Optimizing the molecular structure and packing style of a crystal by intramolecular cyclization from picrylhydrazone to indazole

Abstract

Crystal engineering has prompted the development of energetic materials in recent years. Obtaining an explosive crystal with an ideal crystal shape and a single polymorph with high packing density and stability needs to be further studied. This work reports the synthesis, structure and energetic properties of picrylhydrazone and indazole derivatives. We presented a cyclization reaction in which the twist structure picrylhydrazones (4 and 6) with a disordered crystal packing style was converted into planar structure indazole derivatives (5 and 7) with a face-to-face crystal packing style. Compared to picrylhydrazones 4 and 6, the indazole derivatives 5 and 7 exhibited increases of 0.04 and 0.05 g cm⁻³ (2.4%, 2.8%), 0.07 and 0.39 kJ g⁻¹ (3.3%, 19.3%), 341 and 309 m s⁻¹ (4.3%, 3.7%), 2.5 and 2.9 GPa (10.9%, 11.7%), 82 and 127 K (17.9%, 27.5%), 9 and 7 J (37.5%, 31.6%) and 64 and 29 N (25.4%, 11.0%) in density, heat of formation (Δ_fH), detonation velocity (D) and pressure (P), decomposition temperature (T_dec), impact sensitivity (IS) and friction sensitivity (FS), respectively. According to the experimental results and theoretical calculations, the structure–property relationship was demonstrated. Among target compounds, indazole derivative 7 exhibits an acceptable detonation performance and good sensitivity (T_dec = 316 °C, IS = 27 J, D = 8518 m s⁻¹). This study enriches future prospects for the molecular design and crystal engineering of energetic materials.