Assembling Nitroamino and Amino Groups on a Pyrazolyl-1,3,4-Oxadiazole Framework for the Construction of High-Performance and Insensitive Energetic Materials

Abstract

The introduction of nitroamino groups onto a nitrogen-rich heterocyclic skeleton is an efficient method for constructing high-performance energetic compounds. In this work, the nitroamino-functionalized compound 4-(5-amino-1,3,4-oxadiazol-2-yl)-N,N′-dinitro-1H-pyrazole-3,5-diamine was synthesized in yields of up to 83%. In addition, its energetic salts were also prepared. All these compounds were characterized by NMR and IR spectroscopy and, in two cases, by single-crystal X-ray diffraction. In addition, the difference in the reactivities of the three amino groups on the pyrazolyloxadiazole system was analyzed by an average local ionization energy analysis. The hydrazinium salt of the diamine exhibits promising detonation properties and good molecular stability, suggesting it has good application potential. Compared with previous works, this strategy gives an improved isolated yield and provides a promising method for the construction of nitroamino-functionalized 1,3,4-oxadiazole derivatives.

Key words

energetic materials - explosives - nitrogen heterocycle - pyrazolyloxadiazoles - detonation - nitroamines

Publication History

Received: 06 September 2023

Accepted after revision: 25 September 2023

Accepted Manuscript online:
25 September 2023

Article published online:
30 October 2023

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• 35 3,5-Diamino-1H-pyrazole-4-carbohydrazide (2) CAUTION! Compounds in this study are potential explosives and might explode or detonate under certain circumstances. The use of appropriate protective equipment (goggles, reaction shields, earplugs, gloves, etc.) is strongly recommended. To a suspension of diamino ester 1 (1.25 g, 8.0 mmol) in H₂O (15 mL) was added 80% N₂H₄·H₂O (0.81 g, 16 mmol), and the resulting mixture was refluxed and stirred for 6 h, then cooled. The precipitate was collected by filtration and washed with H₂O (10 mL) to give a white solid; yield: 1.07 g (86%); mp 217 °C. IR (KBr): 3350, 3305, 3127, 1670, 1554, 1496, 1443, 1339, 1206, 1146, 1034, 950, 871, 804, 775 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 10.53 (s, 1 H), 7.89 (s, 1 H), 5.26 (s, 4 H), 4.19 (s, 2 H). ¹³C NMR (75 MHz, DMSO-d₆): δ = 166.0, 151.2, 84.9. Anal. Calcd for C₄H₈N₆O: C, 30.77, H, 5.16, N, 53.82. Found: C, 30.91, H, 5.24, N, 53.67. 3,5-Diamino-4-(5-amino-1,3,4-oxadiazol-2-yl)-1H-pyrazole (3) BrCN (0.95 g, 9.0 mmol) and KHCO₃ (0.91 g, 9.0 mmol) were sequentially added in small portions to a suspension of 2 (1.25 g, 8.0 mmol) in a mixture of EtOH (15 mL) and H₂O (15 mL), and the resulting mixture was refluxed for 6 h, then cooled. The precipitate was collected by filtration and washed with H₂O (20 mL) to give a pale-yellow solid; yield: 1.16 g (80%). T_d (onset): 261 °C. IR (KBr): 3376, 3293, 3106, 1666, 1617, 1588, 1480, 1366, 1320, 1098, 1012, 961, 719, 619 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 10.73 (s, 1 H), 6.72 (s, 2 H), 5.14 (s, 4 H). ¹³C NMR (75 MHz, DCl): δ = 154.5, 149.7, 149.3, 71.6. Anal. Calcd for C₅H₇N₇O: C, 33.15, H, 3.89, N, 54.12. Found: C, 33.26, H, 3.81, N, 54.25. 4-(5-Amino-1,3,4-oxadiazol-2-yl)-N,N′-dinitro-1H-pyrazole-3,5-diamine (4) KNO₃ (1.01 g, 10.0 mmol) was added to concd H₂SO₄ (5 mL) at 0 ℃, then compound 3 (0.36 g, 2.0 mmol) was added and the mixture was stirred at 0 ℃ for 2 h. The resulting mixture was poured onto ice-water (50 mL), and the precipitate was collected by filtration and washed with H₂O (20 mL) to give a yellow solid; yield: 0.45 g (83%); T_d (onset): 136 °C. IR (KBr): 3596, 3356, 3127, 1731, 1651, 1597, 1556, 1476, 1383, 1288, 1214, 1029, 867, 697 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 7.69 (s, 2 H). ¹³C NMR (75 MHz, DCl): δ = 155.5, 147.6, 135.2, 92.5. Anal. Calcd for C₅H₅N₉O₅: C, 22.15, H, 1.86, N, 46.49. Found: C, 22.31, H, 1.78, N, 46.35. Salts 5–7; General Procedure To a suspension of 4 (0.27 g, 1.0 mmol) in MeOH (10 mL) was added 25–28% aq NH₃ (0.5 mL), 50% aq NH₂OH (0.5 mL) or 80% N₂H₄·H₂O (0.5 mL), and the resulting solution was stirred at r.t. for 2 h. The precipitate was collected by filtration and washed with MeOH (5 mL) and Et₂O (5 mL). 5 Pale-yellow solid; yield: 0.24 g (76%). T_d (onset): 159 °C. IR (KBr): 3376, 3015, 1663, 1586, 1413, 1285, 1166, 1115, 983, 867, 747, 617, 567 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 6.73 (s, 2 H), 3.61 (br). ¹³C NMR (75 MHz, DMSO-d₆): δ = 162.7, 154.0, 148.9, 87.7. Anal. Calcd for C₅H₁₁N₁₁O₅: C, 19.68, H, 3.63, N, 50.48. Found: C, 19.89, H, 3.51, N, 50.61. 6 Yellow solid; yield: 0.26 g (70%). T_d (onset): 141°C. IR (KBr): 3387, 2921, 2631, 1682, 1611, 1488, 1388, 1292, 1078, 992, 857, 744, 561 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 7.99 (s, 8 H), 6.92 (s, 2 H). ¹³C NMR (75 MHz, DMSO-d₆): δ = 162.9, 153.1, 145.5, 89.2. Anal. Calcd for C₅H₁₁N₁₁O₇: C, 17.81, H, 3.29, N, 45.69. Found: C, 17.96, H, 3.36, N, 45.51. 7 Yellow solid; yield: 0.27 g (73%). T_d (onset): 164 °C. IR (KBr): 3276, 3090, 1664, 1572, 1368, 1292, 1213, 1166, 1063, 968, 879, 846, 759, 702 cm^–1. ¹H NMR (300 MHz, DMSO-d₆): δ = 6.79 (s, 2 H), 5.52 (s, 10 H). ¹³C NMR (75 MHz, DMSO-d₆): δ = 162.7, 154.1, 148.9, 87.6. Anal. Calcd for C₅H₁₃N₁₃O₅: C, 17.91, H, 3.91, N, 54.32. Found: C, 17.78, H, 3.83, N, 54.47.