (BrF 3 ), and arsenic pentoxide (As 2  O 5 ); the use of fluorine nitrate (FNO 3 ); or metathesis reactions from other AsF 6   -  salts. These reactions generally were run in metal cylinders or quartz vessels and were multi-step. 
     In all cases, the process is made more expensive and chemically complicated by the fact that the reagents are incompatable with Pyrex™ glass. NO 2  F and HF cannot be made to contact Pyrex™ glass because silicon tetrafluoride (SiF 4 ), which would contaminate the product, is also generated. The reagents in this new process, are compatible with dry Pyrex™ glass under the reaction conditions, and no SiF 4 , or SiF 6   2-   salts contaminate the product. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of this invention to provide a new and novel process for the preparation of simple nitronium salts. 
     Another object is to provide a single-step process for the preparation of simple nitronium salts. 
     A further object of this invention is to provide a process for the preparation of simple nitronium salts which may be carried out in a glass vessel. 
     Yet another object of this invention is to provide a single step process for the preparation of nitronium hexafluoroarsenate which may be carried out in a glass vessel. 
     A still further object of this invention is to provide a process for the preparation of intercalants for covalent materials. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by reacting nitrogen dioxide, a simple nitronium salt former, and fluorine gas to yield a simple nitronium salt. Equimolar amounts of nitrogen dioxide and the simple nitronium salt former are condensed into a vessel. An at least stoichiometric amount of fluorine gas is admitted into the vessel and the vessel allowed to warm until a reaction occurs to form a simple nitronium salt. The nitronium salt is then separated. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To carry out the process of this invention, nitrogen dioxide is condensed into a vessel. Typically, this vessel is immersed in liquid nitrogen so that the vessel is at a temperature of about -196° C. An essentially equimolar amount of a simple nitronium salt former is also condensed into the vessel. In this description and the claims that follow, the term &#34;simple nitronium salt former&#34; refers to a compound of the formula MF n , wherein M is selected from the group consisting of arsenic, boron, and phosphorus, and n is an integer equal to the maximum valence state of the substance M. An at least stoichiometric amount of fluorine gas and preferably a stoichiometric excess is admitted into the vessel. The vessel and its contents are allowed to warm to room temperature behind a shield, whereupon a reaction occurs during this warming to form the simple nitronium salt of the salt former, which appears as a solid. The excess fluorine is then removed. The vessel is then typically taken into a Dry Box containing a dry oxygen-free (&lt;5 ppm) inert gas, typically argon, and the product isolated and removed from the vessel. 
     For the sake of simplicity and economy, the process is best carried out using a dry reaction vessel made of Pyrex™ glass. The vacuum system used as well as the Pyrex™ vessel should be pretreated to accomodate the handling of fluorine gas. The Pyrex™ vessel is typically pretreated by admitting fluorine gas into the dry vessel, exposing the vessel to bright sunlight for about one hour, then removing the fluorine. As implied above, any equipment used for handling either the reactants or the product should be as dry as possible. 
     In this description and the claims that follow, the term simple nitronium salt refers to a compound having the formula NO 2  MF y  wherein M has the meaning stated above and y is an integer equal to n+1. Attempts at forming the complex salt (NO 2 ) 2  SiF 6  resulted in an impure product. Although the process described herein has been applied as yet only to the formation of nitronium hexafluoroarsenate (NO 2  AsF 6 ), it is believed that the process is widely applicable to simple nitronium salts analagous to NO 2  AsF 6 . 
     It is imperative that essentially equimolar amounts of the simple nitronium salt former and nitrogen dioxide be employed. An excess of one or the other could lead to side reactions and a resultant contamination of the final product. 
     Further, it is desirable that an excess of fluorine gas be employed to ensure that the reaction, NO 2  +MF n  +1/2F 2  →NO 2  MF y , continues to completion. Preferably, the excess should be about 10 molar percent over the stoichiometric amount and most preferably an at least equimolar amount of fluorine gas is added. A large excess over the equimolar amount of fluorine gas would be wasteful but not harmful to the process. 
     In addition, the nitrogen dioxide reagent should be carefully purified to remove essentially all traces of nitrogen oxide (NO). Traces of NO may form side products, most likely FNO, that may contaminate the product. 
     It should also be noted that the reaction which forms the product is highly exothermic. Therefore, extreme caution should be used to avoid an explosion, especially if greater than 6 mmol of the product are to be made. 
    
    
     EXAMPLES 
     Having described the invention in general, the following examples are being given to illustrate the principles of the invention and are not intended to limit the scope of the invention in any manner. 
     EXAMPLE 1 
     (nitronium hexafluoroarsenate) 
     Fluorine was pretreated to remove hydrogen fluoride and silicon fluoride (SiF 4 ) impurities and was handled in a passivated copper vacuum line designed for fluorine use. 
     Arsenic pentafluoride and NO 2  were carefully purified by trap-to-trap fractional condensation on a standard Pyrex™ vacuum line (10 -  6 mm Hg) equipped with Teflon™ glass valves. To purify the AsF 5 , the less volatile impurities (HF and arsenic trifluoride (AsF 3 )) were condensed at -95° C. (toluene slush-cooled with liquid nitrogen); the AsF 5  was condensed at -126° C. (methylcyclohexane slush-cooled with liquid nitrogen), and the more volatile SiF 4  was condensed at -196° C. The NO 2  was purified by collecting the NO 2  in a -78° C. bath (acetone-Dry Ice™) and passing the more volatile NO into a -196° C. bath. The trap-to-trap distillations were repeated 3 times in each case. 
     A Pyrex™ reaction vessel with a volume of approximately 390 ml was constructed from a 350 ml Pyrex™ round-bottom flask to which a 29/42 standard taper male joint was connected to the neck, and a Teflon™/glass stopcock, which had a 29/42 standard taper female joint, attached. The standard taper joints were lubricated with Fluorolube™ grease. The vessel was attached to the glass vacuum line using Swagelok™ connectors, evacuated under high vacuum, and dried under dynamic vacuum by heating with a flame from a torch. When the vessel cooled, the valve was closed and the vessel was transferred to the copper line and connected with Swagelok™ connectors. The interspace was evacuated and approximately 100 Torr of F 2  was added to the vessel to passivate the surface, then the stopcock was closed. The vessel was removed from the vacuum line and placed in the sunlight for approximately 1 hour. The vessel was reattached to the copper vacuum line and the interspace evacuated. The F 2  was removed, and the stopcock closed. 
     The vessel was taken from the copper line and attached to the glass vacuum line with Swagelok™ connectors. The interspace was evacuated and the base of the vessel was cooled to -196° C. The stopcock valve was opened and the NO 2  and AsF 5  were added in layers. First, the NO 2  (5.84 mmol) was slowly condensed onto the bottom of the vessel, followed by the AsF 5  (5.84 mmol). The reaction vessel stopcock was closed and the vessel, while being maintained at -196° C., was removed from the glass line and reconnected to the copper line. The interspace was evacuated and the entire bulb of the vessel was cooled to -196° C. Excess F 2  (6.0 mmol) was slowly and carefully admitted into the vessel. While maintaining the temperature of the vessel at -196° C., the stopcock on the vessel was closed and the vessel removed from the copper line. The liquid nitrogen coolant was discarded and a cold Dewar flask placed around the reaction vessel. The Dewar flask and the vessel were then placed behind a shield and allowed to slowly warm to ambient temperature by placing towels around the neck of the Dewar flask. As the flask warmed to room temperature (˜2 hr) a copious amount of white solid formed. The vessel was attached to the copper line, the interspace was evacuated; and the excess F 2  and unreacted reagents were removed under dynamic vacuum. After pumping for approximately 15 minutes, the stopcock valve was closed and the vessel was disconnected from the vacuum line and transferred into a dry box. The stopcock on the vessel was opened; the standard taper joints disconnected and the grease removed from the standard taper joint. Approximately 1.01 g (75% yield) of product was scraped into a tared Kel-F™ vessel equipped with a stainless steel Swagelok™ cap and Teflon™ ferrules. The product was identified and its purity determined by infrared spectra (Nujol™ and Fluorolube™ mulls), Raman spectrum and Debye-Scherrer X-ray powder pattern (no extraneous peaks observed). 
     EXAMPLE 2 
     (nitronium hexafluorophosphate) 
     The process of example 1 is carried out substituting phosphorus pentafluoride (PF 5 ) for arsenic pentafluoride. 
     EXAMPLE 3 
     (nitronium tetrafluoroborate) 
     The process is carried out as in example 1, substituting boron trifluoride (BF 3 ) for arsenic pentafluoride. 
     Obviously, many modifications and variations are possible in the 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.