N‐Alkylation of 4‐Hydroxy‐3,5‐Dinitropyrazole: A Facile Route for the Synthesis of Insensitive Energetic Materials

ABSTRACT Behind the synthesis of any new high‐energy density material is the objective to achieve equilibrium between energy and stability, which becomes laborious because of their adversarial nature. Nitropyrazoles, with their viable syntheses and easy modifiability, have been adopted typically as backbones to achieve this goal. However, the acidic nature of pyrazole N H in nitropyrazoles limits their usefulness despite exhibiting appealing characteristics. 4‐Hydroxy‐3,5‐dinitropyrazole (HODNP) is an interesting skeleton because of the presence of the hydroxy functionality in between two nitro groups, which not only provides additional oxygen content for better energy output but also possesses the quality of exhibiting strong hydrogen bonding interactions and tuning of properties via salt formation. N‐alkylation is the most straightforward technique for the protection of the unwanted acidic proton, in addition to making them more physically stable. In this work, we have synthesized N ‐methylated and N , N’ ‐ethylene‐bridged derivatives ( 2 and 7 ) of HODNP. Mono and dicationic salts 3 ‐ 6 and 8 ‐ 11 were also synthesized to boost their performance further. N‐methylated neutral compound 2 and its ammonium salt were confirmed through 15 N NMR analysis, and 2 was also confirmed with single‐crystal analysis. All the compounds showed excellent physical stability (IS >40 J, FS > 360 N), and most of the compounds showed good thermal stability (T d > 180°C). The detonation characteristics were found to be similar to the ones found for 1,3,5‐triamino‐2,4,6‐trinitrobenzene (D v = 7704–8172 m/s, p = 22.4–27.3 GPa).