Initial Decomposition Mechanism of NH3OH+N5- Crystal under Thermal and Shock Loading: A First-Principles Study

NH3OH+N5- is a novel energetic material (EM) which has attracted much interest for its promising performances, including high energy density, high density, low sensitivity, and low toxicity. In this study, the initial decomposition mechanism of NH3OH+N5- crystal was investigated under thermal and shock loading by molecular dynamics simulation. First, programmed heating and constant temperature simulations were carried out by molecular dynamics simulation on the basis of density functional theory (DFT-MD). Results indicated that the initial decomposition reactions of NH3OH+N5- could be described by three reactions: proton transfer, ring-opening reaction, and cation decomposition and recombination, and three pathways of ring-opening reaction were found, including the ring-opening of N5-, HN5, and H2N6. The first two reactions are the main pathways that produce N2 molecules. Furthermore, we carried out DFT-MD simulations to study the shock decomposition behaviors of NH3OH+N5-, and three initial steps were proposed: N5-, HN5, and N6 ring-opening. The fewer N5- and HN5 ring-opening reactions were found during the shock simulation, accompanied by a significant change in the N5- bond angle. What's more, the transition states of decomposition reactions were investigated through quantum chemical calculations. The results revealed that the proton transfer reaction exhibits lower activation barriers compared to ring-opening reactions, and proton transfer would accelerate ring-opening reactions. In addition, the ring-opening reaction is the main energy-releasing reaction in the early stages of the decomposition. This work could promote the comprehension of the decomposition mechanism and energy release regularity of N5- ions.

Pentazolate Anion: A Rare and Preferred Five-Membered Ligand for Constructing Pentasil-Zeolite Topology Architectures

As the fourth full-nitrogen structure, the pentazolate anion (cyclo-N5 - ) was highly coveted for decades. In 2017, the first air-stable non-metal pentazolate salt, (N5 )6 (H3 O)3 (NH4 )4 Cl, was obtained, representing a milestone in this field. As the latest member of the azole family, cyclo-N5 - is comprised of five nitrogen atoms. Although significant attention has been paid to the potential of cyclo-N5 - as an energetic material, its poor thermostability hinders any practical application. However, the unique ring structure and multiple coordination capability of cyclo-N5 - provide a platform for the fabrication of various structures, among which pentasil-zeolite topologies are the most intriguing. In addition, the introduction of structure-directing auxiliaries enables the self-assembly of diverse topological architectures, potentially imparting cyclo-N5 - with the potential to impact wide-ranging areas of coordination chemistry and topology. In this minireview, different pentasil-zeolite topologies based on metal-pentazolate frameworks are evaluated. To date, three zeolitic and zeolite-like topologies have been reported, namely the melanophlogite (MEP), chibaite (MTN), and unj topologies. The MEP topology consists of two nanocages, Na20 N60 and Na24 N60 , whereas the MTN topology contains Na20 N60 and Na28 N80 nanocages. Furthermore, the unj topology features multiple homochiral channels consisting of two helical chains. Various possible strategies for obtaining additional pentasil-zeolite topologies are also discussed.

Thermal stability of azole-rich energetic compounds: their structure, density, enthalpy of formation and energetic properties

Energetic compounds, as a type of special material, are widely used in the fields of national defense, aerospace and exploration. Their research and production have received growing attention. Thermal stability is a crucial factor for the safety of energetic materials. Azole-rich energetic compounds have emerged as a research hotspot in recent years owing to their excellent properties. Due to the aromaticity of unsaturated azoles, many azole-rich energetic compounds have significant thermal stability, which is one of the properties that researchers focus on. This review presents a comprehensive summary of the physicochemical and energetic properties of various energetic materials, highlighting the relationship between thermal stability and the structural, physicochemical, and energetic properties of azole-rich energetic compounds. To improve the thermal stability of compounds, five aspects can be considered, including functional group modification, bridging, preparation of energetic salts, energetic metal-organic frameworks (EMOFs) and co-crystals. It was demonstrated that increasing the strength and number of hydrogen bonds of azoles and expanding the π-π stacking area are the key factors to improve thermal stability, which provides a valuable way to develop energetic materials with higher energy and thermal stability.

Three-Dimensional Metal-Organic Frameworks as Super Heat-Resistant Explosives: Potassium 4,4'-Oxybis[3,3'-(5-tetrazol)]furazan and Potassium (1,2,4-Triazol-3-yl)tetrazole

Heat-resistant explosives play an irreplaceable role in specialized applications. Two energetic metal-organic frameworks (EMOFs), potassium 4,4'-oxybis[3,3'-(5-tetrazol)]furazan and potassium (1,2,4-triazol-3-yl)tetrazole, featuring a three-dimensional metal-organic framework structure, were first synthesized and characterized by chemical (¹H NMR, ¹³C NMR, MS, IR spectroscopy, and single-crystal XRD) and physicochemical analyses (sensitivity toward friction, impact, electrostatic, and DSC-TGA test). The new 3D EMOFs were found to show high thermostability, highly positive heat of formation, and suitable sensitivities. The Hirshfeld surface was further analyzed in order to explore the effect on sensitivities. Their detonation properties (detonation velocity, detonation pressure, etc.) were calculated by the EXPLO5 program. K₂NTT exhibits extremely high decomposition temperatures of up to 361 °C; meanwhile, its detonation performance is comparable to that of TATB and other energetic potassium salts, which makes it a promising heat-resistant explosive.

Energetic Salts of Sensitive N,N'-(3,5-Dinitropyrazine-2,6-diyl)dinitramide Stabilized through Three-Dimensional Intermolecular Interactions

N-nitration of 2,6-diamino-3,5-dinitropyrazine (ANPZ) leads to a sensitive energetic compound N,N'-(3,5-dinitropyrazine-2,6-diyl)dinitramide. This nitro(nitroamino) compound was stabilized by synthesizing energetic salts, dipotassium (3,5-dinitropyrazine-2,6-diyl)bis(nitroamide) (3) and diammonium (3,5-dinitropyrazine-2,6-diyl)bis(nitroamide) (4). Compounds 3 and 4 are fully characterized by single-crystal X-ray diffraction. Compound 3 exhibits a three-dimensional energetic metal-organic framework (3D EMOF) structure and an outstanding overall performance by combining high experimental density (2.10 g cm^-3), good thermal stability (T_d(onset) = 220 °C), and good calculated performance of detonation (D = 8300 m s^-1, P = 29.9 GPa). Compound 4 has acceptable thermal stability (155 °C), moderate experimental density (1.73 g cm^-3), and good calculated performance of detonation (D = 8624 m s^-1, P = 30.8 GPa). The sensitivities of compounds 3 and 4 toward impact and friction were determined following standard methods (BAM). The energetic character of compounds 3 and 4 was determined using red-hot needle and heated plate tests. The results highlight a 3D EMOF (3) based on a six-membered heterocycle as a potential energetic material.

Controllable Structural Modulation: Assembling Variable Dimension Energetic Metal-Organic Frameworks via Free Protons

Herein, we provide an efficient strategy for constructing three-dimensional (3D) energetic coordination polymers (ECPs), namely, metal-organic frameworks (EMOFs), avoiding solvent coordination without changing the organic ligands or metal nodes. Three ECPs with the same ligand and metal center, namely, two-dimensional (2D) layer ECP [Pb(HOBTT)(H₂O)₂]_n (1), 3D solvent-free EMOFs [Pb(HOBTT)]_n (2), and dense [Pb₃(OBTT)₂]_n (3) (H₃OBTT = 4,5-bis(1-hydroxytetrazol-5-yl)-2H-1,2,3-triazole), were rationally designed and synthesized via free protons. As expected, the theoretical density of 3 (4.080 g·cm^-3) is greater than those of 2 (3.299 g·cm^-3) and 1 (3.055 g·cm^-3). Thermal stabilities indicate that their decomposition temperature exceeds 300 °C. Theoretical calculations show that the detonation performance of 3 is better than that of 1 and 2. The detonation performance of 1-3 was further proven by laser irradiation.

Energetic bimetallic complexes as catalysts affect the thermal decomposition of ammonium perchlorate

Two bimetallic complexes of 4-hydroxy-3,5-dinitropyrazole, [K₂Mn(DNPO)₂(H₂O)₄]_n·2H₂O (BMEP-1) and [K₂Zn(DNPO)₂(H₂O)₆]_n (BMEP-2), were synthesized and characterized by IR spectroscopy and elemental analysis. The crystal structures of BMEP-1 and BMEP-2 were determined by single-crystal X-ray diffraction. It is noteworthy that these complexes presented different metal-organic frameworks. The thermal behaviors of BMEP-1 and BMEP-2 were investigated by differential scanning calorimetry and thermogravimetric analysis measurements. These bimetallic complexes exhibited high thermal stability (348.0 °C and 331.0 °C) due to their large coordination bonds and three-dimensional interconnected structure. The catalytic performances of BMEP-1 and BMEP-2 on the thermal decomposition of ammonium perchlorate were investigated by TGA-DSC, TGA-FTIR, and non-isothermal kinetic analyses. The results showed that BMEP-1 and BMEP-2 exhibited excellent catalytic performance in the thermal decomposition of ammonium perchlorate. Notably, there was only a single exothermic peak at 302.6 °C and 318.6 °C, and the activation energy values of ammonium perchlorate decreased to 123.88 kJ mol^-1 and 128.43 kJ mol^-1, respectively. TGA-FTIR results showed that BMEP-1 and BMEP-2, as effective components of catalysis, will promote the production of H₂O, N₂O, NO₂, and HCl in advance, during the thermal decomposition of ammonium perchlorate. BMEP-1 and BMEP-2 are expected to be two candidate additives for the catalytic decomposition of ammonium perchlorate in composite solid propellants.

Hydrogen Bond and π-π Stacking Interaction: Stabilization Mechanism of Two Metal Cyclo-N5 --Containing Energetic Materials

In recent years, cyclo-N₅ ^- has attracted extensive attention because all-nitrogen high-energy-density materials (HEDMs) have been expected to reach a TNT equivalent of over 3.0. However, for cyclo-N₅ ^--containing HEDMs, the stabilization mechanism has remained enigmatic. In this study, two typical cyclo-N₅ ^--containing metal hydrates, [Na(H₂O)(N₅)]·2H₂O (Na-cyclo-N₅ ^-) and [Mg(H₂O)₆(N₅)₂]·4H₂O (Mg-cyclo-N₅ ^-), are selected to gain insights into the factors affecting their stability by the first-principles method. Both binding/lattice energy calculations and density of states analysis show that Mg-cyclo-N₅ ^- is more stable than Na-cyclo-N₅ ^-. Hydrogen bonding is the main stabilization mechanism for stabilizing crystals and cyclo-N₅ ^-. Two types of hydrogen bonds, O-H···O and O-H···N, are clarified, which construct a 3D hydrogen bond network in Mg-cyclo-N₅ ^- and an intralayer 2D hydrogen bond network in Na-cyclo-N₅ ^-. Moreover, nonuniform stress causes distortion of cyclo-N₅ ^-. Comparing the two samples, the distortion degree of cyclo-N₅ ^- is higher in Na-cyclo-N₅ ^-, which indicates that cyclo-N₅ ^- decomposition is easier. These findings will enhance the future prospects for the design and synthesis of cyclo-N₅ ^--containing HEDMs.

Symmetrical cyclo-N5- hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts

Pairing different cations (R⁺) to stabilize cyclo-N₅^- is the main synthesis path for non-metallic cyclo-pentazolate (cyclo-N₅^-) salts. As novel energetic materials (EMs), crystalline packing-force of cyclo-N₅^- salts has been a puzzle, and whether cyclo-N₅^- is protonated also is a controversial issue. In this paper, four non-metallic cyclo-N₅^- salts, PHAC, N₂H₅N₅, NH₃OHN₅, and NH₄N₅, are quantitatively studied by coupling first-principle method and bond-strength analyzing technology. Different from the traditional CHON-EMs (molecular crystal) and azide-EMs (ionic crystal), the four salts are stabilized by 3D hydrogen bond (HB) networks. One new type of hydrogen bond, protonated HB (p-H, R-H⋯N₅^-), is discovered to be a key stabilizing factor for cyclo-N₅^-. Proton competition mechanism between R and cyclo-N₅^- in p-H HB showed that cyclo-N₅^- cannot be protonated into HN₅. In general, p-H HB can be adopted to estimate the stability of novel non-metallic cyclo-N₅^- EMs. Such findings have great significance for future design and performance prediction of novel cyclo-N₅^- EMs in both theoretical and experimental aspects.

Pentazolate coordination polymers self-assembled by in situ generated [Pb4(OH)4]4+ cubic cations trapping cyclo-N5−

A series of cyclo-N5−-based lead-containing energetic coordination polymers [Pb(OH)]4(N5)4 (1), [Pb3(N5)3(H2O)9(NO3)]4(N5)8(H2O)5 (2), [Pb(OH)]4(N5)3(NO3)(H2O)3 (3), and [Pb(OH)]4(N5)3(ClO4)(H2O) (4) were synthesized by self-assembly and characterized by single-crystal X-ray diffraction, powder X-ray diffraction,...

A novel all-nitrogen molecular crystal N16 as a promising high-energy density material

All-nitrogen solids, if successfully synthesized, are ideal high energy density materials because they store a great amount of energy and produce only harmless N2 gas upon decomposition. Currently, the only...

[Na4(N5)4(H2O)2]•H2O•2MeOH: a honeycomb-like sodium-pentazolate-framework with helical chains

A new NaN5 framework [Na4(N5)4(H2O)2]•H2O•2MeOH was synthesized by changing the content of auxiliary ligand. It features an UNJ-type zeolite topology with double right-handed helix chains construct enclosed channels filled with...

Reaction Mechanism and Energetics of Decomposition of Tetrakis(1,3-dimethyltetrazol-5-imidoperchloratomanganese(II)) from Quantum-Mechanics-based Reactive Dynamics

Energetic materials (EMs) are central to construction, space exploration, and defense, but over the past 100 years, their capabilities have improved only minimally as they approach the CHNO energetic ceiling, the maximum energy density possible for EMs based on molecular carbon-hydrogen-nitrogen-oxygen compounds. To breach this ceiling, we experimentally explored redox-frustrated hybrid energetic materials (RFH EMs) in which metal atoms covalently connect a strongly reducing fuel ligand (e.g., tetrazole) to a strong oxidizer (e.g., ClO₄). In this Article, we examine the reaction mechanisms involved in the thermal decomposition of an RFH EM, [Mn(Me₂TzN)(ClO₄]₄ (3, Tz = tetrazole). We use quantum-mechanical molecular reaction dynamics simulations to uncover the atomistic reaction mechanisms underlying this decomposition. We discover a novel initiation mechanism involving oxygen atom transfer from perchlorate to manganese, generating energy that promotes the fission of tetrazole into chemically stable species such as diazomethane, diazenes, triazenes, and methyl azides, which further undergo exothermic decomposition to finally form stable N₂, H₂O, CO, CO₂, Mn-based clusters, and additional incompletely combusted products.

Synthesis and characterization of promising insensitive energetic salts based on 3-amino-5-hydrazinopyrazole

The development of green energetic materials is based on environmental friendliness, safety and performance improvement. It is of great significance to design and synthesize new nitrogen rich salts for a new generation of green energetic materials. In the present work, a series of 3-amino-5-hydrazinopyrazole energetic salts comprising energetic anions were synthesized and were characterized using elemental analysis, IR spectroscopy and differential scanning calorimetry (DSC). Compounds 1-5 were further confirmed by single crystal X-ray diffraction and the sensitivities were measured by the standard BAM methods. Additionally, the structure-property relationship was elucidated from the experimental results and theoretical calculations. Energetic salts of 2 and 5 exhibited high heat of formation (5, 1160.06 kJ mol-1), high decomposition temperature (2, 172 °C; 5, 186 °C), excellent detonation performance (2, Dv, 9076 m s-1, P 34.1 GPa; 5, Dv, 8974 m s-1, P 31.9 GPa), moderate sensitivity towards outer stimuli and high nitrogen contents (2, 41.03%; 5, 63.84%). This work increases future prospects for the design of insensitive and novel high-energy green energetic material.

Energy density of high-pressure nitrogen-rich MNx compounds

In the past decade, a large number of nitrogen-rich MN_x compounds have been discovered under high-pressure conditions. In this work, we have evaluated the energy densities of MN_x structures with thermodynamic and dynamical stability through first-principles calculations. The results show that the energy densities of MN_x consisting of alkali metals and cyclo-N₅^- are less than ∼0.5 TNT equivalence, whereas the group-III metal nitrides have high-energy density regardless of the type of nitrogen oligomers in the structures. To clarify the energy density difference for MN_x composed of different impurities, bivariate Pearson correlation analysis is performed, which reveals that the high-energy density of MN_x is related to the large N density, small M atomic radius, short M-N bond length, small MN_x ionicity, long N-N bond length and large M formal oxidation state. According to this correlation, H, Be, B, Al, Si and P elements have been proposed as the candidate impurities to synthesize high-energy density MN_x.

Recent advances in synthesis and crystal structures of metal pentazolate salts

Pentazole is a good nitrogen-donor ligand to prepare intriguing structures. Herein, the synthesis and crystal structures of all reported metal pentazolate salts were summarized and the future prospects in this field were also discussed.

Stabilization of pentazolate anions in the high-pressure compounds Na2N5 and NaN5 and in the sodium pentazolate framework NaN5·N2

Synthesis and characterization of nitrogen-rich materials is important for the design of novel high energy density materials due to extremely energetic low-order nitrogen-nitrogen bonds. The balance between the energy output and stability may be achieved if polynitrogen units are stabilized by resonance as in cyclo-N5- pentazolate salts. Here we demonstrate the synthesis of three oxygen-free pentazolate salts Na2N5, NaN5 and NaN5·N2 from sodium azide NaN3 and molecular nitrogen N2 at ∼50 GPa. NaN5·N2 is a metal-pentazolate framework (MPF) obtained via a self-templated synthesis method with nitrogen molecules being incorporated into the nanochannels of the MPF. Such self-assembled MPFs may be common in a variety of ionic pentazolate compounds. The formation of Na2N5 demonstrates that the cyclo-N5 group can accommodate more than one electron and indicates the great accessible compositional diversity of pentazolate salts.

Crystalline Structures and Energetic Properties of Lithium Pentazolate under Ambient Conditions

Recently, it has been reported that high-pressure synthesized lithium pentazolates could be quenched down to ambient conditions. However, the crystalline structures of LiN5 under ambient conditions are still ambiguous. In this work, the structures of LiN5 compound were directly explored at atmospheric pressure by using a new constrain structure search method. By using this method, three new allotropes were confirmed, and they show lower energy than the previous reported LiN5 phases. Both their thermodynamic and dynamic stability were confirmed through formation enthalpies, phonon spectrum, and ab initio molecular dynamics simulations under ambient conditions. Moreover, these three allotropes show similar formation enthalpies and properties, which suggests that it is hard to obtain a single LiN5 phase, which is well consistent with the experimental phenomenon. Furthermore, because of their low formation energy, all of them possess low energy density when they directly decompose to Li3N and nitrogen (0.52 kJ/g). Instead, the decomposed energy could be further improved to 3.78 kJ/g when they decompose under an oxygen-rich environment.

Properties and Promise of Catenated Nitrogen Systems As High-Energy-Density Materials

The properties of catenated nitrogen molecules, molecules containing internal chains of bonded nitrogen atoms, is of fundamental scientific interest in chemical structure and bonding, as nitrogen is uniquely situated in the periodic table to form kinetically stable compounds often with chemically stable N-N bonds but which are thermodynamically unstable in that the formation of stable multiply bonded N₂ is usually thermodynamically preferable. This unique placement in the periodic table makes catenated nitrogen compounds of interest for development of high-energy-density materials, including explosives for defense and construction purposes, as well as propellants for missile propulsion and for space exploration. This review, designed for a chemical audience, describes foundational subjects, methods, and metrics relevant to the energetic materials community and provides an overview of important classes of catenated nitrogen compounds ranging from theoretical investigation of hypothetical molecules to the practical application of real-world energetic materials. The review is intended to provide detailed chemical insight into the synthesis and decomposition of such materials as well as foundational knowledge of energetic science new to most chemists.

Stabilization of the High-Energy-Density CuN5 Salts under Ambient Conditions by a Ligand Effect

A series of excellent works have demonstrated that high-nitrogen-content metal pentazolate (cyclo-N5 -) compounds could be stabilized by high pressure. However, under ambient conditions, low stability precludes their synthesis and application in the field of high-energy-density material. In this work, by using a constrained structure search method, we predicted two new structures as P212121-CuN5 and P21/c-CuN5 containing cyclo-N5 - with strong N-N and Cu-N bonds. In both structures, cyclo-N5 - form four coordination with the Cu+ ligand, which increases the structural stability by lowering the disturbance to the aromaticity of cyclo-N5 -. The calculated results show that the P212121-CuN5 and P21/c-CuN5 structures exhibit high dynamic and thermal stability up to 400 K, indicating that they can be stabilized under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and the pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising high-energy candidates in the field of explosive combustion.

Prediction of stable energetic beryllium pentazolate salt under ambient conditions

The most stable BeN₁₀ salt was directly predicted at atmospheric pressure, and the energy density is up to 5.36 kJ g⁻¹.

Theoretical Insights on the High Pressure Behavior of Pentazolate Anion Complex [Co(H2O)4(N5)2]·4H2O

Periodic dispersion corrected density functional theory (DFT) calculations were carried out to examine the Hirshfeld surface, two dimensional (2D) fingerprint plots, crystal structure, molecular structure and density of state of all-nitrogen pentazolate anion complex [Co(H2O)4(N5)2]·4H2O under hydrostatic pressure from 0 to 20 GPa. The GGA/PW91-OBS method was applied in the present study. The intercontacts in [Co(H2O)4(N5)2]·4H2O were analyzed by Hirshfeld surfaces and 2D fingerprint plots. With ascending pressure, the lattice constants, compression rates, bond lengths, bond angles, and density of states change irregularly. Under 11.5, 13.0 and 15.8 GPa, covalent interaction competition is obvious between Co-N and Co-O bonds. It is possible to achieve orderly modification and regulation of the internal structure of [Co(H2O)4(N5)2]·4H2O by applied pressure. This is in accordance with the results from density of states analysis. The external compression causes the nonuniformity of electron density and the differential covalent interaction between pentazolate anion, coordinated water and atom Co. It is of great significance to interpret inter/intramolecular interaction and structural stability of [Co(H2O)4(N5)2]·4H2O and provide theoretical guidance for the design of metal complexes of all-nitrogen pentazolate anion.

Syntheses of Energetic cyclo-Pentazolate Salts

Three energetic salts of cyclo-N₅ ^- were synthesized via a metathesis reaction of barium pentazolate and sulfates which was driven by the precipitation of BaSO₄ . All the energetic cyclo-N₅ ^- salts were characterized by single-crystal X-ray diffraction, infrared (IR), ¹ H and ¹³ C multinuclear NMR spectroscopies, thermal analysis (TGA and DSC), and elemental analysis. The salts exhibit relatively good detonation performance with low sensitivities and good thermal stabilities. This new method opens the door to exploring more pentazolate anion-containing high-performance energetic materials.

Theoretical perspective on the reaction mechanism from arylpentazenes to arylpentazoles: new insights into the enhancement of cyclo-N5 production

A thorough theoretical exploration of the formation mechanism of arylpentazole (cyclo-N₅), the key precursor of cyclo-N₅⁻, has been performed.

Crystal structures, thermodynamics and accelerating thermal decomposition of RDX: two new energetic coordination polymers based on a Y-shaped ligand of tris(5-aminotetrazole)triazine

Two new energetic coordination polymers were prepared under hydrothermal condition. They have good detonation velocity, detonation pressure and effective acceleration effect toward the thermal decomposition of RDX (1,3,5-trinitro-1,3,5-triazinane).

A kinetically persistent isomer found for pentazole: a global potential energy surface survey

After the pentazole with a 103-year-old research history, the second N₅R isomer with reasonable kinetic stability was found computationally.

Origin of the considerably high thermal stability of cyclo-N5− containing salts at ambient conditions

Ionization, conjugation, hydrogen bonding, coordination bonding and π–π stacking consolidate the cyclo-N₅⁻ caged in salt crystals.

To Be or Not To Be Protonated: cyclo-N5- in Crystal and Solvent

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.

Recent advances in the syntheses and properties of polynitrogen pentazolate anion cyclo-N5− and its derivatives

This review summarizes recent developments and advances in pentazole chemistry, including substituted-pentazole precursors, strategies for the preparation of pentazolate anion, derivatives of pentazolate anion and their bonding properties.