Introduction

High energy density materials (HEDMs) have seen extensive use in explosives, propellants, and pyrotechnics, and the defense industry has realized rapid development1. With increasing demand for HEDMs in modern weapons, traditional CHON-based energetic compounds (2,4,6-trinitrotoluene, 1,3,5-trinitro-1,3,5-triazacyclohexane, 1,3,5,7-tetranitro-1,3,5,7-tetraazacycloocatane, and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurzitane, etc.) are facing the bottleneck of limited theoretical energies2.

One of the latest developments in HEDMs is high nitrogen and polynitrogen compounds3. The energy contents of these materials come from the high enthalpies of formation for extended single- and double-bonded nitrogen systems (N─N, N═N, N─C, N═C, N─O, etc.)4. Nitrogen chemistry is very different from carbon chemistry. Increasing the bond order of nitrogen leads to a higher average bond energy (BE), rather than a lower bond energy as seen for carbon (BEN≡N > 2 × BEN=N > 3 × BEN−N; BEC≡C < 2 × BEC=C < 3 × BEC−C; Supplementary Table 14)5. In practice, this implies that carbon compounds form stable polymers, whereas nitrogen compounds form stable monomers. Natural allotropic modifications of nitrogen, except for molecular nitrogen (N2), have not been found6. Due to this special property, the formation of N2 (triple bond) from single and double nitrogen bonds releases large amounts of energy7. This approach is attractive for use in HEDMs, but it also means that polynitrogen compounds are metastable, making their syntheses and characterization extremely challenging.

To date, only three stable polynitrogen ions, namely, N3-, N5+, and c-N5-, have been bulk synthesized at room temperature and pressure. The azide anion N3[-8 was first isolated by T. Curtius in 1890. After 109 years, a salt containing the N5+ cation was synthesized in 1999 for the first time9. Then, the stable, aromatic, and planar c-N5- anion was discovered in 2017, and polynitrogen chemistry expanded10,11,12. Within 5 years, c-N5- had been used in the syntheses of various energetic materials, including metallic salts, complexes, metal-pentazolate frameworks, coordination polymers, organic salts, and energetic cocrystals13,14,15,16,17,18,19,20,21,22,23,24,25. Unfortunately, the stabilities and high energy contents are contradictory, especially for catenated nitrogen systems. Almost all compounds containing c-N5- decompose at 80–130 °C26.

For more than half a century, many attempts have been made to inrease the thermal stability of c-N5-, such as the construction of arylpentazole, three-dimensional (3D) metal-pentazolate frameworks15,21 or cocrystals22. The thermal stabilities of these materials continue to increase, surpassing those of N5+ derivatives (22–70 °C)9,27,28,29, but still have not reached 150 °C. It is important to increase the thermal stabilities before these materials can be included in advanced energetic materials. Otherwise, poor thermal stabilities may become a key limiting factor in future applications.

The esthetically pleasing 3D lithium-pentazolate framework we synthesized in 202121 is the most thermally stable (133 °C) of all metal-c-N5- compounds. It consists of lithium and c-N5- ions, wherein the c-N5- cyclic ligands are preternaturally stabilized in this inorganic open framework featuring two types of nanocavities (Li20(N5)12 and Li24(N5)12) formed with strong coordination bonds (Fig. 1a). However, biguanidinium pentazolate19 has the highest thermal decomposition temperature (125 °C) among all metal-free c-N5- salts (Fig. 1b). Two biguanidinium cations form dimers through paired N9─H9B…N8 (2.06 Å) hydrogen bonds, which are surrounded by 14 N─Hs and no hydrogen bond acceptor sites are exposed (Supplementary Fig. 9). However, c-N5- as a hydrogen bonding acceptor is fixed in the channel between dimers without any bonding interactions between them. The relatively high thermal stability of this compound is attributed to extensive hydrogen bonding.

Fig. 1: Structures of relatively thermally stable c-N5- containing compounds.
figure 1

a Lithium-pentazolate framework. b Biguanidinium pentazolate. c c-N5-@HOF (this work).

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As an emerging class of functional porous materials, hydrogen-bonded organic frameworks (HOFs) assembled from organic units with intermolecular hydrogen bonds have shown great potential for various applications30,31,32. In 2021, Zhang et al. designed and constructed an energetic NF@HOF (NF: nitroformate ion) by self-assembly, and the nitroformate guest ion showed a decomposition temperature of 200 °C, which was the first thermal stability above 180 °C33. Like the nitroformate ion, the c-N5- anion is actually a good hydrogen bond acceptor. If c-N5- guest anions occupied the pores of the designed HOF via paired hydrogen bonds and were connected by many N…H hydrogen bonds, the thermal stability of the encapsulated c-N5- would likely improve markedly.

In this work, a HOF that stabilized c-N5- was designed and constructed through self-assembly of melaminium pentazolate (MaN5) and 3,6,7-triamine-7H-[1,2,4]triazole[4,3-b][1,2,4]triazole (TATOT), wherein c-N5- was captured in pores with diameters of 5.4 Å via formation of symmetrical c-N5- hydrogen bonds. Under the combined effects of hydrogen bonding, π–π interactions, and the framework structure, the c-N5- in c-N5-@HOF exhibited a lower electrostatic potentials (ESPs), weaker distortion, a higher Laplacian bond order, and greater aromaticity than other c-N5- derivatives. These factors resulted in c-N5-@HOF thermal stability at 153 °C (onset), surpassing those of all reported c-N5- based compounds. The detonation velocity and pressure of c-N5-@HOF were calculated as 8.029 km s−1 and 24.6 GPa, respectively, which were higher than those of TATOT+N5- (7.791 km s−1, 24.6 GPa) and 2,4,6-trinitrotoluene (6.881 km s−1, 19.5 GPa). In addition, c-N5-@HOF was insensitive to impact and friction. Encapsulation of c-N5- in HOFs will open the door to stable c-N5- and overcome the problems of poor thermal stability and high sensitivity.

Results

Syntheses

Silver pentazolate (AgN5)19, melamine hydrochloride34 and TATOT35 were synthesized according to previously reported procedures. MaN5 was obtained by the metathesis of AgN5 with melamine hydrochloride. MaN5 and TATOT were used to assemble a new compound (c-N5-@HOF, Fig. 1c) in water at 40 °C. All of the newly synthesized compounds were characterized with nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, high-resolution mass spectrometry, differential scanning calorimetry (DSC), thermogravimetric analysis (TG), and elemental analyses (Supplementary Figs. 24–27). For detailed descriptions of the synthetic processes and the analytical data, please refer to the Methods section. c-N5-@HOF was stable in air and could be stored for extended periods (see Supplementary Note 10 and Figs 29–30). In addition, it was soluble in water and dimethyl sulfoxide, slightly soluble in methanol and ethanol, but insoluble in dichloromethane and ethyl acetate.

Single-crystal structure

The single-crystal structures of MaN5 and c-N5-@HOF were determined with X-ray diffraction (see Supplementary Data 1–2). The crystallographic data and data collection parameters, selected bond lengths, bond angles, and hydrogen bonds are listed in Supplementary Tables 1–10.

MaN5 crystallized with melamine and water molecules in the triclinic P−1 space group, and the density was 1.579 g cm−3 at 296 K (Supplementary Fig. 1). Each unit had four melaminium cations, four c-N5- anions, four neutral melamine molecules, and four water molecules. Along the b-axis, the crystal was stacked with N─H…N and O─H…N hydrogen bonds to form a framework structure with 1D pores. However, a 3D cube layer was stacked along the c-axis (Supplementary Fig. 2). The melaminium cations, c-N5- anions, and neutral melamine molecules were connected with many N─H…N hydrogen bonds to form wavy planes (Supplementary Fig. 2b) containing two types of rings (labeled in rose red and orange; composed of three melaminium cations, three neutral melamine molecules, and two water molecules). Water molecules served as key nodes for ring formation via a pair of N─H…O and O─H…N hydrogen bonds (orange ring: 1.8771(20) Å and 2.1796(253) Å; rose red ring: 1.9334(20) Å and 2.0963(259) Å). Melaminium cations and neutral melamine molecules are arranged alternately in each ring and were connected by paired hydrogen bonds (2.0625(20)−2.2147(21) Å) formed between the amino groups and N atoms of the triazine rings. Two types of c-N5- anions were embedded in the two types of ring centers and stabilized by five and six hydrogen bonds from species on the ring (c-N5- (N1-N5): 2.3245(30)–2.6069(22) Å (N─H…N); c-N5- (N6-N10): 2.257(2)–2.4908(20) Å (N─H…N), 1.9201(192) Å (O─H…N)), respectively (Supplementary Figs. 3, 4).

c-N5-@HOF crystallized in the monoclinic space group C2/c with eight molecules per unit cell (Z = 8) and a density of 1.688 g cm−3 at 100 K (CCDC 2286232). The asymmetric unit was crystallographically independent with one neutral melamine molecule, one TATOT+ cation, and one c-N5- anion (Supplementary Fig. 5). As shown in Fig. 2, the neutral melamine molecules and TATOT+ cations were connected through many paired N─H…N hydrogen bonds to form a plane (Fig. 2a) containing 3 + 3 membered rings (composed of three neutral melamine molecules, three TATOT+ cations, and twelve hydrogen bonds) (Fig. 2b). Each ring shared all of its edges with the six surrounding rings. The lengths of the hydrogen bonds on each 3 + 3 membered ring ranged from 1.9654(19) Å to 2.2779(184) Å. It is interesting that the longest and shortest hydrogen bonds were paired between the TATOT+ cation and the melamine molecule. The two pairs of hydrogen bonds between the three melamine molecules had lengths of 2.1131(205) Å and 2.1658(22) Å, respectively, and each pair of hydrogen bonds was of equal length (Supplementary Fig. 7). Along the b-axis, HOF showed layer-by-layer packing (Fig. 2a) with interlayer distances of ~3.2 Å. Two weak N…H9A hydrogen bonds with lengths of 2.7305(390) and 2.8400(371) Å connected adjacent layers to form a positive 3D hydrogen-bonded organic framework with pore diameters of ~5.4 Å (Fig. 2c and Supplementary Fig. 6).

Fig. 2: Structure of the HOF.
figure 2

a Layer-by-layer packing of the HOF. b Arrangement of 3 + 3 membered rings in a single layer. c HOF packing diagram with 1D pore diameters of 5.4 Å.

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The c-N5- anions were located at the centers of 3 + 3 membered rings and were coplanar with the rings (Fig. 3a–c). Each c-N5- anion was connected to seven hydrogen bonds, with bond lengths ranging from 1.9650(22) to 2.693(3) Å. Notably, only two meta-N atoms of c-N5- formed two hydrogen bonds: N16 formed two 2.5156(23) Å and 2.5790(291) Å hydrogen bonds with two melamine molecules; N19 formed two 2.6817(308) Å and 2.693(3) Å hydrogen bonds with two TATOT+ cations (Supplementary Fig. 8). The four hydrogen bonds connecting these two nitrogen atoms were weaker than the single hydrogen bonds connecting each of the other nitrogen atoms, so the force on the c-N5- ring in all directions was relatively uniform, which increased the stability of c-N5-. In addition to the hydrogen bonding interactions on each layer, there should also be π–π interactions between adjacent c-N5- rings in adjacent layers (interlayer distance: ~3.2 Å), which further study confirmed. Along the c-axis, the centers of the c-N5- ions arranged in parallel in 1D pores were located on the central axes of the pores, and hydrogen bonding between the framework and c-N5- in the pores was quite dense (Fig. 3d).

Fig. 3: Structure of c-N5-@HOF.
figure 3

a Layer-by-layer packing of the c-N5-@HOF. b The position of c-N5- in the 3 + 3 membered ring and the hydrogen bonds (green dotted lines) accepted. c Arrangement of c-N5- anions in a single layer. d Packing diagram for c-N5-@HOF.

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Thermal Stability

The thermal stability of c-N5-@HOF was determined with TG-DSC under a N2 atmosphere with a scan rate of 5 °C min−1. As shown in (Fig. 4a, c-N5-@HOF exhibited a weak exothermic peak at 161 °C followed by an obvious endothermic peak (172 °C), which corresponded to the weight loss stage starting at 133 °C on the TG curve. Based on the mass spectra (Supplementary Fig. 28) of the gaseous products (mainly N2 (m/z = 28)), we concluded that c-N5- decomposed during this process. This is the first example in which the onset decomposition temperature of a c-N5- compound13,14,15,16,17,18,19,20,21,22,23,24,25 exceeded 135 °C (Fig. 4c and Supplementary Table 11); it reached an unexpected value of 153 °C, indicating that the HOF significantly increased the thermal stability of c-N5-. The intense exothermic peak at 249 °C, which corresponded to a weight loss starting at 225 °C, was attributed to decomposition of the TATOT+ cation35. This decomposition process produced peaks at m/z = 28, 32, 16, and 27, which indicated the production of N2 (g), NH2-NH2 (g), CH4 (g), and CHN (g), respectively. Melamine had the best thermal stability, and its endothermic sublimation peak was observed at 350 °C, which was consistent with the data (345–352 °C) reported in the literature36,37.

Fig. 4: Thermal stability of c-N5-@HOF.
figure 4

a TG-DSC curves for c-N5-@HOF generated at a heating rate of 5 °C min−1 under a dry nitrogen atmosphere over the temperature range 40–400 °C. b Decomposition temperatures of most reported c-N5- based metal salts, CPs (coordination polymers), MPFs (metal-pentazolate frameworks), organic salts, and cocrystals.

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The mechanical sensitivities of the c-N5-@HOF were measured with the standard BAM method. It proved to be insensitive to impact and friction (IS > 40 J and FS > 360 N), highlighting the potential for application of energetic HOFs in chemical coatings desensitization of sensitive HEDMs. The insensitivity and good thermal stability of c-N5- were the joint results of electrostatic attractions between the positive hydrogen-bonded organic framework and the negative c-N5- rings, hydrogen bonding stabilization of c-N5- by N─Hs exposed on the inner wall of each channel, and π–π interactions between adjacent c-N5- rings. A strengthened framework constructed from many paired moderate to strong hydrogen bonds is required for high stability, which was confirmed by NCI calculations38 with Multiwfn39 (Fig. 5 and Supplementary Fig. 10). All of the nitrogen atoms of c-N5- were surrounded by weak to moderate hydrogen bonds. In addition, there were substantially offset face-to-face π–π interactions between the c-N5- rings and the ligands (melamine molecules and fused TATOT+ cations) of HOF in adjacent layers. These weak attractions created a tightly layered π–π stacked c-N5-@HOF, which resisted the influence of external heat and other factors, thus enabling the monomers to remain stably bound.

Fig. 5: Noncovalent interaction (NCI) plots of c-N5-@HOF.
figure 5

The surfaces are colored on a blue-green-red scale indicating strong attractive interactions, weak attractive, and strong non-bonded overlap, respectively (upper, layer-layer viewpoint, down, layer-layer overlap viewpoint).

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The interactions discussed above were also confirmed with a Hirshfeld surface40 analysis within the suite of tools in CrystalExplorer1741. The H…N and N…H interactions designated hydrogen bonds, while C…N, N…C, C…C, and N…N interactions designated π–π interactions (Fig. 6 and Supplementary Fig. 17). The hydrogen bonding interactions appeared as red dots on the plate Hirshfeld surface edges and were mainly distributed within the layers (Fig. 6c, e), and a few weak hydrogen bonds were distributed on the surface of the layer (Fig. 6d). These interactions (51.8 % of the total weak interactions) strengthened the 3D structure of the framework along with the π–π interactions (20.7 %) (Fig. 6f). In addition, there were also some weak H…H interactions (22.6 %) between layers, which were blue areas on the Hirshfeld surface few close contacts. As shown in Fig. 6a, a pair of remarkable spikes with di + de slightly less than 2 on the bottom left in the 2D fingerprint plot indicated the presence of strong N…H hydrogen bonds, as with reported c-N5- compounds19. Consistent with the findings of previous studies on CHON energetic molecules42, these hydrogen bonds and π–π interactions were also important characteristics that were insensitive to c-N5-@HOF.

Fig. 6: Hirshfeld surface analysis of c-N5-@HOF.
figure 6

a 2D fingerprint plot associated with the Hirshfeld surface. b Pie charts for the individual atomic contact percentages contributing to the Hirshfeld surface in c-N5-@HOF. c–f Hirshfeld surfaces for the asymmetric unit, 2D layer, and layer by layer packing.

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Organic salts and cocrystals of c-N5- with thermal decomposition temperatures ≥ 120 °C, including biguanidinium pentazolate (Td = 125 °C)19, TATOT+N5- ∙ TATOT cocrystal (Td = 122 °C)43, TATOT+N5- (Td = 121 °C)19, 3,5-diamino-4-nitro-1H-pyrazol-2-ium pentazolate (Td = 120 °C)44, and 4-nitro-(3,4-diamino-1,2,4-triazolium-5-yl)-1H-pyrazolium pentazolate (Td = 120 °C)45, were selected for comparison. The ESPs46 were calculated at the MP2/6-311++G(d,p) level and compared with that of c-N5-@HOF (Supplementary Figs. 11–16). A surface local maximum was observed above the center of the c-N5- ring for each compound. The maximum value of the ESP in c-N5-@HOF (−34.37 kcal mol−1) was lower than those of the other five representative compounds (−33.68 to −24.43 kcal mol−1). A lower ESP implies greater stability47, so a strengthened framework and various weak interactions indicated a stabilized c-N5-. In terms of the Hirshfeld surfaces of the c-N5- ring in c-N5-@HOF and the other five compounds used for comparison, the N…H interactions (hydrogen bonds) of c-N5- in c-N5-@HOF accounted for the largest proportion (83.4% vs 55.2–82.4%) and greatest intensity (Supplementary Figs. 18–23). In addition, compared with those of biguanidinium pentazolate, the hydrogen bonds of c-N5- in c-N5-@HOF were more symmetrical, and the face-to-face π–π domains were larger and more abundant (Supplementary Fig. 10). These results also indirectly indicated that symmetrical c-N5- hydrogen bonding48 was the main factor for stabilization of c-N5-, while face-to-face π–π stacking interactions played auxiliary roles in promoting stability.

Previous studies have shown that more symmetrical and homogeneous c-N5- hydrogen bonds are less likely to distort the c-N5- ring and lead to greater stability48. The undistorted c-N5- is a perfect regular pentagon, and theoretically, the most stable structure has five N─N bonds of 1.3175 Å (calculated at the M062X/6-31+G(d,p) level by Gaussian 0949). On this basis, the standard deviation for the N─N bond lengths in c-N5-@HOF was calculated as 6.522 × 10−3 (Supplementary Table 15). This means that from the perspective of bond length, the distortion of c-N5- in c-N5-@HOF was weaker than those in the TATOT+N5- ∙ TATOT cocrystal (22.616 × 10−3), TATOT+N5- (9.183 × 10−3), 3,5-diamino-4-nitro-1H-pyrazol-2-ium pentazolate (18.049 × 10−3), and 4-nitro-(3,4-diamino-1,2,4-triazolium-5-yl)-1H-pyrazolium pentazolate (8.434 × 10−3). Then, to analyze the effects of distortion on the stability of c-N5-, in Supplementary Table 16, the Laplacian bond order (LBO) of the c-N5- in c-N5-@HOF was calculated with Gaussian 0949 at the B3LYP/6-31+G(d,p) level and Multiwfn39 to determine the covalent bond strength. A comparison of the five pentazolate compounds with thermal decomposition temperatures ≥120 °C revealed that in the c-N5-@HOF treatment, the minimum LBO value was 1.0621, which was greater than that of biguanidinium pentazolate (1.0571), TATOT+N5- ∙ TATOT cocrystal (0.9998), and 3,5-diamino-4-nitro-1H-pyrazol-2-ium pentazolate (0.9027). As is well known, a larger LBO means a higher bond dissociation energy50 and a more stable c-N5-, which is consistent with the trend for experimental thermal stabilities. In addition, the most widely used nuclear independent chemical shift (NICS)51 method for diagnosing aromaticity was used to analyze the aromaticities of c-N5- in c-N5-@HOF and five other compounds. As shown in Supplementary Table 17, c-N5-@HOF exhibited the greatest aromaticity (NICSZZ(1): −45.33; NICSZZ(−1): −45.34). According to the analyses of bond lengths and LBO, the c-N5- rings in c-N5-@HOF were slightly distorted. Aromaticity is an important factor in maintaining the stability of c-N5-, so the c-N5- rings in c-N5-@HOF required greater aromaticity to maintain stability.

Energetic performance

As lightweight functional materials, HOFs are essentially low-density porous media without metals, and their densities are generally less than 1 g cm−3 31. However, in this study, c-N5-@HOF was constructed with energetic nitrogen-rich organic ligands instead of the commonly used arenes, and the density was 1.640 g cm−3 at 298 K. This density was greater than those of TATOT+N5- (1.615 g cm−3), TATOT+N5- ∙ TATOT cocrystal (1.638 g cm−3), and most nonmetallic c-N5- compounds13,14,15,16,17,18,19,20,21,22,23,24,25. The heat of formation for c-N5-@HOF was estimated as 787.7 kJ mol−1 according to the literature52. Using the measured density at room temperature (1.640 g cm−3) and the calculated heat of formation, the detonation velocity (D) and pressure (P) of c-N5-@HOF (Supplementary Note 9 and Tables 12–13) were calculated with EXPLO5 V6.05.04 software53. The D and P values (8.029 km s−1, 24.6 GPa) were greater than those of TATOT+N5- (7.791 km s−1, 24.6 GPa)19 and 2,4,6-trinitrotoluene (6.881 km s−1, 19.5 GPa)54. Although the detonation energy of c-N5-@HOF was lower than those of explosives such as 1,3,5-trinitro-1,3,5-triazacyclohexane and 1,3,5,7-tetranitro-1,3,5,7-tetraazacycloocatane, its thermal stability was one step closer to the application standard, and it is also a candidate insensitive material.

Discussion

A nitrogen-rich HOF with 1D pores was designed and obtained by self-assembly, and c-N5- anions occupied the pores and were stabilized by many hydrogen bonds with the HOF, π–π interactions between the five-membered rings, and attraction of the anions with the cationic HOF. These factors resulted in c-N5-@HOF thermal stability up to 153 °C, surpassing those of all reported c-N5- based compounds. We believe that this strategy will open the door to stable c-N5- and overcome the problem of poor c-N5- thermal stability by encapsulating it in HOFs. It is likely that HOFs with increasingly high energies, composed of energetic N─H- and/or O─H-rich ligands, will be used to capture and stabilize c-N5- while exhibiting excellent thermal stabilities and improved detonation properties.

Materials and methods

Safety precautions

Although no explosions or hazards were observed during the preparation and handling of these compounds in this work, all of the compounds investigated are potentially energetic materials. Scratching or scraping of these energetic materials must be avoided. Laboratories and personnel must be properly grounded, and safety equipment such as protective gloves and coats, face shields, and explosion-proof baffles are recommended. In addition, all of the compounds must be synthesized only on small scales.

General methods

All reagents and solvents were purchased from Sigma-Aldrich, Aladdin, and Energy Chemical (analytical grade) and were used as received.

1H NMR and 13C NMR spectra were recorded using a 500 MHz (Bruker AVANCE III 500) nuclear magnetic resonance spectrometer operating at 500 and 125.72 MHz, respectively. Chemical shifts in the 1H NMR and 13C NMR spectra are reported relative to Me4Si as external standard. Single-crystal X-ray diffraction was conducted with a Bruker D8 QUEST PHOTON 100 diffractometer using Mo-Kα radiation (λ = 0.71073 Å) (please refer to Supplementary Note 1 for detailed information). High-resolution mass spectra (electrospray ionization) were recorded with a Thermo Fisher Q Exactive Focus high-resolution mass spectrometer operated in splitless mode. The samples were dissolved in methanol (10 ppm), and introduced via a syringe pump at 30 μL min−1. The instrument was run with an electrospray voltage of 3.0 kV. DSC plots were acquired with a differential scanning calorimeter (NETZSCH DSC 204 F1 Phoenix) at a scan rate of 5 °C min−1 in perforated Al containers under a nitrogen flow of 60 mL min−1. TG-DSC measurements were performed with a NETZSCH STA 449 F5 Jupiter system at a heating rate of 5 °C min−1 under a nitrogen atmosphere. TG/DTA-MS was performed with a Rigaku Thermo Plus EV2/Thermo mass Photothermal Analyzer at a heating rate of 5 °C min−1 under a nitrogen atmosphere. IR spectra were recorded with a Thermo Nicolet iS10 spectrometer equipped with a Thermo Scientific Smart iTR diamond ATR accessory. Elemental analyses were carried out with a Vario EL III CHNOS elemental analyzer. Densities were measured at 25 °C with an Anton Paar Ultrapyc 5000 gas pycnometer. Impact and friction sensitivity measurements were performed with a standard BAM Fall hammer and a BAM friction tester.

Synthetic procedures

AgN5, melamine hydrochloride, and TATOT are synthesized according to the literature19,34,35.

MaN5

A solution of melamine hydrochloride (730 mg, 4.5 mmol) in distilled water (10 mL) was added dropwise to a suspension of AgN5 (800 mg, 4.5 mmol) in distilled water (15 mL). The mixture was reacted in the dark for 0.5 h, after which the mixture was centrifuged and filtered. The obtained filtrate was concentrated under reduced pressure to give the desired MaN5 (797 mg, yield: 90 %). The solid products were recrystallized in methanol/water to obtain white crystals. Td, onset: 110 °C (DSC). 1H NMR: δ 7.72 ppm; 13C NMR: δ 159.79 ppm. HRMS (m/z): [M]+: 127.07 (Ma+); [M]-: 70.01 (c-N5-). IR (ATR): ṽ 3337, 3155, 1681, 1702, 1632, 1537, 1456, 1394, 1218, 1171, 1010, 880, 813, 781, 750, 718 cm−1. Elemental analysis for C3H7N11 (197.166): calcd C 18.28, H 3.58, N 78.15 %. Found: C 18.22, H 3.55, N 78.21 %. IS: >40J; FS: >360N.

c-N5 -@HOF

MaN5 (197 mg, 1 mmol) and TATOT (254 mg, 1 mmol) were dissolved in 10 mL of distilled water, and the reaction mixture was slowly heated to 40 °C and stirred for 2 h. After the reaction, the mixture was cooled to room temperature and filtered, and the filtrate was slowly evaporated at room temperature for 3–4 days to obtain slightly yellow crystals. Yield: 70 %. Td, onset: 153 °C (TG). 1H NMR: δ 9.62, 8.23, 8.03, 7.28, 5.88 ppm; 13C NMR: δ 164.06, 159.96, 148.23, 142.42 ppm. HRMS (m/z): [M]+: 127.073 (Ma+), 155.050 (TATOT + H+) [M]-: 70.015 (c-N5-). IR (ATR): ṽ 3411, 3301, 3115, 2028, 1687, 1635, 1537, 1455, 1414, 1263, 1216, 1159, 1099, 1021, 939, 842, 811, 723, 702 cm−1. Elemental analysis for C6H13N19 (351.303): calcd C 20.51, H 3.73, N 75.76 %. Found: C 20.55, H 3.80, N 75.69 %. IS: >40J; FS: >360N.