Design and Synthesis of a Series of CL-20 Cocrystals: Six-membered

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Design and Synthesis of a Series of CL-20 Cocrystals: Six-membered Symmetrical N-Heterocyclic Compounds as Effective Coformers Teng Fei, Penghao Lv, Yujia Liu, Chunlin He, Chenghui Sun, and Siping Pang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01923 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Crystal Growth & Design

Design and Synthesis of a Series of CL-20 Cocrystals: Six-membered Symmetrical N-Heterocyclic Compounds as Effective Coformers Teng Fei, † Penghao Lv, † Yujia Liu, † Chunlin He, †,‡ Chenghui Sun *†‡ and Siping Pang *†‡

†

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 10081, China.

‡ Key

Laboratory for Ministry of Education of High Energy Density Materials, Beijing Institute of

Technology, Beijing, 100081, China.

ABSTRACT. Cocrystallization of energetic compounds has considered as an effective method to adjust the properties through the selection of coformers in recent, particularly to improve the detonation properties and reduce the mechanical sensitivity of CL-20. In this research, a reliable and promising design strategy to synthesize CL-20-based cocrystals was presented, where i) the coformer should shows local structural similarity to CL-20 i.e., five- or six-membered symmetric heterocycles; ii) the molecular structure of coformer should contain electron donating groups. In order to verify the adaptability of this design strategy, pyrazine, 2,6-dimethoxy-3,5-dinitropyrazine, and 2,5dimethylpyrazine were selected as the templates of six-membered symmetrical N-heterocyclic compounds for cocrystallization with CL-20. Fortunately, based on the results of the experiments and the crystal analysis, three compounds were successfully cocrystallized with CL-20. Moreover, two cocrystals with the same components and different stoichiometric ratios were formed by pyrazine and CL-20. In addition, properties of the designed cocrystals, including thermal stability, detonation 1 ACS Paragon Plus Environment

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properties and sensitivities were also studied in detail. Cocrystallization of CL-20 with the abovementioned coformers demonstrates the appropriateness of six-membered symmetrical N-heterocyclic compounds as coformers. In light of this finding, the safety and detonation properties of CL-20 can be modulated by employing suitable six-membered N-heterocyclic compounds as coformers.

INTRODUCTION

The contradictory relationship between high energy and low sensitivity poses a challenge in the research of energetic materials. The possibility of resolving this contradiction effectively is directly related to the development trend of these materials.1-2 To resolve this issue effectively, a number of approaches have been attempted in recent years, such as designing a reasonable energetic formula, synthesizing new insensitive compounds, 3-5 or using cocrystallization techniques.6-8 Among these, the cocrystallization technique has been accepted as an effective and mainstream method to develop highenergy insensitive energetic compounds. The formation of an energetic cocrystal not only improves the properties and overcomes the disadvantages of existing energetic materials, but also helps in obtaining unexpected benefits.9-10 Currently, certain amounts of energetic cocrystals have been obtained, which have created the potential to tune properties with energetic materials. Among energetic cocrystals, CL-20-based cocrystals are most intensively researched. Moreover, CL-2011 is currently the most powerful energetic compound. However, its military, industry, and civil applications have been limited by its relatively high sensitivity to external stimuli for a long time. To overcome this shortcoming, some novel cocrystals9,

12-16

or solvates10,

17

based on CL-20 were

synthesized, and relevant adjustments were made to the properties of CL-20. From the perspective of 2 ACS Paragon Plus Environment

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molecular structure, the rigid cage structure of CL-20 can be regarded to consist of three Nheterocycles by the densification of C–C or C–N bonds.18 Moreover, CL-20 is advantageous for cocrystals owing to its high flexibility by the free rotation of its six NO2 groups.19 Until today, certain amounts of CL-20-based cocrystals have been obtained, thereby enabling the development of energetic materials. However, the preparation of CL-20-based energetic cocrystals does not follow any rules. This is largely attributed to the lack of predictability in the synthesis of CL-20-based cocrystals, such as in the choice of coformer.20 Presently, the progress of research on novel CL-20-based cocrystals has been very slow, probably owing to the primary reliance of energetic cocrystals on scientific intuition and trial-and-error. Therefore, research on the reasonable choices of coformers of CL-20-based cocrystals can resolve this problem. Based on the above-mentioned consideration, we attempt to introduce a new design strategy for CL-20-based cocrystals, where i) the coformers have local structural similarity with CL-20, i.e., five or six-membered symmetric heterocycles and ii) the molecular structure of the conformer contains electron donating groups. We believe that this strategy can promote the development of CL-20-based cocrystals and reduce the sensitivity of high energy density compounds. Based on our interest in finding new energetic cocrystals, especially CL-20-based cocrystals, we explored the cocrystallization of CL-20 with pyrazine and its derivatives as coformers. We conducted cocrystallization trials with three types of six-membered symmetrical N-heterocyclic coformers (Figure 1). The crystal structures, physicochemical and detonation properties, and intermolecular interactions of these cocrystals were investigated. The crystal structures were significant in developing novel CL-20-based cocrystals and provided an insight into the design of N-heterocyclic energetic cocrystals. 3 ACS Paragon Plus Environment


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Figure 1. Cocrystal 1-1: 2:3 CL-20/pyrazine cocrystal; Cocrystal 1-2: 1:2 CL-20/pyrazine cocrystal; Cocrystal 2: 1:2 CL-20/2,6-dimethoxy-3,5-dinitropyrazine cocrystal; Cocrystal 3: 2:3 CL-20/2,5-dimethylpyrazine cocrystal.

RESULTS AND DISCUSSION.

The emphasis and difficulty of the cocrystallization technique lie in still the unable inability to effectively predict effectively the intermolecular interactions between two potential cocrystallization partners at the theoretical level. Moreover, the strength of hydrogen bonding as well as and the molecular geometric matching ability between two components also play a significant role that can’t be ignored in the formation of a cocrystal. According to Matzger et al.’s work,

7, 21-22

most of the

maximum ESP sites on the molecular surfaces were determined to be the sets of hydrogen bond acceptor and hydrogen bond donor sites, describing the potential intermolecular interaction sites. To the best of our knowledge, the p-benzoquinone (maximum ESP: −29.1 kcal mol−1) naphthoquinone (maximum ESP: −31.1 kcal mol−1)

20

20

and 1,4-

are known to form cocrystals with CL-20 by 4

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hydrogen bonding and other intermolecular interactions. Although these two coformers are not belong to N-heterocyclic compounds, they have the aromatic structure of a six-membered ring. Therefore, using the calculated maximum ESP of p-benzoquinone and 1,4-naphthoquinone as reference value, we calculated the maximum ESP of pyrazine derivatives in this study to examine whether they have the their abilities ability to form cocrystals with CL-20. The coformers selected for in this study (pyrazine, 2,6-dimethoxy-3,5-dinitropyrazine, and 2,5-dimethylpyrazine) have a similar maximum ESP values (−30.43, −41.71, and −31.20 kcal mol−1, respectively) with that of p-benzoquinone and 1,4naphthoquinone (Figure 2), suggesting that they may interact with CL-20 through hydrogen bonding interactions. However, because of the complexity of the cocrystallization, these estimates should be regarded as suggestive rather than conclusive results.

Figure 2. ESP of the coformers pyrazine, 2,6-dimethoxy-3,5-dinitropyrazine and 2,5-dimethylpyrazine mapped onto the moxlecular surfaces of ρ = 0.001 au and minimum ESP (unit in kcal mol-1).

In this study, the cocrystallization experiments with CL-20 and the above three coformers were successful. The four novel cocrystals of CL-20 with pyrazine presented in this study are (1-1 and 1-2) two variations of stoichiometric cocrystals, (2) 2,6-dimethoxy-3,5-dinitropyrazine, and (3) 2,55 ACS Paragon Plus Environment


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dimethylpyrazine. Cocrystals 1-1, 1-2, 2, and 3 are composed of CL-20 with pyrazine in the ratio 2:3, with pyrazine in the ratio 1:2, with 2,6-dimethoxy-3,5-dinitropyrazine in the ratio 1:2, and with 2,5dimethylpyrazine in the ratio 2:3, respectively. These cocrystals were formed from their saturated organic solutions (the solvents were dried over molecular sieve before using; refer Supporting Information SI 1). The crystal structures of the four CL-20-based cocrystals were ascertained by powder X-ray diffraction (PXRD) patterns (refer Supporting Information SI 2), which are evidently different from those of pure CL-20, indicating the formation of new crystalline phases. The main crystallographic information of the four cocrystals is listed in Table 1. Cocrystals 1-1, 1-2, 2 and 3 have densities of 1.723, 1.732, 1.800, and 1.632 g cm−3, respectively, at room temperature (296 K), which are lower than that of pure CL-20. Table 1. Main Crystallographic Information for CL-20 Cocrystals in This Study. cocrystal

CL-20 / 1-1

CL-20 / 1-2

CL-20 / 2

CL-20 / 3

formula

C12H12N15O12

C14H14N16O12

C18H18N20O24

C15H18N15O12

MW, g mol-1

558.37

598.41

898.52

600.44

stoichiometry

1:1.5

1:2

1:2

1:1.5

temperature, K

296 K

153 K

296 K

153 K

morphology

block

block

block

block

conformation of CL-20

γ

β

γ

β

color

colorless

colorless

colorless

colorless

space group

P-1

P21/c

C2/c

P21/n

a, Å

8.0801(3)

22.277(5)

15.939(4)

12.784(3)

b, Å

9.1554(3)

14.334(3)

17.350(6)

8.0826(16)

c, Å

14.9417(5)

15.125(3)

13.021(3)

23.207(5)

α, °

99.036(1)

90

90

90

β, °

91.673(1)

109.30(3)

112.976(9)

92.10(3)

γ, °

99.140(1)

90

90

90

V, Å3

1076.10(6)

4558.3(19)

3315.2(16)

2396.3(8)

Z

2

8

4

4

density, g cm-3

1.723

1.744

1.800

1.664

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Interestingly, two novel different stoichiometric cocrystals of CL-20 with pyrazine in the ratios of 2:3 forming triclinic 1-1 and 1:2 forming monoclinic 1-2 were obtained. Cocrystals 1-1 and 1-2 were formed initially using saturated ethyl acetate and ethyl acetate/methanol solutions, respectively, through the slow evaporation of the solvents. These findings indicate the role played by the nature of the solvent in product formation. The formation of different stoichiometric cocrystals not only depends on the mole ratio of the raw material but also on the polar aprotic and protic solvents used. The observations made are as follows: (i) cocrystal 1-1 does not convert into 1-2 by the further addition of methanol; (ii) cocrystal 2 cannot be formed only in the presence of protic solvents; and (iii) wet solvents do not lead to the formation of cocrystals 1-1 and 1-2. Although different stoichiometric cocrystals are the main issue in cocrystal engineering, Braga et al.23 suggested that cocrystals containing strong hydrogen bonded units could result in cocrystals with different stoichiometries. The solvates of CL-20/H2O2 also prove this statement.10 In this study, we suggest that the synergistic effect between hydrogen bonding and NO2−π interactions have similar effects. The formation of both 1-1 and 1-2 depends on the hydrogen bonding interactions between pyrazine and the NO2 groups of CL-20, and the NO2−π interactions between the NO2 groups in CL-20 and pyrazine. In cocrystals 1-1 and 1-2, CL-20 cocrystallizes with favorable γ and undesired β forms, respectively. There are no significant pyrazine-pyrazine interactions in 1-1 and 1-2. By analyzing the structure of the two cocrystals, it is evident that their formation mainly depends on two main types of intermolecular forces. As shown in Figure 2A and 3A, the first type of the intermolecular interactions is C−H⋯O hydrogen bonding interactions, which occur between the oxygen atoms in the NO2 groups of CL-20 and the hydrogen atoms in pyrazine. The hydrogen bond length in cocrystal 1-1 is 2.705 Å, 7 ACS Paragon Plus Environment


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while it ranges from 2.484 to 2.715 Å in cocrystal 1-2. The second type of motif is the NO2−π interactions at distances of 3.220 and 3.209 Å, which are formed by the NO2 groups of CL-20 and the heterocyclic rings in pyrazine. The formation and stabilization of cocrystals 1-2 and 1-2 not only depend on the directional hydrogen bonding and NO2−π interactions, but also on their molecular shapes and polarities.24 The packing styles of cocrystals 1-1 and 1-2 are sandwich and channel stacking, respectively (Figure 3B and 4B). Therefore, the above-mentioned two types of interactions associated with sandwich and channel stacking may contribute to the stability of cocrystals 1-1 and 1-2, thereby supporting the creation of low sensitivity energetic materials.25

Figure 3. The intermolecular interactions of the cocrystal 1-1. (A) The CH···O hydrogen bonding interaction (green lines) and NO2−π interaction (yellpw line) between CL-20 and pyrazine in 1-1. (B) The sandwich stacking type of cocrystal 1-1 which viewed down along the a axis, for clarity, pyrazine molecules were coloured red.


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Figure 4. The intermolecular interactions of the cocrystal 1-2. (A) The CH···O hydrogen bonding interaction (green lines) and NO2−π interaction (yellpw line) between CL-20 and pyrazine in 1-2. (B) The channel stacking type of cocrystal 1-2 which viewed down along the c axis, for clarity, pyrazine molecules were coloured green.

In cocrystal 2, CL-20 cocrystallizes with 2,6-dimethoxy-3,5-dinitropyrazine and exists in the γ form. With the same intermolecular interactions as in cocrystals 1-1 and 1-2, cocrystal 2 primarily depends on two motifs based on its structural analysis. The hydrogen bonding interactions occurred between the hydrogen atoms of 2,6-dimethoxy-3,5-dinitropyrazine rings and the oxygen atoms in the NO2 groups of CL-20 with bond lengths at 2.403 and 2.625 Å (Figure 5A). On the other hand, the NO2−π interactions, with distances ranging from 2.951 to 3.213 Å, were formed between the NO2 groups on CL-20 and the heterocyclic rings on pyrazine (Figure 5B). As shown in Figure 5C, a crystal cell of cocrystal 2 is constituted by two 2,6-dimethoxy-3,5-dinitropyrazine molecules and one CL-20 molecule, forming a repeating unit of the crystal structure. The crystal cells repeat in the crystal structure and extend endlessly to form the cocrystal.26-27 It was found that cocrystal


2

belongs to 9


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channel stacking according to the research on the packing types of CL-20-based cocrystals. In this type of stacking, 2,6-dimethoxy-3,5-dinitropyrazine molecules are homogeneously arranged as numerous channels, and the CL-20 molecules are evenly distributed in the channels. Therefore, the CL-20 molecules inserted in the channels are completely isolated from the 2,6-dimethoxy-3,5-dinitropyrazine molecules. Using this packing type, high sensitivity of CL-20 can be effectively reduced.

Figure 5. The intermolecular interactions of the cocrysta 2. (A) The CH···O hydrogen bonding interaction (green lines) between CL-20 and 2,6-dimethoxy-3,5-dinitropyrazine in cocrystal 2. (B) The NO2−π interaction (yelloe lines) between CL-20 and 2,6-dimethoxy-3,5-dinitropyrazine in cocrystal 2. (C) The channels stacking type of cocrystal 2 which viewed down along the c axis, for clarity, 2,6-dimethoxy-3,5-dinitropyrazine molecules were coloured purple.

With no NO2−π interactions available, the formation of cocrystal 3 primarily depends on a series of hydrogen bonding interactions between the oxygen atoms in NO2 groups and the hydrogen atoms in 2,5-dimethylpyrazine, and the hydrogen bonding interactions between the CL-20 molecules. Figure 6A shows the hydrogen bonding interaction between the NO2 groups of the CL-20 molecule and the hydrogen atoms on methyl moiety and the pyrazine ring of the 2,5-dimethylpyrazine molecule with 10 ACS Paragon Plus Environment

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distances ranging from 2.562 to 2.659 Å. With respect to the CL-20 intermolecular hydrogen bonding interactions, Figure 6B exhibits three kinds of hydrogen bonding interactions with distances ranging from 2.431 to 2.643 Å. However, according to the crystal analysis of cocrystal 3, NO2−π and π−π interactions between coformers were not found. In addition, its packing style is sandwich stacking,19 as viewed along the b axis, where the homogeneous molecules of any layer are pliantly arranged (the layers of CL-20 were completely separated by the layers of 2,5-dimethylpyrazine).

Figure 6. The intermolecular interactions of the cocrystal 3. (A) The CH···O hydrogen bonding interaction (green lines) between CL-20 and 2,5-dimethylpyrazine in 3. (B) The CH···O hydrogen bonding interaction (green lines) between the CL-20 molecules each other in cocrystal 3. (C) The sandwich stacking type of cocrystal 3 which viewed down along the b axis, for clarity, 2,5-dimethylpyrazine molecules were coloured yellow.

The calculation of crystal packing coefficient (PC) is an approach to evaluate the efficiency of crystal packing in a cell, which can be calculated according to an existing empirical formula (refer Supporting Information SI 6.1). The PCs of the four cocrystals were calculated to be 72.5, 73.5, 76.0, 11 ACS Paragon Plus Environment


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and 71.5 %, respectively. For the above four cocrystals, PC is directly proportional to density, which means, higher the PC, higher is the density. However, the PCs of the four cocrystals are lower than those of the pure components γ-CL-20 (75 %) and β-CL-20 (79 %). The main reason for this undesirable result can be attributed to the directional hydrogen bonding and the NO2−π interactions between CL-20 and conformer molecules. The distance between the two components of the cocrystals is increased greatly by using directional interactions as driving forces, which directly destroys the maximum accumulation of crystals close packing, thereby causing a precipitous decrease in the density of the cocrystals. Similarly, changes in packing style also affect the PC to some extent. In addition, the unavailability of π–π stacking is another reason for the decrease in the density. Notably, at an approximate PC and with the same components, 1-2 exhibits a slightly higher density than 1-1, indicating that density can be modified by adjusting the stoichiometric ratio of the cocrystal components. In the research of cocrystals, Hirshfeld surface analysis is an effective tool to study crystal packing.28 The 2D-fingerprinting of the crystals in this study and the associated Hirshfeld surfaces were employed by using the suite of tools of Crystal Explorer.29 As shown in Figure 7, the 2Dfingerprint plots of the four cocrystals are fairly different, among which the O⋯H (42.5, 48.9, 48.0, and 51.8 %), O⋯N (16.0, 5.1, 12.4, and 15.1 %), and O⋯O (22.3, 21.8, 25.4, and 21.0 %) contacts dominate the intermolecular interactions. The largest population of the O⋯H contacts plays a dominant role, with approximately 50.0 % of the total weak interactions, suggesting that hydrogen bonding interactions are the major driving force in the formation of CL-20-based cocrystals.30 In contrast, the second largest population of contacts is attributed to the O⋯O contacts, which can be assigned to the interactions between the six NO2 groups surrounding the rigid hexaazaisowurtzitane cage. 30 Finally, 12 ACS Paragon Plus Environment

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the O⋯N contacts contribute to about a quarter of the intermolecular interactions, which only occurred between the NO2 groups of CL-20 molecules and the nitrogen atoms of N-heterocyclic molecules. Additionally, with respect to the other contacts, O⋯N and C⋯N contribute quite less towards intermolecular interactions.

Figure 7. (A) Two-dimensional fingerprint plots and Hirshfeld surface of the CL-20 molecule in cocrystal 1-1; (B) Two-dimensional fingerprint plots and Hirshfeld surface of the CL-20 molecule in cocrystal 1-2; (A) Twodimensional fingerprint plots and Hirshfeld surface of the CL-20 molecule in cocrystal 2; (A) Two-dimensional fingerprint plots and Hirshfeld surface of the CL-20 molecule in cocrystal 3.

Thermal stability, detonation properties, and sensitivity are the key research components for energetic materials and cocrystals. Thermal stabilities were determined by differential scanning calorimetry at a heating rate of 10 °C min−1 in N2. The decomposition temperatures of cocrystals 1-1, 1-2, and 3 (225, 233, and 235 °C) are slightly higher than that of CL-20 (219 °C), indicating excellent thermal stabilities (refer Supporting Information SI 3). Cocrystal 2 has a melting point of 177 °C before 13 ACS Paragon Plus Environment


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it decomposes at 193 °C. The above results demonstrate that cocrystallization has the advantage of modulating the thermal stability of CL-20 within a certain range. The impact and friction sensitivities of the four cocrystals were measured using a BAM fall hammer BFH-10 and FSKM-10 BAM friction apparatus, respectively.31 Cocrystals 1-1, 1-2, 2, and 3 have impact sensitivities of 20, 24, 18, and 30 J, respectively, and friction sensitivities of 168, 192, 158, and 174 N, respectively. Because the abundant intermolecular interactions increase the stability of the crystal structure in CL-20-based cocrystals, their sensitivities are higher than that of CL-20 (impact sensitivity: 4 J; friction sensitivity: 48 N). Therefore, the above results suggest that the mechanical sensitivity of a high power explosive can be decreased by introducing non-energetic insensitive coformers through cocrystallization techniques. Using the measured ambient temperature density and the calculated heat of formation, the detonation properties of the four cocrystals were calculated using the EXPLO5 (v6.01) program (refer Supporting Information SI 6.3).32 The detonation properties of cocrystals will inevitably suffer losses by the incorporation of another non-energetic coformer. Cocrystals 1-1, 1-2, 2, and 3 have detonation velocities of 8195, 8166, 8352, and 7789 m s−1, respectively, and pressures of 28.01, 27.18, 30.75, and 24.62 GPa, respectively, which are less than those of pure CL-20 (9706 m s-1 and 45.2 GPa). This is attributed to the decrease in the density of the cocrystals and the worsening of oxygen balance, thereby making it more negative. Moreover, CL-20-based cocrystals also exhibit detonation properties exceeding that of TNT (7303 m s-1, 21.3 GPa) owing to the energetic superiority of CL-20. In future studies, the loss of explosive power can be overcome by cocrystallization with energetic six-membered symmetrical N-heterocyclic compounds. 14 ACS Paragon Plus Environment

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CONCLUSIONS

In summary, four novel CL-20-based energetic cocrystals were prepared based on an innovative design strategy, where (i) the coformers have local structural similarity with CL-20 and (ii) the molecular structure of the conformer contains electron donating groups. According to the experimental and crystal analysis results, two different stoichiometric cocrystals composed of pyrazine and CL-20 were obtained. All the cocrystals were fully characterized using PXRD, elemental analysis, and single crystal X-ray diffraction. The thermal stabilities, mechanical sensitivities, and detonation properties of the four cocrystals were also investigated. Although their detonation properties are diluted to a certain extent compared to pure CL-20, they exhibit insensitivity towards impact and friction. We envision that the above design strategy can serve as an emerging principle to develop versatile CL-20-based cocrystals for various applications. In addition, the method of adjusting the molar ratio can offer insights into effectively constructing energetic cocrystals for applications in different fields and pertinently tuning their properties. However, the incorporation of the above-mentioned coformers will result in a substantial decrease in the detonation properties, which can be overcome by cocrystallization with energetic six-membered symmetrical N-heterocyclic compounds in future studies.

ASSOCIATED CONTENT

Supporting Information.



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The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, PXRD patterns, crystallographic data, DSC curves, impact sensitivity tests and detonation property evaluations.

Accession Codes

CCDC 11878548, 1845210-1845212 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

C. H. Sun, email: [email protected]

S. P. Pang, email: [email protected]

Author Contributions

S. Pang, C. Sun and T. Fei designed the study. T. Fei and P. Lv performed most of the reactions and measurements and finished the theoretical calculations. C. He performed the crystallographic structural analysis. T. Fei prepared the manuscript. Y. Liu gave important suggestions on the paper. All authors discussed and commented on the manuscript. 16 ACS Paragon Plus Environment

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ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Grant No. 21576026, 21702017) and SKLST (BIT) QNKT18-03. We thank Prof. Chaoyang Zhang and Dr. Guangrui Liu (Institute of Chemical Materials, China Academy of Engineering Physics) for help of theoretical calculation.

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Briefs. A reliable design strategy to synthesis CL-20 based cocrystals was introduced, namely, using sixmembered symmetrical N-heterocyclic compounds as effective coformers. In addition, four novel CL-20 based cocrystals were synthesized under this strategy.


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Design and Synthesis of a Series of CL-20 Cocrystals: Six-membered

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