Trinitromethyl- and nitramino-substituted triazolo-pyridazines: synthesis and energetic performance

Abstract

We synthesized a series of ring-fused triazolo-pyridazine energetic compounds with trinitromethyl, nitramine, and hydrazine groups and thoroughly characterised them using NMR, IR, mass spectrometry, and TGA–DSC analysis. Furthermore, single-crystal X-ray analysis supports the structure of compounds 3 and 7. Among all, compound 6 shows exceptional thermal stability (T_d = 353 °C), good density (1.77 g cm⁻³), good VOD (6710 m s⁻¹), and high insensitivity towards impact (40 J) and friction (360 N), which makes it a super heat-resistant explosive.

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High energy density materials (HEDMs) are essential to modern technology and industry, offering high performance with quick energy release, efficiency, and precision.^1,2 Their role spans critical sectors such as defense, space exploration, construction, mining, and safety, contributing to advancements in science, technology, and engineering while maintaining safety and operational control.^3–5 Balancing the trade-off between high energy and low sensitivity in the design of new energetic molecules is a critical challenge in the field of energetic materials.^5–7 Over the past few decades, various strategies have been devised to develop advanced HEDMs including using energetic salts, energetic zwitterionic salts, cocrystals, energetic coordination compounds and energetic metal–organic frameworks (EMOFs).^8–12 In recent years, bicyclic or polycyclic fused heterocyclic compounds have been studied as building blocks to construct new HEDMs.^13,14 Nitrogen-rich energetic materials with conjugated fused rings containing favourable aromaticity have garnered attention due to their promising balance between energy output, sensitivity, and thermal stability.^15–17 Compared to traditional high explosives like cyclo-1,3,5-trimethylene-2,4,6-trinitamine (RDX) and 1,3,5,7-tetranitrotetraazacyclooctane (HMX), fused-ring energetic compounds are typically planar, conjugated molecules.¹⁸ These compounds often exhibit high density and superior thermal stability, owing to strong hydrogen bonding and π–π interactions.¹³ Additionally, the high nitrogen content of the N-heterocyclic fused ring skeleton ensures a significant presence of C–N, and N–N bonds within the molecular structure, which contributes to a high positive heat of formation in the target energetic molecules.^19,20 Therefore, the development of fused-ring energetic materials is considered a promising approach to address the energy-safety trade-off, offering a balanced combination of thermostability, and insensitive next-generation HEDMs. Over the past few years, numerous fused nitrogen-rich heterocycles, such as imidazolo-pyridazine,²¹ tetrazolo-pyridazine,²² triazolo-pyridazine,²³ and tetrazino-tetrazine²⁴ have been reported. Energy-rich functional groups, such as nitro (–NO₂), nitrato (–ONO₂), nitramino (–NHNO₂), geminal dinitro (–CH(NO₂)₂), and trinitromethane (–C(NO₂)₃) are commonly introduced in HEDMs as they enhance key properties like density, nitrogen content, and oxygen balance, thereby boosting the overall energetic performance.^25–30 The trinitromethane group (–C(NO₂)₃) is highly valued in the field of energetic materials due to its ability to significantly enhance the energy density and oxidizing potential.²⁸ One of the key advantages is its high oxygen content, as it contains three nitro groups that contribute a substantial amount of oxygen, thereby improving the oxygen balance of compounds. This leads to more efficient combustion and detonation, making it ideal for high-performance explosives and propellants.

Keeping this in mind, we propose the synthesis of a series of triazolo-pyridazine fused energetic compounds functionalized with –NH₂, –NHNH₂, –NHNO₂, and –C(NO₂)₃ groups derived from commercially available 3-chloro-6-hydrazineylpyridazine. Among the synthesized compounds, compound 3 with –C(NO₂)₃ functionality reveals good energetic performance with an impact sensitivity (IS) of 2.5 J, suggesting its potential as a metal-free primary explosive. Furthermore, compound 6 with –NHNO₂ functionality displays exceptional thermal stability up to 353 °C, respectively, highlighting its potential as a super heat-resistant explosive.

Ethyl 2-(6-chloro-[1,2,4]triazolo[4,3-b]pyridazin-3-yl)acetate (2) was synthesized via the cyclization of 3-chloro-6-hydrazineylpyridazine (1) with 3-ethoxy-3-iminopropionic acid ethyl ester hydrochloride in acetonitrile, using BF₃·OEt₂ as a catalyst. The nitration of compound 2 was performed using conc. sulfuric acid and 100% nitric acid, resulting in 6-chloro-3-(trinitromethyl)-[1,2,4]triazolo[4,3-b]pyridazine (3) in a 79% yield. Finally, compound 3 was treated with nitrogen-rich bases such as hydrazine hydrate, hydroxylamine hydrate, and aqueous ammonia in ethanol at room temperature for 6 hours to obtain hydrazine (4a), hydroxylamine (4b), and ammonia (4c) salts in quantitative yields as shown in Scheme 1.

Scheme 1 Preparation of fused energetic compounds 3 and 7 and energetic salts of compound 3.

Furthermore, compound 1 reacted with cyanogen bromide (CNBr) in 1N HCl at room temperature for 24 hours, affording 6-chloro-[1,2,4]triazolo[4,3-b]pyridazin-3-amine (5) with a 76% yield. The nitration of compound 5 was subsequently performed using concentrated H₂SO₄ and 100% HNO₃, producing N-(6-chloro-[1,2,4]triazolo[4,3-b]pyridazin-3-yl)nitramide (6) with an 86% yield. When compound 6 was treated with 80% hydrazine monohydrate at 100 °C overnight, the reaction unexpectedly yielded 6-hydrazineyl-[1,2,4]triazolo[4,3-b]pyridazin-3-amine (7) in a 70% yield, instead of the anticipated N-(6-hydrazineyl-[1,2,4]triazolo[4,3-b]pyridazin-3-yl)nitramide. This result highlights a preferential reaction pathway involving the removal of the nitro group, likely driven by the reducing environment provided by hydrazine under these conditions.

Crystals suitable for X-ray single crystal diffraction analysis of compounds 3 and 7 were obtained through slow evaporation at room temperature. For compound 3, a 3 : 1 methanol-to-dimethyl sulfoxide solution was used, while for compound 7, dimethyl sulfoxide alone served as the solvent. Compound 3 crystallizes in the triclinic crystal system and belongs to the P1̄ space group having a crystal density of 1.89 g cm⁻³ at 100 K. In the asymmetric unit, four molecules of compound 3 are present, as shown in Table S1 (ESI†). All the ring atoms and C6 are coplanar to each other. The trinitromethyl groups linked to C5 are arranged in a tetrahedral geometry. The interlayer distance between the two planes is 3.557 Å, as depicted in Fig. 1b.

Fig. 1 (a) Crystal structure of 3. (b) Interlayer distance between two layers of compound 3. (c) Crystal packing along the a-axis for compound 3.

Compound 7 crystallizes in the orthorhombic space group Pbca with a crystal density of 1.63 g cm⁻³ at 100 K. It has eight molecules per unit cell, as shown in Table S2 (ESI†). The molecular structure of compound 7 is planar, as depicted in Fig. 2c. A strong hydrogen bonding interaction between the hydrogen atom of the hydrazine (–NHNH₂) group and the nitrogen atom of the triazole ring leads to the formation of a helical-like packing diagram extended via a hydrogen bonding network along the b-axis (Fig. 2b).

Fig. 2 (a) Molecular structure of 7. (b) Packing diagram of 7 along the b-axis (blue dotted lines show hydrogen bonding interactions). (c) Planarity observed in 7. (d) Wave-like packing diagram and interlayer distance of 7 along the a-axis.

Additionally, along the a-axis, the molecules exhibit well-ordered wave-like layer-by-layer stacking with an interlayer distance of approximately 3.540 Å (Fig. 2d). These strong hydrogen bonds and π–π stacking interactions contribute to the enhanced molecular stability and insensitivity of compound 7.

The physicochemical and energetic properties of all the synthesized compounds, such as density, thermal stability, detonation performance, and mechanical sensitivity, were comprehensively evaluated and are presented in Table 1. Density is one of the most important factors in determining the performance of energetic compounds. Experimental densities were determined using a gas pycnometer (Ultrapyc 5000 with meso cells) in a helium atmosphere at 25 °C. Each compound was measured three times to ensure accuracy, and the average values are reported in Table 1. The densities of the synthesized compounds range from 1.60 to 1.84 g cm⁻³. All geometry optimizations were performed using the Gaussian 09 suite of programs³¹ and heats of formation (HOFs) were computed based on the isodesmic reaction approach, revealing values between −156.31 and 565.5 kJ mol⁻¹. Detonation velocity (VOD) and detonation pressure (DP) were calculated using EXPLO5 v7.01.01 software, based on the experimental density and computed HOF data. The synthesized compounds exhibited moderate performance, with VODs ranging from 6624 to 7554 m s⁻¹ and DP from 16.35 GPa to 20.43 GPa. Thermal stability, crucial for the safe handling of energetic compounds during synthesis, storage, and transport was assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 5 °C min⁻¹ in a nitrogen atmosphere. The onset decomposition temperatures of the compounds 3, 4a–c, 6, and 7 ranged from 126 to 353 °C. Among the synthesized compounds, compound 6 (T_d = 353 °C) demonstrated notable thermal stability, surpassing the benchmark heat-resistant explosive HNS (T_d = 318 °C) and closely rivalling PYX (T_d = 357 °C).

Table 1 Physicochemical and energetic parameters of compounds 3–7 in comparison with LA, DDNP, PYX, and HNS

Compound	T d [°C]	D [g cm^−3]	HOF^c [kJ mol^−1]	VOD^d [ms^−1]	DP^e [GPa]	IS^f [J]	FS^g [N]
a Onset decomposition temperature (DSC, 5 °C min^−1). b Density measured by a gas pycnometer (25 °C). c Computed HOF. d Detonation velocity. e Detonation pressure. f Impact sensitivity. g Friction sensitivity. h Ref. 15. i Ref. 32. j Ref. 33.
3	126	1.84	−156.31	7188	20.43	2.5	254
4a	194	1.80	−4.14	7025	18.48	25	360
4b	156	1.73	−92.8	6798	17.60	10	360
4c	197	1.79	−142.7	6624	16.35	20	360
6	353	1.77	377.67	6710	17.68	40	360
7	183	1.60	565.5	7554	20.34	40	360
LA^h	315	4.80	450.1	5920	33.8	2.5–4	0.1–1
DDNP^h	157	1.72	321	6900	24.2	2.2^j	24.7
PYX^i	357	1.76	43.7	7448	24.2	15.4^j	360
HNS^i	318	1.75	78.2	7164	21.6	10.7^j	240

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Impact and friction sensitivities were evaluated using BAM Fall Hammer and Friction Tester instruments. These sensitivities are crucial safety parameters, as they indicate how easily an energetic material can be accidentally initiated during impact and friction. The measured impact sensitivities of the compounds 3, 4a–c, 6 and 7 range from 2.5 to 40 J, while their friction sensitivities fall between 254 and 360 N. Among these compounds, the neutral compound 3 exhibits relatively higher susceptibility to impact (IS = 2.5 J) owing to the presence of a –C(NO₂)₃ functionality; however, it still demonstrates better impact resistance than DDNP (IS = 2.2 J). Importantly, all compounds display insensitivity to friction, with FS values consistently in the range of 254 to 360 N.

To gain deeper insight into how non-bonded interactions affect the physical properties of crystalline materials, we conducted a detailed analysis using Hirshfeld surfaces, 2D fingerprints, and the percentage contribution of individual close contacts of compounds 3 and 7via Crystal Explorer Software (version 17).³⁴ The results from the Hirshfeld surface analysis were visually represented with colour codes: blue signified distances longer than the van der Waals radii, red indicated shorter distances, and white denoted distances equal to the sum of the van der Waals radii. The red and blue regions correspond to high and low populations of close contacts, respectively. Specifically, red spots highlighted intermolecular O⋯H/H⋯O and N⋯H/H⋯N interactions. In compound 3, O⋯H interactions contributed 19%, while N⋯H interactions accounted for 9%. In contrast, compound 7 exhibited a significantly higher contribution of N⋯H interactions at 45%, highlighting the strength of intermolecular hydrogen bonding, beneficial to lowering the impact sensitivity. Additionally, compound 3 displayed various other interactions, including N⋯O (16%), Cl⋯N (4%), C⋯O (8%), O⋯O (16%), and Cl⋯O (15%). For compound 7, notable interactions included N⋯C (7%), N⋯N (5%), and H⋯H (36%), as illustrated in Fig. 3.

Fig. 3 (a) and (d) Hirshfeld surfaces of 3 and 7, respectively. (b) and (e) 2D fingerprint plots in the crystal stacking for 3 and 7, respectively. (c) and (f) Pie charts showing the percent contribution of individual atomic contacts to the Hirshfeld surfaces of 3 and 7, respectively.

In summary, we successfully synthesized compounds 3, 6, and 7, along with the energetic salts 4a–4c derived from compound 3, using commercially available and cost-effective starting materials with good yields. All the newly synthesized compounds were fully characterized with IR, multinuclear NMR, and TGA–DSC measurements. Additionally, the structure of compounds 3 and 7 is further supported by X-ray diffraction analysis. All the synthesized compounds showed good density (1.60–1.84 g cm⁻³), moderate VOD (6624–7554 m s⁻¹) and DP (16.35–20.43 GPa), good thermal decomposition (126–353 °C) and acceptable sensitivity toward impact (2.5–40 J) and friction (FS > 254 N). Compound 3 has good density (1.84 g cm⁻³) and detonation velocity (7188 m s⁻¹) with high impact sensitivity (2.5 J), and can be utilised as a metal-free primary explosive. Additionally, compound 6 exhibits remarkable thermal stability (T_d = 353 °C), underscoring its suitability as a super heat-resistant explosive. We have demonstrated the potential of a fused triazolo-pyridazine backbone substituted with trinitromethyl (–C(NO₂)₃), nitramine (–NHNO₂), and hydrazine (–NHNH₂) groups and hope that our contribution will further encourage researchers to extend their studies in the energetic materials field with nitrogen-rich fused molecular frameworks.

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

S. N. thanks IIT Kanpur for funding and infrastructure. A. K. Y. thanks IIT Kanpur for the FARE Fellowship and infrastructure. S. D. is grateful for the financial support from the core research grant (ANRF-CRG/2023/000007), the Anusandhan National Research Foundation-Science and Engineering Research Board, Department of Science and Technology, Government of India.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2441109 and 2441110. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5cc02368h

‡ These authors (SN and AKY) contributed equally.

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