Cation-Engineered Tuning of Stability and Energetic Properties in Pentazolate Cocrystals

Abstract

Cocrystallization has been widely embraced as a strategy to enhance the properties of pentazolate cocrystals. While existing studies indicate that pentazolate cocrystals comprising different cations exhibit distinct structural and energetic properties, systematic investigations into cation-driven structure–property relationships remain scarce. Here, a series of pentazolate cocrystals containing NH₄⁺ (ammonium cation), N₂H₅⁺ (hydrazine cation), and NH₃OH⁺ (hydroxylammonium cation) groups were successfully synthesized and fully characterized. Single crystal X-ray diffraction analysis revealed that the NH₄⁺ compound presents a relatively disordered stacking model, while N₂H₅⁺ and NH₃OH⁺ compounds exhibit relatively ordered face-to-face stacking and cross-like stacking patterns, respectively. All cocrystals demonstrated excellent thermal stability, with compound 1 exhibiting a higher onset decomposition temperature (T_d = 114.2 °C) compared to that of NH₄N₅ (T_d = 102.0 °C). The calculated enthalpies of formation (262.3–337.7 kJ·mol^–1) and detonation velocities (8.56–9.06 km·s^–1) correlate strongly with the cation types. Furthermore, cocrystals 1–3 presented high insensitivity (IS > 50 J, FS > 300 N), with values significantly lower than those of RDX (IS = 7.4 J, FS = 120 N). The results indicate that NH₄⁺ can improve the thermal stability and insensitivity of pentazolate cocrystals, while N₂H₅⁺ and NH₃OH⁺ play crucial roles in improving the detonation properties. Notably, cocrystal 2 holds great potential for applications in explosives and propellants, which achieves a balance of high energy, thermal stability, and insensitivity. These findings provide valuable insights for the development of the next generation of high-energy pentazolate cocrystals.