Thermostable Insensitive Energetic Materials Based on a Triazolopyridine Fused Framework with Alternating Nitro and Amine Groups

Abstract

In this work, we designed and synthesized a series of novel triazolopyridine fused-ring compounds with alternating nitro and amine groups. Three compounds showed remarkable thermal stability at 256, 310, and 294 °C, respectively, and a low mechanical sensitivity [impact sensitivity (IS) = 40 J, friction sensitivity (FS) = 324 N; IS = 35 J, FS = 240 N; and IS > 40 J, FS = 324 N, respectively]. Significantly, two of these compounds exhibited a better detonation performance [detonation velocity (D) = 8200 and 8335 m s^–1, Detonation pressure (P) = 25.6 and 27.2 GPa, respectively] than the widely used heat-resistant explosive hexanitrostilbene (HNS; D = 7612 m s^–1, P = 24.3 GPa). Additionally, a nitramine derivative displayed a detonation performance (D = 8569 m s^–1, P = 31.3 GPa) similar to that of the high-energy explosive RDX. The superior properties of the materials were further confirmed by X-ray diffraction analysis and by several theoretical calculations (ESP, LOL–π, Hirshfeld surfaces, RDG, and NCI analyses). These results indicated that the three compounds might be potential candidates for use as heat-resistant energetic materials.

The development of energetic materials has been an area of intense research for many decades, due to their widespread applications in military and civilian fields.[1] [2] [3] Among these engineering materials, heat-resistant explosives with onset thermal decomposition temperatures above 250 °C have gained momentum as a result of their ability to withstand extreme conditions of temperature and pressure during their storage and use in such activities as rocket propulsion and oil-well drilling.[4,5] Through extensive research, several heat-resistant explosives with high thermal stabilities have been identified. These include 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), hexanitrostilbene (HNS), 3,5-dinitropyridine-2,4,6-triamine (TANPy), and 3,5-dinitro-N ²,N ⁶-bis(2,4,6-trinitrophenyl)pyridine-2,6-diamine (PYX) (Figure [1a]).[6] [7] [8] [9] Their structures predominantly consist of conjugated benzene rings or N-hetarenes. The components in these structural motifs are arranged with alternating nitro groups and amino groups or hydrogen atoms that provide multiple molecular hydrogen bonds. However, their detonation velocities are usually unsatisfactory (about 8000 m/s) as a result of their relatively low heats of formation (ΔH _f < 78.2 kJ mol^–1).

In recent years, fused-ring molecular backbones have gained widespread attention as novel energetic molecules, owing to their elevated heats of formation and their stable conjugated structure.[10] [11] [12] [13] [14] [15] [16] [17] [18] Recently, the group of Tang and Shreeve developed a novel compound, 3,8-dinitropyrazolo[5,1-c][1,2,4]triazine-4,7-diamine (DNPTA), with two pairs of vicinal amino and nitro groups on the fused pyrazolotriazine ring. This compound exhibits a remarkable thermal stability of 355 °C and low mechanical sensitivities.[12] Therefore, the design and development of fused-ring energetic materials offers a promising strategy to satisfy the demands for high thermal stability and high-energy properties. Among the various types of fused-ring energetic materials, those containing a nitrogen-rich triazole ring have become popular because of their high heats of formation and high densities, which result in a high detonation performance (Figure1b).[19] [20] [21] [22] However, fused-ring energetic compounds containing a triazole or other nitrogen-rich heterocycle (e.g., triazine, tetrazine, or diazine) that are characterized by detonation velocities exceeding 8000 m/s tend to exhibit relatively low decomposition temperatures. This can be attributed to a decrease in modification sites with increasing nitrogen content in the fused ring. Consequently, this makes it difficult to introduce stabilizing groups (NH₂) and leads to a reduction in the number of hydrogen bonds. Ultimately, these factors lead to lower decomposition temperatures.[23] [24] [25]

In comparison, the pyridine ring offers more modification sites than other heteroarenes; consequently, the pyridine ring favors the formation of an alternating nitro and amine group structure.[8] [26] [27] For instance, the pyridine-based heat-resistant explosive TANPy possesses a high decomposition temperature of 353 °C.[8] Therefore, both triazole and pyridine rings are ideal structural units for the design of heat-resistant explosives. Based on these potential advantages, we designed and synthesized a number of heat-resistant explosives by using a fused triazolopyridine as the skeleton, with remarkable onset decomposition temperatures of up to 310 °C and a low mechanical sensitivity (Figure [1c]). Additionally, a nitramine group was added to the core skeleton to improve the oxygen balance and detonation performance, which resulted in a high detonation velocity of 8569 m/s. Furthermore, we conducted relevant theoretical calculations to probe the prospective correlation between structural characteristics and molecular stability.

The synthetic routes to the target compounds are shown in Scheme [1]. Compounds 1 and 6 were synthesized by following the reported procedures.[26] [28] The aminated compound 2 was obtained by carefully controlling the reaction of amine 1 with aqueous ammonia; subsequent treatment with hydrazine monohydrate gave the hydrazine-substituted compound 3. Compound 3 was then modified with cyanogen bromide (BrCN) to produce the [1,2,4]triazolo[4,3-a]pyridine-based compound 4 in a high yield of 87%. In addition, oxidation of compound 4 with a mixture of potassium peroxomonosulfate (Oxone) and 20% H₂SO₄ gave compound 5 in an appreciable yield (86%). Compound 7 was obtained by treating chloro amine 6 with hydrazine monohydrate. Interestingly, compound 7 reacted with BrCN to form the Dimroth rearrangement product 6,8-dinitro[1,2,4]triazolo[1,5-a]pyridine-2,7-diamine (8), the structure of which was confirmed by a single-crystal X-ray diffraction analysis.[29] Finally, compound 8 was nitrated with a mixture of concentrated H₂SO₄ and fuming HNO₃ to give the corresponding nitramine 9. These synthetic methods contribute to the advancement of synthetic approaches for creating new fused-ring energetic molecules.

A single-crystal X-ray diffraction analysis was conducted to elucidate the molecular structures and intra- and intermolecular interactions of compounds 4, 8, and 9 (Figure [2]).[29] Single crystals of the compounds were grown from N,N-dimethylformamide (DMF) solution at atmospheric temperature. 4·DMF crystallizes in a monoclinic crystal system with a P2₁/c space group and four molecules per unit cell (Z = 4). The amino and nitro groups and the fused-ring framework in 4 have small torsion angles (<8.4°) as a result of the contributions of five intramolecular hydrogen bonds and the conjugated fused-ring framework. As a result of the intermolecular hydrogen bonds between compound 4 and DMF, compound 4 exhibits a wavelike layered stacking framework in which the individual layers are separated by DMF (Figure [2c]).

Compound 8 crystallizes in a monoclinic crystal system with a P2₁/c space group and a calculated crystal density of 1.856 g/cm³ at 298 K, with four molecules per unit cell (Z = 4). Compound 8 also exhibits small torsion angles except for O3–N4–C4–C3 [15.9(5)°] and O4–N4–C4–C5 [14.5(5)°] (Figure [2d]). Notably, the C3–N3 [1.330(5) Å] and C6–N6 [1.320(5) Å] bond lengths are shorter than those of a typical C=N double bond (1.34–1.38 Å), indicating a strong conjugation effect from the fused ring.[30] In addition, an abundance of intermolecular hydrogen bonds leads to the formation of a two-dimensional (2D) structure. Moreover, compound 8 displays a wavelike layered stacking with an average interlayer distance of 3.14(5) Å. The abundant hydrogen bonds and the layered stacking substantially contribute to an improvement in molecular stability and a reduction in the sensitivity of compound 8.

Crystal 9·2DMF had a monoclinic crystal system with a P2₁/c space group and four molecules per unit cell (Z = 4). All the atoms in compound 9 are almost coplanar, with small torsion angles [<6.8(4)°] (Figure [2g]). Additionally, compound 9 also displays a wavelike and face-to-face layered stacking with an average interlayer distance of 3.22(5) Å.

To study the thermal behavior of the samples, the thermal stabilities of the energetic compounds 4, 5, 8, and 9 were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure [3]) at a heating rate of 5 °C/min under nitrogen. The DSC curve of compound 4 showed a narrow and high exothermic peak, with an onset temperature of 256 °C and a peak temperature of 262 °C. This exothermic peak corresponds to the sharp weight loss observed at approximately 262 °C in the TG curve. Compound 5 showed a high onset temperature of 310 °C and a peak temperature of 341 °C, which are comparable to those of the heat-resistant energetic material HNS (318 °C). Moreover, compound 8 displayed two exothermal peaks in its DSC curve, with an onset temperature of 294 °C and a peak temperature of 341 °C. These thermal events aligned with the two mass-loss stages in the corresponding TG curve. Compound 9, with the nitramine group, exhibited an onset decomposition temperature of 120 °C.

The physicochemical properties of the compounds were measured or calculated, and are summarized in Table [1]. The densities of these compounds were determined from the crystal density or by using a gas pycnometer at room temperature, and ranged from 1.764 to 1.859 g/cm³. Notably, compound 9 with the nitramine group displayed the highest density (1.859 g/cm³), surpassing that of the typical explosive RDX (1.800 g/cm³). Furthermore, the heats of formation were calculated by using the Gaussian 09 suite of programs; as expected, compounds 4, 8, and 9 exhibited positive heats of formation (157.3–275.6 kJ/mol), which are higher than those of the heat-resistant explosives TATB, HNS and LLM-105 (ΔH _f = –139.7, 78.2, and 11 kJ/mol, respectively). Based on the measured densities and calculated heats of formation for compounds 4, 5, 8, and 9, the detonation performances were calculated by using EXPLO5 (version 6.02). Compounds 4, 8, and 9 displayed higher detonation velocities (D = 8200, 8335, and 8568 m/s, respectively) than TATB or HNS (D = 8179 and 7612 m/s, respectively). Furthermore, the detonation pressures (P) of the synthesized compounds ranged from 22.9 to 31.3 GPa. The initial performance safety of these compounds was assessed by evaluating their impact sensitivity (IS) and friction sensitivity (FS) properties using the standard BAM fall hammer and BAM friction tester, respectively. As shown in Table [1], compounds 4, 5, and 8 exhibited low mechanical sensitivities (IS ≥ 35 J; FS ≥ 240 N), surpassing those of typical explosives such as RDX and HNS (IS = 7.4 and 5 J; FS = 120 and 240 N, respectively), while remaining comparable to the values for LLM-105 (IS = 20 J; FS = 360 N). The lower mechanical sensitivities and high decomposition temperatures of compounds 4, 5, and 8 suggest that they are promising candidates for the development of low-sensitivity and heat-resistant energetic materials.

To investigate the internal factors that influence the thermal stabilities and sensitivity of compounds 4, 5, 8, and 9, electrostatic potential (ESP) surfaces and localized orbital locator–π (LOL–π) analyses were conducted by using the Multiwfn program (Version 3.8).[32] [33] Generally, more-negative ESP values result in lower impact sensitivities.[34,35] Interestingly, compounds 4, 5, and 8 exhibited more-negative ESP values than compound 9, which contained a nitramine group (Figures [4a–d]). Furthermore, the LOL–π diagrams revealed that the π-electrons in compound 8 were distributed throughout the fused-ring skeletons, as well as on the amino and nitro groups, forming larger conjugated structures than those of compound 9 with the nitramine group (Figures [4g] and 4h). In general, an exceptional conjugation structure contributes to increased molecular stability, leading to a high thermostability.

Hirshfeld surface, 2D fingerprint plot, reduced density gradient (RDG), and noncovalent interaction (NCI) plot analyses are frequently used to investigate intermolecular interactions, specifically intermolecular hydrogen-bond interactions and π–π stacking interactions.[36] Extensive research has been conducted to comprehend the impact of these intermolecular interactions on thermal stabilities and sensitivity.[37] [38] [39] [40] [41] It is generally observed that the presence of abundant intermolecular hydrogen bonds and strong π–π interactions tends to enhance molecular stability and reduce mechanical sensitivity. The properties of compound 8 have been analyzed by using these theoretical calculations (Figure [5]). In the Hirshfeld surfaces, the red and blue regions indicate strong and weak interactions, respectively.[35] As shown in Figure [5a], the red dots were primarily concentrated on the edges of the molecular surface, indicating strong hydrogen-bond interactions with neighboring molecules. In contrast, the blue and white spots were located on the surface, representing weak π-stacking interactions, such as O–O, O–C, N–C, and N–O. Significantly, the high percentage of hydrogen-bond interactions (such as C···H, O···H, and N···H) contributed 59% of the total of weak interactions, leading to a reduction in the mechanical sensitivity of compound 8 (Figure [5c]). Furthermore, RDG and NCI analyses aid an intuitive understanding of intermolecular interactions and their corresponding distribution regions. As shown in Figure [5d], the green region indicates that the intermolecular interactions of compound 8 involved van der Waals interactions. This observation aligns with the appearance of a green patch between two molecular layers in the NCI diagram (Figure [5e]), indicating the existence of a strong face-to-face π–π stacking interaction between adjacent parallel molecules.[42] These interactions significantly contribute to an enhancement in the molecular stability and reduce the sensitivity of compound 8. The calculation results were in good agreement with the experimental results, demonstrating a sound reliability of the calculated results. Furthermore, these comprehensive theoretical calculations provide a strong theoretical foundation for the design and development of new insensitive heat-resistant explosives with fused-ring skeletons.

In summary, we have successfully designed and developed a series of novel, low-sensitivity, heat-resistant energy materials based on a fused triazolopyridine framework.[43] In this work, we found that compounds 4, 5, and 8 showed excellent thermal stabilities, low mechanical sensitivities, and suitable detonation performances. These superior properties are attributed to their good conjugation contributions, strong face-to-face π–π stacking, and extensive intra- and intermolecular hydrogen-bonding interactions through the alternating nitro and amine structure, as revealed by X-ray diffraction analyses and several theoretical calculations (ESP, LOL–π, Hirshfeld surfaces, RDG, and NCI analyses). Moreover, compound 9, with a high density (1.859 g/cm) and a positive heat of formation (275.6 kJ/mol), exhibited a similar detonation performance (D = 8568 m/s, P = 31.3 GPa) to the well-known high-energy explosive RDX (D = 8801 m/s, P = 33.6 GPa). Importantly, the stable triazolopyridine fused-ring backbone and the alternating nitro and amine structure can serve as blueprints for the design and development of new heat-resistant energetic materials with excellent properties.

Conflict of Interest

The authors declare no conflict of interest.

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• 43 CAUTION! These new compounds are energetic materials with a high potential for explosion under certain conditions. It is therefore essential to take appropriate protective measures at all times when handling these materials.Amine 1 (2.53 g, 10 mmol) was dissolved in CH₂Cl₂ (100 mL) and 25% aq NH₃ (3 mL) in EtOH (30 mL) was added dropwise at 0 °C. The mixture was allowed to react for 24 h and then the solids were collected by filtration and washed with H₂O and MeOH. The crude product was crystallized from DMSO–EtOAc to give a faintly yellow solid; yield: 1.65 g (71%).IR (KBr): 3416, 3285, 3167, 1596, 1522, 1283, 1021, 969, 871, 831, 786, 629 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 8.75–8.89 (m, 4 H, NH₂). ¹³C NMR (100 MHz, DMSO-d₆): δ = 113.7, 123.0, 147.3, 147.6, 154.4. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₅H₃ClN₅O₄: 231.9879; found; 231.9876.6-Hydrazino-3,5-dinitropyridine-2,4-diamine (3) A solution of N₂H₄·H₂O (0.7 mL, 15 mmol) in EtOH (10 mL) was slowly added to a mixture of 2 (1.16 g, 5 mmol) and EtOH (30 mL). The mixture was stirred for 3 h and then filtered. The resulting solid was washed sequentially with H₂O and EtOH and dried to give a yellow solid; yield: 1.02 g (95%).IR (KBr pellet): 3393, 3343, 3242, 3181, 1615, 1542, 1268, 1201, 1059, 973, 784, 697, 545 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 5.46 (s, 2 H, NH₂), 8.66 (d, J = 157.2 Hz, 2 H, NH₂), 10.47 (d, J = 72.8 Hz, 2 H, NH₂), 10.88 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 108.8, 109.5, 148.3, 151.7, 155.7. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₅H₆N₇O₄: 228.0487; found: 228.0488.6,8-Dinitro[1,2,4]triazolo[4,3-a]pyridine-3,5,7-triamine (4) A solution of BrCN (508.8 mg, 4.8 mmol) in MeCN (14 mL) was added to a suspension of 3 (890 mg, 4 mmol) in MeOH (14 mL) and H₂O (21 mL) at r.t. The mixture was heated to 50 °C for 12 h, then cooled to r.t. The solid was collected by filtration, washed successively with H₂O and EtOH, and dried to give a yellow powder; yield: 888 mg (87%).IR (KBr ): 3370, 3322, 3213, 1673, 1628, 1597, 1525, 1474, 1397, 1270, 1215, 1172, 1081, 961, 881, 690 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 8.17 (s, 2 H, NH₂), 10.60–10.67 (m, 3 H, NH₂ and C=NH), 14.11 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 104.0, 110.6, 138.2, 148.7, 150.3, 151.8. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₅N₈O₄: 53.0439; found: 253.0449. Anal. Calcd for C₆H₆N₈O₄ (254.16): C, 28.35; H, 2.38; N, 44.09. Found: C, 28.14; H, 2.56; N, 41.15.3,7-Diamino-6,8-dinitro[1,2,4]triazolo[4,3-a]pyridin-5(1H)-one (5) A mixture of 4 (254 mg, 1 mmol) and 20% H₂SO₄ (6 mL) was treated with potassium peroxomonosulfate (Oxone) (770 mg, 2.5 mmol) at r.t. The mixture was heated to 90 °C with stirring for 12 h, then cooled to r.t. The resulting mixture was filtered, and the solids were washed successively with H₂O and EtOH, then dried to afford a yellow solid; yield: 216 mg (85%).IR (KBr pellet): 3478, 3333, 3249, 1609, 1509, 1282, 1228, 1175, 1108, 778, 693, 593 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 7.42 (s, 2 H, NH₂), 9.55 (s, 1 H, NH₂), 10.04 (s, 1 H, NH₂), 14.10 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 104.6, 113.7, 139.1, 150.6, 151.4, 153.0. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₄N₇O₅: 254.0279; found. 254.0270. Anal. Calcd for C₆H₅N₇O₅ (255.15): C, 28.24; H, 1.98; N, 38.43. Found: C, 28.98; H, 2.60; N, 35.24.6,8-Dinitro[1,2,4]triazolo[4,3-a]pyridine-3,5,7-triamine (7) A solution of N₂H₄·H₂O (0.7 mL, 15 mmol) in EtOH (10 mL) was added slowly to a mixture of amine 6 (1.09 g, 5 mmol) and EtOH (30 mL) at r.t., and the mixture was allowed to react for 3 h, The resulting solid was washed sequentially with H₂O and EtOH, then dried to give a green solid; yield: 1.0 g (94%).IR (KBr): 3408, 3371, 3338, 3286, 3251, 1610, 1550, 1495, 1381, 1268, 1205, 1120, 1067, 962, 777, 709, 653 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 5.94 (s, 2 H, NH₂), 8.94 (s, 1 H, CH), 9.71 (d, J = 15.48 Hz, 2 H, NH₂), 11.43 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 111.8, 121.0, 148.5, 148.8, 153.7. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₅H₇N₆O₄: 215.0523; found: 215.0526.6,8-Dinitro[1,2,4]triazolo[1,5-a]pyridine-2,7-diamine (8) A solution of BrCN (508.8 mg, 4.8 mmol) in MeCN (14 mL) was added to a suspension of 7 (856 mg, 4 mmol) in MeOH (14 mL) and H₂O (21 mL) at r.t., and the mixture was stirred at 50 °C for 12 h. The mixture was then cooled to r.t. and filtered to give a solid that was washed successively with H₂O and EtOH, then dried to give an orange powder; yield: 812.6 mg (85%).IR (KBr pellet): 3415, 3298, 3159, 3049, 1645, 1547, 1494, 1432, 1289, 1042, 900, 776, 608 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 7.03 (s, 2 H, NH₂), 9.22 (s, 2 H, NH₂), 9.68 (s, 1 H, CH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 115.0, 123.7, 134.0, 143.9, 149.7, 169.0. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₆H₆N₇O₄: 240.0476; found: 240.0477. Anal. Calcd for C₆H₅N₇O₄ (240.16): C, 30.01; H, 2.52; N, 40.83. Found: C, 30.44; H, 2.16; N, 37.77.N²,6,8-Trinitro[1,2,4]triazolo[1,5-a]pyridine-2,7-diamine (9) 8 (478 mg, 2 mmol) was added in portions to a mixture of concd H₂SO₄ (4 mL) and fuming HNO₃ (4 mL) at 0 °C. The mixture was stirred for 24 h at 0 °C, then poured onto ice. The resulting precipitate was collected by filtration, washed successively with H₂O and EtOH, and dried to give a yellow solid; yield: 528 mg (93%).IR (KBr pellet): 3429, 3312, 1674, 1625, 1537, 1445, 1291, 1252, 899, 775, 646 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 9.28 (s, 2 H, NH₂), 10.19 (s, 1 H, CH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 116.5, 128.4, 136.5, 144.1, 148.5, 159.7. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₃N₈O₆: 283.0181; found: 283.0181. Anal. Calcd for C₆H₄N₈O₆ (284.15): C, 25.36; H, 1.42; N, 39.44. Found: C, 25.39; H, 2.65; N, 35.38.

Corresponding Author

Publication History

Received: 31 August 2023

Accepted after revision: 25 September 2023

Article published online:
27 October 2023

© 2023. Thieme. All rights reserved

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• References and Notes

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• 43 CAUTION! These new compounds are energetic materials with a high potential for explosion under certain conditions. It is therefore essential to take appropriate protective measures at all times when handling these materials.Amine 1 (2.53 g, 10 mmol) was dissolved in CH₂Cl₂ (100 mL) and 25% aq NH₃ (3 mL) in EtOH (30 mL) was added dropwise at 0 °C. The mixture was allowed to react for 24 h and then the solids were collected by filtration and washed with H₂O and MeOH. The crude product was crystallized from DMSO–EtOAc to give a faintly yellow solid; yield: 1.65 g (71%).IR (KBr): 3416, 3285, 3167, 1596, 1522, 1283, 1021, 969, 871, 831, 786, 629 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 8.75–8.89 (m, 4 H, NH₂). ¹³C NMR (100 MHz, DMSO-d₆): δ = 113.7, 123.0, 147.3, 147.6, 154.4. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₅H₃ClN₅O₄: 231.9879; found; 231.9876.6-Hydrazino-3,5-dinitropyridine-2,4-diamine (3) A solution of N₂H₄·H₂O (0.7 mL, 15 mmol) in EtOH (10 mL) was slowly added to a mixture of 2 (1.16 g, 5 mmol) and EtOH (30 mL). The mixture was stirred for 3 h and then filtered. The resulting solid was washed sequentially with H₂O and EtOH and dried to give a yellow solid; yield: 1.02 g (95%).IR (KBr pellet): 3393, 3343, 3242, 3181, 1615, 1542, 1268, 1201, 1059, 973, 784, 697, 545 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 5.46 (s, 2 H, NH₂), 8.66 (d, J = 157.2 Hz, 2 H, NH₂), 10.47 (d, J = 72.8 Hz, 2 H, NH₂), 10.88 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 108.8, 109.5, 148.3, 151.7, 155.7. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₅H₆N₇O₄: 228.0487; found: 228.0488.6,8-Dinitro[1,2,4]triazolo[4,3-a]pyridine-3,5,7-triamine (4) A solution of BrCN (508.8 mg, 4.8 mmol) in MeCN (14 mL) was added to a suspension of 3 (890 mg, 4 mmol) in MeOH (14 mL) and H₂O (21 mL) at r.t. The mixture was heated to 50 °C for 12 h, then cooled to r.t. The solid was collected by filtration, washed successively with H₂O and EtOH, and dried to give a yellow powder; yield: 888 mg (87%).IR (KBr ): 3370, 3322, 3213, 1673, 1628, 1597, 1525, 1474, 1397, 1270, 1215, 1172, 1081, 961, 881, 690 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 8.17 (s, 2 H, NH₂), 10.60–10.67 (m, 3 H, NH₂ and C=NH), 14.11 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 104.0, 110.6, 138.2, 148.7, 150.3, 151.8. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₅N₈O₄: 53.0439; found: 253.0449. Anal. Calcd for C₆H₆N₈O₄ (254.16): C, 28.35; H, 2.38; N, 44.09. Found: C, 28.14; H, 2.56; N, 41.15.3,7-Diamino-6,8-dinitro[1,2,4]triazolo[4,3-a]pyridin-5(1H)-one (5) A mixture of 4 (254 mg, 1 mmol) and 20% H₂SO₄ (6 mL) was treated with potassium peroxomonosulfate (Oxone) (770 mg, 2.5 mmol) at r.t. The mixture was heated to 90 °C with stirring for 12 h, then cooled to r.t. The resulting mixture was filtered, and the solids were washed successively with H₂O and EtOH, then dried to afford a yellow solid; yield: 216 mg (85%).IR (KBr pellet): 3478, 3333, 3249, 1609, 1509, 1282, 1228, 1175, 1108, 778, 693, 593 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 7.42 (s, 2 H, NH₂), 9.55 (s, 1 H, NH₂), 10.04 (s, 1 H, NH₂), 14.10 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 104.6, 113.7, 139.1, 150.6, 151.4, 153.0. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₄N₇O₅: 254.0279; found. 254.0270. Anal. Calcd for C₆H₅N₇O₅ (255.15): C, 28.24; H, 1.98; N, 38.43. Found: C, 28.98; H, 2.60; N, 35.24.6,8-Dinitro[1,2,4]triazolo[4,3-a]pyridine-3,5,7-triamine (7) A solution of N₂H₄·H₂O (0.7 mL, 15 mmol) in EtOH (10 mL) was added slowly to a mixture of amine 6 (1.09 g, 5 mmol) and EtOH (30 mL) at r.t., and the mixture was allowed to react for 3 h, The resulting solid was washed sequentially with H₂O and EtOH, then dried to give a green solid; yield: 1.0 g (94%).IR (KBr): 3408, 3371, 3338, 3286, 3251, 1610, 1550, 1495, 1381, 1268, 1205, 1120, 1067, 962, 777, 709, 653 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 5.94 (s, 2 H, NH₂), 8.94 (s, 1 H, CH), 9.71 (d, J = 15.48 Hz, 2 H, NH₂), 11.43 (s, 1 H, NH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 111.8, 121.0, 148.5, 148.8, 153.7. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₅H₇N₆O₄: 215.0523; found: 215.0526.6,8-Dinitro[1,2,4]triazolo[1,5-a]pyridine-2,7-diamine (8) A solution of BrCN (508.8 mg, 4.8 mmol) in MeCN (14 mL) was added to a suspension of 7 (856 mg, 4 mmol) in MeOH (14 mL) and H₂O (21 mL) at r.t., and the mixture was stirred at 50 °C for 12 h. The mixture was then cooled to r.t. and filtered to give a solid that was washed successively with H₂O and EtOH, then dried to give an orange powder; yield: 812.6 mg (85%).IR (KBr pellet): 3415, 3298, 3159, 3049, 1645, 1547, 1494, 1432, 1289, 1042, 900, 776, 608 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 7.03 (s, 2 H, NH₂), 9.22 (s, 2 H, NH₂), 9.68 (s, 1 H, CH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 115.0, 123.7, 134.0, 143.9, 149.7, 169.0. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₆H₆N₇O₄: 240.0476; found: 240.0477. Anal. Calcd for C₆H₅N₇O₄ (240.16): C, 30.01; H, 2.52; N, 40.83. Found: C, 30.44; H, 2.16; N, 37.77.N²,6,8-Trinitro[1,2,4]triazolo[1,5-a]pyridine-2,7-diamine (9) 8 (478 mg, 2 mmol) was added in portions to a mixture of concd H₂SO₄ (4 mL) and fuming HNO₃ (4 mL) at 0 °C. The mixture was stirred for 24 h at 0 °C, then poured onto ice. The resulting precipitate was collected by filtration, washed successively with H₂O and EtOH, and dried to give a yellow solid; yield: 528 mg (93%).IR (KBr pellet): 3429, 3312, 1674, 1625, 1537, 1445, 1291, 1252, 899, 775, 646 cm^–1. ¹H NMR (400 MHz, DMSO-d₆): δ = 9.28 (s, 2 H, NH₂), 10.19 (s, 1 H, CH). ¹³C NMR (100 MHz, DMSO-d₆): δ = 116.5, 128.4, 136.5, 144.1, 148.5, 159.7. HRMS (ESI^–): m/z [M – H]⁺ calcd for C₆H₃N₈O₆: 283.0181; found: 283.0181. Anal. Calcd for C₆H₄N₈O₆ (284.15): C, 25.36; H, 1.42; N, 39.44. Found: C, 25.39; H, 2.65; N, 35.38.

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