3D-Cube Layer Stacking: A Promising Strategy for High-Performance

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3D-cube layer stacking: A promising strategy for high-performance insensitive energetic materials Qi Sun, Cheng Shen, Xin Li, Qiuhan Lin, and Ming Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01264 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Crystal Growth & Design

3D-cube layer stacking: A promising strategy for high-performance insensitive energetic materials Qi Sun, Cheng Shen, Xin Li, Qiuhan Lin*, and Ming Lu* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.

ABSTRACT: It is desirable to consider the molecular design and crystal configuration in the research of energetic materials. We discovered an interesting layer stacking crystal configuration, that is different from previous 2D-plane layer stacking, and termed it as “3D-cube layer stacking”. This new configuration, resulting from the unusual U-shaped molecular structures and vast H-bonding interactions, breaks through the limitations of the planar molecular structures in 2D-plane layer stacking. Compound 4, which features such characteristics, exhibits excellent energetic performance (D: 9043 m s-1; P: 35.6 GPa) and acceptable sensitivities (IS: 16 J; FS: 180 N). These positive results indicate that 3D-cube layer stacking may open new avenues for the design of energetic materials.

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Introduction The extensive use of explosives in military and civilian fields promotes their development and innovation. The preparation of new advanced energetic materials is of great significance in the materials field.1-4 Nowadays, materials with high energy density are designed not only in pursuit of excellent detonation performance, but also for low sensitivity, good thermostability, and low environmental impact. In the design of energetic molecules, combining different nitrogen-containing heterocyclic rings into one molecule is a promising strategy for developing advanced energetic materials.5,6 1,2,5-oxadiazole (furazan) and tetrazole are the most considered five-membered rings. Furazan has a high content of nitrogen and oxygen. In addition, colorful energetic groups such as nitro, nitramino, and dinitromethyl groups, can be added to the rings to increase the detonation performance. Tetrazole has a nitrogen content of 80%, which can impart compounds with a good heat of formation. Several complexes consisting of furazan and tetrazole rings have been reported.7,8 However, simple molecular designs cannot satisfy the increasing demand for new advanced energetic materials because of the unsatisfactory balance between high detonation properties and good mechanical sensitivity. To achieve this harmony, crystal configurations and weak interactions must be considered. Layer stacking is a special crystal configuration style, and hydrogen-bonding interaction is one of the representative weak interactions.9-11 Based on the crystal data of 5-(5-amino-2H-1,2,3triazol-4-yl)-1H-tetrazole12 (Figure 1), extensive intermolecular hydrogen bonds connect numerous molecules to form layers, which then assemble to form the entire packing system. This layer-by-layer assembly of energetic molecules is an effective method that maintains an optimal


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balance between performance and stability. Many representative energetic materials, such as 2,4,6-triamino-1,3,5-trinitrobenzene (TATB) and 1,1-diamino-2,2-dinitroethene (FOX-7), also exhibit such a layered crystal configuration.13,14 The key feature of this packing style is its planar molecular structure. However, most energetic molecules are not planar,15-18 which greatly limits the applicability of layer stacking and the development of advanced energetic materials. Therefore, a way to break through this limitation is highly sought. Herein, we report the synthesis, structures, and energetic performance of N,N'-methylenebis(N-(4-(2H-tetrazol-5-yl)1,2,5-oxadiazol-3-yl)nitramide) (4) and 4,4'-(methylenebis(nitroazanediyl))bis(1,2,5-oxadiazole3-carboxylic acid) (5), which exhibit the unusual U-shaped molecular structures and unique 3Dcube layer stacking.

Figure 1. Comparison of 2D-plane and 3D-cube layer stacking.


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Synthesis

Scheme 1. Synthesis paths to 4 and 5.

The synthesis routes to 4 and 5 are shown in Scheme 1. The methylene-linked compound 2 was obtained by the reaction of 3-amino-4-cyanofurazan19 (1) with 37% aqueous formaldehyde solution under acidic condition. N,N'-methylenebis(N-(4-cyano-1,2,5-oxadiazol-3-yl)nitramide) (3) was prepared by the nitration of 2 with a mixture of 100% nitric acid and acetic anhydride. Using zinc bromide as a catalyst, 3 was treated with sodium azide in a mixture of CH3OH and H2O to form 4. Unexpectedly, the tetrazole rings were converted to the carboxy groups of 5, when 4 was treated with 100% nitric acid and trifluoroacetic anhydride. Such a reaction has rarely been reported and may provide a new method for preparing heterocyclic carboxylic acid compounds.


4

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Structure discussions Compounds 3, 4 and 5 were further examined through single-crystal X-ray diffraction to find their potential relationship between molecular structures and configuration packing styles. Compound 3 crystallizes in the monoclinic space group P21/c, whereas compounds 4 and 5 both crystallize in the triclinic space group P-1. Calculated densities of them at 173 K are 1.700, 1.858, and 1.805 g cm-3, respectively. Compounds 3, 4 and 5 all exhibit U-shaped molecular structures (Figure 2(a), (c) and (e), respectively) owing to the sp3 hybridization of the central carbon atoms (C1 in the structures). The bond angles of N1-C1-N2 in 3, 4, and 5 are 110.76°, 112.75° and 112.94°, respectively. Compared with 3, the formation of a tetrazole ring and carboxy group results in an increase in the bond angle.


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Figure 2. (a), (c) and (e) Molecular structures of 3, 4 and 5. (b) Packing diagram of 3. (d) and (f) 3D-cube layer stacking of 4 and 5 viewed down the b axis.


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Figure 3. (a) and (b) Hydrogen bonds for 4, as seen from different directions (c) Hydrogen bonds for 4 between two molecules, two unit cells, and two layers.

The crystal packing diagrams of 4 and 5, shown in Figure 2(d) and 2(f), reveal three 3D-cube layers and no 2D-plane layers. As shown in Figure 3, the packing mechanism of 4 can be described as follows: two molecules assemble face to face to form a unit cell, then several unit cells arrange side by side to form a layer, and finally the layers stack into a stratiform structure to form the entire packing system. The U-shaped molecular structure is the primary feature resulting in such a novel configuration, and it overcomes the limitations of planar molecular structures in 2D-plane layer stacking. The closed 3D-cube system is held together by the intermolecular hydrogen bonds between the molecules, unit cells, and layers (the sky-blue lines in Figure 3 represent hydrogen bonds and detailed information is provided in ESI). The difference in packing styles between 4 and 5 is that the unit cell in 5 includes four crystal water molecules. These water molecules are in the middle of the two U-shaped molecules and act as linkages to form the unit cell. It is notable that the hydrogen bond concentrations between the


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layers in 4 and 5 is much higher than that between previously reported 2D-plane layers,20 which contributes to higher density, better detonation performance, and lower sensitivity. Although 3 exhibits the U-shaped molecular structure, it does not undergo the layer stacking due to its lack of hydrogen-bonding interactions. Energetic properties

Table 1 Physiochemical and energetic properties of 3, 4 and 5 compared with TNT and RDX. Compd

Tda[°C]

ISb[J]

FSc[N]

∆Hfd[kJ mol-1/ kJ g-1]

de[g cm-3]

Df[m s-1]

Pg[GPa]

3

132

7

100

981.6/3.0

1.67

8291

28.0

4

154

16

180

1426.3/3.5

1.83

9043

35.6

5

115

36

300

-188.3/-0.48

1.77

8307

30.7

TNTh

295

15

353

-59.4/-0.26

1.65

6881

19.5

RDXh

230

7.4

120

92.6/0.42

1.80

8795

34.9

a

Thermal decomposition temperature (DSC under N2, onset temperature 5 °C min-1). b Impact

sensitivity. c Friction sensitivity. d Heat of formation (Gaussian 0921). e Crystal density at 298 K. ρ298K = ρT/(1+αv(298-T)); αv = 1.5×10-4 K-1.

f

Detonation velocity (EXPLO 522).

g

Detonation

pressure. h Ref 23.


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Figure 4. DSC plots for 3, 4 and 5.

To investigate whether 3D-cube layer stacking could improve the properties of energetic compounds, thermal stabilities, sensitivities, and detonation performances of 3, 4 and 5 were determined (Table 1). 4 demonstrates a decomposition temperature of 154 °C, which is 22 °C more than that of 3 due to extensive intermolecular hydrogen-bonding interactions. 5 decomposes at 115 °C for the first time, and undergos a violent decomposition at 140 °C (Figure 4). According to the mechanical sensitivity test24, 5 is the most insensitive because of the carboxyl groups and crystal waters. In consideration of molecular structures of 3 and 4, 4 should be more sensitive than 3 due to the formation of tetrazole rings with a high nitrogen content. However, it is surprising that the test results show 4 is much less sensitive than 3. Such an interesting result indicates that hydrogen-bonding interactions in 3D-cube layer stacking play an important role in decreasing the sensitivity. 3 and 4 both exhibit positive heats of formation (∆Hf


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= 3.0 and 3.5 kJ g-1, respectively); the introduction of tetrazole rings increases ∆Hf by 0.5 kJ g-1, which potentially leads to better detonation performance. The carboxy groups and crystal waters lead to the negative heat of formation of 5.

the same linkage O NO2 N N N

O

O 2N N N N N O N

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N O2N N N

H2N N H

O

N

O

N

O

N

NO2 N N

N

NH O2N N

the same heterocyclic rings

N 7

O

N

d: 1.62 g cm-3 D: 8135 m s-1 P: 26.8 GPa

H N N N N

N

H N

-3 N d: 1.75 g cm -1 N D: 8477 m s P: 28.0 GPa

N N

O N

d: 1.77 g cm-3 D: 8182 m s-1 P: 27.7 GPa

NO2 NH -3 NH2 d: 1.64 g cm N N D: 8126 m s-1 H P: 25.3 GPa N N N N O O 8

N H N

N N

O

N

N 9

O

N

N

N O N

O

N N N 10 O

H N

N d: 1.74 g cm-3 N D: 7977 m s-1 P: 26.5 GPa

Scheme 2. 4 shows better energetic performance than compounds with the same linkage or same heterocyclic rings.

Compared with 3, compounds 4 and 5 show increases of 0.16 and 0.10 g cm-3 in density, 752 and 18 m s-1 in detonation velocity, and 7.6 and 2.7 GPa in detonation pressure because of the formation of the tetrazole ring, carboxylic group and layer stacking. It is noteworthy that 4 possesses the highest density (d = 1.83 g cm-3) and best detonation performance (D = 9043 m s-1, P = 35.6 GPa) as compared with compounds containing the same bisnitramide-methylene linkage25 and heterocyclic rings26-28, this may have resulted from the molecular components and 3D-cube layer stacking (Scheme 2). 4 exhibits a much better detonation performance than 7, they differ in parts of the tetrazole rings and 1,2,4-oxadiazole rings in their molecular components.


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Tetrazole rings provide a higher heat of formation than the 1,2,4-oxadiazole rings do, and introducing tetrazole rings into molecules can effectively increase their nitrogen content.29,30 As compared with 9, the differences of density (∆d = 0.08 g cm-3) and detonation performance (∆D = 566 m s-1, ∆P = 7.6 GPa) arise from the differences of linkages and crystal configurations. Although the azo group can impart energetic compounds with high heats of formation, the bisnitramide-methylene linkage can not only provide two nitro groups, but also improves the oxygen balance of the compounds. Moreover, the absence of crystal layer stacking in 9 is the main reason for the differences in performance. The energetic evaluation also indicates that 4 exhibits a higher density, detonation velocity and pressure, and lower impact and friction sensitivities than those of the powerful explosive RDX. In addition, all of 3, 4 and 5 show better detonation performance than the traditional explosive TNT. Conclusion In summary, an interesting 3D-cube layer stacking was presented. As supported by X-ray data, 4 and 5 exhibit an unusual U-shaped molecular structure that overcomes the limitations of previously reported planar molecular structures having 2D-plane layer stacking. Extensive intermolecular hydrogen-bonding interactions promote the unique 3D-cube layer stacking. Compared to compounds with similar structures, 4 possesses the highest density and best detonation performance. The molecular components and 3D-cube layer stacking also accord a fine harmony between the high detonation performance and low sensitivity of 4. These encouraging results indicate that this unique 3D-cube layer stacking could be a promising strategy for the design of energetic materials. Experimental section


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Syntheses 4,4'-(methylenebis(azanediyl))bis(1,2,5-oxadiazole-3-carbonitrile) 2: Aqueous formaldehyde (37%, 0.81 g) and concentrated hydrochloric acid (0.4 ml) were added to a solution of 1 (1.10 g, 10 mmol) in 25 ml water at 60 °C. The reaction mixture was stirred at 80 °C for 0.5 h. The white precipitate was collected when still hot, washed with water and dried in air to obtain 2 (1.02 g, yield: 88%). Tm: 156 °C, Td: 237 °C. 1H NMR (DMSO-d6, 500 MHz): δ 8.58 (t, 2H, NH), 4.76 (t, 2H, CH2) ppm.

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C NMR (DMSO-d6, 500 MHz): δ 155.82 125.26 107.65 53.02 ppm. IR

(KBr): ν 3420 3374 2988 2261 1587 1534 1383 1361 1219 1117 1002 928 860 719 655 585 cm1

. Elemental analysis calcd for C7H4N8O2 (232.16): C 36.21, H 1.74, N 48.27 %; found: C 36.44,

H 1.63, N 48.61 %. N,N'-methylenebis(N-(4-cyano-1,2,5-oxadiazol-3-yl)nitramide) 3: 2 (0.23 g, 1 mmol) was added to a mixture of Ac2O (2 ml) and 100% HNO3 (1 ml) at 0 °C. With the temperature rise to 20 °C, the solution was poured into ice water. The white precipitate was filtered, washed with a mixture of water (50 ml) and ethanol (5 ml) to obtain 3 (0.24 g, 73% yield). Td: 132 °C. 1H NMR (DMSO-d6, 500 MHz): δ 6.76 (s, 2H, CH2) ppm.

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C NMR (DMSO-d6, 500 MHz): δ 151.06

131.19 106.37 64.80 ppm. IR (KBr): ν 1755 1598 1547 1423 1403 1281 1121 1080 1020 902 745 731 665 610 cm-1. Elemental analysis calcd for C7H2N10O6 (322.16): C 26.10, H 0.63, N 43.48 %; found: C 26.46, H 0.83, N 43.07 %. N,N'-methylenebis(N-(4-(2H-tetrazol-5-yl)-1,2,5-oxadiazol-3-yl)nitramide) 4: 3 (3.22 g, 10 mmol), ZnBr2 (2.25 g, 10 mmol) and NaN3 (1.63 g, 25 mmol) was added to a mixture of water (15 ml) and methanol (25 ml), and stirred at 40 °C for 2 h. It was then cooled and concentrated hydrochloric acid was added to adjust the pH to 2, the mixture was stirred for another 0.5 h.


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Finally, the mixture was concentrated in vacuo, methanol was removed and the precipitate was collected, washed with hydrochloric acid (2 %) and water to obtain 4 (2.49 g, 69% yield). Td: 154 °C. 1H NMR (DMSO-d6, 500 MHz): δ 6.65 (s, 2H, CH2), 4.98 (2H, NH) ppm.

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C NMR

(DMSO-d6, 500 MHz): δ 150.35 148.97 144.23 66.60 ppm. IR (KBr): 3376 2988 1585 1405 1276 1190 1078 1023 1001 881 750 730 642 608 cm-1. Elemental analysis calcd for C7H4N16O6 (408.22): C 20.60, H 0.99, N 54.90 %; found: C 20.96, H 0.87, N 54.67 %. 4,4'-(methylenebis(nitroazanediyl))bis(1,2,5-oxadiazole-3-carboxylic acid) 5: 4 (0.408 g, 1 mmol) was suspended in (CF3CO)2O (2 ml) at -5 °C, 95% HNO3 (1 ml) was added to the mixture slowly. The temperature was slowly warmed to 25 °C for 2 h. The white precipitate was filtered, washed with (CF3CO)2O to obtain 5 (0.30 g, 83.3% yield). Td: 115 °C. 1H NMR (DMSO-d6, 500 MHz): δ 6.89 (s, 2H, COOH), 6.59 (s, 2H, CH2) ppm.

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C NMR (DMSO-d6,

500 MHz): δ 160.83 146.96 120.60 64.11 ppm. IR (KBr): 3272 2447 1728 1581 1403 1280 1210 1115 1084 1013 902 749 702 cm-1. Elemental analysis calcd for C7H4N8O10 (360.16): C 23.34, H 1.12, N 31.11 %; found: C 23.48, H 1.07, N 30.94 %.


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ASSOCIATED CONTENT CCDC 1569238-1569240 Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx General analysis methods and safety precautions. Details of X-ray crystallography, including additional figures, crystallographic data, and bond lengths and angles data for 3, 4 and 5. Details of hydrogen-bonding interactions for 3, 4 and 5. Theoretical calculations, including calculation methods, isodesmic reactions, and enthalpies of the gas-phase species. IR and

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C spectra for

target compounds.

AUTHOR INFORMATION Corresponding Author *Qiuhan Lin, Email: [email protected]. ORCID: 0000-0002-8909-5434. *Ming Lu, Email: [email protected]. ORCID: 0000-0003-3007-7773. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC, No. 51374131 and NSAF, No. U1530101). The authors greatly thank Dr Zaichao Zhang for his help of crystal structures.


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For Table of Contents Use Only 3D-cube layer stacking: A promising strategy for high-performance insensitive energetic materials Qi Sun, Cheng Shen, Xin Li, Qiuhan Lin*, and Ming Lu*

3D-cube layer stacking, which resulted from the unusual U-shaped molecular structure and extensive hydrogen bonds, was discovered. Compared with compounds containing similar structures, the new prepared compound 4 possesses the highest density and best detonation performance.


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3D-Cube Layer Stacking: A Promising Strategy for High-Performance

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