Five Energetic Cocrystals of BTF by Intermolecular Hydrogen Bond

Dec 13, 2012 - This paper owns an important consideration in the design of future BTF and other explosive cocrystals, and the result provides some fea...
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Five Energetic Cocrystals of BTF by Intermolecular Hydrogen Bond and π‑Stacking Interactions Haobin Zhang,†,§ Changyan Guo,†,‡,§ Xiaochuan Wang,† Jinjiang Xu,† Xuan He,† Yu Liu,† Xiaofeng Liu,† Hui Huang,*,† and Jie Sun*,† †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, People’s Republic of China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Five novel BTF (benzotrifuroxan) cocrystals, possessing a similar density to RDX (1,3,5-trinitrohexahydro1,3,5-triazine), have been prepared and reported first. Their single-crystal structures are presented and discussed. Interactions between cocrystal formers are discussed with shifts in the IR spectra providing additional support for the presence of various interactions. Hydrogen-bonding and π-stacking interactions are found to be the most prominent. Especially, the interactions between electron-poor π-systems of BTF and electron-rich groups of other cocrystal formers such as nitro groups of TNB exist commonly in all five novel cocrystals. This kind of interaction can be a more potential driving force for energetic cocrystals, since explosives with poor active hydrogen bonds are usually hard to form cocrystals with other explosives for the lack of strong intermolecular interactions. Because of the changes in structure, the physicochemical characteristics including density and melting point together with energetic properties of BTF altered after cocrystallization. All of the densities are between both of the cocrystal formers. Cocrystals of BTF with TNT and TNB have impact sensitivities between those of both cocrystal formers, while the remaining three cocrystals (BTF/TNA, BTF/MATNB, and BTF/TNAZ) all are more sensitive than either cocrystal former. It indicates that a cocrystal with TNT or TNB can reduce the shock sensitivity of BTF; especially, the cocrystal BTF/TNB not only has a lower sensitivity than RDX but also equal energetic properties, which potentially improve the viability of BTF in explosive applications. This paper owns an important consideration in the design of future BTF and other explosive cocrystals, and the result provides some feasibility to improve the application of the high explosive BTF.

1. INTRODUCTION A cocrystal is defined as a crystal that is built up out of two or more neutral molecular components that are, in their pure forms, solid at ambient conditions.1−3 It is well documented that cocrystallization offers great potential to dramatically alter many of the physicochemical properties of crystalline solids.4−6 During the past decades, considerable efforts have been devoted to apply the cocrystallization techniques to improve factors such as the dissolution rate, thermal stability, and bioavailability in the pharmaceutical industry.7−11 Encouraged by the great success of pharmaceutical cocrystallization, the development of new functional materials in other fields by cocrystallization techniques is attracting more and more interest. Energetic materials, especially explosives, are highly considered for their energy and sensitivity to accidental initiation. However, the energy and sensitivity of explosives usually conflict with each other, and very few high explosives with better sensitivity have been known and applied. To obtain explosives with better integrated performance, the common strategy is to develop new explosive compounds, but this is difficult to achieve in a finite time. A novel and intriguing © 2012 American Chemical Society

approach that has the opportunity to ameliorate the viability of candidate high explosives is the utility of cocrystallization. Seventeen cocrystals of TNT (2,4,6-trinitrotoluene) with a range of aromatic coformers were reported by Landenberger et al.12 The results revealed an alteration of key properties including density, melting point, and decomposition temperature as compared with TNT. Recently, the cocrystals of HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) with a wide variety of cocrystal formers have been reported, which afford a tremendous reduction in sensitivity as compared with pure HMX.13 David et al. investigated four solvates of CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane), which make a reduction in the sensitivity and energy of CL-20.14 Although these cocrystals exhibit some deduction in energy, they actually show more deduction in the sensitivity, which indicates that the application of a cocrystal engineering method in energetic materials to promote the stability of high explosives is desirable. If a high explosive cocrystallizes with insensitive explosives, the Received: September 16, 2012 Revised: November 21, 2012 Published: December 13, 2012 679

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phase. For further understanding of the cocrystals, single-crystal X-ray diffraction is employed to identify the crystal structures. 2.1. Structure of BTF Cocrystals. The crystallographic data of BTF cocrystals obtained by a X-ray single-crystal diffraction meter are presented in Table 1. They have been deposited at the Cambridge Crystallographic Data Centre (CCDC) and the deposition numbers are as follows: BTF/ MATNB cocrystal, 882012; BTF/TNA cocrystal, 882013; BTF/TNB cocrystal, 882014; BTF/TNT cocrystal, 882015; and BTF/TNAZ cocrystal, 898253, respectively. Because of the similar molecular structures of TNB, TNT, TNA, and MATNB, their cocrystals with BTF have similar structures, and we discuss the crystal structures of those four cocrystals together. With a different arrangement, the crystal structure of BTF/TNAZ cocrystal is analyzed alone. 2.1.1. BTF/TNB, BTF/TNT, BTF/TNA, and BTF/MATNB Cocrystals. The crystal structures of those four cocrystals viewed along the c-axis are shown in Figure 3. They show similar zigzag crystal structures, in which the molecules arrange in alternate layers of BTF and corresponding coformers, but as compared with the structure of the BTF in Figure 3, there is much difference. This implies that due to the intermolecular noncovalent interactions, the original molecular arrangement of BTF has been changed by the formation of the cocrystal, resulting in a new structure. There might be several kinds of intermolecular interactions between BTF and coformers, intermolecular hydrogen bond interactions, p−π stacking interactions (donor−acceptor interactions), and π−π stacking interactions, as shown in Figures 4−8. Because of the strong polarizing effects of oxygen atoms and nitrogen, the sixmembered ring of BTF becomes an electron-poor π-system,13 while all of the coformers have relatively electron-rich nitro groups, which is easy to form p−π stacking interactions with BTF according to the difference of electronegativity,20 leading to the formation of cocrystals. Here, we take the BTF/MATNB cocrystal as an example to explain the intermolecular interactions that drive the formation of those cocrystals. As shown in Figure 4, a crystal cell of the BTF/MATNB cocrystal is made up of two MATNB molecules (denoted as A and C, respectively) and two BTF molecules (denoted as B and D, respectively), and the molecules A and B as well as C and D are stacked by p−π stacking interactions, together constituting a repeating unit of the crystal structure. The building units repeat in the crystal structure and extend unlimited to the surroundings to form the cocrystal.21,22 The p−π stacking interaction between A and B is similar as that between C and D, and the only difference is the direction of the arrangement in space (as shown in Figure 4b). To test the p−π stacking interaction between BTF and MATNB, infrared spectra of the BTF/MATNB cocrystal and that of coformers (Figure S2 in the Supporting Information) are employed. As compared with that in BTF and MATNB crystals, the CN stretching band of BTF in the cocrystal shifts from 1649.1 to 1658.3 cm−1, while the O−NO antisymmetric vibration band of MATNB in the cocrystal shifts from 1526.0 to 1537.0 cm−1. This is because the nitro group of MATNB and the electronpoor π-system of BTF interact with each other and make a big conjugate p−π stacking system, which leads to the increase of electronic density of the π-system and nitro group. Consequently, the force constants of CN bond and nitro group strengthen, and the wavenumber of their infrared adsorption peak increases. For both, the force constants of the CN bond and nitro group increase, and it is reasonable to consider that

sensitivity may be deducted without a remarkable decrease in its energy. Fortunately, Bolton et al. prepared a cocrystal of TNT and CL-20, which improves the security of CL-20 while barely reducing its energy,15 and a high power explosive cocrystal of CL20:HMX, which has been predicted to exhibit greater power and similar sensitivity to that of the current military standard explosive HMX.16 These works have convinced us that cocrystallization engineering is an effective way to improve the integrated performance of explosives. Benzotrifuroxan (BTF) is a zero-hydrogen explosive with zero oxygen balance, high energetic density, and better initial detonation property and can be used as an igniter powder and high explosive.17,18 Besides, BTF is also one of the most powerful explosive available in the commercial field currently. However, because of the high sensitivity, the use of BTF is strongly restricted. Focus on this problem, our group concentrate on the properties of BTF research. By cocrystallizing with another insensitive explosive, it may improve the sensitivity of BTF without obviously decreasing the energy and make BTF with a better integrated performance.19 In this paper, we cocrystallized BTF with other five explosives and investigated the crystal structure, cocrystallization mechanism, intermolecular interactions, and their influence on the cocrystal performance. The five explosives used to cocrystallize with BTF are 1,3,5-trinitrobenzene (TNB), TNT, 2,4,6-trinitroaniline (TNA), 2,4,6-trinitrobenzene methylamine (MATNB), and 1,3,3-trinitroazetidine (TNAZ), as shown in Figure 1. Because

Figure 1. Molecular structure of BTF and coformers discussed in this study: BTF, TNB, TNT, TNA, MATNB, and TNAZ.

of the different molecular polarity and electronic distribution, strong intermolecular interactions between BTF and other explosive molecules, such as hydrogen bond and π-stacking interactions, drive the BTF molecules to rearrange to form a new crystal with the coformers during the cocrystallization. We have obtained the cocrystal mechanism and regularity, which will provide a good guide for the performance improvement of BTF and other explosives.

2. RESULTS AND DISCUSSION Five novel cocrystals containing a 1:1 molar ratio of components were prepared by evaporating acetone at room temperature over 2 days. The cocrystals have more regular crystal morphologies than that of BTF (Figure 2). The powder X-ray diffraction patterns of the cocrystals are different from those of the coformers (Figures S10−S15 in the Supporting Information), which confirms the formation of a new cocrystal 680

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Figure 2. Microscope images of BTF (a) and five cocrystals: BTF/MATNB (b), BTF/TNA (c), BTF/TNT (d), BTF/TNB (e), and BTF/TNAZ (f) cocrystals.

Table 1. Crystal Data and Structure Refinement Results for the Five Cocrystals and the BTF Crystal BTF/MATNB formula MW (g/mol) stoichiometry morphology temp (K) crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) volume (Å3) Z independent reflections final R indexes [I ≥ 2σ(I)] final R indexes (all data)

C13H6N10O12 494.28 1: 1 flake 145.00 (10) monoclinic P21/c 9.3316(3) 12.6044(3) 15.4760(4) 90.00 90.619(3) 90.00 1820.17(9) 4 3721 [R(int) = 0.0231] R1 = 0.0753, wR2 = 0.2024 R1 = 0.0968, wR2 = 0.2209

BTF/TNA C12H4N10O12 480.25 1: 1 prismatic 135.00 (10) monoclinic P21/c 9.4022(2) 12.6746(3) 14.3271(4) 90.00 97.406(3) 90.00 1693.10(8) 4 2986 [R(int) = 0.0264] R1 = 0.0730, wR2 = 0.1947 R1 = 0.0853, wR2 = 0.2079

BTF/TNT C13H5N9O12 479.26 1: 1 flake 145.00 (10) triclinic P1̅ 9.3377(4) 12.8957(6) 14.7285(7) 88.514(4) 84.163(4) 88.942(4) 1763.54(13) 4 7212 [R(int) = 0.0214] R1 = 0.1085, wR2 = 0.2592 R1 = 0.1445, wR2 = 0.2841

BTF/TNB C12H3N9O12 465.23 1: 1 irregular bulk 293.15 monoclinic P21/c 9.5491(4) 12.5670(5) 14.4542(6) 90.00 99.530(4) 90.00 1710.62(12) 4 3489 [R(int) = 0.0161] R1 = 0.0456, wR2 = 0.0984 R1 = 0.0784, wR2 = 0.1185

BTF/TNAZ C10H4N9O12 444.22 1: 1 irregular bulk 293.15 triclinic P1̅ 6.7469(5) 10.3885(6) 12.3483(11) 70.745(6) 88.771(7) 80.123(6) 804.39(11) 2 3672 [R(int) = 0.0179] R1 = 0.0838, wR2 = 0.1897 R1 = 0.1077, wR2 = 0.2056

BTF20 C6H6O6

flake 283−303 orthorhombic Pna21 6.923(1) 19.516(1) 6.518(1) 90 90 90 880.642 4

within the crystal itself. For further understanding of the hydrogen bond, we investigated the bond lengths and bond angles, as shown in Table 2. It is clear that the amino group (−NH) from MATNB forms a strong hydrogen bond with one of the O atoms (O5) from BTF. Although the bond angle is 147.28°, the especially short bond length of 2.228 Å is enough to make it a strong hydrogen bond. So, the MATNB and BTF really formed a strong intermolecular hydrogen bond. In the BTF/TNA cocrystal, the antisymmetric −NH2 stretching band (3436.5 cm−1) and symmetric −NH2 stretching band (3324.2 cm−1) of TNA shift to higher frequency for 19.16 and 20.83 cm−1, which indicates that the NH2 group forms strong hydrogen bonds with BTF, with one strong hydrogen bond (bond lengths and bond angles are 2.405 Å and 148.03°) and one secondary strong hydrogen bond (2.443 Å and 119.66°), making a contribution to the formation of cocrystals. However, for most explosives, there is no active hydrogen, and it can not form a strong hydrogen bond with other explosives, such as in the cocrystal BTF/TNB and BTF/TNT. In the two cocrystals,

for the BTF and MATNB, a stronger p−π stacking interaction in the cocrystal takes place than that in the individual crystal. Similar phenomena are also found in the BTF/TNA, BTF/ TNT, and BTF/TNB cocrystals. The CN stretching band of BTF shifts from 1649.1 to the region of 1655.0−1660.0 cm−1, which has a change of 6−11 cm−1 to the higher frequency. Also, the antisymmetric −NO2 stretching vibrations of TNA, TNT, and TNB increased for 4−6 cm−1 by the p−π interaction. These changes confirm the formation of a p−π stacking interaction and reveal that it can be a common interaction in these four BTF cocrystals, even in other explosive cocrystals. In addition to the p−π stacking interaction, the intermolecular hydrogen bond also plays an important role in the cocrystal. The formation of the hydrogen bond can be distinguished obviously from the infrared spectra. The stretching vibration band of −CH in MATNB shifts from 3112.7 to 3091.4 cm−1, and the −NH stretching band shifts from 3318.9 to 3308.6 cm−1. It indicates that in the cocrystal, MATNB forms a stronger hydrogen bond with BTF than 681

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Figure 3. Crystal structures of BTF/MATNB (a), BTF/TNA (b), BTF/TNT (c), BTF/TNB (d), and BTF/TNAZ (e) cocrystals and BTF (f) viewed along the c-axes.

Figure 5. Repeat units of the BTF/MATNB cocrystal connected to one another by means of a hydrogen bond and the π−π stacking interaction between BTF molecules themselves.

the BTF molecules still separated into two groups, and in each group, the molecules parallel to each other and so do MATNB molecules. This should result from the π−π stacking interaction. For both the BTF and the MATNB molecules, the six-membered carbon rings connect with intense electronwithdrawing groups, resulting in electron-poor centers and electron-rich borders. If one electron-poor center interacts with another electron-rich border, each molecule will take what it needs and become a stable system. However, with just one center interacting with one border, the two molecules connecting with each other just at a point can easily revolve or rotate along the point. So, just the π−π stacking interaction between an electron-poor center and an electron-rich border can not stabilize a system. Furthermore, other center and borders exposed into air can interact with other molecules, and if other molecules build into the crystals, it will be more stable. In the four cocrystals, the π−π-stacked BTF molecules interact with other coformer (such as MATNB) molecules, while the

Figure 4. Crystal structure of the BTF/MATNB cocrystal. (a) The repeat units containing four molecules of A, B, C, and D arrange in the b-axis. (b) The spatial direction of the A, B, C, and D molecules. (c) The p−π stacking interaction between A and B as well as C and D.

the nearest distance between the H atom of TNB or TNT and the O atom of BTF is further than that in the BTF/MATNB and BTF/TNA cocrystals, and the C−H bond is less active to be polarized, so the TNB or TNT molecules can not form strong hydrogen bonds with BTF molecules. In the four cocrystals, there is another important intermolecular interaction: the π−π stacking interaction. From Figure 3f, it is clear that the BTF molecules arrange in alternate layers with different orientations, and in each layer, the molecules parallel to each other. In the MATNB/cocrystal, 682

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Figure 6. Crystal structure of the BTF/TNA cocrystal. The repeat units with four molecules of A, B, C, and D arranged in the b-axis (a) and the hydrogen bond and π−π between BTF molecules themselves found in the cocrystal (b).

Figure 7. Crystal structure of the BTF/TNT cocrystal. The repeat units with four molecules of A, B, C, and D arranged in the b-axis (a) and the hydrogen bond and π−π between BTF molecules themselves found in the cocrystal (b).

Figure 8. Crystal structure of the BTF/TNB cocrystal. The repeat units with four molecules of A, B, C, and D arranged in the b-axis (a) and the hydrogen bond and π−π between BTF molecules themselves found in the cocrystal (b).

π−π-stacked MATNB molecules interact with MATNB molecules under the p−π stacking interaction, and they support the cocrystal together. If in a pure crystal, such as in the BTF crystal, the exposed center and borders can just interact with other BTF molecules and lead to a T-shape stacking. Figure 11 shows the crystal structure and intermolecular interactions of BTF. The BTF molecules are stacked in T shapes and unlimitedly extend to the surroundings along the a-axis to constitute a huge network structure (Figure 11a,b), which is stacked in columns parallel to the c-axis with significant π−π

stacking interactions between BTF (Figure 11c,d).23 It is can be seen that the crystals of molecules with an electron-poor center and an electron-rich border usually stack into a T-shape but not the expected parallel ones. To the contrary, cocrystals with one electron-poor center and one electron-rich center usually stack into parallel layers, just as in the 17 cocrystals of TNT reported by Landenberger et al.12 In the four cocrystals, the BTF and another coformer construct into a unit under the p−π stacking interaction. The repeated units connected to one another by means of a strong 683

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between the nitro group and the electron-poor six-membered ring of BTF, and the π−π stacking interaction between BTF itself. The repeated units composed of molecules A and B by p−π stacking interaction (Figure 9) are linked into a zigzag structure along the c-axis (Figure 10) under intermolecular

Table 2. Bond Lengths and Angles of Intermolecular Hydrogen Bonds Found in the Five Cocrystals cocrystal

interaction

bond length (Å)

bond angle (deg)

BTF/MATNB

C13−H13C···O10 C9−H9···O3 C13−H13C···O8 N7−H7···O5 C11−H11···O1 C13−H13B···O8 N7−H7A···O1 C11−H11···O5 C9−H9···O3 C11−H11···O7 N7−H7B···O5 C26−H26A···O11 C15−H15···O4 C26−H26B···O4 C17−H17···O6 C24−H24···O18 C26−H26B···O14 C12−H12···O5 C8−H8···O12 C10−H10···O3 C7−H7A···O12

2.703 2.626 2.820 2.228 2.507 2.992 2.405 2.761 2.669 2.653 2.443 2.965 2.676 2.507 2.874 2.722 2.882 2.743 2.560 2.837 2.742

162.46 158.08 114.75 147.28 173.89 114.75 148.03 164.84 171.50 117.56 119.66 132.95 159.67 125.26 168.18 120.52 156.07 154.58 125.44 175.03 170.04

BTF/TNA

BTF/TNT

BTF/TNB

BTF/TNAZ

Figure 10. Hydrogen bond and π−π interaction between BTF molecules themselves found in the BTF/TNAZ cocrystal.

hydrogen bonds and π−π stacking interactions, and these two interactions make the antisymmetric −NO 2 stretching frequency of TNAZ shift to higher frequency for 20−30 cm−1 (Figure S4 in the Supporting Information). Another significant shift in the C−H stretching frequency of benzene ring and −CH2 can further explain the formation of new intermolecular interactions. Although the molecular structure of TNAZ is different from that of the other four explosives, the cocrystal structures are similar, and the main force driving two kinds of molecules cocrystal into the same crystal cell is just a p−π stacking interaction. Comparing the structures of fivecocrystals with BTF, it is obvious that the structure of BTF is largely changed by the addition of another substance. Because of the strong intermolecular interactions between BTF and other explosive molecules, the T-shaped arrangements are broken, and it rearranges at a different direction together with another explosive by the difference of electronegativity.25 The results indicate that there is a certain regularity for the formation of the five cocrystals. Because the six-membered ring of BTF has an electron-poor π-system, we speculate that these substances with electron-rich groups can form a cocrystal with BTF.

hydrogen bond or π−π stacking interaction and extended to the surroundings to form cocrystals.24 Because of the different molecular polarity and electronic distribution, strong intermolecular interactions between BTF and other explosive molecules drive the BTF molecules to arrange at a different direction and form a new crystal with the coformer during the cocrystallization. In conclusion, the formation of a cocrystal is a joint result of several intermolecular interactions. Traditionally, a hydrogen bond is considered as an important driving force during cocrystals formation and used to be the first selection for cocrystal design. However, the explosives molecules are usually short of active hydrogen atoms and can not form strong hydrogen bonds. It seems that explosives are difficult to form cocrystals. However, it does not matter, for the explosive molecules can usually form strong p−π stacking and π−π stacking interactions, which can still make two kinds of molecules cocrystallize into one crystal cell. 2.1.2. BTF/TNAZ Cocrystal. In the cocrystal of BTF/TNAZ, the crystal structure is formed by alternately arranged TNAZ and BTF molecules, and the driving force for the formation of cocrystal is also a hydrogen bond, the p−π stacking interaction

Figure 9. Crystal structure of the BTF/TNAZ cocrystal. The repeat units with two molecules of A and B arranged in the a-axis (a) and the p−π interaction between A and B (b). 684

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2.2. Cocrystal Properties. To evaluate whether the cocrystal engineering is an effective way to improve the integrated performance of explosives, the physicochemical properties of the five cocrystals are investigated. First, differential scanning calorimetry (DSC) experiments were carried out to study the melting points of the five cocrystals, as shown in Table 3, to evaluate the thermal safety properties of Table 3. Properties of the Five Cocrystals and Their Coformers coformer name

Tm (°C)

BTF TNT TNB TNA MATNB TNAZ

197.4 80.5 122.9 184.2 109.0 100.7

cocrystal

H50 (cm)

ρ (g/cm3)

Tm (°C)

H50 (cm)

ρ (g/cm3)

21 59 77.8 61.9 46.4 28−29

1.901 1.654 1.69 1.77 1.642 1.84

132.6 189.0 205.8 171.3 164.5

36.2 42.2