4-Amino-1,2,3-triazine 2-oxide: a promising structural unit for the design and synthesis of novel energetic materials with good thermal stability and low impact sensitivity

Abstract

A new [6,6]-fused ring energetic molecule, 4-amino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (1), was designed and synthesised using 4-amino-1,2,3-triazine 2-oxide as the basic skeleton unit. Subsequently, an amino group was incorporated into the corresponding position of compound 1via a vicarious nucleophilic substitution (VNS) reaction, resulting in the formation of 4,5-diamino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (2). Despite the introduction of adjacent C–NO₂/C–NH₂ blocks into the molecular structure of compound 2, which is generally accepted to contribute to the increase in the thermal decomposition temperature of energetic molecules, the results of thermal analysis demonstrated that the thermal decomposition temperature of compound 2 (T_d = 285 °C) was lower than that of its precursor (T_d = 311 °C). This suggested that the incorporation of adjacent C–NO₂/C–NH₂ blocks into the molecular structure did not inevitably lead to the formation of novel energetic molecules with enhanced thermal decomposition temperatures. To elucidate the mechanism behind this phenomenon, the structures of compounds 1 and 2 were subjected to detailed analysis using X-ray diffraction and quantum chemical calculations. Both 1 and 2 displayed high resistance to mechanical impact and were prepared using straightforward methods. The aforementioned results suggested that both 1 and 2 can be employed as heat-resistant, insensitive energetic materials.

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Introduction

Energetic materials are specialized functional materials that can rapidly release energy through their own chemical reactions when stimulated, without the need for external substances. Since their inception, energetic materials have found extensive applications in both civil and military sectors. As industries such as aerospace propulsion, military weaponry, and the exploitation of underground resources continue to evolve, new demands for energetic materials with good thermal stability have emerged.^1–4 To achieve new energetic molecules with good thermal stability, a common strategy is to introduce heat-resistant units into the molecular structure (Fig. 1). A typical case is the good thermal stability of 1,3,5-trinitrobenzene,^5,6 which led researchers to introduce picryl groups into the molecular structures, resulting in various heat-resistant explosives such as 2,2′,4,4′,6,6′-hexanitrostilbene (HNS),⁷ 2,6-bis-(picrylamino)-3,5-dinitropyridine (PYX)⁸ and 5,5′-bis-(2,4,6-trinitrophenyl)-2,2′-bi(1,3,4-oxadiazole) (TKX-55).⁹ Despite their excellent thermal stability, energetic molecules with picryl groups pose safety risks during storage and transportation due to their high mechanical sensitivity. Another feasible strategy to obtain novel energetic materials with good thermal stability is to introduce adjacent nitro and amino structures into the molecular backbone. It is postulated that the introduction of adjacent C–NO₂/C–NH₂ blocks into the molecular skeleton may facilitate the formation of intramolecular hydrogen bonds, which could potentially enhance thermal stability (Fig. 1).¹⁰ To date, this strategy has been adopted to design and synthesize a variety of new energetic molecules with good thermal stability. For instance, 4-amino-3,7-dinitro-[1,2,4]triazolo[5,1-c][1,2,4]triazine¹¹ and 4-amino-3,7,8-trinitropyrazolo-[5,1-c][1,2,4]triazine^12,13 present good thermal stability.

Fig. 1 Design strategies for heat-resistant energetic molecules.

In previous years, two heat-resistant and insensitive energetic materials were obtained by amination and acidification/methylation reactions of 6-nitro-7-azido-pyrazol[3,4-d][1,2,3]triazine-2-oxide (ICM-103).^14,15 Subsequently, 4-nitro-7-azido-pyrazolo [4,3-d][1,2,3]triazine-2-oxide (NAPTO) was treated with NH₃ and a novel energetic material was obtained with good thermal stability and low mechanical sensitivity.^16,17 Upon comparing the molecular structures of these new energetic materials, it was discovered that all of them contained fused 4-amino-1,2,3-triazine 2-oxide. Inspired by these results, it was believed that 4-amino-1,2,3-triazine 2-oxide is a promising structural unit. By conducting molecular design and synthesis around this structural unit, it was considered possible to obtain new energetic molecules with both good thermal stability and low impact sensitivity.

Based on these considerations, 4-azido-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide is a suitable precursor. The synthesis of this precursor is relatively straightforward, with the 4-amino-1,2,3-triazine 2-oxide structural unit being introduced after a simple functional group transformation. Furthermore, the benzene ring of the precursor molecule can be readily modified at multiple sites. The amino group can be introduced at the ortho position of the nitro group through the vicarious nucleophilic substitution (VNS) reaction, thereby forming intramolecular hydrogen bonds which enhance the thermal stability of the target molecule.

To confirm the possibility of the hypothesis, the synthesis of target compound 2 (4,5-diamino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide) was studied. As illustrated in Scheme 1, the target compound was synthesized through a straightforward two-step process. The resulting compounds were fully characterized using high-resolution mass spectrometry, infrared spectroscopy, and ¹H and ¹³C NMR spectroscopy (see the ESI†). The thermal decomposition temperatures of compounds 1 and 2 were determined by TG-DSC analysis. Surprisingly, the TG-DSC test results showed that the thermal decomposition temperature of 2 is 285 °C, which is lower than the thermal decomposition temperature of its precursor 1 (311 °C). This is a unique case where the formation of adjacent C–NO₂/C–NH₂ blocks does not increase the thermal decomposition temperature of the target molecule as expected (Fig. 1). To elucidate the mechanism underlying this phenomenon, X-ray diffraction analysis and quantum chemical calculations were employed to investigate the structures of 1 and 2. Novel insights were obtained into the relationship between the structure of energetic molecules and their thermal stability.

Scheme 1 Routes for the synthesis of compounds 1 and 2.

Results and discussion

Synthesis

The synthetic routes of 1 and 2 are shown in Scheme 1. Precursor 4-azido-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide was readily synthesized by reaction according to a known literature report.¹⁸ Then the fused-ring precursor was reacted with ammonia in acetonitrile to yield the amination product 4-amino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (1). Subsequently, 4,5-diamino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (2) was obtained as a brown solid via a vicarious nucleophilic substitution (VNS) reaction between compound 1 and hydroxylamine. Characterization of all new compounds was accomplished via NMR spectroscopy, HRMS, IR spectroscopy, and single-crystal X-ray diffraction. Crystallographic data and data collection parameters can be found in the ESI.†

Single-crystal analysis

To determine the molecular structures and investigate the intermolecular interactions of the prepared energetic derivatives, X-ray single-crystal analysis was conducted. All the crystal data and CCDC numbers of compounds 1 and 2 are reported in the ESI.† Specifically, their single crystals were grown from solutions of acetonitrile or methanol. As shown in Fig. 2, compound 1 crystallized in the monoclinic space group P2₁/c with a calculated density of 1. 502 g cm⁻³ at 296 K. Fig. 2 illustrates that the unit consists of one molecule of compound 1 and one acetonitrile molecule. Notably, its fused-ring backbone is nearly coplanar, which is supported by the torsion angles C1–C2–C3–C4 = 178.07° and C7–C2–C3–N4 = −177.97°. Additionally, the formation of intramolecular C–H⋯O interactions with lengths of 2.4216 Å and 2.4063 Å is observed. The insertion of acetonitrile molecules further enriched intermolecular interactions but diminished the crystal density. Owing to the π–π interaction, the crystal packing of compound 1·MeCN formed a wave-like layered arrangement.

Fig. 2 (a)–(d) Molecule structure, H-bonding networks, and crystal packing of 1·MeCN. (e)–(h) Molecule structure, H-bonding networks, and crystal packing of 2.

The crystal of compound 2 belonged to the P2₁2₁2₁ space group and orthorhombic system and tetragonal crystal system with four molecules per unit cell (Z = 4). The crystal density of this compound was measured to be 1.877 g cm⁻³ at 102 K. It is worth noting that the benzene ring and triazine ring of compound 2 are not coplanar, and there is a torsion angle of 8.23° between them. This may be caused by the repulsion between the two amino groups. Additionally, the formation of an intramolecular N–H⋯O hydrogen bond with a length of 1.9952 Å is observed. The introduction of the amino group not only forms a strong intramolecular hydrogen bond in compound 2, but also enhances the intermolecular hydrogen bonds and increases crystal density. In weak interaction networks, intermolecular hydrogen bonds with lengths of 2.5717 Å and 2.5464 Å were present with neighboring molecules. Furthermore, due to the presence of other weak interactions, the crystal packing of compound 2 also produced a wave-like layered arrangement.

To gain insight into the interactions among the target molecules contained in the single crystal, a two-dimensional (2D) fingerprint based on Hirshfeld surfaces was employed to analyse intramolecular interactions. Fig. 3 shows that red spots are typically located on the edges of Hirshfeld surfaces, indicating strong O⋯H and N⋯H interactions. Blue regions, represented by π–π accumulations, generally spread over the molecule plane.¹⁹ This result provides additional evidence for the planar molecule structure. Additionally, two crystals are dominated by O⋯H (30.3% in 1·MeCN and 36.3% in 2) and N⋯H (14.9% in 1·MeCN and 9.4% in 2) interactions, indicating the crucial role of hydrogen bonds in crystal self-assembly. Correspondingly, the proportion of C⋯O (18.7% in 1·MeCN and 17.2% in 2) and N⋯O (7.1% in 1·MeCN and 12.7% in 2) interactions in 1·MeCN and 2 is also high, which indicates the existence of strong π–π or p–π interactions between molecules. The insensitivity of compounds 1 and 2 may be attributed to both hydrogen bonding interactions and other weak interactions π–π interactions, helping to reduce the mechanical sensitivity of compound 2.

Fig. 3 (a) Hirshfeld surfaces of 1. (b) The 2D fingerprint plots of 1. (c) Atomic interaction proportions of 1. (d) Hirshfeld surfaces of 2. (e) The 2D fingerprint plots of 2. (f) Atomic interaction proportions of 2.

Additionally, interaction region indicator (IRI) analysis²⁰ was employed to study the interaction effects within the intermolecular framework of 1 and 2. As shown in Fig. 4 the green isosurface between the central molecule and the surrounding fragments suggests the presence of strong π–π and p–π interactions in the crystal. The high planarity of 1 facilitates the formation of conjugated systems and π–π stacking, thereby improving thermal stability. In addition, the closer crystal packing of compound 2 increases the density, ensuring its satisfactory energy performance. Furthermore, intramolecular hydrogen bonds (HBs) between adjacent nitro and amino groups can be observed in the Fig. 4. These intramolecular HB interactions, along with the π–π interactions, help to reduce the mechanical sensitivity of compound 2.

Fig. 4 Noncovalent interactions analysis of compounds 1 and 2.

Physicochemical and energetic properties

Thermal stability is one of the most important physicochemical parameters of energetic materials because it is related to whether energetic materials can be stored stably for a long time.

Thus, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to investigate the thermal properties of compounds 1 and 2 at a heating rate of 10 K min⁻¹ under an N₂ atmosphere. As shown in Fig. 5, both compounds 1 and 2 exhibit good thermal stability. Furthermore, it was observed that in the TG-DSC curve of compound 1, in addition to the exothermic peak corresponding to the thermal decomposition process, there is another exothermic peak and an endothermic peak, whose corresponding temperatures are 163 °C and 235 °C, respectively.

Fig. 5 (a) TG-DSC curves of 1 (heating rates of 10 K min⁻¹). (b) TG-DSC curves of compound 1 after heating and cooling treatment (heating rates of 10 K min⁻¹). (c) TG-DSC curves of 2 (heating rates of 10 K min⁻¹). (d) Comparison of ¹³C-NMR spectra of compound 1 before and after heating.

To rule out the possibility that the process corresponding to these two peaks is a chemical reaction, compound 1 was subjected to heating at the corresponding temperature. Once the sample had been sufficiently cooled, its ¹³C NMR spectrum was recorded and compared with that of the unheated sample. The results are presented in Fig. 5d. The ¹³C NMR of compound 1 demonstrated no change before and after heating, indicating that the process corresponding to these two peaks is a phase transition process rather than a chemical reaction. Furthermore, a TG-DSC test was conducted on the cooled sample once more, and it was found that the exothermic peak at 163 °C had disappeared (Fig. 5b). This indicates that when heated to this temperature, compound 1 undergoes a transition from a metastable phase to a more stable phase.^21,22 Compound 1 undergoes thermal decomposition at 311 °C, with a peak temperature of 345 °C, indicating that it features a thermal stability comparable to that of the classic heat-resistant explosive HNS.⁷ In general, introducing amino groups at adjacent positions of the nitro group will form intramolecular and intermolecular hydrogen bonds, which is beneficial to improve thermal stability. However, the initial thermal decomposition temperature of the amination product 2 is 285 °C, which is lower than that of compound 1. Fig. 2 shows that the benzene ring and triazine ring of compound 2 are not coplanar but have a torsion angle. This may be due to the repulsion between the amino group of the benzene ring and that of the triazine ring. In contrast, compound 1 has a fused-ring skeleton with good planarity, resulting in a more complete delocalization of its π electrons compared to compound 2. This could be a possible reason for the better thermal stability of compound 1.

Energy is another important evaluation index of energetic materials. Detonation velocity (D) and detonation pressure (P) are commonly used to indicate the energy properties of energetic materials. The values of D and P are related to the density and the heat of formation. Through the utilization of a gas pycnometer and operating at room temperature, the experimentally determined density of compound 1 was measured to 1.69 g cm⁻³. The theoretical density of compound 2 at room temperature is 1.84 g cm⁻³, which was calculated based on the Sun.²³ The Gaussian 09 program²⁴ was used to calculate the heats of formation for compounds 1 and 2, which were found to be 275.89 kJ mol⁻¹ and 283.69 kJ mol⁻¹, respectively. The corresponding detonation velocities and detonation pressures were calculated by using EXPLO5 (version 6.05).²⁵ As shown in Table 1, the energetic performance of compound 1 (D = 7606 m s⁻¹ and P = 22.68 GPa) is compared to that of HNS (D = 7612 m s⁻¹ and P = 24.30 GPa). In comparison to 1, the energy performance of 2 (D = 8314 m s⁻¹ and P = 28.28 GPa) improved due to the increase in density. BAM tests were used to evaluate the impact sensitivity (IS) and friction sensitivity (FS) of 1 and 2. Given the potential impact of the sample size on the results of mechanical sensitivity testing, a laser particle size analyser was employed to ascertain the particle size distribution of compounds 1 and 2. The particle size distribution is presented in the ESI.† The D₅₀ of compound 1 was 20.91 μm, the D₉₀ was 33.68 μm, and the IS of compound 1 at this grain size is 32 J, with a FS of 144 N. The D₅₀ of compound 2 was 5.97 μm, while the D₉₀ was 10.59 μm. In accordance with the aforementioned particle size distribution, the IS of compound 2 was determined to be 37 J, while the FS was determined to be 216 N. The IS of 1 and 2 is higher than that of HNS, indicating the potential application of these two energetic molecules as novel insensitive energetic materials with good thermal stability.

Table 1 Physicochemical and energetic performances of compounds 1 and 2 in comparison with HNS

Compound	ρ (g cm^−1)	T d (°C)^c	ΔfHm^d (kJ mol^−1)	D (m s^−1)^e	P (GPa)^f	IS (J)^g	FS (N)^h
a Measured density using a gas pycnometer at room temperature. b Crystal density recalculated to 298 K: ρ298 K = ρT − 0.188 × (298 − T)/1000. c Onset thermal decomposition temperature (10 K min^−1). d Calculated heat of formation. e Detonation velocity calculated using EXPLO5/6.02. f Detonation pressure calculated using EXPLO5/6.02. g Impact sensitivity measured using a standard BAM fall-hammer. h Friction sensitivity measured using a BAM friction tester. i Ref. 7.
1	1.69^a	311	275.89	7606	22.68	32	144
2	1.84^b	285	283.69	8314	28.25	37	216
HNS^i	1.74	318	78.2	7612	24.30	5	240

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Thermostability and molecular stability analysis

A decrease in the thermal decomposition temperature was shown when an amino group was introduced into the benzene ring of 1. This may be related to π electron conjugation and aromaticity. To gain a deeper understanding of the conjugation of 1 and 2, the Gaussian 09 and Multiwfn 3.8 (ref. 26) software tools were used to investigate the π-electron delocalization and aromaticity. Planarity is an important structural characteristic of molecules that strongly correlates with the degree of conjugation. The results of single crystal X-ray diffraction demonstrate that the fused-ring skeleton of compound 1 had an almost perfect planar arrangement. However, the benzene ring and triazine ring of compound 2 exhibited a certain torsion angle, which reduced the planarity of the fused-ring skeleton. The planar arrangement of 1 provided significant benefits in improving intramolecular conjugation. Due to its flat structure, the π electrons in 1 have increased mobility, allowing them to participate in an uninterrupted π–π stacking or π-conjugated system. The molecule's electronic stability is enhanced by the presence of a π-conjugated system, resulting in remarkable aromaticity. In addition, the aromaticity was quantified using nuclear independent chemical shifts (NICS) analysis,²⁷ which is a commonly used technique for assessing aromaticity. NICS is defined as the negative value of the absolute magnetic shielding calculated at the center of the ring. As shown in Fig. 6, the NICS value of the triazine ring in 1 is comparable to that of 2. However, the NICS value of the benzene ring in 1 is more negative than that of 2, indicating that the benzene ring in 1 has a stronger magnetic field shielding capability and exhibits more pronounced aromaticity.

Fig. 6 Calculated NICS values for compounds 1 and 2.

To better reflect the difference in aromaticity between 1 and 2, the Multiwfn software tool was used to calculate the multicenter bond order (MBO).^28,29 This parameter was commonly used to describe the aromaticity of compounds. The MBO of compound 1 was 0.5799, while that of compound 2 was 0.5614. This confirmed that compound 1 exhibited stronger aromatic characteristics than compound 2. A stronger conjugation is generally achieved with a higher degree of π-electron delocalization. By using the Multiwfn tool, the π electron isosurfaces of 1 and 2 with a value of 0.44 were visualized based on the local orbital locator (LOL) theory.^30,31Fig. 7 shows that the π electron distribution of the fused ring skeleton in compound 1 is more continuous than that of compound 2, indicating that 1 exhibited more significant π electron delocalization and thus accounted for the better thermal stability of 1.

Fig. 7 Quinazoline skeleton features and LOL-π-electron delocalization isosurface maps of 1 and 2.

In addition to the aromaticity of the molecular skeleton, the strength of the trigger bond of the energetic compound is also an important factor affecting its thermal decomposition temperature. The trigger bond has the least energy and is the most easily broken, resulting in further chemical reactions such as decomposition or explosion. For energetic materials, the N–NO₂ or C–NO₂ bonds are usually the trigger bonds.^32,33 To gain a comprehensive understanding of the thermal decomposition temperature differences between compounds 1 and 2, the Laplacian bond order (LBO)³⁴ of the trigger bonds was calculated. As shown in Table 2, the Laplace bond order of the C–NO₂ bond in compound 2 (0.74869/0.72392) is lower than that in compound 1 (0.75489/0.745815), indicating that the strength of the C–NO₂ bond in compound 2 is weaker.

Table 2 Calculated structural parameters of compounds 1 and 2

No	Planarity of the molecular skeleton	NICS	MBO	LOL-π	LBO of C–NO2 bond	The maximum of ESPs
1	Planarity	−19.91/−9.19	0.5799	Continuous	0.7549/0.7458	73.38
2	Non-planarity	−12.84/−10.52	0.5614	Discontinuous	0.7487/0.7239	63.97

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Furthermore, electrostatic surface potentials (ESPs) were calculated to precisely determine the static distribution of the molecule.³⁵Fig. 8 illustrates positive and negative potentials with red and blue regions, respectively. The maximum and minimum are represented by orange and blue dots. In general, molecular electrostatic surface potentials (ESPs) around nitrogen and oxygen atoms tend to be relatively negative, as lone pair electrons contribute negatively to ESPs. Additionally, the maximum electrostatic potential is concentrated near hydrogen atoms. This is due to the lower electronegativity of hydrogen in comparison to nitrogen and oxygen atoms. As shown in Fig. 8, here are typically blue regions surrounding the nitro groups in compounds 1 and 2. According to previous research, a higher positive electrostatic potential (ESP) corresponds to a larger region, which is consistent with higher mechanical sensitivities.³⁶ Compared to compound 2, compound 1 has a higher electropositive potential, which corresponds to higher experimental mechanical sensitivity.

Fig. 8 ESP-mapped molecular van der Waals (VDW) surfaces of 1 and 2.

In conclusion, the molecular structures of compounds 1 and 2 were elucidated via single-crystal X-ray diffraction, and the electronic structure differences between 1 and 2 were investigated through quantum chemical calculations. The results demonstrated that the introduction of –NH₂ on the benzene ring resulted in a repulsive interaction with –NH₂ on the triazine ring, leading to a distortion of the fused-ring skeleton. The planarity of the molecular skeleton in compound 2 deteriorates, making its π electron conjugation less sufficient than compound 1, which reduces the molecular stability caused by π electron delocalization. Furthermore, the Laplace bond order calculations indicate that the introduction of –NH₂ on the benzene ring results in a reduction in the strength of the C–NO₂ bond in compound 2. This implies that under thermal action, the C–NO₂ bond of compound 2 is more susceptible to rupture. Although the introduction of amino groups on the benzene ring is conducive to the formation of intramolecular hydrogen bonds, which is generally considered to be an effective strategy to increase the thermal decomposition temperature; this still cannot eliminate the adverse effects on thermal stability caused by the distortion of the molecular skeleton and the weakening of the C–NO₂ bond. The aforementioned factors result in the thermal decomposition temperature of compound 2 being lower than that of compound 1.

Conclusions

Two [6,6] fused-ring energetic molecules 4-amino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (1) and 4,5-diamino-6,8-dinitrobenzo[d][1,2,3]triazine 2-oxide (2) with 4-amino-1,2,3-triazine 2-oxide structures were designed and synthesized. A comparison of the thermal decomposition temperatures of compounds 1 and 2 revealed that the construction of adjacent C–NO₂/C–NH₂ blocks does not invariably result in an increase in the thermal decomposition temperature of energetic molecules. Single crystal X-ray diffraction and theoretical calculations consistently demonstrate that the reduction in the thermal decomposition temperature of compound 2 is a consequence of the introduction of the amino group, which causes the fused ring skeleton to distort, thereby reducing the aromaticity and the bonding strength of the C–NO₂ bond. This is in contrast to the conventional under-standing that previous studies have shown that the introduction of adjacent C–NO₂/C–NH₂ blocks leads to better thermal stability. Hence, the long-standing view on the relationship between energetic molecular structures and thermal stability is challenged. Compounds 1 and 2 exhibited lower impact sensitivity than that of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS). This suggests that both 1 and 2 can be employed as insensitive heat-resistant energetic materials while indicating that 4-amino-1,2,3-triazine 2-oxide has potential application value as a promising structural unit for the design and synthesis of novel energetic materials with good thermal stability and low impact sensitivity.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 1 and 2 have been deposited at the CCDC under 2354368 for 1 and 2354370 for 2.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22375190 and 21975231).

Notes and references

1. D. Larcher and J. M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem., 2015, 7(1), 19–29.
2. G. Erick, H. Assed and M. Nathália, Application of Explosives in the Oil Industry, Int. J. Oil, Gas Coal Technol., 2013, 1(2), 16–22.
3. H. Li, L. Zhang, N. Petrutik, K. Wang, Q. Ma, D. Shem-Tov, F. Zhao and M. Gozin, Molecular and Crystal Features of Thermostable Energetic Materials: Guidelines for Architecture of “Bridged” Compounds, ACS Cent. Sci., 2020, 6(1), 54–75.
4. Q. Wang, S. Wang, X. Feng, L. Wu, G. Zhang, M. Zhou, B. Wang and L. Yang, A Heat-Resistant and Energetic Metal-Organic Framework Assembled by Chelating Ligand, ACS Appl. Mater. Interfaces, 2017, 9(43), 37542–37547.
5. S. Zeman, Thermal stabilities of polynitroaromatic compounds and their derivatives, Thermochim. Acta, 1979, 31(3), 269–283.
6. S. Zeman, Possibilities of applying Piloyan method of determination of decomposition activation energies in differential thermal analysis of polynitroaromatic compounds and their derivatives: Part IV. 1,3,5-Trinitrobenzene, 2,2′,4,4′,6,6′-hexanitrobiphenyl, 2,2′,2′′,4,4′,4′′,6,6′,6′′-nonanitro-m-terphenyl, 1,4,5,8-tetranitronaphthalene and 2,4,6-tripicryl-l, 3,5-triazine, J. Therm. Anal. Calorim., 1980, 19(2), 207–214.
7. T. Rieckmann, S. Völker, L. Lichtblau and R. Schirra, Investigation on the thermal stability of hexanitrostilbene by thermal analysis and multivariate regression, Chem. Eng. Sci., 2001, 56(4), 1327–1335.
8. T. M. Klapötke, J. Stierstorfer, M. Weyrauther and T. G. Witkowski, Synthesis and Investigation of 2,6-Bis(picrylamino)-3,5-dinitro-pyridine (PYX) and Its Salts, Chem. - Eur. J., 2016, 22(25), 8619–8626.
9. T. M. Klapötke and T. G. Witkowski, 5,5′-Bis(2,4,6-trinitrophen-yl)-2,2′-bi(1,3,4-oxadiazole) (TKX-55): Thermally Stable Explosive with Outstanding Properties, ChemPlusChem, 2016, 81(4), 357–360.
10. Y. Tang, C. He, G. H. Imler, D. A. Parrish and J. M. Shreeve, Aminonitro Groups Surrounding a Fused Pyrazolotriazine Ring: A Superior Thermally Stable and Insensitive Energetic Material, ACS Appl. Energy Mater., 2019, 2, 2263–2267.
11. M. C. Schulze, B. L. Scott and D. E. Chavez, A high density pyrazolo-triazine explosive (PTX), J. Mater. Chem. A, 2015, 3, 17963–17965.
12. D. G. Piercey, D. E. Chavez, B. L. Scott, G. H. Imler and D. A. Parrish, An Energetic Triazolo-1,2,4-Triazine and its N-Oxide, Angew. Chem., Int. Ed., 2016, 55, 15315–15318.
13. D. Kumar, G. H. Imler, D. A. Parrish and J. M. Shreeve, A highly stable and insensitive fused, triazolo-triazine explosive (TTX), Chem. - Eur. J., 2017, 23, 1743–1747.
14. M. Deng, Y. Feng, W. Zhang, X. Qi and Q. Zhang, A green metal-free fused-ring initiating substance, Nat. Commun., 2019, 10, 1339.
15. M. Deng, F. Chen, S. Song and Q. Zhang, From the sensitive primary explosive ICM-103 to insensitive heat-resistant energetic materials through a local azide-to-amino structural modification strategy, Chem. Eng. J., 2022, 429, 132172.
16. Y. Feng, M. Deng, S. Song, S. Chen, Q. Zhang and J. M. Shreeve, Construction of an unusual two-dimensional layered structure for fused-ring energetic materials with high energy and good stability, Engineering, 2020, 6, 1006–1012.
17. Y. Cao, Z. Cai, J. Shi, Q. Zhang, Y. Liu and W. Zhang, A heat-resistant and insensitive energetic material based on the pyrazolo-triazine framework, Energ. Mater. Front., 2022, 3, 26–31.
18. J. Shi, H. Xia and S. Song, et al., Structure-activity relationship analysis and molecular structure construction of fused tricyclic energetic molecules containing flexible ring, Energ. Mater. Front., 2023, 4(3), 194–201.
19. M. A. Spackman and D. Jayatilaka, Hirshfeld surface analysis, CrystEngComm, 2009, 11, 19–32.
20. T. Lu and Q. Chen, Interaction region indicator: a simple real space function clearly revealing both chemical bonds and weak interactions, Chem.: Methods, 2021, 1(5), 231–239.
21. J. S. Aronhime, Application of thermal analysis (DSC) in the study of polymorphic transformations, Thermochim. Acta, 1988, 134, 1–14.
22. M. Leităo, J. Canotilho and M. Cruz, et al., Study of polymorphism from DSC melting curves; polymorphs of terfenadine, J. Therm. Anal. Calorim., 2002, 68(2), 397–412.
23. C. Sun, Thermal Expansion of Organic Crystals and Precision of Calculated Crystal Density: A Survey of Cambridge Crystal Database, J. Pharm. Sci., 2007, 96, 1043–1052.
24. M. J. Frisch, et al., Gaussian 09, Revision D. 01, Gaussian Inc., Wallingford, CT, 2009.
25. M. S. I. Zagreb, EXPLO5 V6.05 software, Croatia.
26. T. Lu and F. Chen, Multiwfn: A multifunctional wavefunction analyzer, J. Comput. Chem., 2012, 33, 580–592.
27. X. Wang, Z. Liu, X. Yan, T. Lu, W. Zheng and W. Xiong, Bonding Character, Electron Delocalization, and Aromaticity of Cyclo[18]Carbon (C₁₈) Precursors, C₁₈-(CO)n (n = 6, 4, and 2): Focusing on the Effect of Carbonyl (–CO) Groups, Chem. - Eur. J., 2022, 28, 202103815.
28. M. Giambiagi, M. S. Giambiagi and K. C. Mundim, Definition of a multicenter bond index, Struct. Chem., 1990, 1, 423–427.
29. E. Matito, An electronic aromaticity index for large rings, Phys. Chem. Chem. Phys., 2016, 18(17), 11839–11846.
30. H. L. Schmider and A. D. Becke, Chemical content of the kinetic energy density, J. Mol. Struct. THEOCHEM, 2000, 527(1–3), 51–61.
31. T. Lu and Q. Chen, A simple method of identifying π orbitals for non-planar systems and a protocol of studying π electronic structure, Theor. Chem. Acc., 2020, 139(2), 25.
32. L. K. Harper, A. L. Shoaf and C. A. Bayse, Predicting Trigger Bonds in Explosive Materials through Wiberg Bond Index Analysis, Chem. Phys. Chem., 2015, 16, 3886–3892.
33. A. L. Shoaf and C. A. Bayse, Trigger Bond Analysis of Nitroaromatic Energetic Materials using Wiberg Bond Indices, J. Comput. Chem., 2018, 39, 1236–1248.
34. T. Lu and F. Chen, Bond Order Analysis Based on the Laplcian of Electron Density in Fuzzy Overlap Space, J. Phys. Chem. A, 2013, 117, 3100–3108.
35. T. Lu and F. Chen, Quantitative Analysis of Molecular Surface Based on Improved Marching Tetrahedra Algorithm, J. Mol. Graphics Modell., 2012, 38, 314–323.
36. X. Li, Q. Sun, Q. Lin and M. Lu, [N–N=N–N]-Linked Fused Triazoles with π–π Stacking and Hydrogen Bonds: Towards Thermally Stable, Insensitive, and Highly Energetic Materials, Chem. Eng. J., 2021, 406, 126817.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2354368 and 2354370. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj04219k

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