An efficient strategy for the preparation of insensitive energetic materials: intramolecular cyclization of picrylhydrazone into indazole derivative

The intramolecular cyclization reaction of picrylhydrazone into an indazole derivative was systematically studied. The optimized conditions, mechanism and the generality of the cyclization reaction from picrylhydrazone to indazole were investigated. Picrylhydrazones 1a–6a and indazole derivatives 1b and 3b–7b were synthesized through a one-step reaction and the crystal structures of 2a, 4a and 5b were characterized by single crystal X-ray diffraction analysis. Among all the compounds, the indazole derivatives exhibit an increase of 24–341 m s−1 (0.3–4.3%), 0.6–3.1 GPa (2.2–14.3%) and 9–82 °C (5.0–44.0%) in detonation performance and thermal stability as well as a decrease of 4–10 J (15.4–33.3%) and 50–120 N (16.1–50.0%) in mechanical sensitivity to impact and friction compared with picrylhydrazones. Furthermore, the structure–property relationship was demonstrated by a combination of fingerprint plots, Hirshfeld surfaces and explosive properties. All those results demonstrated that this study can enrich the future prospects of the design of energetic materials.