Synthesis\nand Characterization of <i>cyclo</i>-Pentazolate Salts of\nNH₄⁺, NH₃OH⁺, N₂H₅⁺, C(NH₂)₃⁺, and N(CH₃)₄⁺

Abstract

A breakthrough in polynitrogen chemistry\nwas recently achieved\nby our bulk synthesis of (N₅)₆(H₃O)₃(NH₄)₄Cl in which the <i>cyclo</i>-pentazolate anions were stabilized extensively by\nhydrogen bridges with the NH₄⁺ and OH₃⁺ cations. Significant efforts have been carried out to\nreplace these nonenergetic cations and the Cl^– anion\nby more energetic cations. In this paper, the metathetical syntheses\nof <i>cyclo</i>-pentazolate salts containing the simple\nnitrogen-rich cations NH₄⁺, NH₃OH⁺, N₂H₅⁺, C­(NH₂)₃⁺, and N­(CH₃)₄⁺ are reported. These salts were characterized by their crystal\nstructures; vibrational, mass, and multinuclear NMR spectra; thermal\nstability measurements; sensitivity data; and performance calculations.\nIt is shown that the <i>cyclo</i>-pentazolates are more\nenergetic than the corresponding azides but are thermally less stable\ndecomposing in the range of 80 °C to 105 °C. As explosives,\nthe hydrazinium and hydroxyl ammonium salts are predicted to match\nthe detonation pressure of RDX but exhibit significantly higher detonation\nvelocities than RDX and HMX with comparable impact and friction sensitivities.\nAlthough the ammonium salt has a lower detonation pressure than RDX,\nits detonation velocity also exceeds those of RDX and HMX. As a rocket\npropellant, the hydrazinium and hydroxyl ammonium salts are predicted\nto exceed the performances of RDX and HMX. The crystal structures\nshow that the <i>cyclo</i>-pentazolate anions are generally\nstabilized by hydrogen bonds to the cations, except for the N­(CH₃)₄⁺ salt which also exhibits strong\ncation-π interactions. This difference in the anion stabilization\nis also detectable in the vibrational spectra which show for the N­(CH₃)₄⁺ salt a decrease in the <i>cyclo</i>-N₅⁻ stretching vibrations of about\n20 cm^–1.