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Pentazole having the molecular formula HN5 is an archetypical five-membered homocyclic inorganic aromatic molecule consisting of five nitrogen atoms. A hydrogen atom is bonded to one of the nitrogens. Even though the molecule does not contain a carbon it appears last in the series of the heterocyclic azole family; the family containing one to five nitrogen atoms. This series of heterocyclic azoles is pyrrole, imidazole, pyrazole, triazole, tetrazole, and the last one is the pentazole. Barring pentazole, the rest of the members of the azole family are heterocyclic organic molecules. The pKa of N(1)H-acidity values of all the azole members are known, except for that of pentazole. In the present work we endeavoured to determine the pKa of pentazole by a graphical method and by performing theoretical DFT calculations.

In this work, three novel nonmetallic pentazole salts (6–8) based on furazan or 4-nitropyrazole were synthesized. Some coplanar groups were introduced into the compounds to improve the planarity of...

Full-nitrogen energetic materials have excellent performance in the quest for higher-energy and greener explosive materials. Herein, the structure, properties under electric field, transition state, and unimolecular decomposition of octaazapentalene (o-N8), azidopentazole (a-N8), and bispentazole (N10) are studied in detail using density functional theory methods. The results show that three molecules are aromatic and each N atom is in the molecular plane. Applied electric field will change the bond length, atomic charge, and rate constant of thermal decomposition. The two pentazole rings of N10 immediately change from being in a plane (180°) to being perpendicular to each other (90.64°–91.81°) when the absolute value of the electric field intensity is greater than 40 × 10–4 a.u. The stability order of the three molecules is o-N8 > a-N8 > N10 judged by the activation energy of the initial decompostion. In addition to decompose to N2 in the azole ring, o-N8 was found to have a new pathway to eliminate N2 in the azide group. Moreover, ab initio molecular dynamics simulations were performed to further reveal the effect of different temperatures on the cleavage of single molecules.

Pentazole is the last member of the azole family. As a five-membered ring constructed from nitrogen, a pentazole skeleton is a good nitrogen-donor ligand, which possesses great potential to prepare intriguing multidimensional structures. In recent years, a series of metal pentazolate salts have been synthesized, which have attracted immense attention due to their interesting crystal structures. In this study, an overview of all the reported metal pentazolate salts is provided; furthermore, the key issues to be solved and future prospects in this field are also discussed.

The more-than-one-century-old arylpentazoles can only be used in situ in generating the pentazole anion due to their unfavourable kinetic stability. We successfully increased the N2-leaving barrier to reach hitherto the highest value of 40.83 kcal mol-1 at the CBS-QB3 level via a newly proposed co-stabilization method, making the broader applications of arylpentazoles feasible.

Pentazole (HN5) has been actively pursued due to its intriguing aesthetic structure and potential applications as ultrahigh energetic materials. It is generally accepted that cyclo-N5– is a highly ...

Stabilization of the pentazole anion only by acidic circumstances entrapment impedes the realization of a full-nitrogen substance; however, compression of nitrogen-rich nitrides has been recommend as an alternative way that has more controllable advantages to acquire the atomic nitrogen states. Through the structure searches are in conjunction with first-principle calculations, moderate pressure stabilized nitrogen-rich zinc nitrides with abundant extended nitrogen structures, e.g., cyclo-N5, infinite -(N4)n- chains, three-point stars N(N3), and N2 dumbbells, are predicted. The resonance between alternating σ bonds and π bonds in poly nitrogen sublattices takes charge of the coexistence of single and double bonds. The Zn(N5)2 salt has a noteworthy energy density (6.57 kJ/g) among the reported binary metal nitrides and synthesized pentazolate hydrates. An excellent Vicker's hardness (34 GPa) and detonation performance is unraveled. Although Zn(N5)2 salt is not expected to be recoverable at ambient conditions, it is worth noting that Zn(N5)2 is found to be stable at a very low pressure of ∼30 GPa, which is only half of those pressures required to synthesize CsN5. We clarified that the metal-centering octahedral pentazolate framework was entrapped by dual ionic-covalent bonds. More importantly, the covalent bonding can effectively enhance the chemical insensitivity and thermal stability, further preventing the autodecomposition of monatomic solid N5- anions into dinitrogen. Meanwhile, a unique topological pseudogap that attached to a metastable phase of ZnN4 salt is exposed for the first time, due to the dual effects of strong covalent sp2 hybridization interaction and the origin of ionic states.

The utility of high energy density materials (HEDMs) comes from their thermodynamic properties which arise from specific structural features that contribute to energy storage. Studies of such structural features seek to increase our understanding of these energy storage mechanisms in order to further enhance their properties. High-nitrogen-containing HEDMs are of particular interest because they are less toxic than traditional HEDMs. Pentazole is the largest of the nitrogen rings which has been synthesized and considered for an HEDM; however, few experimental studies exist due to the difficulty involved in the synthesis, and most previous theoretical studies employed composite methods where lower level geometries were used with higher level methods. Here, the decomposition reaction of pentazole is studied. Geometries, fundamental frequencies, and energies for each of the stationary points of the decomposition pathway are computed using ab initio methods up to CCSDT(Q). Decomposition rates are calculated over a range of temperatures using canonical transition state theory in order to determine the kinetic stability of pentazole. Based on the present results, it would be difficult for pentazole to act as an HEDM, requiring temperatures close to 200 K to achieve a suitable level of stability.

High nitrogen energetic compounds have always been a hot spot in energetic materials. In this work, we provide a new approach for the design of promising energetic molecules containing pentazole. Attractive energetic compounds include 5-amino-3-nitro-1H-1,2,4-triazole (ANTA) and 5-nitro-1,2,4-triazol-3-one(NTO) are used to effectively combine with pentazole to form a series of pentazole derivatives. Then, the NH2, NO2 or NF2 groups were introduced into the system to further adjust the property. Herein, the structures and densities of designed compounds as well as the heats of formation, detonation properties and impact sensitivities were predicted based on density functional theory (DFT). The results show that all ten designed molecules have excellent densities (1.81 g/cm3 to 1.97 g/cm3) and high heats of formation (621.66 kJ/mol to 1374.63 kJ/mol). Furthermore, detonation performances of compounds A3 (P = 41.16 GPa and D = 9.45 km/s) and A4 (P = 43.90 GPa and D = 9.69 km/s) are superior to 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and lower impact sensitivity than HMX. It exhibited that they could be taken as promising candidates of high-energy density materials. This work provides a worthy way to explore the energetic compounds with excellent performance based on pentazole.

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second σ-aromaticity in addition to the prototypical π-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional π-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid−base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N5¯, tetrazole anion N4C1H1¯, and 1,2,4-triazole anion N3C2H2¯) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid−base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

The pentazolate anion cyclo-N5- has received considerable attention from researchers, most metallic and non-metallic salts synthesized from pentazole have decomposition temperatures exceeding 100 °C. The author thinks, anhydrous metallic pentazolates provides prospects for wide application in the field of green, non-toxic, high-energy primary explosives. Hydrazine pentazolate and hydroxylamine pentazolate which have nitrogen contents of 95% are regarded as non-metallic pentazolates with the best performance and application prospects. Optimize synthesis processes of pentazole and functional group-modified pentazolate compounds will be the key direction in pentazole chemistry.

Cyclopentazolate anions ( cyclo -N5−) have been receiving ever-increasing attention as component of energetic explosives since recent fulfilment of the first stable sample in solid phase and ambient conditions. Herein, we present a new strategy to utilize deflagration reactions of cobalt pentazolate in combination with explosive poly(ionic liquid) (EPIL) for the preparation of Co@N-doped carbon materials with homogeneously distributed cobalt nanoparticle encapsulated by the layers of N-doped carbon sheets. The resultant 5%Co(N5)2-EPIL-900 exhibits high electrocatalytic activities, excellent stability and tolerance to CH3OH towards oxygen reduction reaction (ORR). Moreover, the present approach provides a novel routine for preparation of functional materials from energetic and newly-emerging cyclo -N5−-derived compounds.

Pentazole anion, the best candidate for full-nitrogen energetic materials, can be isolated only from acidic solution for unclear reasons, which hinders the high-yield realization of a full-nitrogen substance with higher energy density. Herein, we report for the first time the discovery of the dual aromaticity (π and σ) of cyclo-N5–, which makes the anion unstable in nature but confers additional stability in acidic surroundings. In addition to the usual π-aromaticity, similar to that of the prototypical benzene, five lone pairs are delocalized in the equatorial plane of cyclo-N5–, forming additional σ-aromaticity. It is the compatible coexistence of the inter-lone-pair repulsion and inter-lone-pair attraction within the σ-aromatic system that makes the naked cyclo-N5– highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N5– become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise, the cyclo-N5– cannot be productively isolated. The dual aromaticity discovered in cyclo-N5– is expected to be universal for pnictogen five-membered ring systems. Accepted version Financial support from the National Natural Science Foundation of China (Nos. 11604017, 11572053, and U1730244), the Science Challenging Program of China (No. TZ2016001), and the National Supercomputing Center (Shenzhen) and helpful discussions with B.C. Hu, M. Gozin, M. Š ob, E. Hayward, C.Y. Zhang, J.G. Zhang and H.H. Zong are gratefully acknowledged.

Nineteen cyclo-N5− containing salts with considerably high thermal stability at ambient conditions were experimentally reported in the past two years. This work focuses upon the origin of the rather high thermal stability of these salts, in combination with the analysis of the crystal packing. In the lattice, cyclo-N5− exists in various manners, i.e., as an independent ring without covalent bonding, as a moiety of zero-, one- or three-dimensional stacking structures, or even as complex zeolite-like architectures. The high structural diversity of cyclo-N5− containing salts originates from the η-σ coordination bonding between cyclo-N5− and various metal atoms, with or without hydrogen bonding. Meanwhile, we conclude that the considerably high thermal stability of the salts is attributed to the ionization and conjugation of cyclo-N5−, and the hydrogen bonding, coordination bonding and π–π stacking between cyclo-N5− and surrounding molecules and/or atoms in the lattice. Among these factors, the ionization is the first premise for stability. Moreover, keeping the high planarity of cyclo-N5− is crucial to maintaining the high thermal stability of the cyclo-N5− salts. Thus, more and more cyclo-N5− salts with high stability are expected to come into being soon, setting a substantial basis for pentazole chemistry research. This insight facilitates guiding the manufacture of new cyclo-N5− containing salts and developing a strategy for stabilizing other unstable chemical species under common conditions.

Modified polycyanostyrene and polynitrostyrene microspheres were designed and synthesized, which proved to be an effective carrier to prepare gold nanoparticles with pentazole gold as a catalyst precursor. The developed gold nanoparticles were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray, X-ray power diffraction and X-ray photoelectron spectrometry. In addition, it was observed that this catalytic system successfully achieved application in the synthesis of cinnoline derivatives and hydration reactions in high yields. Mechanism investigation was conducted to better understand these transformations and catalyst systems. To the best of our knowledge, this is the first example of utilizing pentazole gold as the catalyst precursor and achieving catalytic applications.

Metal–metal multiple bonds based polynitrogen complex clusters may become a class of high-energy, insensitive and green primary explosives. Five complexes containing chromium (Cr≡Cr), molybdenum (Mo≡Mo), tungsten (W≡W), technetium (Tc≡Tc) and rhenium (Re≡Re) metal multiple bonds with pentazole ligands were theoretically built. The metal–metal multiple bonds, metal-pentazole coordination bonds, molecular static and dynamic polarizability were investigated with density functional theory M06-L/LanL2DZ, The Wiberg bond order sequence of metal–metal is Mo–Mo > Cr–Cr > W–W > Re–Re > Tc–Tc Bidentate coordination bonds were formed between each metal–metal edge and two pentazole ligands, which strengthen the coordination bonds, and Wiberg bond order sequence of metal-pentazole is Re–N > W–N > Tc–N > Mo–N > Cr–N. The gaseous and solid phase formation enthalpies, enthalpies of sublimation and theoretical density of five complexes were predicted. As candidates for primary explosives, their detonation heats were calculated and the descending order is Re4(N5)8 > W4(N5)8 > Tc4(N5)8 > Mo4(N5)8 > Cr4(N5)8. M4N40 clusters may be a class of promising green primary explosives.

A three-dimensional bowl-shaped molecular container based on pentazole was first synthesized. These containers are sealed and linked by the assembled “molecular plane”. Each container has an ovoid cavity occupied by one DMSO guest molecule. The self-assembly of this molecular container will provide opportunities for the use of pentazole in supramolecular chemistry.

The all-nitrogen cyclo-pentazolate (c-N5ˉ) anion has attracted interest because of its promising application in high-energy materials. Now, nonmetallic pentazolate salts were prepared by assembling cyclo-N5ˉ and cations but it was shown to be challenging to balance excellent detonation performance and good stability, which results in restrictions for practical applications. However, we find the layer-by-layer crystal stacking structure can enhance the sensitivity of pentazolate salts. Herein, we designed and synthesized two targeted cyclo-pentazolate salts with layered stacking mode in their crystal structures based on two isomeric cations: 3,4,5-triamino-1,2,4-triazolium and 3-hydrazino-4-amino-1,2,4-triazolium. The results demonstrate that the aromaticity of triazole and pentazole rings arouse the π−π interaction force, promoting the formation of a layer-by-layer crystal stacking structure. Two compounds show very low sensitivity characteristics (FS>40 and IS>360) and good detonation properties (detonation velocity: 3, Dv= 8796m⋅s−1; 4, Dv=8574m⋅s−1. detonation pressure: 3, P = 26.6 GPa; 4, P = 25.1 GPa). Computational analysis based on noncovalent interaction index, localized orbital locator-π and electron density difference provide a better understanding about the effects of layer-by-layer stacking structure on the stabilities. These findings contribute to preparation of both high-energy and low sensitivity pentazolate salts.

Pentazole (HN5) and its anion ( cyclo-N5-) have been elusive for nearly a century because of the unstable N5 ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N5- in (N5)6(H3O)3(NH4)4Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N5- in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N5)6(H3O)3(NH4)4Cl → (N5)5(N5H)(H2O)(H3O)2(NH4)4Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N5- due to the temperature effect; consequently, both deprotonated and protonated cyclo-N5- exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N5-, H2O, and NH3, and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N5- in DMSO solvent. Our AIMD simulations reconcile the recent controversy.

Coordination to form polymer is emerging as a new technology for modifying or enhancing the properties of the existed energetic substances in energetic materials area. In this work, guanidine cation CN3H6+ (Gu) and 3-amino-1,2,4-triazole C2H4N4 (ATz) were crystallized into NaN5 and two novel energetic coordination polymers (CPs), (NaN5)5[(CH6-N3)N5](N5)3– ( 1 ) and (NaN5)2(C2H4N4) ( 2 ) were prepared respectively via a self-assembly process. The crystal structure reveals the co-existence of the chelating pentazole anion and organic component in the solid state. In polymer 1 , Na+ and N5– were coordinated to form a cage structure in which guanidine cation [C(NH2)3]+ was trapped; for polymer 2 , a mixed-ligand system was observed; N5– and ATz coordinate separately with Na+ and form two independent but interweaved nets. In this way, coordination polymer has been successfully utilized to modify specific properties of energetic materials through crystallization. Benefiting from the coordination and weak interactions, the decomposition temperatures of both polymers increase from 111°C (1D structure [Na(H2O)(N5)]∙2H2O) to 118.4 and 126.5°C respectively. Moreover, no crystallized H2O was generated in products to afford the anhydrous compounds of pentazole salts with high heats of formation (> 800 kJ mol–1). Compared to traditional energetic materials, the advantage in heats of formation is still obvious for the cyclo -N5– based CPs, which highlights cyclo -N5– as a promising energetic precursor for high energy density materials (HEDMs).

The pentazolate anion, as a polynitrogen species, holds great promise as a high-energy density material for explosive or propulsion applications. Designing pentazole complexes that contain minimal non-energetic components is desirable in order to increase the material’s energy density. Here, we report a solvent-free pentazolate complex, AgN5, and a 3D energetic-framework, [Ag(NH3)2]+[Ag3(N5)4]ˉ, constructed from silver and cyclo-N5ˉ. The complexes are stable up to 90 °C and only Ag and N2 are observed as the final decomposition products. Efforts to isolate pure AgN5 were unsuccessful due to partial photolytical and/or thermal-decomposition to AgN3. Convincing evidence for the formation of AgN5 as the original reaction product is presented. The isolation of a cyclo-N5ˉ complex, devoid of stabilizing molecules and ions, such as H2O, H3O+, and NH4+, constitutes a major advance in pentazole chemistry.

The experimental detection and synthesis of pentazole (HN5 ) and its anion (cyclo-N5- ) have been actively pursued for the past hundred years. The synthesis of an aesthetic three-dimensional metal-pentazolate framework (denoted as MPF-1) is presented. It consists of sodium ions and cyclo-N5- anions in which the isolated cyclo-N5- anions are preternaturally stabilized in this inorganic open framework featuring two types of nanocages (Na20 N60 and Na24 N60 ) through strong metal coordination bonds. The compound MPF-1 is indefinitely stable at room temperature and exhibits high thermal stability relative to the reported cyclo-N5- salts. This finding offers a new approach to create metal-pentazolate frameworks (MPFs) and enables the future exploration of interesting pentazole chemistry and also related functional materials.

From a 1D sodium-N5- framework, a new zeolite-like metal-organic framework (MOF) with a fascinating 3D structure was successfully constructed. It exhibited an enhanced thermal stability with a decomposition temperature (onset) of 129 °C and an enhanced coordination ability (five-coordination) of cyclo-N5- in weak alkaline conditions. The 3D MOF with a bulky size offers new opportunities not only for the formation of porous materials but also to control the balance between the performance and stability of polynitrogen materials.

The pentazolate anion, or cyclo-N5−, which is a five-membered ring composed solely of nitrogen atoms, has a unique structure among polynitrogen compounds. Cyclo-N5− is receiving ever-increasing levels of attention because of its potential ability to store large amounts of energy compared to the azide ion, its environmentally friendly decomposition products, and its carbon- and hydrogen-free composition, which are promising characteristics for advancing the field of high-energy-density materials (HEDMs), that include explosives, oxidisers, and propellants in closed environments. In this review, we provide a detailed introduction to cyclo-N5− and cover the following topics: (1) substituted pentazoles as precursors of cyclo-N5−, with a focus on the syntheses and stabilities of substituted pentazole derivatives; (2) routes to cyclo-N5− through cleavage of C–N bonds in substituted pentazoles, during which competitive reactions between pentazole decomposition and C–N bond cleavage need to be considered to ensure a successful outcome; (3) complexes of cyclo-N5−, summarising recent progress toward producing cyclo-N5−-based complexes through the assembly of isolated cyclo-N5− with both metallic and nonmetallic components; and (4) interactions between cyclo-N5− and metal cations and non-metal species, as well as factors that influence the stability of these complexes; in particular, the thermal stabilities of prepared cyclo-N5− salts are discussed. This review summarises recent studies and is intended to improve the understanding of polynitrogen chemistry while supporting further research into its potential application as an efficient, safe, and environmentally friendly HEDM.

The structural features, energetic characteristics, and dynamics behaviors for series of pentazole anion salts under various intensities of external electric field (0.0–0.5 V/A) were investigated by molecular dynamics simulations. The results show that there exist significant conformational changes for (NH4)N5 in 0.2 V/A and [N(CH3)4]N5 in 0.5 V/A. The sudden conformational changes resulted in sudden changes in energetic properties, dynamics behaviors, and so on. The comprehensive analysis on N-N bond length distribution, potential energy and cohesive energy density indicates that external electric field can improve thermodynamics stability. The analysis on diffusion behavior suggests that external electric field restrict the diffusion of N5− anion. Our studies innovatively introduced external electric field by Perl script and provide a new avenue to tune and improve the performances of energetic materials by external electric field.

Benzylpentazole (B) and its derivatives (B-NH 2 ~ B-NF 2 ) with electron-donating group (−NH2, −NHMe, −NMe2, and−OH) and electron-withdrawing group (−NO2, −CN, −CF3 and −NF2) were studied using density functional theory to assess their potentials for replacing phenylpentazole (PhN 5 ) as cyclo-N5 − precursor. The pyrolysis mechanisms of the N5 ring and the C─N bond connecting pentazole and benzyl, bond lengths, and atomic charge have been investigated for revealing the effects of substituents on stability and assessing the possibilities of benzylpentazoles as cyclo-N5 − precursor. Substituents especially electron-withdrawing groups lower the N5 ring stability and the C─N bond stability of B simultaneously. Compared to PhN 5 , benzylpentazoles mostly possess higher activation energy for the N5 ring decomposition (81.2 ~ 101.7 kJ/mol), lower C─N bond dissociation energy (385.6 ~ 452.8 kJ/mol), and higher chemical stability (7.83 ~ 8.49 eV). B and B-NMe 2 with the highest N5 ring stability, appropriate C─N bond stability, and chemical stability are the most potential candidates for replacing PhN 5 as cyclo-N5 − precursor.

The cyclopentazolate anion, N5–, has been researched extensively over the years and detected in the gas phase more than a decade ago, but was only recently measured in solution. The process whereby aryl pentazole reduction leads to the production of N5– is still not fully understood. Here, the production of N5– in solution was investigated using isotopic labeling techniques while implementing changes to the synthesis methodologies. 15N labeled phenyl pentazole produced appropriately labeled phenyl pentazole radical anions and N5– which, upon collision induced dissociation, produced the expected N3– signals. Changing to higher purity solvent and less coated Na metal allowed for a much more rapid pace, with experiments taking less time. However, the best yields were obtained with heavily coated metal and much longer reaction times. Utilization of a vacuum line and ultrapure solvents led to no products being detected, indicating the importance of a sodium passivation layer in this reaction and the possibilit...

Metal complexes of the pentazole anion exhibit multiple coordination modes, through ionic, covalent and hydrogen-bonding interactions, and good thermal stability with onset decomposition temperatures greater than 100 °C. Polynitrogen compounds can decompose to N2 with an extraordinarily large energy release, which makes them promising candidate materials for explosives but difficult to produce in a stable form. Compounds containing five-membered all-nitrogen rings have attracted particular interest in the search for a stable polynitrogen molecule. Yuangang Xu et al. report five metal complexes containing the pentazole anion, cyclo--N5−, four of which exhibit good thermal stability and a range of different bonding interactions for stabilization. Given their energetic properties and stability, and the adaptability of the cyclo-N5− species in terms of its bonding interactions, these complexes might lead to the development of a new class of high-energy-density materials and of other unusual polynitrogen complexes. Singly or doubly bonded polynitrogen compounds can decompose to dinitrogen (N2) with an extremely large energy release. This makes them attractive as potential explosives or propellants1,2,3, but also challenging to produce in a stable form. Polynitrogen materials containing nitrogen as the only element exist in the form of high-pressure polymeric phases4,5,6, but under ambient conditions even metastability is realized only in the presence of other elements that provide stabilization. An early example is the molecule phenylpentazole, with a five-membered all-nitrogen ring, which was first reported in the 1900s7 and characterized in the 1950s8,9. Salts containing the azide anion (N3−)10,11,12 or pentazenium cation (N5+)13 are also known, with compounds containing the pentazole anion, cyclo-N5−, a more recent addition14,15,16. Very recently, a bulk material containing this species was reported17 and then used to prepare the first example of a solid-state metal–N5 complex18. Here we report the synthesis and characterization of five metal pentazolate hydrate complexes [Na(H2O)(N5)]·2H2O, [M(H2O)4(N5)2]·4H2O (M = Mn, Fe and Co) and [Mg(H2O)6(N5)2]·4H2O that, with the exception of the Co complex, exhibit good thermal stability with onset decomposition temperatures greater than 100 °C. For this series we find that the N5− ion can coordinate to the metal cation through either ionic or covalent interactions, and is stabilized through hydrogen-bonding interactions with water. Given their energetic properties and stability, pentazole–metal complexes might potentially serve as a new class of high-energy density materials19 or enable the development of such materials containing only nitrogen20,21,22,23. We also anticipate that the adaptability of the N5− ion in terms of its bonding interactions will enable the exploration of inorganic nitrogen analogues of metallocenes24 and other unusual polynitrogen complexes.