A thermally resilient high-performing metal-free primary explosive

Abstract

Lead azide (LA), though discovered centuries ago, remains the most widely used primary explosive due to its reliable initiation performance. However, its toxicity and environmental persistence present serious health and ecological concerns. This has driven the pursuit of lead-free, environmentally benign, thermally stable, and intrinsically safe alternatives with high initiating capability. Herein, we report the design and synthesis of a novel organic metal free primary explosive 6-azido-8H-tetrazolo[1,5-b][1,2,3]triazolo[4,5-d]pyridazine (3) with a tricyclic fused-ring scaffold, prepared from commercially available reagents in high yield. Its planar structure having high nitrogen content and various noncovalent interactions imparts high density, controlled sensitivity, and excellent priming ability, while being free from metals and perchlorates. Compound 3 exhibits exceptional thermal stability with a decomposition temperature of 216 °C, the highest reported for any metal-free primary explosive. It also shows excellent environmental resistance, low cost, and scalability. Notably, in a detonation test, compound 3 initiates 500 mg of RDX with a minimum primary charge (MPC) of 10 mg, matching the performance of LA (10 mg) and outperforming DDNP (70 mg). These features position compound 3 as a leading candidate for next-generation, safer, thermally stable metal free primary explosives and a potential replacement of benchmark explosives such as DDNP, lead azide, and many recently reported primary explosives.

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Introduction

Primary explosives are a class of highly reactive compounds that detonate with minimal external stimuli, such as impact, shock, heat, flame, friction, or an electric spark. Their rapid deflagration-to-detonation transition (DDT) makes them essential for initiating secondary explosives.^1–4 Mercury fulminate (MF) was the first widely utilized primary explosive but was later replaced by lead azide (LA) and lead styphnate (LS), which remain prevalent to date due to their reliable DDT performance.^5–7 However, LA and LS are highly susceptible to accidental detonation and pose significant environmental and health hazards due to their lead content as shown in Fig. 1a.⁸ Consequently, the pursuit of safer, more environmentally sustainable alternatives has become a pressing research priority.

Fig. 1 (a) Traditional and some metal-based primary explosives. (b) Previously reported organic primary explosives and this work.

Over the years, the quest for environmentally benign primary explosives has catalysed extensive research into diverse categories of energetic materials, including transition metal-based primary explosives,^9–13 potassium-based primary explosives,^14–24 and organic compounds.^25–36 Among transition metal-based candidates, copper(I) 5-nitrotetrazolate (DBX-1) has emerged as the most viable successor to lead azide (LA) due to its optimal ignition sensitivity, exceptional detonation efficacy, and superior output performance, coupled with commendable thermal stability and chemical compatibility.^34,37,38 However, its tendency to decompose in the presence of periodate salts significantly impairs its explosive efficacy. While potassium-based primary explosives offer improved safety, they are limited by complex synthesis methods, high production costs, and weak priming efficiency.^{13,16,20,23,39} Additionally, metal-based explosives are highly sensitive to external factors such as heat, friction, impact, and electrical discharge, making them prone to accidental detonation during handling, storage, and transport as depicted in Fig. 1a. Their instability, when exposed to moisture and temperature fluctuations, further increases the risk of decomposition, potentially leading to unintended explosions. Therefore, the development of metal-free primary explosives has become a critical scientific challenge and an urgent necessity.

Nitrogen-rich energetic compounds have emerged as promising alternatives to traditional explosives, offering a more environmentally friendly option while maintaining high energy output per unit mass. These materials typically undergo initiation through the cleavage of covalent bonds such as C–N, N–O, or N–N, which leads to rapid energy release and eventual detonation.^2,3 As a result, the design of energetic materials based on nitrogen-rich heterocycles has become a widely adopted and effective strategy.

Particularly, high-nitrogen compounds with fused ring structures have garnered significant attention in the field of energetic materials. Their distinctive structural features, including a high density of C–N, N–N, and N=N bonds, pronounced ring strain, and extended conjugation, contribute to elevated heats of formation and remarkable energetic performance.^35,40–44 Moreover, the intrinsic planarity of these fused systems facilitates efficient molecular packing, thereby enhancing crystal density.

In recent years, numerous new primary and secondary energetic materials have been developed, most of which are based on nitrogen-rich heterocyclic scaffolds. Some representative examples of these compounds are listed in the SI (Table S3), while a selection of some known organic primary explosives is illustrated in Fig. 1b, including 2-diazo-4,6-dinitrophenol (DDNP),²⁵ 1-(5-tetrazolyl)-3-guanyl tetrazene hydrate (TGTH),^1,26 1,3,5-triazido-2,4,6-trinitrobenzene (TATNB),^28,34 3,6-diazido-1,2,4,5-tetrazine (DiAT),²⁹ 4-azido-5-nitro-7H-pyrazolo[3,4-d][1,2,3]triazine 2-oxide (ICM-103),³⁰ 6-azido-7,8-dinitrotetrazolo[1,5-b]pyridazine (ANTP),³¹ (E)-1,2-bis(3-azido-5-(trifluoromethyl)-4H-1,2,4-triazol-4-yl)diazene (BATTD),³² and 1,8-diazidobis([1,2,4]triazolo)[4,3-b:3′,4′-f][1,2,4,5]tetrazine (DATTY).³³ But none of these metal free primary explosives have demonstrated thermal stability exceeding 170 °C. Additionally, most organic alternatives developed for primary explosive applications face significant limitations, including excessive sensitivity, complex synthesis routes, or inadequate stability, thereby restricting their large-scale implementation. DDNP, for instance, suffers from three major shortcomings: (1) extreme sensitivity to impact, friction, and electrostatic discharge, making it highly prone to accidental initiation; (2) pronounced photosensitivity, leading to decomposition and darkening upon light exposure, which compromises product integrity; and (3) environmental toxicity, as nitrophenols are designated as priority pollutants by both the US Environmental Protection Agency and China's Ministry of Ecology and Environment.⁴⁵ Other organic alternatives also exhibit critical deficiencies. TGTH is unstable in boiling water,⁴⁶ while TATNB degrades under sunlight, undergoing nitrogen elimination followed by intramolecular rearrangement to form benzotrifurozan (BTF), a secondary explosive.⁴⁷ DiAT is compromised by exceptionally low thermal stability and extreme sensitivity to impact, spark, and friction, making it highly susceptible to accidental detonation. Additionally, several other recent candidates like ICM-103, ANTP, and BATTD suffer from relatively high minimum primary charge (MPC) values of 60 mg, 40 mg, and 40 mg, respectively, limiting their efficiency as primary explosives as shown in Fig. 1b. DATTY, despite exhibiting an impressive detonation velocity of 8835 m s⁻¹, is critically hindered by an extremely high impact sensitivity (0.5 J). Moreover, its synthesis is hindered by a laborious multi-step precursor preparation, further complicating its practical applicability.

Taking all these factors into account, we have successfully synthesized a primary explosive featuring a tricyclic fused ring system 6-azido-8H-tetrazolo[1,5-b][1,2,3]triazolo[4,5-d]pyridazine (3), which can be obtained in high yields in a straightforward manner from inexpensive starting materials. Remarkably, this energetic compound shows a well-balanced primary explosive performance such as high initiating capability with a minimum primary charge (MPC) of 10 mg and high thermal decomposition temperature of 216 °C, the highest reported for a metal-free primary explosive. Additionally, 4,7-dihydrazineyl-2H[1,2,3]triazolo[4,5-d]pyridazine (2) demonstrates good energetic performance (VOD = 8649 m s⁻¹), high insensitivity to impact and friction, and excellent thermal stability at 312 °C, making it a promising candidate for an insensitive, heat-resistant secondary explosive. Comprehensive characterization of the synthesized compounds was conducted using multiple spectroscopic techniques, including infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), and high-resolution mass spectrometry (HRMS). Thermal stability assessment was performed through thermogravimetric analysis and differential scanning calorimetry (TGA-DSC). Furthermore, single-crystal X-ray diffraction (SC-XRD) confirmed the molecular structure of compound 3. Compound 3 exhibits superior properties that eclipse those of the industrially employed primary explosive DDNP and other primary explosives like lead azide. Furthermore, its streamlined synthetic route, cost-efficient production, high yield, and easy scalability underscore its significance as a highly valuable contender for real-world defence and civilian applications.

Results and discussion

Synthesis

Compound 2H[1,2,3]triazolo[4,5-d]pyridazine-4,7-diamine (1) was prepared following a literature-reported method.⁴⁸ It was then refluxed with hydrazine hydrate for 15 hours, yielding compound 4,7-dihydrazineyl-2H[1,2,3]triazolo[4,5-d]pyridazine (2) with an 82% yield. The subsequent reaction of compound 2 with sodium nitrite in an acidic medium for 12 hours at room temperature afforded a solid precipitate, which was filtered, washed with water, and air-dried to obtain 6-azido-8H-tetrazolo[1,5-b][1,2,3]triazolo[4,5-d]pyridazine (3) in 85% yield as shown in Scheme 1.

Scheme 1 Synthesis of 6-azido-8H-tetrazolo[1,5-b][1,2,3]triazolo[4,5-d]pyridazine (3).

Crystal structure

Suitable crystals of compound 3 were obtained by slow evaporation of methanol and DMSO in a 3 : 1 ratio at room temperature. It crystallizes in the P₂₁₂₁₂₁ space group with four molecular moieties per unit cell and a calculated density of 1.839 g cm⁻³ at 100 K. The molecular structure of compound 3 with a tricyclic fused ring system is completely planar. The azide groups have linear and coplanar arrangements with a heterocycle-fused framework, as illustrated in Fig. 2b. Fig. 2c shows the herringbone packing diagram of compound 3 extending through a hydrogen bonding network between the NH of the triazole and the ring nitrogen. Fig. 2d shows the stacked packing diagram of compound 3 along the c-axis. Fig. 3a and b shows the front and side view of the enlarged structure of a single layer of a packing diagram of compound 3. Fig. 3c shows the layer-by-layer wave like packing diagram of compound 3 along the a-axis.

Fig. 2 (a) The crystal structure of compound 3. (b) The planarity of compound 3. (c) The herringbone packing arrangement of compound 3. (d) Stacked packing diagram of compound 3 along the c-axis.

Fig. 3 (a and b) Front and side view of the enlarged structure of a single layer of a packing diagram of compound 3 respectively. (c) Packing diagram of compound 3 along the a-axis.

Physicochemical properties

Density is a critical parameter for energetic materials, significantly impacting their detonation performance, energy output, and the efficiency of initiating explosive devices (IEDs). The densities of compounds 2 and 3 were measured using a gas pycnometer under a helium atmosphere at 25 °C, yielding values of 1.71 g cm⁻³ for 2 and 1.80 g cm⁻³ for 3. Notably, the density of compound 3 of 1.80 g cm⁻³, exceeds that of DDNP (1.71 g cm⁻³) and is close to that of previously reported organic primary explosives ANTP and DATTY (1.82 g cm⁻³), as presented in Table 1.

Table 1 Physicochemical and energetic properties of the newly synthesized compounds along with previously reported benchmark primary explosives

Compds	T d [°C]	ρ [g cm^−3]	HOF^c [kJ mol^−1/kJ g^−1]	DP^d [GPa]	VOD^e [m s^−1]	IS^f [J]	FS^g [N]
a Onset decomposition temperature (DSC, 5 °C min^−1). b Pycnometer density at 25 °C. c Heat of formation calculated experimentally using bomb calorimetry. d Detonation pressure calculated with EXPLO5 v7.01.01. e Detonation velocity calculated with EXPLO5 v7.01.01. f Impact sensitivity. g Friction sensitivity. h Ref. 31. i Ref. 32. j Ref. 33. k Ref. 30. l Ref. 25. m Ref. 49.
2	312	1.71	682.3/3.76	25.8	8649	>40	>360
3	216	1.80	836.8/4.11	23.9	8016	3	10
ANTP^h	163	1.82	811.2/3.65	31.5	8746	5	120
BATTD^i	145	1.88	126.1/0.33	23.9	7862	3	5
DATTY^j	151.5	1.82	1595.3/6.53	30.7	8835	0.5	5
LA^k	315	4.80	450.1/1.55	33.8	5920	2.5–4	0.1–1
DDNP^l	157	1.72	327.0/1.56	24.7	6900	1	24.7
ICM-103^k	160	1.86	744.7/3.33	35.1	9111	4	60
HNS^m	318	1.75	78.2/0.17	21.6	7164	5	240
TATB^m	360	1.94	−139.5/−0.54	32.4	8114	>40	>360

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An essential characteristic of an ideal primary explosive is its ability to withstand harsh environmental conditions, particularly resistance to moisture and light. DDNP, however, is highly photosensitive and undergoes rapid degradation upon light exposure, which significantly compromises its initiating efficiency. In sharp contrast, compound 3 demonstrates outstanding environmental stability, and it remains unaffected by both moisture and light, maintaining its high purity even after extended exposure to sunlight (8 hours). This exceptional resilience clearly sets compound 3 apart from DDNP.

Additionally, compound 3 exhibits extremely low water solubility, with only 0.03 g dissolving in 100 mL of water at 25 °C, significantly lower than reference primary explosives such as ICM-103 (0.08 g) and DDNP (0.10 g) under identical conditions. Another critical factor in assessing the practical applicability of a primary explosive is its hygroscopicity, which influences handling, long-term storage, and operational reliability. To evaluate this, a dry sample of compound 3 was placed in a humidity-controlled chamber (68% relative humidity at 25 °C) for 24 hours. The compound exhibited minimal moisture absorption, with a water uptake of just 0.16 wt%, notably lower than DDNP's 0.36 wt%. These results indicate that compound 3 is essentially non-hygroscopic, enhancing its suitability for real-world applications.

Furthermore, its long-term thermal stability was assessed by exposing the compound to 120 °C in an oven under atmospheric pressure for 15 hours. The mass loss was negligible (<2 mg per ∼0.5 g sample), and no visible physical changes were observed, as illustrated in Fig. 4. To comprehensively evaluate its environmental and long-term stability, infrared (IR) and differential scanning calorimetry (DSC) analyses were performed under various conditions: immediately after synthesis, after 8 hours of direct sunlight exposure, following 15 hours in an oven at 120 °C, after 24 hours in a humidity chamber, and after two months of ambient storage. Remarkably, the IR and DSC profiles remained unchanged across all scenarios, demonstrating excellent resistance to photodegradation, thermal stress, and moisture-induced alterations (for detailed spectra refer to SI, Fig. S9–S18). This consistency highlights the compound's strong potential for reliable long-term storage and operational use.

Fig. 4 IR and DSC plots of compound 3 under various conditions.

Supporting these findings, the ¹³C NMR spectra of compound 3 were also recorded under the same set of conditions. As shown in Fig. 5, the spectra remained unchanged in all cases, further confirming the compound's outstanding environmental and thermal stability, an essential characteristic of a dependable and durable primary explosive.

Fig. 5 ¹³C NMR spectra of compound 3 under various conditions.

Thermal stability and sensitivity

Thermal stability is a crucial factor for primary explosives, particularly as storage and operational temperatures for munitions can exceed 70–80 °C, and in some cases even 100 °C. The thermal endurance of compound 3 was meticulously examined via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a controlled heating increment of 5 °C min⁻¹. Notably, compound 3 exhibited an exceptional onset decomposition temperature of 216 °C, marking the highest recorded value (to the best of our knowledge) for a metal-free organic primary explosive and surpassing the thermal thresholds of the benchmark organic primary explosive DDNP (T_d = 157 °C), and previously reported potential organic primary explosives such as ICM-103 (T_d = 160 °C), ANTP (T_d = 163 °C), BATTD (T_d = 145 °C), DATTY (T_d = 151.5 °C), TGTH (T_d = 142 °C), DiAT (T_d = 130 °C), TATNB (T_d = 168 °C), and the perovskite based primary explosive {(C₆H₁₄N₂)₂[Na(NH₄)(IO₄)₆]}_n (DPPE-1) (T_d = 161 °C)⁵⁰ as shown in Fig. 6 and other previously reported primary explosives as listed in Table S3, SI.

Fig. 6 Thermal stability of compound 3 with comparison to the metal free primary explosives.

From a safety and reliability perspective, the susceptibility of primary explosives to mechanical perturbations such as impact and friction plays a crucial role in assessing their ignition behaviour during synthesis, transit, storage, and general handling. Compound 3 exhibits a well-balanced sensitivity profile, with an impact sensitivity of 3 J and a friction sensitivity of 10 N. Compared to LA (IS = 2.5–4 J, FS = 0.1–1 N), and DATTY (IS = 0.5 J, FS = 5 N) compound 3 offers a better safety profile with respect to impact, and friction sensitivity as listed in Table 1. Also, the impact sensitivity of compound 3 is better than that of DDNP (IS = 1 J, FS = 24.7 N) but less robust in terms of friction sensitivity. These data, as summarized in Table 1, reinforce the ability of compound 3 to effectively balance sensitivity and performance, establishing it as a viable and highly reliable candidate for real-world primary explosive applications.

To better understand the relationship between the structure of compound 3 and its physicochemical properties, Hirshfeld surface analysis, two-dimensional (2D) fingerprint plots, and the percentage contributions of close contacts were examined via Crystal explorer software⁵¹ as shown in Fig. 7. The Hirshfeld surfaces of compound 3 exhibit a nearly planar conformation. Strikingly, red spots are primarily observed on the lateral faces of the molecular plates rather than the front faces, suggesting that intermolecular interactions predominantly occur through the external atoms (H and N) that envelop the molecules. Fig. 7c illustrates the relative percentage contributions of various atomic contacts in compound 3. Among these, N⋯N close contacts account for 61% of the total weak interactions. The N⋯H interactions account for 18%, and are the main contributors to hydrogen bonding, which enhances structural stability. Meanwhile, C⋯N close contacts also constitute 18% of the total weak interactions, playing a crucial role in π–π stacking, which serves as a stabilizing interaction.

Fig. 7 (a) Hirshfeld surface of 3. (b) 2D fingerprint plots in the crystal stacking for 3. (c) Pie chart showing % contribution of individual atomic contacts to the Hirshfeld surface. (d) ESP-mapped molecular vdW surface of 3.

The substantial proportion of N⋯N interactions is a key factor in the high sensitivity of compound 3, as these interactions contribute to destabilization. In contrast, the molecular stability of compound 3 can be attributed to the strong intra- and intermolecular N⋯H and C⋯N interactions, which reinforce its structural integrity.

To better understand the relationship between structural characteristics and sensitivity of compound 3 (as shown in Fig. 7d), an electrostatic potential (ESP) analysis was conducted using Gaussian 09 (ref. 52) and Multiwfn 3.8 software⁵³ and visualized using VMD software. This method provides valuable insight into how electrostatic interactions across the molecular surface contribute to the compound's reactivity and potential impact sensitivity.

In Fig. 7d, the cyan and red spheres mapped on the van der Waals (vdW) surface represent the local maxima and minima of ESP, respectively. The red regions highlight electron-deficient (electropositive) zones, while the blue regions indicate electron-rich (electronegative) sites. The most positive and negative ESP values observed were +71.61 kcal mol⁻¹ and –39.15 kcal mol⁻¹, respectively. These extreme values correspond to regions of intense electrostatic charge concentration, with cyan spheres denoting highly positive potentials and red spheres marking negative charge accumulation.

Such ESP distribution is significant in the context of impact sensitivity, a key parameter in evaluating energetic materials. Previous studies have shown that compounds with large electropositive surface areas often correlate with higher sensitivity, as these regions can attract negatively charged particles or impact stimuli, thereby facilitating decomposition pathways.³³ In compound 3, the pronounced positive ESP observed on the upper molecular surface is primarily due to the presence of the electron-withdrawing azido group, which leads to localized charge depletion. This distribution aligns with the experimentally observed high sensitivity, characterized by an impact sensitivity of 3 J and friction sensitivity of 10 N, confirming the potential use of compound 3 as a primary explosive.

Heat of combustion

The heat of combustion is a vital parameter for evaluating the energetic performance of explosives. The constant-volume combustion energy of the compounds was measured using a high-precision oxygen bomb calorimeter (Parr 6200). For sample preparation, tablets were formed by mixing each compound with benzoic acid in a 1 : 3 mass ratio (150 mg of compound and 450 mg of benzoic acid). These tablets were sealed in a combustion bomb and ignited in a pure oxygen atmosphere.

The internal energy of combustion (ΔcU) was determined to be −3263.68 kJ mol⁻¹ for compound 2 (C₄H₇N₉, MW = 181.16 g mol⁻¹) and −2566.89 kJ mol⁻¹ for compound 3 (C₄HN₁₁, MW = 203.12 g mol⁻¹). The enthalpy of combustion (ΔcH°) was calculated using the relation ΔcH = ΔcU + ΔnRT, where Δn is the change in the number of moles of gaseous products and reactants, R = 8.314 J mol⁻¹ K⁻¹, and T = 298 K. Based on this, the enthalpies of combustion were found to be −3256.86 kJ mol⁻¹ for compound 2 and −2553.88 kJ mol⁻¹ for compound 3.

The balanced combustion equations for compounds 2 and 3 are given below:

C₄H₇N₉ + 5.75O₂ → 4CO₂(g) + 3.5H₂O(l) + 4.5N₂(g) (1)

C₄HN₁₁ + 4.25O₂ → 4CO₂(g) + 0.5H₂O(l) + 5.5N₂(g) (2)

The standard enthalpy of formation (Δ_fH°) for compound 2 and 3 was calculated to be 682.38 kJ mol⁻¹ and 836.88 kJ mol⁻¹, respectively, using Hess's Law, as outlined in eqn (3) and (4). These calculations were based on the known standard enthalpies of formation for H₂O (l) at −285.83 kJ mol⁻¹ and CO₂ (g) at −393.51 kJ mol⁻¹.

Δ_fH°[2, s] = 4Δ_fH°[CO₂, g] + 3.5Δ_fH°[H₂O, l] − ΔcH°[2, s] (3)

Δ_fH°[3, s] = 4Δ_fH°[CO₂, g] + 0.5Δ_fH°[H₂O, l] − ΔcH°[3, s] (4)

Both compounds 2 and 3 exhibit high positive heats of formation, measured to be 682.38 and 836.88 kJ mol⁻¹, respectively. This can be attributed to their fused-ring structures and the abundance of N–N and C–N bonds within their molecular frameworks. Notably, the heat of formation of compound 3 significantly exceeds that of several benchmark primary explosives, including LA (450.1 kJ mol⁻¹), DDNP (327.0 kJ mol⁻¹), ICM-103 (744.7 kJ mol⁻¹), as well as numerous other reported compounds, as summarized in Table 1. To validate this, we calculated the heat of formation for both compounds using Gaussian software at the B3LYP/6-311G(d,p) level of theory as detailed in the SI. The obtained values are 631.5 and 1022.9 kJ mol⁻¹ for compounds 2 and 3, respectively, which closely align with the experimental values listed in Table 1.

Detonation properties

The detonation properties, particularly detonation velocity and pressure, were thoroughly investigated by EXPLO-5 software v7.01.01 using the experimental density and heat of formation. Both compounds 2 and 3 exhibit a compelling combination of energetic performance and environmental stability, positioning them as strong contenders for next-generation explosive materials.

Compound 2 demonstrates a high detonation velocity of 8649 m s⁻¹ and a detonation pressure of 25.8 GPa, surpassing well-known heat-resistant explosives such as HNS (7164 m s⁻¹) and TATB (8114 m s⁻¹), as shown in Table 1. It also offers excellent thermal stability, with a decomposition temperature of 312 °C, closely matching that of HNS (318 °C). In terms of sensitivity, it shows outstanding resistance to mechanical stimuli, with an impact sensitivity >40 J and friction sensitivity >360 N, far better than HNS (IS = 5 J) and on par with TATB (IS > 40 J). These balanced characteristics highlight its potential as an insensitive, heat-resistant secondary explosive, with performance and safety surpassing those of current benchmarks.

Compound 3 also delivers impressive results, with a detonation velocity of 8016 m s⁻¹ and a detonation pressure of 23.9 GPa. These values notably exceed those of traditional primary explosives like LA (5920 m s⁻¹) and DDNP (6900 m s⁻¹), as listed in Table 1. Coupled with its exceptional environmental and thermal stability, compound 3 offers a safer and more effective alternative for use in primers and detonators, suitable for both military and civilian applications.

We predicted the potential detonation products of neat compound 3 and its mixtures with RDX using the Explo-5 software, as detailed in the SI. Compound 3 predominantly decomposes into N₂, which accounts for 73.7% of the detonation products, followed by carbon (23%) and NH₂ (2%). Interestingly, when mixed with RDX in a 10 : 90% ratio, the primary detonation products shift to N₂ (41%), CO₂ (13%), H₂O (13%), and CH₂O₂ (20%), with carbon reducing significantly to just 5%. Additionally, when the ratio of compound 3 to RDX is reduced to 2 : 98% (i.e., 10 mg of compound 3 and 500 mg of RDX, used for the initiation ability test), the carbon content further decreases to 3.9%, while the major products become N₂ (38%), CO₂ (14%), H₂O (11%), and CH₂O₂ (29%). These results clearly indicate that using compound 3 as a primary explosive to initiate a secondary explosive significantly reduces the release of toxic byproducts. For comparison, we have also predicted the potential detonation products of several previously reported primary explosives, both in their pure form and in mixtures with RDX (10% primary explosive: 90% RDX), as presented in Table S4.

Hot needle and initiating ability tests

A hot needle (HN) test was conducted to assess the ignition ability of the newly synthesized compound 3. Upon placing a heated copper needle on a 10 mg sample of compound 3, it ignited instantaneously with a pronounced sound, exhibiting an ignition delay of 2 ms, as shown in Fig. 8. The ignition delay (ID) is defined as the time elapsed from the placement of a hot needle on the sample to the appearance of the first visible flame.

Fig. 8 Hot needle test of compound 3.

Initiating efficiency is one of the most crucial parameters for assessing a primary explosive's ability to reliably initiate secondary explosives. In this study, the initiating efficiency of compound 3 was evaluated using the Minimum Primary Charge (MPC) test. This method involved detonating compound 3 against a 5 mm thick lead plate, with pure RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) used as the secondary explosive. The efficiency was determined by identifying the minimum quantity of the primary explosive required to perforate the lead plate. In the MPC test, measured amounts of compound 3 were placed in a stainless-steel cap and ignited using a pyrotechnic igniter (see Fig. 9). Notably, the lead plate consistently exhibited a hole measuring 3.9 cm (measured via an inner diameter caliper), even when the priming charge was reduced from 40 mg to 10 mg. This established the MPC of compound 3 as 10 mg, which is significantly lower than that of the benchmark organic primary explosive DDNP (MPC = 70 mg).^25,30 Charges below 10 mg were found to be insufficient for complete coverage of the RDX surface, thereby affecting reliable initiation. Additionally, compound 3 demonstrated excellent stability over time. After two months of storage under ambient conditions, a repeat test using 10 mg of aged compound 3 with 500 mg of RDX produced hole dimensions identical to those generated using a freshly prepared material, confirming its sustained initiating performance.

Fig. 9 Illustration of the initiation capability test using compound 3 and lead azide (LA) as primary explosives, demonstrating the ejection of lead plates from the hole using 500 mg of RDX as the secondary explosive.

For better comparison, a test conducted with 10 mg of lead azide (LA) and 500 mg of RDX produced a slightly smaller hole of 3.7 cm, closely matching the performance of compound 3. These results suggest that the initiating efficiency of compound 3 is comparable to that of Pb(N₃)₂, as both produced lead plate perforations approximately similar in diameter, with compound 3 (3.9 cm) being slightly better than LA (3.7 cm) (Fig. 9). Overall, these findings underscore the remarkable initiating power of compound 3 as a metal-free primary explosive and its strong potential for use in both military and civilian explosive applications.

Conclusion

In conclusion, we report for the first time a straightforward, cost-effective, and high-yielding synthesis of the tricyclic fused organic primary explosive, compound 3. Single-crystal X-ray diffraction analysis confirms its planar molecular geometry. Compound 3 exhibits a high measured density (1.80 g cm⁻³), exceptional thermal stability, the highest reported to date for metal-free primary explosives (216 °C), a positive heat of formation (836.8 kJ mol⁻¹), and good detonation performance (VOD = 8016 m s⁻¹; DP = 23.9 GPa). In detonation testing, compound 3 successfully initiated 500 mg of RDX with an ultralow minimum primary charge (MPC) of just 10 mg, matching that of lead azide (LA) and significantly outperforming benchmark organic primary explosive DDNP (70 mg). It also exhibited rapid ignition upon contact with a hot needle. From a practical perspective, compound 3 satisfies all industrial criteria for safer primary explosives, including cost-effective synthesis, scalability, excellent priming capability, high thermal stability, and acceptable mechanical sensitivities. Overall, the discovery of compound 3 marks a notable advancement in the field of energetic materials, addressing the critical need for safer, metal-free primary explosives for both military and civilian applications.

Author contributions

Sonali Kukreja: conceptualization; methodology; investigation, validation; formal analysis; writing – original draft. Abhishek Kumar Yadav: conceptualization; methodology; investigation, validation; formal analysis; writing – original draft. Srinivas Dharavath: conceptualization; methodology; validation, supervision; funding acquisition; software; project administration; writing – original draft; writing – review & editing. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

CCDC 2445403 contains the supplementary crystallographic data for this paper.⁵⁴

The data underlying this study are available in the published article and its supporting information (SI). Supplementary information: general experimental procedures; X-ray crystallographic data, and NMR, IR, HRMS and TGA-DSC spectra. See DOI: https://doi.org/10.1039/d5ta07800h.

Acknowledgements

SK thanks IIT Kanpur for a research fellowship and the infrastructure. AKY thanks IIT Kanpur for a FARE Fellowship and for the infrastructure. SD is grateful for the financial support from the core research grant (CRG/2023/000007), Science and Engineering Research Board, Department of Science and Technology, Government of India.

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Footnote

† These authors contributed equally.

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