Efficient synthesis of 3,6-dinitropyrazolo[4,3-c]pyrazole via an in situ diazotization strategy: Structural and detonation performance characterization

3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP) is an important fused heterocyclic energetic compound. A series of DNPP derivatives and their corresponding ionic salts with excellent properties have been reported, demonstrating significant application potential. However, the traditional synthetic route for DNPP presents inherent safety concerns due to the accumulation and isolation of hazardous diazo intermediates, which severely restrict its scalable and environmentally benign production. To address this issue, we proposed a novel synthetic strategy based on the in situ generation of diazonium salts. Starting from 3,5-dimethylpyrazole, DNPP was synthesized via a sequential process involving nitrosation, reduction, acetylation, cyclization, hydrolysis, nitration, oxidation, and decarboxylative nitration. This process increased the overall yield of the key intermediate, 3-methylpyrazolo[4,3-c]pyrazole, from 24.1% to 53.9%. The improved route avoids the accumulation of hazardous intermediates and offers advantages including operational simplicity, high safety, environmental benignity, excellent yield, and facile work-up, thereby demonstrating good scalability. Furthermore, the reaction mechanism was investigated using density functional theory (DFT) calculations. The single-crystal structure of DNPP was elucidated. Based on electrostatic potential and Hirshfeld surface analyses, correlations were established among its charge distribution, intermolecular interactions, stability, and chemical modifiability. Its detonation performance was experimentally evaluated and validated via the laser micro-detonation technique and initiation capability test. This study lays a foundation for the scale-up production and practical application of DNPP and its derivatives and also provides guidance for the synthesis and development of fused heterocyclic energetic compounds with superior performance.