Single-step nitration of N,N′-bispyrrolylmethane: polynitro derivatives as insensitive energetic materials

Abstract

A series of N,N′-methylene-bridged oligonitro bispyrrolyl methane derivatives were synthesised and explored as potential thermostable and insensitive high-energy materials. A mixture of penta- and tetra- nitro derivatives (1–4) was obtained through a single-step controlled nitration using KNO₃/H₂SO₄, while regioisomeric tetranitro compounds (2–4) were selectively synthesised using Cu(NO₃)₂·3H₂O/acetic anhydride. The structural diversity imparted by the methylene bridge influenced the molecular conformation, packing, and resulting properties. Single-crystal X-ray diffraction confirmed a compact packing in compound 1 and extensive C–H⋯O hydrogen bonding in compounds 2–4, which correlated with their measured densities (1.730–1.841 g cm⁻³) and thermal behaviour. Compounds 3 and 4 displayed excellent thermal stability (T_d > 300 °C) and detonation performances (D_v = 7.52 and 7.15 km s⁻¹) comparable to those of HNS, along with high mechanical insensitivity (IS > 40 J, FS > 360 N). Compound 1 exhibited the highest detonation velocity (8.11 km s⁻¹) and decomposed around 260 °C, indicating a trade-off between performance and thermal stability. Noncovalent interaction (NCI) and Hirshfeld surface analyses revealed the role of intermolecular interactions in stabilising molecular frameworks and insensitivity. We also explored the possible reason behind the failure in achieving the octanitro-derivative of bispyrrolylmethane using a synthetic and computational approach.