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 that of TATB (1,3,5-triamino-2,4,6-trinitrobenzene), represents a promising thermally stable explosive. It demonstrates excellent thermal stability (T_d = 309 °C) alongside low impact and friction sensitivities (IS > 40 J and FS > 360 N). Furthermore, it exhibits high density (ρ = 1.892 g cm⁻³) and favourable detonation properties (D = 8461 m s⁻¹ and 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 compounds 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 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 and P = 30.72 GPa) are comparable to those of RDX (1,3,5-trinitro-1,3,5-triazinane) (D = 8795 m s⁻¹ and P = 34.9 GPa), while CF-2 (D = 8862 m s⁻¹ and P = 32.91 GPa) rivals FOX-7 (1,1-diamino-2,2-dinitroethylene) (D = 8870 m s⁻¹ and P = 34.0 GPa). Concurrently, both compounds demonstrate excellent thermal stability and low sensitivity (CF-2: T_d = 224 °C, IS = 30 J, and FS = 252 N; CF-3: T_d = 230 °C, IS = 25 J, and 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).