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Three Energetic HNS Cocrystals Regularly Constructed by H-bonding, #-Stacking and van der Waals Interactions Yu Liu, Shichun Li, Jinjiang Xu, Hongli Zhang, Yuxiang Guan, Huaiyu Jiang, Shiliang Huang, Hui Huang, and Zeshan Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00019 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018
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Crystal Growth & Design
Three Energetic HNS Cocrystals Regularly Constructed by Hbonding, π- Stacking and van der Waals Interactions Yu Liu,[a], [b] Shichun Li,[b] Jinjiang Xu,[b] Hongli Zhang,[c] Yuxiang Guan,[b] Huaiyu Jiang,[b] Shiliang Huang,*[b] Hui Huang[b] and Zeshan Wang*[a] 5
[a]
School of Chemical Engineeing, Nanjing University of Science&Technology, Nanjing, 210094, P. R. China; Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621900, P. R. China. [c] The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu, 610065, P. R. China. [b]
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KEYWORDS: Energetic cocrystal, HNS, H-bonding, π-Stacking
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ABSTRACT: Three new energetic HNS cocrystals, HNS/BP, HNS/BPE and HNS/BPA have been synthesized. A good geometric match and rich H-bonds have been observed between the selected coformer and HNS molecules in all the three cocrystals. According to similar configuration and arrangement of HNS-coformer H-bonds, coformer-coformer π-staking and HNS-HNS H-bonding interactions, the three cocrystals have a common cocrystal architecture and show high thermal stability and improved sensitivity. This study is helpful for understanding the formation mechanism of energetic cocrystals and the design of new energetic cocrystals.
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Cocrystallization is an effective crystal engineering strategy has molecules for 2,2',4,4',6,6'-hexanitrostilbene (HNS), an excellent been widely used in many fields such as pharmaceuticals,1,2 nonheat-resistant explosive, we have synthesized three new HNS cocrystals, HNS/BP (BP=4,4’-Bipyridine), HNS/BPE linear optics,3,4 ferroelectrics5,6 and organic electronics.7,8 Recent(BPE=trans-1,2-Bis(4-pyridyl)ethylene) and HNS/BPA ly, the application of cocrystallization on energetic materials including explosives, propellants and pyrotechnics has drawn much 55 (BPA=1,2-Bis(4-pyridyl)ethane). They have similar supramolecuattention.9-12 The cocrystals of 2,4,6-trinitrotoluene (TNT), lar structures regularly constructed H-bonding, π-stacking and van 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and 2,4,6,8,10,12der Waals interactions. After cocrystallization, the HNS cocrystals hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) have clearexhibit remarkably improved impact sensitivity, compared to pure ly demonstrated that the physicochemical properties and functionHNS crystals. alities of energetic materials can be dramatically changed by cocrystallization.13-19 More recently, a series of energetic cocrystals were synthesized by incorporating the primary explosive diacetone diperoxide (DADP) with trihalotrinitrobenzene explosives. After cocrystallization, the density, oxygen balance and stability of DADP have been dramatically improved.20,21 However, in contrast to the fast-spreading application of cocrystallization in the pharmaceutical industry, the development of energetic cocrystals is relatively slow. As defined primarily by nitro groups, a solitary moiety that offers very few predictable interactions for cocrystallization, energetic materials have been proven difficult to form cocrystals.15,22 In this case, establishing sufficiently strong intermolecular interactions such as hydrogen bonding and π-stacking between energetic materials and the coformers is critical for the Figure 1. Molecules of (a) HNS, (b) 4,4-Bipyridine, (c) trans-1,2-Bis(4design of energetic cocrystals. pyridyl)ethylene and (d) 1,2-Bis(4-pyridyl)ethane. Besides, these non-covalent intermolecular interactions can also generate dramatic changes on the properties of energetic materials, 60 The three HNS cocrystals were synthesized by dissolving a especially sensitivity. For example, theoretical calculations indimixture of HNS and the coformer with a molar ratio of 1:1 in cated that the layered π-stacking can buffer effectively against acetonitrile at temperature of 70°C with continuously stirring. The external mechanical stimuli and make the explosives involved solution was then naturally cool down to room temperature and insensitivity.23,24 The increased degree of H-bonding is hypothekept in atmosphere for a period of several days. After filtration sized to be the surprisingly low sensitivity of HMX/CL-20 cocrys- 65 and drying at room temperature, needle cocrystals were obtained, tal.25,26 The significantly different halogen−peroxide interactions as shown in the Supporting Information Fig. S1. All the structures in DADP/TITNB cocrystal demonstrate for the first time an enerwere solved by single crystal X-ray diffractions.28 getic cocrystal that is less sensitive to impact than either of its pure components.21,27 Here, by a selection of proper coformer 1
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Figure 3. The coformer-coformer and HNS-HNS intermolecular interactions in HNS/BP, HNS/BPE and HNS/BPA cocrystals. (a) π···π stacking of BP in HNS/BP; (b) slipped π-stacking of BPE in HNS/BPE; (c) C–H···π interaction in HNS/BPA; (d) H-bonds between HNS molecules in all the three cocrystals.
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Figure 2. H-bonds between HNS and coformer molecules in (a) HNS/BP, (b) HNS/BPE and (c) HNS/BPA cocrystals. The molecule was represented in ball-stick model and the H-bonds by purple dash line.
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The molecule structure of the three coformers, BP, BPE and BPA can be considered as two pyridine rings directly connected, connected by –CH=CH– and lined by –CH2–CH2– groups, respectively, which provide a good symmetric match with HNS molecules. As an example, in HNS/BP, the N-atom and neighboring two H-atoms on the pyridine ring make a D-A-D (D=hydrogen donner, A=hydrogen acceptor) sequence, while the neighboring two nitro groups and the H-atom in-between them on the aromatic ring of HNS molecule form an A-D-A sequence. Therefore, in each side of the BP molecule, three near parallel H-bonds can be formed between the pyridine ring and one HNS molecule and a one-dimensional (1D) zigzag H-bonded HNS-BP chain was formed in the cocrystal structure, as shown in Fig.2a and Fig.S3 in Supporting Information. The distance of D···A ranges from 3.27 Å to 3.44 Å. The details of the hydrogen bonds were listed in Table 2s. As the BPE and BPA molecule have a similar configuration with BP molecule, similar HNS-BPE and HNS-BPA H-bonds as well as 1D zigzag H-bonded HNS-BPE and HNS-BPA chains can be found in HNS/BPE and HNS/BPA cocrystal structures (see Figure 2a, 2b and Fig. S3 in Supporting Information). This symmetric match between the pyridyl and dinitrobenzene units can be also seen in literature. For instance, 3,5-diaminobenzoic acid and aminopyrazine molecules have similar H-bonding interactions in their cocrystal structure29 with the distance of D···A ranges from 3.30 Å to 3.59 Å (see Fig. S4 in Supporting Information). Based on the aromatic property of pyridine ring, π-staking interactions were observed between neighboring coformer molecules in all the three cocrystals. In HNS/BP, the BP molecules stack with their pyridine rings almost on top of each other, as shown in Figure 3a. The central distance of the pyridine rings is 3.48Å and the N···C distance is 3.48Å (Figure 3a). In HNS/BPE, the neighboring two BPE molecules are also parallel. However, in contrast with the BP molecules in HNS/BPE, there is a slip between the neighboring BPE molecules, which results a central distance of 4.28Å and a N···H distance of 3.57Å for the pyridine 2
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rings (Figure 3b). In HNS/BPA, due to the –CH2-CH2– group connection between the pyridine rings, the whole BPA molecule is not flat. The π-staking interaction between the BPA molecules has a conformation of C–H···π with a N···H distance of 2.67Å and the central distance of pyridine rings 4.57Å, as shown in Figure 3c. These different π-stacking interactions can be also seen in the pure crystals of the coformer and other nitrogen-containing aromatic rings.30-32 Similar H-bonding interactions have been found between the HNS molecules in all the three cocrystals. The central –CH=CH– group of one HNS molecule were H-bonded to the –NO2 group of two neighboring HNS molecules, which is the same as the HNSHNS H-bonds in pure HNS crystals, as show in Figure 3d. The details of the HNS-HNS H-bonds can be seen in Table S2 in Supporting Information. According to the similar intermolecular interactions, the three cocrystals structures have, topologically, a common architecture. In all the cocrystals, the HNS-coformer H-bonds arrange in the [510] direction, while the HNS-HNS H-bonds and HNS-HNS πstaking interaction arrange along the b-axis. Consequently, a twodimensional (2D) HNS-coformer layer was formed in the ab-plane, as shown in Figure 4a. These 2D layers stack along the c-axis, resulting a three-dimensional (3D) supermolecular structure (Figure 4b). The adjacent layers are symmetrically related by the cglide and the interlayer interaction dominates by van der Waals interaction. The thermal stability of the cocrystals was investigated by TGDSC measurement, as shown in Figure 5. For HNS/BP, the first weight loss of 26.8% accompanies with an endothermic peak at 206.5 °C, which indicates a decomposition of the cocrystal struc-
Figure 4. The architecture of the HNS/BP, HNS/BPE and HNS/BPA cocrystals. (a) The HNS and conformer molecules interact by HNScoformer H-bonds along the [510] direction and coformer-coformer πstaking interactions and HNS-HNS H-bonds along the [010] direction, forming a HNS-coformer layer on the ab-plane. (b) These HNS-conformer layers stack along the c-axis to form the 3D supermolecular structure of the HNS/BP, HNS/BPE and HNS/BPA cocrystals.
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Crystal Growth & Design
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Figure 5. TG-DSC measurement of (a) HNS/BP, (b) HNS/BPE, (c) HNS/BPA and (d) raw HNS in air flow and a heat rate of 10 °C/min.
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ture and release of the BP molecules (calculated value 28.6%). As for HNS/BPE, the weight loss is 28.0% (calculated value 28.8%) and the cocrystal decomposition temperature is 245.7 °C. Different with HNS/BP, an exothermic peak at 256.9 °C was found, following the endothermic peak at 245.7 °C. This means the BPE molecules partly decompose during the collapse of the cocrystal structure. For HNS/BPA, the cocrystal structure decomposition was caused mainly by the decomposition of BPA molecules, as evidenced by the exothermic peak at 211.2 °C. The high thermal stability of the HNS cocrystals is considered to origin from the rich intermolecular interactions especially the hydrogen bonding between HNS and the coformer molecules. However, for all the three cocrystals, the decomposition of the HNS molecules following the decomposition of cocrystal structure is very similar to that in pure HNS crystal (Figure 5d). In order to analyze the safety of the cocrystals, impact, spark and friction sensitivity tests were performed and the results were listed in Table 1. The drop height of 50% explosion probability (H50) of HNS/BP, HNS/BPE and HNS/BPA are 106.9 cm, 100.9 cm and 44.20 cm, respectively, which are much higher than that of raw HNS crystals (22 cm). This can be attributed to the introduction of the relatively strong intermolecular interactions, mainly the HNS-coformer H-bonding and coformer-coformer π-stacking interactions. As described above, the conformer-coformer C–H···π interaction in HNS/BPA is relatively weaker than the π-stacking interactions in the other two cocrystals. The remarkable increase Table 1. The impact, spark and friction sensitivity of HNS/BP, HNS/BPE, HNS/BPA and raw HNS. Sample
Impact sensitivity (H50/cm)
Spark sensitivity (E50/J)
Friction sensitivity
HNS/BP
106.9
0.304
44%
HNS/BPE
100.9
2.86
24%
HNS/BPA
44.20
3.99
0%
Raw HNS
22
1.11
28%
of the impact sensitivity from HNS/BP and HNS/BPE to HNS/BPA may supplies a good evidence for importance of πstacking in reducing the sensitivity of energetic cocrystals. More30 over, the HNS/BPE and HNS/BPA have insensitive properties towards the spark and friction stimuli. Especially, the friction 3
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sensitivity of HNS/BPA achieved 0%. However, as the insensitivity of explosives is very complicate and the mechanism has not yet been fully understood, more investigations including theoretical and experimental studies are needed for clearly explaining the correlation between the insensitivity and the intermolecular interactions in such HNS cocrystals. In summary, we have synthesized three new energetic HNS cocrystals, HNS/BP, HNS/BPE and HNS/BPA. A good geometric match and rich H-bonds have been observed between the selected coformer and HNS molecules in all the three cocrystals. Besides the HNS-coformer H-bonds, coformer-coformer π-staking and HNS-HNS H-bonding interactions were also found in the cocrystal structures. According to the similar regular arrangement of intermolecular interactions, including the HNS-coformer Hbonding, coformer-coformer π-staking and HNS-HNS H-bonding, all the three cocrystals structure have a common architecture. The high thermal stability and improved sensitivity of the cocrystals was considered to origin from the rich and relatively strong intermolecular interactions. Although the introduction of these nonenergetic coformers is considered to reduce the explosive performance of HNS, these results still suggest that the safety performance of HNS can be significantly adjusted by cocrystallization. This study will be helpful for the understanding the formation mechanism of energetic cocrystals and the design of new energetic cocrystals.
ASSOCIATED CONTENT
Supporting Information The experimental and simulated PXRD patterns of HNS/BP, 60 HNS/BPE and HNS/BPA; SEM images; crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org. Accession Codes CCDC 1814315-1814317 contains the supplementary crystallo65 graphic 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. 70
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] (S. H.). * E-mail:
[email protected] (Z. W.).
Notes 75 The authors declare no competing financial interest.
ACKNOWLEDGMENT
This project is supported by National Natural Science Foundation of China (No. 11572295, 11704349, 11372290) and Presidential Foundation of CAEP (Grant No. 201501018). We also thank Dr. 80 Tao Yang in Chongqing University for the data collection of single crystal X-ray diffractionst.
REFERENCES
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Crystal Growth & Design
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Tree Energetic HNS Cocrystals Regularly Constructed by H-bonding, πStacking and van der Waals Interactions Yu Liu,[a], [b] Shichun Li,[b] Jinjiang Xu,[b] Hongli Zhang,[c] Yuxiang Guan,[b] Huaiyu Jiang,[b] Shiliang *[b] 5 Huang, Hui Huang[b] and Zeshan Wang*[a] Synopsis 3D supermolecular structure of the energetic HNS/BP, HNS/BPE and HNS/BPA cocrystals was regularly built from the intralayer HNS-coformer H-bonding, HNS-HNS H-bonding and coformer-coformer π-staking interactions as well as the interlayer van der Waals interactions.
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