A Dual Strategy of Peripheral Modification and Skeleton Fusion for Pyrazolo[3,4-b]pyridine Coplanar Fused Insensitive High-Energy Materials

Abstract

Fused energetic materials are regarded as highly promising energetic compounds, yet their practical application is often hindered by cumbersome synthetic routes. In this study, a commercially available starting material, 4,6-dichloro-1H-pyrazolo[3,4-b]pyridine (1), was employed to successfully synthesise a series of energetic compounds via straightforward reactions. Among these,3,5-dinitro-1H-pyrazolo[3,4-b]pyridine-4,6-diamine (CF-1), exhibiting the characteristic "NH2-NO2-NH2-NO2" arrangement similar to "TATB", represents a promising thermally stable explosive. It demonstrates excellent thermal stability (Td = 309 oC) alongside low impact and friction sensitivities (IS > 40J, FS > 360N). Furthermore, it exhibits high density (ρ = 1.892 g×cm-3) and favourable detonation properties (D=8461 m×s-1, P = 28.27 GPa). This study further employed innovative molecular design strategies to achieve azide-mediated cyclisation of compound 6-chloro-3,5-dinitro-1H-pyrazolo[3,4-b]pyridine-4-amine (3) and N-O functionalisation of compound CF-1, successfully synthesising the tricyclic compound 4,6-dinitro-8H-pyrazolo[4,3-e]tetrazolo[1,5-a]pyridine-5-amine (CF-2) and 4,6-diamin-3, 5-dinitro-1H-pyrazolo[3,4-b]pyridine 7-oxide (CF-3). Both compounds CF-2 (ρ = 1.928 g×cm-3) and CF-3 (ρ = 1.886 g×cm-3) exhibit high density. The detonation velocity and pressure of CF-3(D = 8579 m×s-1, P = 30.72 GPa) are comparable to RDX(D = 8795 m×s-1, P = 34.9 GPa), while CF-2(D = 8862 m×s-1, P = 32.91 GPa) rivals FOX-7(D = 8870 m×s-1, P = 34.0 GPa). Concurrently, both compounds demonstrate excellent thermal stability and low sensitivity ( CF-2: Td = 224 oC, IS = 30 J, FS =252 N ; CF-3: Td = 230 oC, IS = 25 J, FS =240 N). Therefore, compounds CF-1, CF-2 and CF-3 all hold promise as potential candidates for novel Insensitive High-Energy Materials (IHEMs).