Synchronous improvement in density and stability: Energetic salts of 4-dinitromethyl-1-nitropyrazole

High-energy-density materials (HEDMs) based on five-membered nitrogen heterocycles functionalized with dinitromethyl groups hold significant potential but face challenges in balancing energy and stability. While dinitromethyl derivatives of azoles and oxadiazoles exhibit high enthalpies of formation and detonation performance, their inherent acidity, mechanical sensitivities, and poor stability limit practical applications. Energetic salts have been widely explored to enhance stability, yet most reported salts suffer from reduced densities compared to their neutral analogs. Herein, we report the synthesis and characterization of a novel meta-substituted dinitromethylpyrazole derivative, 4-dinitromethyl-1-nitropyrazole (5), and its ammonium (6a), hydrazinium (6b), and hydroxylammonium (6c) salts. Remarkably, all three salts exhibit superior densities (1.81–1.89 g cm⁻³), surpassing that of neutral 5 (1.74 g cm⁻³), while maintaining enhanced thermal stabilities and reduced mechanical sensitivities. Single-crystal X-ray diffraction and theoretical calculations jointly reveal that the structural transformation from a twisted neutral molecule to a planar dinitromethyl anion, driven by deprotonation, induces a synergistic interplay of intermolecular forces. This transition fosters robust hydrogen-bond networks, intensified π-π stacking interactions, and elevated packing coefficients, while simultaneously enhancing aromaticity and conjugation effects. Concurrently, destabilizing O···O repulsions are significantly reduced. These combined structural and electronic modifications collectively contribute to superior density and thermochemical stability, overcoming the typical density loss observed in analogous systems. This work challenges the conventional limitations of ionic HEDMs and provides a strategic approach to designing high-density, low-sensitivity energetic salts through rational substitution and crystal engineering. The results highlight the critical role of molecular geometry and counterion selection in achieving balanced energetic performance, offering new avenues for advancing HEDM development.