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DOI: 10.1039/D4CC03260H (Communication) Chem. Commun., 2024, Advance Article

Construction of heterocycle-triazolotriazine framework energetic compounds: towards novel high-performance explosives†

Pengju Yangab, Xiaoxiao Zhengc, Guojie Zhangd, Caijing Leia, Guangbin Cheng*a and Hongwei Yang*a
aSchool of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu, China. E-mail: gcheng@mail.njust.edu.cn; hyang@mail.njust.edu.cn; Fax: (+86) 25 8430 3286A
bTianyuan (Hangzhou) New Material Technology Co., Ltd, Hangzhou, Zhejiang, China
cZhejiang Dayang Biotechnology Group Co., Ltd, Hangzhou, Zhejiang, China
dSchool of Materials and Chemistry, Southwest University of Science and Technology, Mianyang, Sichuan, China

Received 30th June 2024 , Accepted 20th August 2024

First published on 22nd August 2024

Abstract

In this paper, three neutral heterocycle-triazolotriazine compounds featuring multiple amino groups and nitro groups were designed and synthesized. Among them, compounds 2 and 6 exhibit high detonation performance (Dv = 8180 m s−1, 8650 m s−1; P = 26.40 GPa, 31.5 GPa), low sensitivities (IS > 40 J, FS > 360 N) and high thermal stabilities (Td = 319 °C, 320 °C) suggesting their potential as alternatives to the traditional thermal-stable explosive HNS (Dv = 7612 m s−1, P = 24.3 GPa, IS = 5 J, FS = 240 N; Td = 318 °C). Meanwhile, compound 4 displays excellent properties (Dv = 8810 m s−1, IS = 15 J, FS = 240 N, Td = 215 °C, ρ = 1.84 g cm−3) which is superior to traditional explosive RDX (Dv = 8795 m s−1, IS = 7.5 J, FS = 120 N, Td = 208 °C, ρ = 1.80 g cm−3) making it a promising candidate as a novel secondary explosive. This research not only advances the field of triazolotriazine-based energetic materials but also explores their potential applications as heat-resistant or high-energy explosives.

Energetic materials (EMs) are a range of sub-stable substances that can release a large amount of energy and gas driven through external stimuli. For a specific energetic material, the organic energetic component usually plays a decisive role in the overall detonation properties. The last few centuries have witnessed the emergence of a large amount of organic energetic compounds.1–15 Some of them (e.g., 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), cyclo-1,3,5,7-tetramethylene-2,4,6,8-tetranitramine (HMX), and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12 hexaazatetracyclo[5.5.0.0.0]dodecane (CL-20)) have been widely applied in military and civilian aspects.16–18 Along with social development and human progress, safety issues gradually drawn the eager attention in the research circle of EMs, resulting in the rapid growth of high-energy low-sensitivity energetic materials, for example, the appearance of insensitive explosives TATB and NTO.8,11 Over time, the criteria for preliminary judging potential high-energy compounds have expanded. Researchers gradually realized the giant influence of the heat of formation (HOF) on the overall detonation properties. Fig. 1a lists the common N-heterocycles including 1,3,4-oxadiazole,19 1,2,4-oxadiazole,20 1,2,5-oxadiazole,21 pyrazole,22 1,2,4-triazole,23 and 1,2,3-triazole.24 And their heats of formation were summarized as 71.76 kJ mol−1, 99.80 kJ mol−1, 215.72 kJ mol−1, 177.4 kJ mol−1, 192.70 kJ mol−1, and 233.71 kJ mol−1 accordingly. By introducing pyrazole, triazole, etc. into the framework, the heat of formation can be obviously increased and the detonation properties could also be improved.
image file: d4cc03260h-f1.tif
Fig. 1 (a) Heat of formation of common N-heterocycles. (b) Reported aromatic energetic compounds. (c) This work.

Heat-resistant explosives characterized by high decomposition temperature (>300 °C) and high energy have garnered global attention due to the rapid advancements in deep-space and deep-sea exploration. Two notable examples of these heat-resistant explosives are HNS and TATB (Fig. 1b). However, as society advances, their application is hindered by lower energy levels and there is a growing demand for higher energy levels in heat-resistant explosives. In response to this need, various strategies have been implemented to synthesize high-energy, heat-resistant explosives.25–29 In previous work (Fig. 1b), our group synthesized a fused-ring compound 3-nitro-[1,2,4]triazolo[5,1-c][1,2,4]triazin-4-amine (I) using nitroacetonitrile sodium salt as the reagent.30 Although its explosive velocity (8329 m s−1) surpasses HNS (7612 m s−1) and TATB (8193 m s−1), the low decomposition temperature (275.6 °C) hinders its potential as a heat-resistant explosive.

In this work, nitropyrazole, nitrotriazole, and amino-oxadiazole are attached with compound I by a carbon–carbon bridge to further improve the comprehensive properties and prepare high-performance explosives. As illustrated in Fig. 1c, the decomposition temperature of compound 2 was increased by 43.5 °C at the cost of the reduced detonation performance (Dv = 8180 m s−1; P = 26.4 GPa), but the excellent overall performance supported its potential as a novel heat-resistant explosive. By introducing the nitrotriazole into the framework the detonation performance (Dv = 8810 m s−1; P = 33.2 GPa) of compound 4 has significantly increased, but the decomposition temperature was obviously decreased due to the poor stability of 1,2,3-triazole. Despite all these, compound 4 displays excellent properties (Dv = 8810 m s−1, IS = 15 J, FS = 240 N, Td = 215 °C, ρ = 1.84 g cm−3) which is superior to traditional explosive RDX (Dv = 8795 m s−1, IS = 7.5 J, FS = 120 N, Td = 208 °C, ρ = 1.80 g cm−3) making it a promising candidate as a novel secondary explosive. Compared with compound I, compound 6 shows comprehensive enhancement in overall properties, indicating its potential as a novel high-energy heat-resistant explosive. This work not only proves the significance of structural renovation in the quest for high-performance explosives but also underscores the significant potential of triazolotriazine compounds in energetic materials.

The detailed synthetic procedure is shown in Scheme S1 (ESI†). 5-(3-nitro-1H-pyrazol-4-yl)-4H-1,2,4-triazol-3-amine (1), 5-(5-nitro-2H-1,2,3-triazol-4-yl)-4H-1,2,4-triazol-3-amine (3) and 4-(5-amino-4H-1,2,4-triazol-3-yl)-1,2,5-oxadiazol-3-amine (5) were prepared according to the previously reported.ref. 24 and 31 Intermediates 2-p, 4-p and 6-p were precipitated as yellow solids by stirring compounds 1, 3, 5 and equimolar sodium nitrite in 1 N hydrochloric acid, followed by stirring the nitroacetonitrile sodium salt in the above solution at a low temperature for about 1 h. The suspensions were warmed to room temperature and stirred for 72 h to precipitate fused-ring compounds 2, 4, and 6 as three brown solids. To lower time cost, the intermediates 2-p, 4-p and 6-p were filtered and stirred in a mixture of water and methanol at elevated temperature to prepare compounds 2, 4, and 6 in 8 h. Through this optimization, the total reaction consumption time can be reduced from 73 hours to 9 hours.

All compounds were fully identified by infrared spectroscopy (IR), 500 M nuclear magnetic resonance spectroscopy (1H and 13C NMR), and elemental analyzer (EA). The crystals of 2, 4·2H2O and 6·DMF were identified by single-crystal X-ray diffractometer. Their energy properties were thoroughly characterized by using EXPLO5, gaussian09, gas pycnometer, a BAM friction tester and a standard BAM Fallhammer. By combining the quantum chemistry, crystal analysis and measured detonation performance, the structure–performance relationship of this series of fused-ring compounds was sufficiently studied.

Interestingly, the amino groups in compound 5 underwent selective diazotization (Scheme S1, ESI†). Only the amino group (designated as amino B) which linked to the 1,2,4-triazole reacted with sodium nitrite, while the amino (designated as amino A) which linked to the 1,2,5-oxadiazole did not. As we know, the selectivity of the reaction is related to the molecule's charge distribution and steric hindrance. Thus, two types of theoretical calculations about the active sites including electrostatic potential surfaces (ESP) and average local ionization energy (ALIE) were carried out to explain the different reactivity of these two amino groups.32,33 The minimal values of ESP for compound 5 were calculated and presented in Fig. 2a, among which three nearby amino A are −21.53 kcal mol−1, −7.82 kcal mol−1, and −13.16 kcal mol−1 while one nearby amino B is −11.03 kcal mol−1. This result suggests that the regions near amino A and B were affected deeper by the negative charge and could be attacked by the electrophilic reagent, which is consistent with the result of ALIE (Fig. 2b). However, the fact which amino group could ultimately be attacked by the electrophilic reagent depends on their activity and the steric hindrance. Thus, the activity of the hydrogen atoms in amino A and B is studied by the nuclear electrostatic potential (NEP). According to the result in Fig. 2c, the hydrogen atoms in amino B have a lower electrostatic potential (weaker binding force for proton, the faster the dehydration), indicating the higher nucleophilic activity of amino B compared with amino A. The non-covalent interaction analysis (NCI) of compound 5 was employed to study the intramolecular weak interaction and the steric hindrance. As shown in Fig. 2d, the green part surrounded by the red ellipse indicates the hydrogen bond and the red part indicates the intramolecular repulsive force between amino A and the triazole ring. Due to the higher nucleophilic activity of amino B and the existence of these interactions between amino A and the triazole ring, electrophile reagents tend to attack amino B preferentially. Besides, the Frontier molecular orbital theory (FMO) including HOMO and LUMO orbitals were applied to further verify the differences in amino activity of compound 5. As shown in Fig. 2e and f, the HOMO orbitals in compound 5 mainly distribute in the vicinity of amino B and the individual atomic contribution of N12 (20.31%) is larger than that of N11 (10.18%), indicating large amounts of electrons, which could be preferably attacked by electrophilic reagents. All these results indicate amino B will be attacked by electrophile reagents, resulting in the formation of compound 6.


image file: d4cc03260h-f2.tif
Fig. 2 (a) ESP analysis of compound 5. (b) ALIE analysis of compound 5. (c) NEP analysis of compound 5. (d) NCI analysis of compound 5. (e) LUMO orbital of compound 5. (f) HOMO orbital of compound 5.

Crystals of compounds 2, 4 and 6 were gained by slowly evaporating the saturated N,N-dimethylformamide (DMF) or water solution. Detailed crystal information about bond distances, bond angles, torsion angels and CCDC numbers is recorded in the ESI.†

The crystal analysis results (Fig. 3) show that the crystals of compound 2 belong to the orthorhombic crystal system, the space group is Pbcn, and the unit cell parameters a = 15.467(3) Å, b = 6.7622(7) Å, c = 20.528(2) Å, α = 90°, β = 90°, γ = 90°. There are eight smallest repeating units in a single cell with a crystal density of 1.808 g cm−3. All atoms of the triazolotriazine ring are coplanar, while the nitropyrazole ring is distorted out of the plane due to steric hindrance, and the torsion angle is 49.2°. Six hydrogen bonds (HBs) are present in the crystals. In these HBs, two intramolecular hydrogen bonds N5–H5A⋯N6 and N5–H5B⋯O1 improve the thermal stability due to enhanced conjugation. Molecules 2 are arranged in two directions in the crystal, stacked face-to-face and extended infinitely, forming the overall stacking diagram (Fig. 3d).


image file: d4cc03260h-f3.tif
Fig. 3 (a) The crystal structure of 2. (b) The packing pattern of 2 in a unit cell. (c) The structure planarity of 2. (d) The overall packing of 2.

4·2H2O crystallizes in the monoclinic space group P21/n with a measured crystal density of 1.772 g cm−3 at 170 K. There are eight smallest repeating units in a cell and the cell parameters a = 9.966(2) Å, b = 13.360(3) Å, c = 18.602(4) Å, α = 90°, β = 94.827(7)°, γ = 90°. All atoms of the triazolotriazine ring are coplanar (Fig. 4c), while 1,2,3-triazole is twisted out of the plane due to steric hindrance with a torsion angle of 25.4°. Due to the presence of water molecules, a large number of HBs are present in the crystals. In these HBs, two intramolecular hydrogen bonds N6–H6B⋯O2 and O8–H8A⋯O1 help to improve the thermal stability because of the enhanced conjugation. A minimal repeating unit is composed of one molecule 4 and two water molecules, which stacks face to face and extends infinitely to form an overall stacking diagram (Fig. 4d).


image file: d4cc03260h-f4.tif
Fig. 4 (a) The crystal structure of 4·2H2O. (b) The packing pattern of 4·2H2O in a unit cell. (c) The structure planarity of 4. (d) The overall packing diagram of 4·2H2O.

The crystal of 6·DMF belongs to the monoclinic space group C2/c with a measured crystal density of 1.581 g cm−3 at 170 K. There are eight molecules in a single cell (Z = 8). Detailed crystal structures and the packing diagrams are shown in Fig. 5a–d. As shown in the figures, all atoms in molecule 6 are coplanar and six types of hydrogen bonds (HBs) are recorded in the crystal information, including N2–H2A⋯O3, N2–H2A⋯N6, N2–H2B⋯O1, N8–H8A⋯O3, N8–H8A⋯N6 and N8–H8B⋯N9. Of these hydrogen bonds, four hydrogen bonds N2–H2A⋯N6, N2–H2B⋯O1, N8–H8A⋯N6 and N8–H8b⋯N9 belongs to intramolecular interactions, which is conducive to enhancing the conjugation of the system, thereby increasing the thermal stability of compound 6. In a single unit cell, eight compound molecules are arranged side by side in two directions, which extend infinitely and form an overall 3D stacking structure (Fig. 5d).


image file: d4cc03260h-f5.tif
Fig. 5 (a) The crystal structure of 6·DMF. (b) The packing pattern of 6·DMF in a unit cell. (c) The structure planarity of 6. (d) The overall packing diagram of 6·DMF.

The mechanical sensitivities including impact sensitivity (IS) and friction sensitivity (FS) were assessed by using the BAM drop hammer test and BAM friction sensitivity meter. According to the result, compounds 2 and 6 are insensitive to external mechanical (IS > 40 J; FS > 360 N) while compound 4 has moderate mechanical sensitivities (IS = 15 J, FS = 240 N) which are superior to that of classic explosive RDX (IS = 7.5 J; FS = 240 N).

The heat of formation (HOF) of 2, 4 and 6 were computed by software Gaussian 09. Because of the high positive enthalpy of formation for pyrazole, 1,2,3-triazole and furazan, the total enthalpies of formation for these compounds are up to 695.0 kJ mol−1, 821.0 kJ mol−1 and 710.5 kJ mol−1, respectively. The densities for compounds 2, 4 and 6 in 298 K were measured by using a gas pycnometer to predict the detonation performance. To obtain relatively accurate solvent-free densities, samples from the same batch for thermal stability were directly used in this measurement. According to the result, all compounds 2, 4 and 6 show decent densities of 1.79 g cm−3, 1.84 g cm−3 and 1.86 g cm−3 which are compared to that of RDX (ρ = 1.80 g cm−3).

The detonation performances were predicted by EXPLO5 (version 6.05). Among them, the detonation properties of 2 are better than that of typical heat-resistant explosive HNS (2: Dv = 8180 m s−1, P = 26.4 Gpa vs. HNS: Dv = 7612 m s−1; P = 24.3 Gpa) while the detonation performances of 4 are similar with that of traditional explosive RDX (4: Dv = 8810 m s−1, P = 33.2 Gpa vs. RDX: Dv = 8795 m s−1, P = 34.9 Gpa). Compound 6 has good detonation performance thanks to the high enthalpy of formation and the high density (Table 1).

Table 1 Properties of compounds 2, 4, 6, I, HNS, and RDX
Compd Tda [°C] ρb [g cm−3] HOFc [kJ mol−1] Dd [m s−1] Pe [GPa] ISf [J] FSg [N]
a Onset decomposition temperature (heating rate: 10 °C min−1).b Measured density at 298 K.c Calculated enthalpy of formation in solid state.d Detonation velocity.e Detonation pressure.f Impact sensitivity.g Friction sensitivity.h Ref. 16i Ref. 20j Ref. 13
2 319 1.79 695.0 8180 26.4 >40 >360
4 215 1.84 821.0 8810 33.2 15 240
6 320 1.86 710.5 8620 31.5 >40 >360
Ih 275.6 1.78 480.6 8329 27.0 >40 >360
HNSi 318 1.75 78.2 7612 24.3 5 240
 
RDXj 204 1.80 70.3 8795 34.9 7.5 120

In this paper, three fused-ring compounds 2, 4 and 6, which combine the high enthalpies of formation of pyrazole, 1,2,3-triazole and furazan and the prominent properties of 5,6-fused triazolotriazine ring were successfully synthesized. Among the three compounds, compound 6 has several desirable characteristics, including a coplanar structure, high decomposition temperature (320 °C), low sensitivity (IS > 40 J, FS > 360 N) and high density (1.86 g cm−3). In addition, it exhibited several unique features, such as greater intramolecular hydrogen bonding, higher heat of formation (710.5 kJ mol−1), enhanced aromaticity and greater detonation performance (Dv = 8620 m s−1, P = 31.5 GPa), rendering it a promising alternative of the traditional thermal-stable explosive HNS (Dv = 7612 m s−1, P = 24.3 GPa, IS = 5 J, FS = 240 N; Td = 318 °C).

This work was supported by the National Natural Science Foundation of China [22075143, 21875110, 22205110], the Natural Science Foundation of Jiangsu Province BK [20220958], and the Fundamental Research Funds for the Central Universities [30922010404]. H. Yang thanks the Qing Lan Project for the grant.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no financial interests that need to declare.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2254742, 2190888 and 2190890. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc03260h
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