Taming cyclo-pentazolate anions with a hydrogen-bonded organic framework

High energy density materials (HEDMs) are critical in explosives, propellants, and pyrotechnics, yet conventional CHON-based compounds have reached an energy bottleneck. Polynitrogen compounds offer high enthalpies of formation due to favorable N–N and N=N bond energetics and the large energy release upon forming N2, but are metastable and challenging to synthesize and stabilize. Three polynitrogen ions (N3−, N5+, and c-N5−) have been prepared in bulk under ambient conditions; among them, the aromatic, planar c-N5 anion has enabled diverse derivatives but typically decomposes at 80–130 °C. Improving c-N5 thermal stability is a key hurdle for practical application. The study asks whether a purpose-built hydrogen-bonded organic framework (HOF) can encapsulate and stabilize c-N5 via symmetrical hydrogen bonding and other noncovalent interactions to surpass prior thermal stability limits while maintaining favorable energetic properties and low sensitivity.

Prior strategies to stabilize c-N5 include arylpentazoles, 3D metal–pentazolate frameworks, coordination polymers, organic salts, and cocrystals. A lithium–pentazolate framework previously achieved one of the highest stabilities among metal–c-N5 compounds (decomposition ~133 °C) via coordination in nanocages. Among metal-free salts, biguanidinium pentazolate reached 125 °C, attributed to extensive hydrogen bonding, though c-N5 itself lacked direct bonding to the dimeric cation environment. Hydrogen-bonded organic frameworks (HOFs) have emerged as functional porous materials; notably, an energetic nitroformate@HOF achieved 200 °C decomposition, demonstrating that encapsulation by hydrogen-bond-rich frameworks can markedly increase thermal stability of energetic anions. Like nitroformate, c-N5 is a good hydrogen-bond acceptor, motivating the design of a nitrogen-rich HOF to host c-N5 through paired/symmetrical N···H hydrogen bonds, π–π stacking, and electrostatic attractions to enhance stability beyond the typical ≤135 °C of prior c-N5 materials.

• Synthesis: Silver pentazolate (AgN5), melamine hydrochloride, and 3,6,7-triamine-7H-[1,2,4]triazole[4,3-b][1,2,4]triazole (TATOT) were prepared per literature. Melaminium pentazolate (MaN5) was obtained by metathesis of AgN5 with melamine hydrochloride in water. c-N5@HOF was assembled by dissolving MaN5 and TATOT (1:1) in water, heating to 40 °C for 2 h, cooling, filtering, and slow evaporation (3–4 days) to yield crystals (70% yield).
• Characterization: 1H/13C NMR, IR (ATR), high-resolution ESI-MS, elemental analysis, DSC, TG-DSC, and TG/DTA-MS. Densities measured at 25 °C by gas pycnometry. Impact and friction sensitivities measured by BAM methods.
• Crystallography: Single-crystal X-ray diffraction determined structures of MaN5 and c-N5@HOF. c-N5@HOF crystallized in monoclinic C2/c (Z=8), density 1.688 g cm−3 (100 K). HOF layers formed 3+3 membered hydrogen-bonded rings (melamine and TATOT+), stacked with ~3.2 Å interlayer distances forming 1D pores of ~5.4 Å. c-N5 anions reside centrally and coplanar within the rings, accepting seven N–H···N hydrogen bonds with selected paired, weaker bonds at meta-N atoms to enforce symmetry.
• Thermal analyses: TG-DSC at 5 °C min−1 under N2; evolved gas analysis by MS (m/z assignments) to identify decomposition products.
• Noncovalent interaction analyses: NCI plots computed with Multiwfn to visualize hydrogen bonds and π–π contacts. Hirshfeld surface analyses (CrystalExplorer) quantified contact contributions (H···N/N···H, C···C, C···N/N···C, H···H, N···N).
• Quantum chemical calculations: Electrostatic potentials (ESPs) at MP2/6-311++G(d,p); Laplacian bond orders (LBOs) at B3LYP/6-31+G(d,p) via Multiwfn; aromaticity by NICSzz(±1). Comparative analyses performed against representative c-N5 salts/cocrystals with Td ≥120 °C.
• Energetic performance: Heat of formation estimated from literature correlations; detonation velocity (D) and pressure (P) calculated with EXPLO5 V6.05.04 using measured density (1.640 g cm−3 at 298 K). Sensitivity measured by BAM (IS and FS).

• Structural encapsulation: c-N5 is centrally located and coplanar within HOF 3+3 membered rings, residing in ~5.4 Å 1D pores with dense, largely symmetrical N–H···N hydrogen bonds and interlayer π–π stacking (~3.2 Å). Each c-N5 accepts seven hydrogen bonds; two meta-N atoms each engage in paired, weaker H-bonds, distributing forces more uniformly on the ring.
• Thermal stability: c-N5@HOF shows an onset decomposition temperature of 153 °C (TG), exceeding all reported c-N5 compounds (generally ≤135 °C). DSC reveals a weak exotherm at 161 °C and endotherm at 172 °C, with MS indicating N2 (m/z 28) evolution from c-N5 decomposition. A stronger exotherm at 249 °C corresponds to TATOT decomposition; melamine sublimation near 350 °C.
• Noncovalent interactions: NCI analyses show widespread weak-to-moderate H-bonding around c-N5 and offset face-to-face π–π interactions between c-N5 and adjacent-layer ligands, forming a tightly layered stack. Hirshfeld analysis indicates hydrogen bonds account for 51.8% of weak interactions and π–π related contacts 20.7%; H···H 22.6%.
• Electronic stabilization metrics: The ESP surface maximum over c-N5 in c-N5@HOF is −34.37 kcal mol−1, lower than in five comparison compounds (−33.68 to −24.43 kcal mol−1), indicating greater stability. The fraction of N···H contacts on c-N5 is highest (83.4%) and most intense among comparators. The minimum LBO for c-N5 bonds is 1.0621 (higher than biguanidinium pentazolate 1.0571; TATOT–N5; TATOT cocrystal 0.9998; and DAFP–N5 0.9027), consistent with stronger bonding and higher stability. Aromaticity is greatest for c-N5@HOF (NICSzz(1) = −45.33; NICSzz(−1) = −45.34). Bond-length deviations indicate lower ring distortion (σ ≈ 6.522×10−3) than in several comparators.
• Physical properties and performance: Density 1.640 g cm−3 (298 K), higher than TATOT–N5 (1.615 g cm−3) and the TATOT–N5 cocrystal (1.638 g cm−3). Calculated detonation velocity and pressure: D = 8.029 km s−1; P = 24.6 GPa, exceeding TNT (6.881 km s−1; 19.5 GPa). Sensitivity: insensitive (IS > 40 J; FS > 360 N). The material is air-stable and soluble in water/DMSO.

Encapsulating c-N5 within a nitrogen-rich, cationic HOF addresses the central challenge of c-N5 thermal instability by combining multiple, cooperative noncovalent interactions: (1) symmetrical N–H···N hydrogen bonds that minimize anisotropic stresses and ring distortion; (2) electrostatic attraction between the cationic framework and anionic c-N5; and (3) interlayer π–π stacking involving c-N5 and aromatic ligands that buttress the layered assembly. Quantitative metrics (lower ESP maximum, higher minimum LBO, greater aromaticity, larger proportion and symmetry of N···H contacts) align with the experimentally observed increase in onset decomposition temperature to 153 °C, surpassing prior c-N5 salts, cocrystals, and frameworks. The framework’s robust, paired hydrogen-bond network strengthens the lattice and confines c-N5 in coplanar, centrally positioned sites that reduce ring distortion—considered critical for stability in polynitrogen systems. Despite the enhanced stability, the system remains an energetic material with respectable calculated detonation performance and low mechanical sensitivity, indicating potential utility as a safer, more thermally robust c-N5-based HEDM. The strategy generalizes: designing HOFs rich in N–H/O–H donors and π-stacking capacity should enable broader stabilization of polynitrogen anions.

The study introduces a hydrogen-bonded organic framework that captures and stabilizes the cyclo-pentazolate anion in 1D pores via symmetrical hydrogen bonding, electrostatic attraction, and π–π stacking. This design achieves a record onset decomposition temperature for c-N5 materials (153 °C), favorable calculated detonation performance (8.029 km s−1; 24.6 GPa), and insensitivity to impact and friction. Electronic structure analyses (ESP, LBO, NICS) and Hirshfeld/NCI results corroborate the stabilization mechanism, highlighting the importance of symmetric hydrogen bonding and reduced ring distortion. Future work should explore HOFs with varied N–H/O–H-rich ligands and pore architectures to further enhance stability and energetic performance, extend the approach to other polynitrogen anions, and experimentally validate performance under operational conditions.

• Scope: The stabilization is demonstrated for a single HOF system (melamine/TATOT-based); generality across other frameworks and guest loadings remains to be tested.
• Thermal and performance metrics: Detonation parameters are computationally estimated (EXPLO5) rather than experimentally validated; thermal stability was assessed under nitrogen at a specific heating rate, which may differ under other conditions.
• Structural environment: While symmetry of H-bonding and π–π interactions is strong, the exact tunability window (e.g., pore size, donor density) and long-term stability under humidity/solvent exposure were not extensively detailed beyond air stability.
• Scale and safety: Syntheses were conducted on small scales with energetic materials; scale-up behavior and processing safety were not addressed.
• Comparative dataset: Although several comparators were analyzed, broader benchmarking across diverse c-N5 systems (including metal frameworks and different organic hosts) would strengthen generalizability.