Facile and selective polynitrations at the 4-pyrazolyl dual backbone: straightforward access to a series of high-density energetic materials†
Nitro-functionalized energetic materials are still needed to meet new safety, performance and chemical accessibility demands. The problem of multiple C-nitrations on N-containing heterocycles was resolved successfully for the 4,4′-bipyrazole scaffold. A progression of gradually functionalized 3-nitro-4,4′-bipyrazole (2), 3,3′-dinitro-4,4′-bipyrazole (3), 3,5-dinitro-4,4′-bipyrazole (4), 3,3′,5-trinitro-4,4′-bipyrazole (5) and 3,3′,5,5′-tetranitro-4,4′-bipyrazole (6) was obtained in excellent yields by highly selective direct nitrations of 4,4′-bipyrazole (1). All synthesized polynitro derivatives 3–6 exhibit high decomposition temperatures of above 290 °C. The introduction of three (5) and four nitro groups (6) into the 4,4′-bipyrazole scaffold yields insensitive and thermally stable high explosives with excellent densities and detonation properties. The anhydrous structures of compounds 2–6 were obtained by low-temperature XRD. In addition, the performance of compounds 5 and 6 was investigated with a small scale shock reactivity test.