Ionization treatment of pyrazolo[4,3-c]pyrazole derivatives to achieve the low sensitivity of high-energy materials†

Low mechanical sensitivity is an evaluation index for the development of safe and stable high-energy materials. Herein, 3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP) and its ionic salts 1–4 with a fused-ring backbone were synthesized based on 6-nitro-pyrazolo[4,3-c]pyrazole-3-carbonic acid. Their crystal structures were determined by single crystal X-ray diffraction, and the abundant hydrogen bonds and π–π stacking in the crystal contribute to decreasing the mechanical sensitivity. The synthesized ionic salts 1–4 present low sensitivity features (FS >360 N and IS >20 J), much lower than that of their precursor DNPP (FS = 160 N and IS = 15 J). The detonation performance test shows that hydroxylammonium ionic salt 3 with a density of 1.863 g cm⁻³ has the highest detonation velocity of 9.06 km s⁻¹ and detonation pressure of 34.47 GPa, which is increased by 8.9% and 20.5% as compared with that of DNPP (8.25 km s⁻¹ and 27.4 GPa). To expand the substituents of the fused-ring backbone, 4,5-diamino-3-(6-nitro-1,4-dihydropyrazolo[4,3-c]pyrazol-3-yl)-4H-1,2,4-triazole (5) was synthesized, and its ionic salt 6 indicated the excellent insensitive features (FS >360 N and IS >40 J). Even though salt 6 contains water of crystallization, it still exhibits excellent detonation performance comparable to RDX.