Investigating the Stabilizing Forces of Pentazolate Salts
Chen Yang, Lei Chen, Wenkang Wu, Chong Zhang, Chengguo Sun, Yang Du, Bingcheng Hu
Abstract
Pentazolate (cyclo-N5–) salts are actively pursued energetic nitrogen-rich compounds due to their potential as propellants and explosives. An in-depth understanding of the stabilizing forces between cyclo-N5– anions and cations is important for designing cyclo-N5– salts and achieving cyclo-N5– salt conversions. Herein, the metathetical syntheses of cyclo-N5– salts (compounds 1–4) containing heterocyclic amino-based cations 3,6-diguanidino-1,2,4,5-tetrazine, 1-guanyl-1,2,4-triazolium, 3,7-diamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazol-2-ium, and 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazol-2-ium are reported. In addition, an energetic cocrystal (compound 5) composed of compound 4 with 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazole was synthesized using cocrystallization techniques. Crystal structures were investigated through geometrical and Hirshfeld analyses and theoretical calculations to reveal the contributions of hydrogen-bonding and π-stacking interactions in stabilizing the pentazolate salts. Compounds 1–3 are stabilized by strong hydrogen-bonding interactions and weak π-stacking interactions. The π-stacking interactions (cation–anion π+–π– contacts) are stronger in 4 and have an important role in promoting the stability of the salt. The binding energy of this π-stacking interaction (−82.4 kcal mol–1) slightly surpasses that of the robust N6-H6A···N1 interaction (−75.1 kcal mol–1). For cocrystal 5, the spatial arrangement of its structural framework differs from that of its precursor, compound 4. The molecular stabilization energy, which increases from −75.1 to −94.3 kcal mol–1 during the conversion of 4–5, primarily arises from strong π-stacking interactions. Further observations indicate that cocrystal 5 has better thermal stability and detonation performance than 4, which establishes noncovalent modification via cocrystallization as an efficient method for forming multicomponent crystal systems and highlights the ability of coformers to modify energetic performance.