Polynitrogen Chemistry and the Pursuit of New High Energy Density Materials.

摘要

The goal of this AFOSR program is the synthesis of novel polynitrogen-derived HEDM compounds, exploiting the synergism between theory and synthesis. Under combined DARPA, AFOSR and NSF sponsorship, we have discovered in 1999 the novel polynitrogen compound, N5(+)AsF6(-). The N5(+) cation is only the third known, homoleptic polynitrogen species that can be prepared and isolated in bulk, the other two being N2 and the azide anion. N5(+)AsF6(-) was found to be only marginally stable. During the past year, the N5(+) cation has successfully been tamed by preparing the stable fluoroantimonate salts. N5(+) SbF6(-) and N5(+)Sb2F11(-). The former is surprisingly stable (up to 70 deg C) and, according to drop weight tests, is essentially insensitive. The crystal structure of N5(-)Sb2F11(-) was determined, and the geometry of N5(+) was found to be in excellent agreement with that predicted by our theoretical calculations. A considerable amount of effort was spent on improving the syntheses of the precursors for the N5(+) salts. This work resulted in the discovery of a new cis-trans isomerization process for N2F2, a disproportionation reaction of N2F2 to give NF4(+)Sb3F16(-) under mild conditions, and several new crystal structures for salts that contain nitrogen fluoride cations. Also, a safer method for producing N5(+) salts was developed to overcome a series of explosions, encountered in the N5SbF6 synthesis. Metathetical reactions were carried out in SO2 and anhydrous HF solutions between N5(+)SbF6(-) and alkali metal azides, perchlorates, and nitrates, in pursuit of N5N3, N5ClO4, and N5N02, respectively. It was also found that the N3(-) anion reacts with S02 under formation of the novel SO2N3(-) anion that was characterized by its crystal structure, vibrational spectroscopy, and theoretical calculations.