A series of energetic metal pentazolate hydrates

Hypergolic zeolitic imidazolate frameworks (ZIFs) as next-generation solid fuels: Unlocking the latent energetic behavior of ZIFs

Hypergolic materials, capable of spontaneous ignition upon contact with an external oxidizer, are of critical importance as fuels and propellants in aerospace applications (e.g., rockets and spacecraft). Currently used hypergolic fuels are highly energetic, toxic, and carcinogenic hydrazine derivatives, inspiring the search for cleaner and safer hypergols. Here, we demonstrate the first strategy to design hypergolic behavior within a metal-organic framework (MOF) platform, by using simple "trigger" functionalities to unlock the latent and generally not recognized energetic properties of zeolitic imidazolate frameworks, a popular class of MOFs. The herein presented six hypergolic MOFs, based on zinc, cobalt, and cadmium, illustrate a uniquely modular platform to develop hypergols free of highly energetic or carcinogenic components, in which varying the metal and linker components enables the modulation of ignition and combustion properties, resulting in excellent hypergolic response evident by ultrashort ignition delays as low as 2 ms.

Super-Efficient Synthesis of Mesh-like Superhydrophobic Nano-Aluminum/Iron (III) Oxide Energetic Films

In this study, a novel superhydrophobic nano-aluminum/iron (III) oxide composite has been prepared by a facile one-step process of electrophoretic deposition, with wide potential applications. The optimal suspension included ethanol, acetyl-acetone, and the additives of fluorotriphenylsilane and perfluorodecyltriethoxysilane. The microstructure, wettability, and exothermic performance were analyzed by field emission scanning electron microcopy (FESEM), X-ray diffraction (XRD), water contact angle measurements, and the differential scanning calorimetry (DSC) technique. The water contact angle and the heat-release of the target composites could reach to ~170° and 2.67 kJ/g, and could still keep stable, after exposure for six months, showing a great stability. These results provided an exquisite synthesis of ideas, for designing other superhydrophobic energetic materials with self-cleaning properties, for real industrial application.

From BTO2− to HBTO− insensitive energetic salt: a route to boost energy

A promising strategy was utilized to boost the detonation performance of insensitive energetic salts.

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.

Host–guest energetic materials constructed by incorporating oxidizing gas molecules into an organic lattice cavity toward achieving highly-energetic and low-sensitivity performance

Powerful oxidizer N₂O was incorporated into an organic lattice cavity through aeration crystallization, and smart host–guest energetic materials with highly-energetic and low-sensitivity performance were obtained.

Boosting the performance of energetic materials through thermally-induced conformational transition

We presented an effective strategy to improve the performance of energetic materials through thermally-induced conformational transition.

Energetic furazan-triazole hybrid with dinitromethyl and nitramino groups: decreasing sensitivity via the formation of a planar anion

Enlarging the skeleton containing two heterocyclic five-membered rings can decrease sensitivity in comparison with most highly nitro-functionalized mono nitrogen-rich rings.

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.

Structural evolution of LiNn+ (n = 2, 4, 6, 8, and 10) clusters: mass spectrometry and theoretical calculations

Mixed nitrogen-lithium cluster cations LiN_n⁺ were generated by laser vaporization and analyzed by time-of-flight mass spectrometry. It is found that LiN₈⁺ has the highest ion abundance among the LiN_n⁺ ions in the mass spectrum. Density functional calculations were conducted to search for the stable structures of the Li–N clusters. The theoretical results show that the most stable isomers of LiN_n⁺ clusters are in the form of Li⁺(N₂)_n/2, and the order of their calculated binding energies is consistent with that of Li–N₂ bond lengths. The most stable structures of LiN_n⁺ evolve from one-dimensional linear type (C_∞v, n = 2; D_∞h, n = 4), to two-dimensional branch type (D_3h, n = 6), then to three-dimensional tetrahedral (T_d, n = 8) and square pyramid (C_4v, n = 10) types. Further natural bond orbital analyses show that electrons are transferred from the lone pair on N_α of every N₂ unit to the empty orbitals of lithium atom in LiN_2–8⁺, while in LiN₁₀⁺, electrons are transferred from the bonding orbital of the Li–N_α bonds to the antibonding orbital of the other Li–N_α bonds. In both cases, the N₂ units become dipoles and strongly interact with Li⁺. The average second-order perturbation stabilization energy for LiN₈⁺ is the highest among the observed LiN_n⁺ clusters. For neutral LiN_2–8 clusters, the most stable isomers were also formed by a Li atom and n/2 number of N₂ units, while that of LiN₁₀ is in the form of Li⁺(N₂)₃(η¹-N₄).

LiN_n⁺ clusters were generated by laser ablation and the LiN₈⁺ with tetrahedral Li⁺(N₂)₄ structure has the highest ion abundance.

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.

2,4,4,6,8,8-Hexanitro-2,6-diazaadamantane: A High-Energy Density Compound with High Stability

A novel high-performance energetic compound of the polynitroazaadamantane family, 2,4,4,6,8,8-hexanitro-2,6-diazaadamantane, was designed and synthesized from 1,5-cyclooctadiene by two routes. Based on the experimental and calculated results, it exhibits a surprisingly high density (1.959 g cm^-3), high thermal stability (onset decomposition temperature of 235 °C), high positive heat of formation, and excellent detonation properties. These fascinating properties, which are comparable to those of CL-20, show great promise for potential applications as a high-energy density material.

Periodic DFT study of structural transformations of cocrystal NTO/TZTN under high pressure

Density functional theory (DFT) periodic calculations were performed to study the geometrical and electronic structures of energetic cocrystal NTO/TZTN under pressures ranging from 0 to 80 GPa. The optimized crystal structure by the GGA/PW91 (Perdew-Wang-91) and dispersion corrections corresponds well with the experimental values under ambient pressure. With the pressure increasing, the lattice constants, unit cell volumes, interatomic distances, H-bond energies, atomic charges, and bond populations of cocrystal NTO/TZTN change gradually. At pressures of 4, 8, and 23 GPa, three structural transformations occurred, shown by the results. The cyclization plays an important role in stabilizing the systems. The increasing pressure contributes to the increase of interaction force gradually. At 4 GPa, a new hydrogen bond O3⋯H5 is formed. At 8 GPa, the formation of eight membered rings is because of the existence of a covalent bond O1-H3 between two NTO molecules. In addition, a covalent interaction is formed between N2 and H4 atoms with the biggest H-bond energy compared to the others. As the pressure reaches 23 GPa, another new hydrogen bond forms between N8 and H5 atoms, which contributes to the formation of a five membered ring between NTO and TZTN. The electrons can move freely according to the results of the density of states between the valence and conduction bands when the pressure is high. This work will provide useful information in understanding the high-pressure effect on the structural transformation.

High energetic polymeric nitrogen sheet confined in a graphene matrix

Polymeric nitrogen, as a potential high-energy-density material (HEDM), has many applications, such as in energy storage systems, explosives and propellants. Nowadays it is very urgent to find a suitable method to stabilize polymeric nitrogen at ambient conditions. Herein, we present a new hybrid structure where polymeric nitrogen sheets are sandwiched between graphene sheets in the form of a three-dimensional crystal. According to ab initio molecular dynamics (AIMD) calculations and phonon spectrum calculations, it is demonstrated that polymeric nitrogen sheets are stable at ambient pressure and temperature. The hybrid material has a higher nitrogen content (the weight ratio of nitrogen is up to 53.84%), and the corresponding energy density is 5.2 kJ g-1. The hybrid material (A7@graphene system) has a satisfactory energy density, detonation velocity and detonation pressure. Importantly, the hybrid material can be preserved up to 450 K, and above this temperature, the polymeric nitrogen sheets break up into polymeric nitrogen chains or nitrogen gases and release tremendous energy. Further calculations reveal that small charge transfer between the polymeric nitrogen sheets and graphene sheets creates a weak electrostatic attraction compared with other hybrid materials, which is just good for the stabilization of the polymeric nitrogen sheets at ambient conditions, and favors energy release in a gentle way. The proposed confinement hybrid material which has a high energy density and a gentle energy release temperature, provides a highly promising method for the capture and application of polymeric nitrogen in a controllable way.

Accelerating the discovery of insensitive high-energy-density materials by a materials genome approach

Finding new high-energy-density materials with desired properties has been intensely-pursued in recent decades. However, the contradictory relationship between high energy and low mechanical sensitivity makes the innovation of insensitive high-energy-density materials an enormous challenge. Here, we show how a materials genome approach can be used to accelerate the discovery of new insensitive high-energy explosives by identification of “genetic” features, rapid molecular design, and screening, as well as experimental synthesis of a target molecule, 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide. This as-synthesized energetic compound exhibits a graphite-like layered crystal structure with a high measured density of 1.95 g cm⁻³, high thermal decomposition temperature of 284 °C, high detonation velocity of 9169 m s⁻¹, and extremely low mechanical sensitivities (impact sensitivity, >60 J and friction sensitivity, >360 N). Besides the considered system of six-member aromatic and hetero-aromatic rings, this materials genome approach can also be applicable to the development of new high-performing energetic materials.

The synthesis of explosive materials that are stable, highly dense, and have low sensitivity to external stimuli is a challenge. Here, the authors use a genomic approach to accelerate the discovery of insensitive high explosive molecules with good detonation and low sensitivity properties.

Roads to pentazolate anion: a theoretical insight

The formation mechanism of pentazolate anion (PZA) is not yet clear. In order to present the possible formation pathways of PZA, the potential energy surfaces of phenylpentazole (PPZ), phenylpentazole radical (PPZ-R), phenylpentazole radical anion (PPZ-RA), PPZ and m-chloroperbenzoic acid (m-CPBA), p-pentazolylphenolate anion (p-PZPolA) and m-CPBA, and p-pentazolylphenol (p-PZPol) and m-CPBA were calculated by the computational electronic structure methods including the hybrid density functional, the double hybrid density functional and the coupled-cluster theories. At the thermodynamic point of view, the cleavages of C-N bonds of PPZ and PPZ-R need to absorb large amounts of heat. Thus, they are not feasible entrance for PZA formation at ambient condition. But excitation of PPZ and deprotonation of PPZ-RA probably happen before cleavage of C-N bond of PPZ at high-energy condition. As to the radical anion mechanism, the high accuracy calculations surveyed that the barrier of PZA formation is probably lower than that of dinitrogen evolution, but the small ionization potential of PPZ-RA gives rise to the unstable ionic pair of sodium PPZ at high temperature. In respect of oxidation mechanism, except for PPZ, the reactions of p-PZPolA and p-PZPol with m-CPBA can form PZA and quinone. The PZA formations have the barriers of about 20 kcal mol-1 which compete with the dinitrogen evolutions. The stabilities of PZA in both solid and gas phases were also studied herein. The proton prefers to transfer to pentazolyl group in the (N5)6(H3O)3(NH4)4Cl system which leads to the dissociation of pentazole ring. The ground states of M(N5)2(H2O)4 (M = Co, Fe and Mn) are high-spin states. The pentazolyl groups confined by the crystal waters in the coordinate compounds can improve the kinetic stability. As to the reactivity of PZA, it can be persistently oxidized by m-CPBA to oxo-PZA and 1,3-oxo-PZA with the barriers of about 20 kcal mol-1.

A DFT study of the structure-property relationships of bistetrazole-based high-nitrogen energetic metal complexes

In this work, six series of new energetic metal complexes were designed. Each complex contained a large, high-energy, high-nitrogen, anionic chelating ligand (either the 5,5'-bistetrazolate anion, the 5,5'-azobistetrazolate anion, or the 5,5'-(hydrazine-1,2-diyl)bis-[1H-tetrazol-1-ide] anion-each of which has a different bridging group), Cu or Ni as the metal atom, and two small complexing agent ligands (NH₃ and/or NH₂NO₂). The molecular and electronic structures, heats of formation, densities, detonation properties, and impact sensitivities of the novel complexes were studied using density functional theory. Furthermore, the effects of varying the large chelating ligand (and thus the bridging group), the small complexing agents, and the metal atom on the structure and properties of the complex were investigated and analyzed in depth. The results show that the particular metal, bridging group, and complexing agents included in the energetic complex influence its structure and properties, but the effects of varying the constituents of the complex are complicated or unclear, and these effects are sometimes intertwined. In addition, the detonation pressures, detonation velocities, and impact sensitivities of the novel complexes ranged from 25.9 to 38.6 GPa, from 7.21 to 8.80 km s^-1, and from 17 to 48 cm, respectively. Five of the complexes (B3, C3, D3, E3, and F3) appear to possess comparable performance to the famous and widely used high explosive 1,3,5-trinitro-1,3,5-triazinane, making these new complexes attractive to energetic materials experimentalists.

Synthesis of C-coordinated O-carboxymethyl chitosan metal complexes and evaluation of their antifungal activity

Based on a condensation reaction, a chitosan-derivative-bearing amino pyridine group was prepared and subsequently followed by coordination with cupric ions, zinc ions and nickel ions to synthesize chitosan metal complexes. The calculations using the density functional theory (DFT) show that the copper ions and nickel ions underwent dsp² hybridization, the zinc ions underwent sp³ hybridization, and they all formed a coordination bond with the carbon atom in the p-π conjugate group. The antifungal properties of O-CSPX-M against Phytophthora capsici (P. capsici), Verticillium alboatrum (V. alboatrum), Botrytis cinerea (B. cinerea) and Rhizoctonia solani (R. solani) were also assayed. Apparently, chitosan metal complexes showed enhanced antifungal activity against four fungi at the tested concentrations compared to that of chitosan. It was shown that Cu complexes can inhibit the growth of P. capsici 100%, and Ni complexes can inhibit the growth of B. cinerea 77.1% at a concentration of 0.4 mg/mL and 0.2 mg/mL, respectively. The pot experiment also verified the result. In addition, the phytotoxicity experiment showed that O-CSPX-M had no obvious toxicity on wheat leaves. This kind of complexes may represent as an attractive direction for chemical modifications of metal fungicides.

Computational evaluation of metal pentazolate frameworks: inorganic analogues of azolate metal-organic frameworks

Pentazolate is the ultimate all-nitrogen, inorganic member of the azolate series of aromatic 5-membered ring anions. As an azolate ligand, it has the potential to form open framework structures with metal ions, that would be inorganic analogues of azolate metal-organic frameworks formed by its congeners. However, while the low stability and elusive nature of the pentazolate ion have so far prevented the synthesis of such frameworks, computational studies have focused on pentazolate exclusively as a ligand that would form discrete metallocene structures. Encouraged by the recent first isolation and structural characterization of pentazolate salts and metal complexes stable at ambient conditions, we now explore the role of pentazolate as a framework-forming ligand. We report a computational periodic density-functional theory evaluation of the energetics and topological preferences of putative metal pentazolate frameworks, which also revealed a topologically novel framework structure.

A carbon-free inorganic-metal complex consisting of an all-nitrogen pentazole anion, a Zn(ii) cation and H2O

A carbon-free inorganic-metal complex [Zn(H₂O)₄(N₅)₂]·4H₂O was synthesized by the ion metathesis of [Na(H₂O)(N₅)]·2H₂O solution with Zn(NO₃)₂·6H₂O. The complex was well characterized by IR and Raman spectroscopy, elemental analysis (EA), powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC). The structure of the complex was confirmed by single-crystal X-ray crystallography and a Zn(ii) ion is coordinated in a quadrilateral bipyramid environment in which the axial position is formed by two nitrogen atoms (N1) from two pentazolate rings (cyclo-N₅^-) and the equatorial plane is formed by four oxygen atoms (O1) from four coordinated water molecules. The thermal analysis of [Zn(H₂O)₄(N₅)₂]·4H₂O reveals that although water plays an important role in stabilizing cyclo-N₅^-, dehydration does not cause immediate decomposition of the anion. However, cyclo-N₅^- decomposed into N₃^- and N₂ gas at 107.9 °C (onset). Based on its chemical compatibility and stability, the complex exhibits promising potential as a modern environmentally-friendly energetic material.

Theoretical study on benzoheterocycle based energetic materials, effect of heterocyclic-fused, conjugation, hydrogen bond, and substitutional group on the detonation performance

In this paper, four series of benzoheterocycle based energetic materials (EMs) have been designed to plan out a strategy to improve the density and safety of EMs, such as combining the insensitive group with aminobenzene ring and the high energetic nitramine explosives, benzo-heterocycle mother ring, designing multi-nitrogen heterocycles with a conjugated system containing N-N and C-N high energy bonds, and hydrogen bonding. Their optimized structure and detonation properties were first calculated and discussed using DFT methods. After calculation, these designed explosives all showed good detonation from 7352 m/s to 8788 m/s. Among them, the compounds with six nitro groups, 1c, 2c, 3c, and 4c, exhibit better performance and rather poor impact sensitivity. However, we found that the compounds with five nitro groups and one amino group have a limited performance reduction and a rapid stability improvement. These four compounds, 1b, 2b, 3b, and 4b, have good detonation performance and better stability. Moreover, the synthesis routes for these four compounds were also designed. The precursor 4-0 and mononitro product 4-1 were successfully synthesized. Their 1H NMR, single crystal, and elemental analysis were also done to verify the structures.

Optimizing the oxygen balance by changing the A-site cations in molecular perovskite high-energetic materials

Two new members of molecular perovskite high-energetic materials exhibit optimized oxygen balances by changing the A-site cations.

Self-assembly of 3D porous architectures from energetic nanoparticles for enhanced energetic performances

3D architectures with porous network of energetic molecules were designed and constructed by introduce a general approach through two-step self-assembly process.

Self-assembled energetic 3D metal–organic framework [Na8(N5)8(H2O)3]n based on cyclo-N5–

A new 3D zeolite-like MOF [Na₈(N₅)₈(H₂O)₃]_n with an enhanced stability has been successfully synthesized by the self-assembly of Na⁺ with cyclo-N₅⁻ ligands.

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.

A novel facile transformation to 1,2-bis(3-nitro-1-(1H-tetrazol-5-yl)-1H-1,2,4-triazol-5-yl)hydrazine salts

A facile and unique conversion from NN group to HN–NH group was discovered, by which 1,2-bis(3-nitro-1-(1H-tetrazol-5-yl)-1H-1,2,4-triazol-5-yl)hydrazine and its energetic salts have been readily prepared as high-energy-density materials.