Synthesis and Characterization of Pyrazolo[3,4-b]pyridine Fused Insensitive High-Energy Materials

Abstract. Fused heterocycle-based energetic materials are recognized as highly promising energetic compounds; yet, their practical deployment is frequently impeded by cumbersome synthetic protocols. In this work, 4,6-dichloro-1H-pyrazolo[3,4-b]pyridine (1), a commercially accessible starting material, was utilized to efficiently synthesize a series of energetic derivatives via straightforward reaction sequences. Among these products, 3,5-dinitro-1H-pyrazolo[3,4-b]pyridine-4,6-diamine (CF-1), featuring a distinctive "NH2-NO2-NH2-NO2" substitution pattern analogous to that of TATB, emerges as a promising thermally stable explosive. It exhibits exceptional thermal stability with onset decomposition temperature (Td) of 309 oC, coupled with low impact sensitivity (IS > 40 J) and friction sensitivity (FS > 360 N). Furthermore, this compound possesses a high crystal density (density: 1.892 g/cm3) and favorable detonation performance, with a detonation velocity (D) of 8461 m/s and a detonation pressure (P) of 28.27 GPa. To expand the structural diversity of this energetic scaffold, innovative molecular design strategies were further implemented herein: specifically, azide-mediated cyclization of 6-chloro-3,5-dinitro-1H-pyrazolo[3,4-b]pyridine-4-amine (3) and N-oxide functionalization of CF-1. These synthetic routes successfully afforded two novel energetic compounds: the tricyclic derivative 4,6-dinitro-8H-pyrazolo[4,3-e]tetrazolo[1,5-a]pyridine-5-amine (CF-2) and 4,6-diamino-3,5-dinitro-1H-pyrazolo[3,4-b]pyridine 7-oxide (CF-3). Both CF-2 (density: 1.928 g/cm3) and CF-3 (density: 1.886 g/cm3) display high crystal densities. Notably, the detonation performance of CF-3 (D = 8579 m/s, P = 30.72 GPa) is comparable to that of RDX (D = 8795 m/s, P = 34.9 GPa), while CF-2 (D = 8862 m/s, P = 32.91 GPa) rivals FOX-7 (D = 8870 m/s, P = 34.0 GPa). Concurrently, both derivatives maintain excellent thermal stability and low mechanical sensitivity: CF-2 shows Td = 224 oC, IS = 30 J, FS = 252 N; CF-3 shows Td = 230 oC, IS = 25 J, FS = 240 N. In addition, CF-2 exhibits a crystal density comparable to those of CL-18 and ANTP, while demonstrating superior thermal stability relative to these reference materials. Collectively, compounds CF-1, CF-2, and CF-3 represent promising candidates for next-generation insensitive high-energy materials (IHEMs).