Balancing Excellent Performance and High Thermal Stability in a

Communication pubs.acs.org/JACS

Balancing Excellent Performance and High Thermal Stability in a Dinitropyrazole Fused 1,2,3,4-Tetrazine Yongxing Tang, Dheeraj Kumar, and Jean’ne M. Shreeve* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States S Supporting Information *

application. Now we report the synthesis and characterization of a non-hydrogen-containing 5/6/5 fused ring energetic, 1,2,9,10-tetranitrodipyrazolo[1,5-d:5′,1′-f][1,2,3,4]tetrazine (6). In the molecular organization, two dinitropyrazole rings are fused to 1,2,3,4-tetrazine with concomitant six catenated nitrogen atoms. Such a molecule shows obvious advantages in thermal stability and excellent energetic performance. We initiated our studies by first synthesizing 4,4′,5,5′tetranitro-2H,2′H-3,3′-bipyrazole (4), which has been reported previously.13 However, no details were given. Bipyrazole (1) was synthesized according to the literature.13 Further nitration of 1 with 100% nitric acid in acetic anhydride gave the di-Nnitro substituted product (2). By heating 2 in benzonitrile at 140 °C, a thermal rearrangement proceeded smoothly to provide the di-C-nitro substituted product (3), which could be converted to 4,4′,5,5′-tetranitro-2H,2′H-3,3′-bipyrazole (4) by nitration of 3 with mixed acid at 100 °C. With 4 in hand, we next focused on the corresponding N-amination reaction using O-p-toluenesulfonylhydroxylamine (THA) and hydroxylamineO-sulfonic acid (HOSA). In both cases, the di-N-amino product (5) could be prepared with the yield from THA higher than that from HOSA. Finally, treatment of 5 with tert-butyl hypochlorite (t-BuOCl) gave the tricyclic fused ring (6), which could be isolated by column chromatography (Scheme 1). The structures of 5 and 6 were determined by standard analytical methods including X-ray crystallography. The 15N NMR spectra of 5 and 6 are shown in Figure 1. Compound 5 possess five well resolved resonances (δ −29.0 (N1), −29.6 (N2), −75.0 (N3), −167.0 (N4), −293.7 (N5) ppm). The signals of two C-nitro groups (N1 and N2) are close to each other. The signal of the amino group (N5) was observed at the highest field as expected. In case of 6, it has four well resolved resonances, the signal for the azo bond (N5, δ = −22.6 ppm) was observed at lowest field. In comparison, the nitrogen atoms N1, N2 and N3 are shifted to higher field while N4 is shifted to lower field. Compound 5 crystallizes in the monoclinic space group P21/ c with four molecular moieties in the unit cell. The calculated density is 1.811 g cm−3 at 173 K. The molecular structure is depicted in Figure 2. The angle between the two pyrazole rings is 49.41 °. The two amino groups are perpendicular to their linked pyrazole ring, which is characteristic for the molecular structure of N-amino substituted compounds. The bond lengths of N2−N5 and N4−N6 are 1.399(2) and 1.395(2) Å, respectively, while that of C3−C4 is 1.455(3) Å.

ABSTRACT: The key to successfully designing highperformance and insensitive energetic compounds for practical applications is through adjusting the molecular organization including both fuel and oxidizer. Now a superior hydrogen-free 5/6/5 fused ring energetic material, 1,2,9,10-tetranitrodipyrazolo[1,5-d:5′,1′-f][1,2,3,4]tetrazine (6) obtained from 4,4′,5,5′-tetranitro-2H,2′H3,3′-bipyrazole (4) by N-amination and N-azo coupling reactions is described. The structures of 5 and 6 were confirmed by single crystal X-ray diffraction measurements. Compound 6 has a remarkable room temperature experimental density of 1.955 g cm−3 and shows excellent detonation performance. In addition, it has a high decomposition temperature of 233 °C. These fascinating properties, which are comparable to those of CL-20, make it very attractive in high performance applications.

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he rich and fascinating chemistry of energetic materials (comprising of propellants and explosives) has placed emphasis on the presence of both fuel and oxidizer for high performance.1 Energetic material scientists have synthesized many explosives having diverse backbones (used as fuel), such as aliphatic,2 aromatic,3 strain-caged4 and nitrogen-rich skeletons,5 having covalently or ionically attached oxidizer groups (−NO2, −ONO2, NO3−, ClO4−, etc.). Nitrogen-rich energetic compounds with high positive heats of formation, high density, good thermal stability and green decomposition products are among the most promising candidates as green energetic materials. However, compounds having both high energetic performance and good thermal/mechanical stability are still rarely observed. Recently, numerous efforts by introducing polynitro functionalities into a nitrogen-rich heterocyclic backbone have led to a series of high-nitrogen energetic compounds,6 energetic salts,7 MOFs8 and cocrystals9 with good balance between physical and energetic properties. Fused compounds, a unique class of large conjugate structures, have emerged as a prime contender to traditional nitrogen rich monoring or poly ring materials.10 On the other hand, compounds containing catenated nitrogen chains also have drawn the attention of energetic scientists since they have high heats of formation.11 However, the low bond energy of the N−N bond in the nonaromatic nitrogen chain leads to low thermal stability. Although a tricyclic fused 1,2,3,4-tetrazine system containing six catanated nitrogen atoms in the conjugated structure has been reported,12 the decomposition temperature is only 138 °C, which limits its practical © 2017 American Chemical Society

Received: August 17, 2017 Published: September 14, 2017 13684

DOI: 10.1021/jacs.7b08789 J. Am. Chem. Soc. 2017, 139, 13684−13687

Communication

Journal of the American Chemical Society Scheme 1. Synthesis of 6

Figure 3. (a) A view of the molecular unit of 6. (b) Unit cell view along the a axis.

(torsion angle: C(4)−C(5)−N(9)−O(5), −20.3(5)°; N(6)− C(6)−N(10)−O(7), 148.6(4)°; O(3)−N(8)−C(2)−C(3), −99.5(5)°; N(1)−C(1)−N(7)−O(1), 15.0(5)°). The bond length (1.472(5) Å) of C(6)−N(10) is the longest one in the four C−NO2 bonds. In addition, since the tricyclic fused ring is formed, the bond lengths of C3−C4, N2−N3 and N4−N5 are shorter than those in 5. The packing structure of 6 is shown as a herringbone pattern view along a axis (Figure 3b). The physicochemical and energetic properties of 5 and 6 are given in Table 1. The thermal stabilities were determined with differential scanning calorimetry (DSC) at a scan rate of 5 °C min−1 (Supporting Information). Both 5 and 6 melt prior to decomposition. Their melting points are 209 and 205 °C, respectively. Compound 5 has a decomposition temperature of 252 °C while 6 decomposes at 233 °C, which is the most thermally stable among 1,2,3,4-tetrazine compounds to the best

Figure 1. 15N NMR spectra of 5 and 6.

Table 1. Physical and Detonation Properties of the Newly Prepared Energetic Materials 5 and 6 Compared with RDX, HMX and CL-20

5 6 RDXh HMXh CL-20h

Figure 2. A view of the molecular unit of 5.

The tricyclic ring compound (6) crystallizes in the orthorhombic space group Pca21 with a crystal density of 2.010 g cm−3 (173 K) and four molecules in the unit cell. The molecular structure of 6 is shown in Figure 3a. As expected, the two pyrazole rings and the 1,2,3,4-tetrazine ring are coplanar. However, the nitro groups are rotated due to steric effects

ρa (g cm−3)

νDb (m s−1)

Pc (GPa)

ΔHfd (kJ g−1)

Tde (°C)

ISf (J)

FSg (N)

1.760 1.955 1.800 1.905 2.038

8504 9631 8795 9144 9706

31.0 44.0 34.9 39.2 45.2

1.33 2.23 0.32 0.25 0.91

252 233 204 275 195

30 10 7.4 7.4 4

360 240 120 120 48

a Density, measured with a gas pycnometer (25 °C). bDetonation velocity calculated with EXPLO5 v6.01. cDetonation pressure calculated with EXPLO5 v6.01. dHeat of formation. eDecomposition temperature (onset temperature at a heating rate of 5 °C min−1). f Impact sensitivity. gFriction sensitivity. hRef 14.

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DOI: 10.1021/jacs.7b08789 J. Am. Chem. Soc. 2017, 139, 13684−13687

Communication

Journal of the American Chemical Society of our knowledge.10 The high thermal stability may be the result of the conjugation of the fused backbone and the nature of the pyrazole ring. Both of them have high heats of formation. The experimental densities were measured with a gas pycnometer. It is worth to note that the experimental density (25 °C) of 6 is 1.955 g cm−3 and crystal density (−173 °C) is 2.010 g cm−3, which is very competitive for nitrogen-rich energetic compounds. The calculated detonation properties are also very fascinating. Compound 5 shows a little lower detonation performance (detonation velocity: 8504 m s−1; detonation pressure: 31.0 GPa) than RDX. The detonation performance (detonation velocity: 9631 m s−1; detonation pressure: 44.0 GPa) of 6 is superior to those of RDX and HMX, and close to CL-20. Meanwhile, in comparison to CL20, 6 is more stable toward impact and friction. In summary, 4,4′,5,5′-tetranitro-2H,2′H-3,3′-bipyrazole (4) was prepared and the detailed synthetic route was given. The N-amination reaction of 4 gave 5. tert-Butyl hypochlorite (tBuOCl) was then used to generate the N-azo 5/6/5 tricyclic fused product 6. Both of 5 and 6 were fully characterized including single crystal X-ray diffraction analysis. They have very attractive energetic properties especially for 6. It has a high density of 1.955 g cm−3 and a good thermal stability (233 °C). Calculated detonation performance for 6 is comparable to CL20, much better than those of RDX and HMX. In addition, the impact and friction sensitivities are less than those of CL-20. These excellent properties indicate 6 is a quite superior energetic explosive. Additionally, the interesting molecular structure of 6 shows a potential design concept for thermally stable and high performance energetic compounds, which could combine a polynitrogen catenated chain in the fused ring as the fuel with nitro groups as the oxidizer.

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Synthesis of 6. Compound 5 (1.03 g, 3.0 mmol) was dissolved in acetonitrile (15 mL) and the solution was cooled to 0 °C using an ice−water bath. Then the solution was treated dropwise with tert-butyl hypochlorite (0.65 g, 6.0 mmol). The reaction mixture was stirred for 30 min and the reaction poured into ethyl acetate (50 mL). The solution was washed with water (5 × 7 mL) and the organic layer dried with sodium sulfate, filtered, and concentrated. Compound 6 (0.46 g, 45%) was isolated as colorless crystals by column chromatography (ethyl acetate:hexane = 1:2). 6: Colorless crystals. Tm 205 °C. Td (onset) 233 °C. 13C NMR (CD3CN) δ 148.9, 122.8, 121.1 ppm. 15N NMR (CD3CN) δ − 22.6 (N5), −33.7 (N1), −33.8 (N2), −89.6 (N3), −143.1 (N4) ppm. IR (KBr) ν̃ = 1701, 1648, 1560, 1457, 1402, 1350, 1331, 1271, 1244, 1211, 1162, 1059, 1047, 967, 949, 891, 867, 826, 806, 765, 755, 719, 620, 551 cm−1. Elemental analysis for C6N8O10 (340.13) Calcd: C 21.19, H 0.00, N 41.18%. Found: C 21.20, H 0.17, N 40.98%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08789. Synthesis, calculation detail, crystal refinements, NMR spectra, DSC curves (PDF) X-ray crystallographic file for 5 (CIF) X-ray crystallographic file for 6 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jean’ne M. Shreeve: 0000-0001-8622-4897 Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (N00014-16-1-2089) and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). We are also grateful to the M. J. Murdock Charitable Trust, Reference No.: 2014120: MNL:11/20/2014 for funds supporting the purchase of a 500 MHz NMR spectrometer. The authors also acknowledge Dr. Richard J. Staples for considerable assistance with crystal structuring.

Caution! Although we have encountered no difficulties during preparation and handling of these compounds, they are potentially explosive energetic materials that may be sensitive to impact and friction. Mechanical actions of these energetic materials, involving scratching or scraping, must be avoided. Manipulations must be carried out by using appropriate standard safety precautions. Synthesis of 5. Method A. A mixture of 4,4′,5,5′-tetranitro2H,2′H-3,3′-bipyrazole (4, 1.33 g, 4.0 mmol) and 1,8-diazabicycloundec-7-ene (DBU) (2.43 g, 8 mmol) in acetonitrile (30 mL) was stirred for 30 min at room temperature. This was followed by the addition of freshly prepared O-p-toluenesulfonylhydroxylamine (2.4 equiv) in dichloromethane (100 mL) in one portion. After stirring for 3 h at room temperature, the solvent was removed by air and the residue was added to water (100 mL) to generate the crude product, which could be recrystallized in water/ethanol to give the pure product (5, 0.91 g, 66%) as a white solid. Method B. Compound 4 (0.33 g, 1.0 mmol) was added to a solution of sodium hydroxide (0.72 g, 18 mmol) and potassium dihydrogen phosphate (2.3 g, 17 mmol) in water (17 mL). The reaction mixture was heated to 50 °C and hydroxylamine-O-sulfonic acid (0.45 g, 4.0 mmol) was added. The mixture was further heated to 60 °C and stirred for 6 h at this temperature. After cooling to room temperature, the precipitate was collected by filtration and washed with water to give 5 (0.12 g, 35%). 5: White solid. Tm 209 °C. Td (onset) 252 °C. 1H NMR (CD3CN) 6.21 (s, 4H) ppm. 13C NMR (CD3CN) δ 145.9, 127.9, 126.6 ppm. 15N NMR (CD3CN) δ − 29.0 (N1), −29.6 (N2), −75.0 (N3), −167.0 (N4), −293.7 (N5) ppm. IR (KBr) ν̃ = 3340, 3224, 1266, 1539, 1496, 1466, 1041, 1355, 1337, 1324, 1116, 1041, 944, 874, 822, 808, 772, 756, 736 cm−1. Elemental analysis for C6H4N10O8 (344.16) Calcd: C 20.94, H 1.17, N 40.70%. Found: C 20.79, H 1.20, N 40.56%.

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DOI: 10.1021/jacs.7b08789 J. Am. Chem. Soc. 2017, 139, 13684−13687