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Synlett 2024; 35(17): 1997-2002
DOI: 10.1055/s-0043-1775365
letter
Energetic Molecules

A π-Stacked Highly Stable, Insensitive, Energy-Containing Material with a Useful Planar Structure

Shuliang Liu
a   School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. of China
,
Yuan Qi
a   School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. of China
,
Peng-cheng Zhang∗
b   Research Institute of Chemical Defense, Academy of Military Sciences, Beijing 102205, P. R. of China
,
Qiuhan Lin∗
a   School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. of China
› Author Affiliations

We are grateful to the National Natural Science Foundation of China (22375095) for funding this work.
› Further Information
 
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Abstract

π-Stacking is common in materials, but different π–π stacking modes remarkably affect the properties and performances of materials. In particular, weak interactions, π-stacking and hydrogen bonding often have a significant impact on the stability and sensitivity of high-energetic compounds. A fused [5,7,5]-tricyclic energetic compound with a conjugated structure has been designed and synthesized. 4H-[1,2,5]Oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine is obtained in 48% yield from 3-amino-4-carboxy-1,2,5-oxadiazole through an efficient two-step reaction. Owing to its layered planar structure and weak π interactions between layers, 4H-[1,2,5]oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine exhibits high thermal stability (T d = 318 °C), low sensitivity (IS = 40 J, FS = 360 N), and relatively excellent detonation performance (D = 7059 ms–1, P = 20.2 GPa). This detonation performance is superior to that of the conventional explosive TNT. The developed procedure provides a new method for the synthesis of fused ring compounds.


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Key words

energetic materials - explosives - nitrogen heterocycles - π-stacking - detonation - planar structure

Energetic materials have played significant roles in defense and civil industries since the modern explosive nitroglycerin was invented by Alfred Nobel.[1] For over a hundred years, energetic materials have witnessed significant development, and the energy density has been improved gradually from the level of 2-methyl-1,3,5-trinitrobenzene (TNT), through to those of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), reaching that of hexanitrohexaazaisowurtzitane (CL-20).[2] With an increase in energy, the exposed safety problems of energetic materials also increase due to the contradiction between the energy and the safety of energetic materials.[3] In designing and synthesizing a new generation of high-energy materials, researchers focus on coordinating the structure and properties via an effective combination and rational arrangement of structural properties, chemical bonds, and crystal engineering to balance the contradiction between the energy and the stability of energetic materials.[4]

In recent years, the contradiction between energy and safety has been balanced by adjusting the structural properties, mainly focusing on the construction of fused N-heterocyclic rings.[5] As the number of fused rings in a molecule increases, the larger the π-conjugated system becomes, and the more abundant are the weak interactions. Among them, π–π stacking can be divided into four categories, including face-to-face, wavy, cross and mixed stacking.[6] Face-to-face crystal stacking is the best type; this is due to the free sliding between layers, which can avoid increases in molecular vibration leading to explosive decomposition, hot-spot generation and eventual explosion.[6] Nitrogen-rich five-membered heterocycles have been widely studied for their high heat of formation (HOF) and environmentally friendly nature.[7] Among five-membered heterocycles, the triazole ring has a high HOF value (267 kJ mol–1), good stability and a conjugated structure.[2d] [8] The triazole ring also has two modified carbon sites and one NH site, which can be used as a basis to design and synthesize various energetic compounds. A furazan ring is an excellent structural unit, and related high-energy compounds based on this system not only maintain excellent detonation performance, but also have good stability (Figure [1]). The oxygen atom of the furazan ring can provide a lone pair electrons to form hydrogen bonds with other atoms, which is beneficial to the stability of compounds.[9]

Zoom Image
Figure 1 Four isomers of oxadiazoles
Zoom Image
Scheme 1 Synthesis of compound 3

In view of the above facts, we have synthesized the [5,7,5]-fused-ring compound 4H-[1,2,5] oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine (3), at the same time, introducing amino groups to form an extensive hydrogen bond network, thereby improving thermal stability and reducing sensitivity (Scheme [1]). The structure of compound 3 was characterized by IR, multinuclear NMR spectroscopy, elemental analysis and other techniques. Based on the crystal data, the relationship between its structural characteristics and sensitivity was explored by Hirshfeld surface, electrostatic potential (ESP) and non-covalent interaction (NCI) analysis.

Zoom Image
Figure 2 Molecular structure and crystal stacking of 3. (a) ORTEP representation of the molecular structure of 3. (b) Planar geometry of 3. (c) Crystal stacking obtained along the c-axis of the crystal cell. (d) Crystal stacking obtained along the a-axis of the crystal cell. (Hydrogen bonds are shown in pink).

The synthetic pathway towards compound 3 is shown in Scheme [1].[10] 3-Amino-4-carboxy-1,2,5-oxadiazole (1) was synthesized by the method described in the literature.[11] Next, 3-amino-4-(4,5-diamino-1,2,4-triazole-3-yl)-furazan (2) was obtained by mixing 3-amino-4-carboxy-1,2,5-oxadiazole (1) with diaminoguanidine salt and adding it to a preheated P2O5/H3PO4 solution, and then heating at reflux for 5 h.[9] [12] An acetonitrile solution of compound 2, cooled to 0 °C, was reacted with trichloroisocyanuric acid (TCCA) for half an hour, and then purified by silica gel chromatography (eluent: EtOAc/petroleum ether = 1:1) to afford the final product 3.[13]

In general, the planarity of a molecule is closely related to its crystal density.[14] Crystals of 3, suitable for single-crystal X-ray diffraction studies, were obtained by slow evaporation from methanol at room temperature. A crystal of dimensions 0.150 × 0.120 × 0.060 mm was selected for structural analysis. Compound crystallized as giant white crystal, containing a molecule of water of crystallization. The molecule crystallized in the monoclinic system and adopted the C2/c space group (Figure [2a]), with 4 molecules in each cell; the calculated density of the crystal at 296 K is 1.69 g/m3. The fused ring shows a nearly flat molecular geometry, and the system is rich in π–π interactions, The C–C, C–N, C=N, N–N and N–O bond lengths in the fused ring are 1.428–1.448, 1.335–1.378, 1.281–1.320, 1.395–1.397 and 1.392–1.397 Å, respectively. The length of the C–N bond is between that of the average of a single bond and the average of a double bond (1.47 Å and 1.22 Å, respectively), which may be due to the large π-conjugated structure of the molecule resulting in a high degree of internal charge separation. The lengths of the intermolecular and intramolecular hydrogen bonds formed by compound 3 with various molecules range from 1.934–2.621 Å, as shown in Table [1] (the intermolecular hydrogen bond interaction in compound 3 is N3–H3A…O2, and the D…A bond length is 2.194 Å). Among them, the A, B and C rings are pairwise planar, which can be seen from the torsion angles of N1–C4–N2–N1 (–177.54°), N1–C3–N4–N5 (–178.73°) and N1–C3–C2–C1 (179.08°), indicating that compound 3 has good planarity.

Table 1 The Hydrogen Bonds Present in Compound 3

D–H…A

d(D–H) [Å]

d(H…A) [Å]

d(D…A) [Å]

∠(D…A) [deg]

C5–H5…O1

0.93

2.62

3.173(2)

118.6°

N3–H3A…O2

0.86

2.19

2.959(3)

147.9°

N3–H3A…O2#2

0.86

2.20

3.043(3)

167.1°

N3–H3B…N8

0.86

2.31

3.157(2)

169.9°

N6–H6…N2

0.86

1.93

2.7939(19)

177.5°

Figures [2c] and d reveal the structural details of the fused-ring compound 3 in different directions. Compound 3 is a network of crystalline π–π deposits that enable it to exhibit excellent energy properties, good shock and friction sensitivity, and a high density (IS = 40 J; FS = 360 N).[15] Based on the large planar layered structure and intramolecular hydrogen bonding, a strong crossed π-stack with an interlayer distance of 3.352 Å is formed (Figure [2c]).

The molecular stability of energetic materials is the main parameter for evaluating their safety during use and storage. Undesirable thermal stability and low insensitivity to extra stimulation are quite detrimental to the extensive use of energetic compounds. Considering this fact, the thermal stability and insensitivity of the synthesized fused-ring compound 3 were determined experimentally.

Differential scanning calorimetry (DSC) was used to determine the thermal stability of compound 3 at a stable heating rate of 5 °C min–1. As shown in Figure [3], compound 3 exhibits excellent thermal stability (Td = 318 °C), a sharp exothermic peak at about 250 °C after melting endothermic heat and a decomposition heat much higher than most other reported high-energy materials, such as RDX (Td = 204 °C). According to the above single-crystal analysis, it can be seen that there are a large number of hydrogen bonds between compound 3 and water molecules, and there are π–π interactions between the upper and lower neighbors (see Figure [5]). The oxygen atoms on the furazan ring tend to form hydrogen bonds with the surrounding atoms, and the average length of these hydrogen bonds is 2.25 Å. Compared with other high-energy compounds (e.g., TNT), compound 3 has a larger number of hydrogen bond networks and π–π interactions, and therefore has better thermal stability.

Zoom Image
Figure 3 DSC curve of compound 3

The mechanical sensitivities, including the impact sensitivity (IS) and the friction sensitivity (FS), of compound 3 were measured by using the standard BAM method.[16] Compound 3 showed excellent insensitivity to impact and frictional stimuli. Its impact sensitivity is 40 J and its friction sensitivity is 360 N, which reveals that it is more stable than traditional explosives, e.g., TNT (15 J, 353 N) and RDX (7.5 J, 120 N),[17] and therefore has the potential to be applied as an insensitive high-energy material. The fused [5,7,5]-tricyclic compound 3 has an experimental density of 1.72 g cm–3. The detonation properties of the target compound were calculated using EXPLO5 (version 6.01)[18] based on experimental densities and calculated enthalpies of formation. The calculated detonation velocity and detonation pressure of fused compound 3 are 7509 ms–1 and 20.2 GPa, respectively (Table [2]). Compound 3 has detonation properties between those of TNT and RDX.

Table 2 Comparison of the Physical Properties of Compound 3 with TNT and RDX

3

TNT

RDX

Ρa (g cm–3)

1.72

1.65

1.80

ΔH f b (kJ mol–1)

606.3

–54.2

70.3

D c (m s–1)

7509

6881

8795

P d (GPa)

20.2

19.5

34.9

T d e (°C)

318

295

204

ISf (J)

40

15

7.5

FSg (N)

360

353

120

a Experimental density.

b Heat of formation.

c Detonation velocity.

d Detonation pressure.

e Thermal decomposition temperature (DSC, 5 °C min–1).

f Impact sensitivity.

g Friction sensitivity.

To further study the connections between the molecular structures, thermal stabilities, and mechanical sensitivities, the intermolecular and intramolecular interactions of 4H-[1,2,5]oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4] triazepin-8-amine (3) were studied via theoretical calculations. The theoretical methods include the Hirshfeld surface,[19] [20] noncovalent interaction (NCI),[21] and molecular electrostatic potential (ESP) analyses.[22] The data required are obtained from calculations using Gaussian 09[23] and Multiwfn.[24]

The two-dimensional (2D) fingerprints and Hirshfeld surfaces of compound 3 were analyzed. The analytical results were obtained using CrystalExplorer 17.5 software.[20] [21] The red dots and blue areas on the Hirshfeld surface represent high and low close contact groups, respectively. As shown in Figure [4a], compound 3 has an approximately planar structure, and the red dots showing close contacts between molecules are mostly located on the side surface of the molecule, indicating the existence of a large number of hydrogen bonds that maintain the stability of compound 3. The O–H and H–O groups and N–H and H–N groups interact closely, showing three spikes in the lower left corner of the two-dimensional fingerprint pattern (Figure [4b]). The proportion of the various contacts on the Hirshfeld surface is reflected in the ring diagram. For compound 3, N–H exposure represents the largest proportion, accounting for 25.2% of compound 3. The oxygen atom in compound 3 acts as the acceptor for the NH2 group hydrogen bond, so the N–H–O hydrogen bond in compound 3 is increased compared to N–H–N. From these data, it can be seen that the proportion of N–H is much higher than the proportion of N–O, C–N, and other weak contacts, which means that most of the interactions present in compound 3 are hydrogen bonds. Since these strong interactions favor the stability of high-energy materials, it is reasonable to understand how compound 3 has excellent thermal stability. In addition, π–π interactions (C–N and N–C and N–O and O–N) also exist in compound 3, accounting for 12.3% of this fused-ring compound 3, which can be further analyzed from non-covalent interaction (NCI) diagrams.

Zoom Image
Figure 4 (a) Hirshfeld surfaces for compound 3 and the individual atomic contact percentage contributions to the Hirshfeld surface. (b) Fingerprint plots for compound 3.
Zoom Image
Figure 5 (a) Analysis of non-covalent interactions of compound 3 (blue, strong attraction; green, weak interaction; red, strong repulsion).(b) Scatter graph.

In order to obtain more information on intramolecular and intermolecular effects and to fully investigate the effects of hydrogen bonds and π–π interactions on the crystal packing, we performed such analyses using Multiwfn.[21] The results are shown in Figure [5a]. Compound 3 shows a basic face-to-face π–π packing, which is beneficial for improved thermal stability and density. For compound 3, the centers of the B and C rings have red isosurfaces, indicating spatial repulsion. In addition, there is a hydrogen bond interaction between compound 3 and H2O, indicating that the stability of compound 3 depends not only on the π–π stacking interactions but also on hydrogen bond interactions.

It is worth noting that there is a large area of π–π stacked equipotential surface between two molecules of compound 3, and this interaction is widely distributed. Compared with TNT and 2,4,6-triamino-1,3,5-trinitrobenzene (TATB), compound 3 has a larger isosurface than TNT and a smaller isosurface than TATB, and a large number of blue patches representing strong hydrogen bonds can be observed for TATB (see Figure S8 in the Supporting Information). This result is partly consistent with the ultimate insensitivity of these high-energy compounds. The molecular reduced density gradient (RDG) scatter plot was also obtained to distinguish the intramolecular hydrogen bond strength at different substituent positions (Figure [5b]). The spike near the x axis at –0.03 represents the intramolecular hydrogen bond interaction, the green region represents the weaker molecular interaction and the van der Waals (vdW) interaction with lower electron density, and the spike in the red region represents the strong spatial effect. It is found to be consistent with the red regions on the three rings in Figure [5a]. In summary, the hydrogen bonds and π–π interactions in compound 3 help to increase its density, stability, and reduce its mechanical sensitivity.

For energetic compounds, their impact sensitivity is closely related to their surface electrostatic potential, hence the electrostatic potential of compound 3 was calculated to further analyze its stability (Figure [6]). The red and blue areas represent electropositivity and electronegativity, respectively, and the yellow and blue dots represent positive and negative electrostatic potentials, respectively. In general, a larger molecular electrostatic potential (ESP) value and a larger electropositive region tend to result in higher sensitivity. As can be seen from Figure [6], the blue dots are mainly concentrated near the furazan and 1,2,4-triazole rings, where the ESP value near the triazole ring is low, –57.79 kcal/mol, indicating that this region is more electron-rich and not easy to break. Two positive ESP values appear near the seven-membered ring, the maximum of which is +43.08 kcal/mol. Therefore, the seven-membered heteroatomic ring is more prone to fracture, which corresponds to the red equivalent surface in the NCI.

Zoom Image
Figure 6 Molecular electrostatic potentials of compound 3

In conclusion, the high-energetic material, 4H-[1,2,5]oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine (3), possessing a planar structure and weak π interactions between layers has been synthesized. The structure of compound 3 was confirmed by IR, multinuclear NMR spectroscopy and elemental analysis. Based on the results of single-crystal X-ray structure analysis, Hirshfeld surface, non-covalent interaction (NCI) and electrostatic potential (ESP) analyses were carried out to further study the structural characteristics and properties of compound 3. The results show that the planar structure of compound 3 leads to the formation of abundant π–π interactions and hydrogen bond networks. 4H-[1,2,5]Oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine (3) has extremely high thermal stability with a decomposition temperature of 318 °C and excellent insensitivity to impact and friction (IS = 40 J; FS = 360 N), and a relatively excellent detonation performance (D = 7560 ms–1, P = 21 GPa).

The presence of the amino substituent affects the oxygen balance and detonation performance of compound 3. Thus, transformation of this amino functionality into a nitramino group via nitration, or its oxidation to a nitro group, is expected to improve the detonation performance of compound 3.


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We thank Lan Yi and Ze Xu (Nanjing University of Science and Technology) for recording the NMR spectra.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0043-1775365.
  • Supporting Information

  • References and Notes

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  • 13 4H-[1,2,5]Oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine (3) Compound 2 (0.99 g, 5.4 mmol) was dissolved in acetonitrile (20 mL), the solution was cooled to 0 °C and TCCA (2.325 g, 10 mmol) was added dropwise. The reaction mixture was further stirred at 0 °C for 30 min. The solution was neutralized with solid Na2CO3 and then filtered to remove insoluble solids. The filter cake was washed several times with acetonitrile and the combined filtrates were concentrated by vacuum evaporation. The crude product obtained after removal of the solvent under vacuum was purified by silica gel chromatography (eluent: EtOAc/petroleum ether = 1:1) to give product 3 as a light yellow solid. Yield: 0.65 g (48%). T d = 318 ℃. IR (KBr): 3202, 3049, 2921, 2832, 1693, 1455, 1413, 1390, 1051, 977, 845, 786, 760, 739, 692 cm–1. 1H NMR (500 MHz, DMSO-d 6): δ = 11.15 (s, 1 H), 4.30 (br s, 1 H), 1.29 (s, 2 H). 13C NMR (125.72 MHz, DMSO-d 6): δ = 155.72, 153.1, 140.24, 135.06, 40.2. Anal. Calcd for C5H4N8O: C, 31.26; H, 2.10; N, 58.32, O, 8.33. Found: C, 31.06; H, 2.40; N, 58.02; O, 8.43.
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Corresponding Authors

Qiuhan Lin
School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology
Nanjing 210094
P. R. of China   
linqh@njust.edu.cn

Peng-cheng Zhang
Research Institute of Chemical Defense, Academy of Military Sciences
Beijing 102205
P. R. of China   

Publication History

Received: 12 December 2023

Accepted after revision: 14 May 2024

Article published online:
02 July 2024

© 2024. Thieme. All rights reserved

Georg Thieme Verlag KG
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  • References and Notes

  • 5 Lei C, Tang J, Zhang Q, Yang H, Cheng G. Org. Lett. 2023; 25: 2461
    CrossrefPubMedSearch in Google Scholar
  • 7 Wang Y, Ye J, Yang N, Ma H, Zhang Y, Guo Z. CrystEngComm 2021; 23: 7635
    CrossrefSearch in Google Scholar
  • 9 Cheng B, Yang F, Zhang J, Zhang L, Zhao P, Lin Q. Cryst. Growth Des. 2023; 23: 1466
    CrossrefSearch in Google Scholar
  • 10 Caution! The target compounds in this article are energetic materials with explosive properties. All synthetic manipulations must be carried out using appropriate equipment (fume hood, safety shield) and protective gear (lab coat, goggles and gloves). Compounds should be synthesized on a small scale and mechanical actions involving scratching or scraping must be avoided.
  • 11 Willer RL, Storey RF, Frisch M, Deschamps JR. J. Heterocycl. Chem. 2012; 49: 227
    CrossrefSearch in Google Scholar
  • 12 3-Amino-4-(4,5-diamino-1,2,4-triazol-3-yl)-furazan (2) Phosphorus pentoxide (10 g, 70.4 mmol) was slowly added to phosphoric acid (85 wt%, 30 g, 260 mmol) and the mixture was then heated to 50 °C with stirring. A mixture of 3-amino-4-carboxy furazan (3.87 g, 30 mmol) and diaminoguanidine monohydrochloride (6.15 g, 49 mmol) was added to the preheated solution. After the addition was complete, the mixture was heated to 120 °C and the evolution of gaseous HCl was observed. The mixture was kept at 120 °C for 5 h and then cooled to room temperature. Ice water (150 mL) was added to the cooled reaction mixture and a white precipitate was formed. The mixture was made basic (pH 8) using conc. NaOH solution and the precipitate was filtered, washed repeatedly with water and air-dried to obtain crude product 2. Yield: 3.4 g (62%). IR (KBr): 3436, 3352, 1690, 1608, 1404, 1380, 1340, 1183, 1045, 990, 907, 769, 688, 581 cm–1. 1H NMR (500 MHz, DMSO-d 6): δ = 5.91 (2 H, s), 6.26 (2 H, s), 6.58 (2 H, s). 13C NMR (125.72 MHz, DMSO-d 6): δ = 157.18, 154.98, 140.27, 136.58. Anal. Calcd for C4H6N8O (182.15): C, 26.38; H, 3.33; N, 61.52. Found: C, 26.46; H, 3.28; N, 61.31.
  • 13 4H-[1,2,5]Oxadiazolo[3,4-e][1,2,4]triazolo[3,4-g][1,2,4]triazepin-8-amine (3) Compound 2 (0.99 g, 5.4 mmol) was dissolved in acetonitrile (20 mL), the solution was cooled to 0 °C and TCCA (2.325 g, 10 mmol) was added dropwise. The reaction mixture was further stirred at 0 °C for 30 min. The solution was neutralized with solid Na2CO3 and then filtered to remove insoluble solids. The filter cake was washed several times with acetonitrile and the combined filtrates were concentrated by vacuum evaporation. The crude product obtained after removal of the solvent under vacuum was purified by silica gel chromatography (eluent: EtOAc/petroleum ether = 1:1) to give product 3 as a light yellow solid. Yield: 0.65 g (48%). T d = 318 ℃. IR (KBr): 3202, 3049, 2921, 2832, 1693, 1455, 1413, 1390, 1051, 977, 845, 786, 760, 739, 692 cm–1. 1H NMR (500 MHz, DMSO-d 6): δ = 11.15 (s, 1 H), 4.30 (br s, 1 H), 1.29 (s, 2 H). 13C NMR (125.72 MHz, DMSO-d 6): δ = 155.72, 153.1, 140.24, 135.06, 40.2. Anal. Calcd for C5H4N8O: C, 31.26; H, 2.10; N, 58.32, O, 8.33. Found: C, 31.06; H, 2.40; N, 58.02; O, 8.43.
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Figure 1 Four isomers of oxadiazoles
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Scheme 1 Synthesis of compound 3
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Figure 2 Molecular structure and crystal stacking of 3. (a) ORTEP representation of the molecular structure of 3. (b) Planar geometry of 3. (c) Crystal stacking obtained along the c-axis of the crystal cell. (d) Crystal stacking obtained along the a-axis of the crystal cell. (Hydrogen bonds are shown in pink).
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Figure 3 DSC curve of compound 3
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Figure 4 (a) Hirshfeld surfaces for compound 3 and the individual atomic contact percentage contributions to the Hirshfeld surface. (b) Fingerprint plots for compound 3.
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Figure 5 (a) Analysis of non-covalent interactions of compound 3 (blue, strong attraction; green, weak interaction; red, strong repulsion).(b) Scatter graph.
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Figure 6 Molecular electrostatic potentials of compound 3