Isomeric cocrystals of CL-20: A Promising Strategy for Development of

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Isomeric Cocrystals of CL-20: A Promising Strategy for Development of High-Performance Explosives Zongwei Yang,† Haojing Wang,‡ Yuan Ma,‡ Qi Huang,† Jichuan Zhang,§ Fude Nie,† Jiaheng Zhang,*,§ and Hongzhen Li*,† †

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, China College of Chemical Engineering and Environment, North University of China, Taiyuan 020051, China § School of Material Sciences and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China ‡

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S Supporting Information *

ABSTRACT: Two novel isomeric cocrystals based on CL-20 (2,4,6,8,10,12hexanitrohexaazaisowurtzitane) and MDNI (1-methyl-dinitroimidazole) isomers including a 1:1 CL-20/2,4-MDNI (1) and a 1:1 CL-20/4,5-MDNI (2) were obtained. These cocrystals have high densities, high predicted detonation properties, and low impact sensitivities with excellent thermal stabilities, for example, 2 (ρ: 1.882 g cm−3; D: 8915 m s−1, P: 35.88 GPa; IS: 11 J) exhibits excellent comprehensive performances. Notably, adopting the isomer as a coformer for constructing energetic cocrystals or even new cocrystal forms and further tuning properties represents a study orientation of explosives that is not yet exploited in the energetic material field.

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At the present time, numerous energetic cocrystals have been reported, including cocrystals with nonenergetic coformers, solvents, and energetic components. However, only a few energeticenergetic cocrystals have been obtained, such as CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane), BTF (benzotrifuroxan), and DADP (diacetone diperoxide) based energetic cocrystals.21−30 From the perspective of crystal engineering, a primary challenge in the rational design of energetic cocrystals is the scarcity of reliable motifs generally used to evaluate the possibility of cocrystal formation. Furthermore, obtaining motifs has been hindered due to a low abundance of different kinds of cocrystal formers that have the potential ability to form new energetic cocrystals. Isomers,31 a common nomenclature in chemistry, represent a class of important molecules that have the same chemical formulas but different chemical structures. Structural isomers32 may have a similar ability to exhibit analogous interactions and facilitate formation of cocrystals with suitable host molecules due to high structural similarity, including similar size and molecular shape. Therefore, it is crucial to use isomeric molecules as a new type of cocrystal formers to achieve reliable motifs for the rational design of energetic cocrystals. Furthermore, the usage of isomers as a coformer for screening energetic cocrystals can open novel avenues of research in energetic materials. The research work can offer insights into

ocrystals are composed of two or more different kinds of molecules in a defined ratio within a crystal lattice which are usually joined together via intermolecular interactions such as hydrogen bonds, π-stacking, or van der Waals forces.1,2 Nowadays, formulation of a cocrystal is emerging as a promising strategy to synthesize novel energetic materials (referred to as energetic cocrystal).3−5 The formation of an energetic cocrystal can effectively tune the physicochemical properties and safety and detonation performances of existing explosives, such as density, melting point, sensitivity, detonation velocity, and pressure.6−10 Therefore, the cocrystallization process provides a potential approach to modify characteristics of energetic materials to develop high-performance energetic cocrystals owing to the generation of novel structures at the molecular level. Therefore, energetic cocrystals have become an area of major research in the field of energetic materials. Isomeric cocrystals have the same molecular formulas with different crystal structures. These cocrystals represent a new solid-state form which usually displays distinct physicochemical properties due to changes in the inherent structures.11,12 In contrast to the pharmaceutical cocrystals, isomerism in energetic cocrystals has not been well-studied. However, the study of solid-state forms of energetic cocrystals is as vital as the study of common energetic cocrystals (not including isomeric, polymorphic, and ternary cocrystals) and can lead to significant differences in the performances of explosives. In recent years, researchers in the field of energetic materials have conducted extensive studies on energetic cocrystals, primarily focused on design, preparation, and characterization.13−20 © XXXX American Chemical Society

Received: July 15, 2018 Revised: September 16, 2018 Published: September 24, 2018 A

DOI: 10.1021/acs.cgd.8b01068 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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as compared with that of the coformers (69.8% for pure 2,4MDNI and 72.8% for pure 4,5-MDNI).33 Notably, at a similar packing coefficient and the same component contents, 2 exhibits slightly higher density than 1, indicating a high efficacy of isomeric cocrystals in density modification. Furthermore, under the similar crystal structure and packing coefficient, a stronger interaction usually may offer a higher density. As shown in Figure 1, the 2D fingerprint plots of both cocrystals (Figure 1a,b) are quite different. The shorter di + de of the middle spike in cocrystal 2 is identified as O−O interactions and suggests stronger weak interation for higher solid-state density. In compound 1, CL-20 cocrystallizes with 2,4-MDNI in monoclinic system with favorable ε form (Figure 2c), whereas in compound 2, it cocrystallizes with 4,5-MDNI in orthorhombic systems with undesired β form (Figure 3c), indicating that isomeric cocrystals may have a potential ability to inhibit crystal transition and stabilize the crystal form. The formations of both cocrystal 1 and 2 depend primarily on two different motifs through structure analyses. One of the significant motifs is the hydrogen bonding interactions, which occur between the hydrogen atoms of CL-20 rings and the oxygen atoms in the nitro groups on the nitroimidazoles with bond lengths ranging from 2.357 to 2.645 Å (Figure 2a and Figure 3a, respectively). Another important motif is the NO2−π interactions with distances ranging from 2.913 to 3.415 Å which are formed by the electron-rich nitro groups and electron-deficient π-systems between the nitro groups on CL-20 and the heterocyclic rings on imidazoles (Figure 2b and Figure 3b, respectively). Furthermore, these interactions provide important avenues for further rational design and development of novel energetic cocrystals even as new cocrystal forms. Also, this pheonomenon suggests that structural isomers may be apt to exhibit similar intermolecular interactions to enrich the library of interaction motifs of energetic materials to exploit more cocrystal forms with suitable host molecules. The thermal behavior of cocrystal 1 and 2 was examined by differential scanning calorimetry (DSC). The DSC traces display clear endothermic peaks at 177 and 115 °C for 1 and 2 (see Supporting Information SI 4) corresponding to the melting points of 1 and 2, respectively, which are substantially higher than those of their respective components: 2,4-MDNI (145 °C) or 4,5-MDNI (75 °C). Relatively dramatic variations in melting points of these materials can be achieved by the formation of this new cocrystal form to regulate the great differences in melting points of pure components. In particular, the melting point of cocrystal 1 is found to be 62 °C higher than that of cocrystal 2 due to the fact that 2,4-MDNI has a higher melting point relative to 4,5-MDNI, suggesting the great potential of isomeric cocrystals to alter the melting points through the formation of new structures. In addition, 1 and 2 both possess similar decomposition temperatures of about 220 °C, indicating excellent thermal stabilities. Sensitivity is one of the crucial properties of energetic materials that is generally used to evaluate their safety prior to practical applications. The impact sensitivity measurements were carried out via BAM fallhammer. The samples were stuck with a 2 kg drop hammer from different heights corresponding to different impact energies. The results are expressed as E50%, the impact energy of 50% detonation probability (Figure 4). The E50% values (see Supporting Information SI 5) for cocrystal 1 and 2 are 9 and 11, respectively, which are substantially higher than those of pure CL-20 (2.5 J) and traditional nitroaromatic cocrystals, such as CL-20/TNT (6 J) and CL-20/DNB

effectively constructing energetic cocrystals, even new cocrystal forms, and significantly tune properties as well as profoundly understand structure−property relationships. Therefore, we use MDNI isomers as new coformers for replacement of traditional nitroaromatic explosives for design CL-20 cocrystals. As is well-known, 1-methyl-2,4-dinitroimidazole (2,4-MDNI) and 1-methyl-4,5-dinitroimidazole (4,5MDNI) are important structural isomers of MDNI belonging to the class of polynitroimidazole explosives. Unlike traditional nitroaromatic explosives previously used as coformers, polynitroimidazole explosives, a class of nitrogen-rich heterocyclic compounds, have attracted growing attention in the energetic material community due to high nitrogen content, high positive enthalpy, and favorable explosive property. Additionally, 2,4-MDNI and 4,5-MDNI (Scheme 1), typical representatives Scheme 1. Chemical Structures of CL-20 (left), 2,4-MDNI (middle), and 4,5-MDNI (right)

of polynitroimidazole explosives, exhibit excellent safety characteristics with higher power relative to traditional aromatic explosives. On the contrary, CL-20 (Scheme 1) with four forms (α, β, γ, and ε) has high power but is relatively more sensitive to impact, friction, and shock waves, which seriously limits its practical applications. Therefore, to dramatically reduce the impact sensitivity of CL-20 and further enrich the kinds of CL-20 cocrystals for the development of high-performance enhanced explosives, we cocrystallize CL-20 with MDNI isomers. Herein, we report two novel energetic-energetic cocrystals based on CL-20 and MDNI isomers. To the best of our knowledge, these cocrystals are the first case of formulation of energetic cocrystals by using isomers as cocrystal coformers. The phenomenon is useful for extending the scope of coformers to screen cocrystals and seek reliable motifs. Two isomeric cocrystals of CL-20/2,4-MDNI (1) and CL-20/4,5MDNI (2) were obtained in ratios of 1:1 and 1:1, respectively. Furthermore, these cocrystals belong to position isomers due to the different positions of nitro groups on imidazole rings of 2,4-MDNI and 4,5-MDNI. Additionally, the developed solid form is the first example of isomerism in energetic cocrystals and is expected to offer an in-depth intriguing strategy to construct high-performance explosives. Cocrystal 1 and 2 were easily formed from ethanol solution by a slow evaporation of solvent (see Supporting Information SI 1). The observed powder X-ray diffraction (PXRD) patterns of both cocrystals are distinct from pure CL-20 and the respective components. Moreover, the experimental and simulated PXRD patterns of cocrystals 1 and 2 are in good agreement, indicating the pure phases of the cocrystals (see Supporting Information SI 2). The crystal structures of 1 and 2 were confirmed by single X-ray diffraction (SXRD) (see Supporting Information SI 3). Both 1 and 2 have high densities of 1.867 g cm−3 and 1.882 g cm−3 at 130 K, respectively, which lie between those of the shared CL-20 and respective coformers but are significantly superior to corresponding coformers: 2,4-MDNI (1.647 g cm−3) and 4,5-MDNI (1.683 g cm−3). This is due to the higher packing coefficient (75%) for 1 and 2 B



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Figure 1. Two-dimensional fingerprint plots in crystal stacking for 1 and 2 as well as the associated Hirshfeld surfaces. The fingerprint plots in crystal stacking for 1 (a) and 2 (b).

Figure 2. Cocrystal 1. (a) CH···O hydrogen bonding interaction in 1. (b) NO2−π interaction between CL-20 and 2,4-MDNI in 1. (c) Super cell viewed down along the c axis.

Figure 3. Cocrystal 2. (a) CH···O hydrogen bonding interaction in 2. (b) NO2−π interaction between CL-20 and 4,5-MDNI in 2. (c) Super cell viewed down along the c axis.

Detonation properties mainly including detonation velocity and pressure are typical representatives of the explosive power and are critical performance parameters that are commonly used to assess energy released during detonation of energetic materials. The detonation properties were predicted using EXPLO5 (v 6.02) based on density and calculated heat of formation.34 The RDX and traditional nitroaromatic cocrystals were used as references. The predicted detonation velocities (Figure 4) and pressures (see Supporting Information SI 6) of cocrystals 1 and 2 were 8839, 34.82, and 8918 m/s, 35.69 GPa,

cocrystal (7 J). The results indicate that impact sensitivity is drastically decreased due to the intermolecular interactions formed through this new cocrystal form. Additionally, intermolecular interactions may increase the stability of the crystal structure and contribute to the insensitivity. Furthermore, both 1 and 2 have impact sensitivity superior to that of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane, 8 J), a commonly used high-energy, low-sensitivity explosive, and thus may act as potential attractive explosive candidates for low vulnerability formulations. C



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zongwei Yang: 0000-0002-7065-2384 Jiaheng Zhang: 0000-0002-2377-9796 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support for this study by the National Natural Science Foundation of China (No.11402236, No.11602240), NASF (U1530262) and the Shenzhen Science and Technology Innovation Committee (JCYJ20151013162733704). We sincerely thank Dr. Jun H. Zhou for heartily helping calculate intermolecular interactions and heats of formations.

Figure 4. Impact sensitivities and predicted detonation velocities for 1−2 and respective components as well as references.

respectively, which are lower than those of pure ε-CL-20 (9722 m/s, 44.45 GPa) and substantially higher than those of pure components 2,4-MDNI (7558 m/s, 22.11 GPa) and 4,5MDNI (7736 m/s, 23.59 GPa). The phenomenon is due to higher densities and nitrogen contents endowed by cocrystallization of CL-20 with polynitroimidazoles. In comparison to 2, 1 is slightly less powerful due to a relatively lower density, which further exhibits that isomeric cocrystals offer a promising method to tune detonation properties of energetic materials by regulation of packing densities of pure components. In addition, the predicted detonation velocities for 1 and 2 exceed RDX (8724 m/s),35 CL-20/TNT cocrystal (8654 m/s), and CL-20/DNB cocrystal (8798 m/s), suggesting that both cocrystal 1 and 2 exhibit excellent detonation performances. Consequently, 1 and 2 possess desirable power with low sensitivity and, thereby, can potentially serve as replacements of RDX. In summary, two isomeric cocrystals formed by cocrystallization of CL-20 with 2,4-MDNI and 4,5-MDNI in both 1:1 ratios have been obtained by using MDNI isomers as coformers. The adoption of isomers as new cocrystal coformers is expected to offer significant insights into reliable motif and new cocrystal form discovery. Cocrystal 1 and 2 both exhibit high densities (>1.86 g cm−3), high calculated detonation velocities (>8800 m/s), and low impact sensitivities (>8 J) with excellent thermal stabilities. In particular, for cocrystals 1 and 2, the drastic differences in properties caused by the developed new cocrystal forms can aid in the formation of promising strategy for in-depth tuning of properties of energetic materials for the design of enhanced explosives.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01068. Experimental methods, PXRD patterns, Crystallographic data, DSC curves, Impact sensitivity tests, and Detonation property evaluations (PDF) Accession Codes

CCDC 1834219−1834220 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. D



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Isomeric cocrystals of CL-20: A Promising Strategy for Development of

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