Nitroimino-tetrazolates and oxy-nitroimino-tetrazolates

Recent Progress on Nitrogen-Rich Energetic Materials Based on Tetrazole Skeleton

Development of nitrogen-rich energetic materials has gained much attention because of their remarkable properties including large nitrogen content and energy density, good thermal stability, low sensitivity, good energetic performance, environmental friendliness and so on. Tetrazole has the highest nitrogen and highest energy contents among the stable azoles. The incorporation of diverse explosophoric groups or substituents into the tetrazole skeleton is beneficial to obtain high-nitrogen energetic materials having excellent energetic performance and suitable sensitivity. In this review, the development of high-nitrogen energetic materials based on tetrazole skeleton is highlighted. Initially, the property and utilization of nitrogen-rich energetic materials are presented. After showing the advantage of the tetrazole skeleton, the high-nitrogen energetic materials based on tetrazole are classified and introduced in detail. Based on different types of energetic materials (EMs), the synthesis and properties of nitrogen-rich energetic materials based on mono-, di-, tri- and tetra-tetrazole are summarized in detail.

Initial Decomposition of DATB Induced by an External Electric Field

1,3-Diamino-2,4,6-trinitrobenzene (DATB), a nitro aromatic explosive with excellent properties, can be detonated by an electric field. Using first-principles calculation, we have investigated the initial decomposition of DATB under an electric field. In the realm of electric fields, the rotation of the nitro group around the benzene ring will cause deformation of the DATB structure. Furthermore, when an electric field is applied along the [100] or [001] direction, the C4-N10/C2-N8 bonds initiate decomposition due to electron excitation. On the contrary, the electric field along the [010] direction has a weak influence on DATB. These, together with electronic structures and infrared spectroscopy, give us a visual perspective of the energy transfer and the decomposition caused by C-N bond breaking.

Phase transition-induced initial decomposition of nitrogen-rich binary CN compound 2,2'-azobis(5-azidotetrazole) and its precursor 2-amino-5-azidotetrazole via tetrazole ring opening under external electric fields: a comparative DFT-D study

A comparative DFT-D study was performed to investigate the external electric field-induced crystal structures, electronic features, Hirshfeld surfaces, vibrational properties and initial decomposition mechanisms of nitrogen-rich binary CN compound 2,2'-azobis(5-azidotetrazole) (C₂N₁₆) and its precursor 2-amino-5-azidotetrazole (CH₂N₈). The results show that there exist phase transitions at the critical points of 0.006 a.u. and 0.008 a.u. for CH₂N₈ and C₂N₁₆, respectively, which are embodied in various properties of these compounds and induce their initial decomposition of the tetrazole ring opening via the breaking of N-N single bonds. The analysis of band gaps and density of states suggests the external electric field-induced enhancing ability for electron transition from the occupied orbitals to empty ones and N-N bond breaking may be the initial decomposition pathway for them. The variations in Hirshfeld surfaces indicate the spatial change and adjustment of non-bonding interactions in the two crystals. The discussions on vibrational properties indicate that IR characteristic peaks of all vibrational modes in the two crystals show a gradual red shift toward a low frequency region. The external electric field-induced initial decomposition pathways of both crystals are tetrazole ring opening via the breaking of a N-N single bond. Our findings provide insights for a comprehensive understanding of external electric field-induced phase transition and initial decomposition mechanisms of nitrogen-rich binary CN energetic compounds.

Phase transition induced by an external electric field as a buffer to facilitate the initial decomposition of a series of catenated nitrogen energetic systems: a first-principles study

The effect of an external electric field on the crystal and electronic structures, Hirshfeld surfaces, hydrogen-bonding network, mechanical properties, vibrational properties and initial decomposition mechanisms of a series of chain-catenated N_x (x = 4, 8, 10) energetic crystals was investigated via a first-principles study. The results indicate that the response behaviors to the external electric field show a great dependence on the nitrogen chain length and the intensity of the external electric field. The critical points of the phase transition were found and are embodied in various properties of all the compounds. Analysis of the electronic structures shows the increasing ability of the electron transition, thereby leading to possible subsequent decomposition reactions. The studies on Hirshfeld surfaces and the hydrogen-bonding network suggest that the external electric field can modify and tune the spatial distribution of the hydrogen-bonding network, thereby affecting the physicochemical properties. Our comprehensive analysis based on the mechanical properties, vibrational features and initial decomposition mechanism reveals that the external electric field can weaken the trigger bonds, reduce the thermal stability, and initiate decomposition. Our findings provide insights into the comprehensive understanding of the effects of an external electric field on energetic materials, especially for polynitrogen chain-catenated and even all-nitrogen compounds.

Design and exploration of 5-nitro-3-trinitromethyl-1H-1,2,4-triazole and its derivatives as energetic materials

According to the fact that 5-nitro-3-trinitromethyl-1H-1,2,4 triazole (NTNMT) is a successful, good explosive, energetic groups such as -CH3, -NH2, -NHNO2, -NO2, -ONO2, -NF2, -CN, -NC, -N3 groups were introduced into NTNMT and their oxygen balance was at about zero. The energetic properties, detonation performance, and sensitivity were studied at the B3LYP/6-31G** level of density functional theory to seek for possible high energy density compounds. The effects of substituent groups on heat of formation (HOF), density ρ, detonation velocity D, detonation pressure P, detonation energy Q, and sensitivity (evaluated using oxygen balance OB, the nitro group charges -QNO2, and bond dissociation energies BDE were studied and discussed. The order of contribution of the substituent groups to ρ, D, and P was -NF2 > -ONO2 > -NO2 > -NHNO2 > -N3 > -NH2 > -NC > -CN > -CH3; while to HOF is -N3 > -NC > -CN > -NO2 > -NF2 > -ONO2 > -NH2 > -NHNO2 > -CH3. The trigger bonds in the pyrolysis process for NTNMT derivatives may be N-NO2, N-NH2, N-NHNO2, C-NO2, or O-NO2 varying with the attachment of different substituents. Results show that NTNMT-NHNO2, -NH2, -CN, and -NC derivatives have high detonation performance and good stability. In a word, the oxygen balance at about zero strategy in this work offers new routes for the improvement in properties and stabilities of energetic materials. In the present paper, several 5-nitro-3-trinitromethyl-1H-1,2,4 triazole (NTNMT) derivatives were designed. Their energetic properties, detonation performance, and sensitivity were studied at the B3LYP/6-31G** level of density functional theory (DFT) to seek for possible high energy density compounds (HEDCs). The different substituents have some changes in the influence on heat of formation (HOF), density ρ, detonation velocity D, detonation pressure P, detonation energy Q, and sensitivity. In a word, the oxygen balance at about zero strategy in this work offers new routes for the improvement in properties and stabilities of energetic materials.

Theoretical studies on the structure and properties of DAT/BTNAT cocrystal under high pressure

The structural, electronic, and absorption properties of 3,5-diamino-1H-1,2,4-triazole (DAT) and 5,5′-bis(trinitromethyl)-3,3′-azo-1H-1,2,4-triazole (BTNAT) cocrystal under hydrostatic compression of 0–100 GPa were investigated by using periodic density functional theory with dispersion correction (DFT-D). The results indicate that a structural transformation occurred at 25 GPa. The structural transformation makes the positions of the molecules rearrange in the cocrystal and improves the stability and planarity. An analysis of the band gap and density of states indicates that the DAT/BTNAT cocrystal becomes more sensitive under compression. The absorption spectra illustrate that the DAT/BTNAT cocrystal has relatively high optical activity with the increasing pressure. Our work may offer some valuable information for understanding the behavior of energetic cocrystals under high pressure.

Time-Domain Terahertz Spectroscopy and Density Functional Theory Studies of Nitro/Nitrogen-Rich Aryl-Tetrazole Derivatives

The paper reports the time-domain THz spectroscopy studies of noncentrosymmetric energetic nitro/nitrogen-rich aryl-tetrazole high-energy molecules. The fingerprint spectra in the THz domain reveal the role of different functional groups attached to position "1" of the tetrazole moiety, which controls the energetic properties. These responses are deliberated through density functional theory (DFT) calculations. The synthesized aryl-tetrazoles exhibit high positive heat of formation (369-744 kJ/mol), high detonation velocities, and pressures (D _v: 7734-8298 m·s^-1; D _p: 24-28 GPa) in comparison to the noncentrosymmetric 2,4,6-trinitrotoluene (TNT). These compounds exhibit variation in the refractive indices and absorption between 0.1 and 2.2 THz range. The DFT studies at the molecular and single-crystal level (using plane wave pseudo potential method) endorse in detecting these bands (with ∼1% deviation). The calculated vibrational frequencies and linear optical properties are found to have good agreement with the experimental data in UV-visible and THz regions.

Azo- and methylene-bridged mixed azoles for stable and insensitive energetic applications

A simple synthetic strategy for the preparation of high nitrogen content azo- and methylene bridged mixed energetic azoles was used.

Synthesis of Thermally Stable and Insensitive Energetic Materials by Incorporating the Tetrazole Functionality into a Fused-Ring 3,6-Dinitropyrazolo-[4,3-c]Pyrazole Framework

A series of fused-ring energetic materials, i.e., 3,6-dinitro-1,4-di(1H-tetrazol-5-yl)-pyrazolo[4,3-c]pyrazole (DNTPP, compound 2) and its ionic derivatives (compounds 3-8), were designed and synthesized in this study. The molecular structures of compounds 2, 3, 6, 7·2H₂O, and 8 were confirmed using single-crystal X-ray diffraction. Their physicochemical and energetic properties, such as density, thermal stability, heat of formation, sensitivity, and detonation properties (e.g., detonation velocity and detonation pressure), were also evaluated. The results indicate that DNTPP and most of its ionic derivatives are extremely thermally stable and insensitive toward mechanical stimuli. In particular, the thermal decomposition temperature of compound 3 is up to 329 °C, while compounds 7 and 8 are very insensitive (impact sensitivity: >20 J; friction sensitivity: >360 N). Compounds 2, 3, and 6 possess good comprehensive properties, including excellent thermal stability, remarkable low sensitivities, and favorable detonation performance. These features show that DNTPP and its ionic derivatives have considerable promise as thermally stable and insensitive energetic materials.

Theoretical studies on the structures, material properties, and IR spectra of polymorphs of 3,4-bis(1H-5-tetrazolyl)furoxan

Theoretical studies on the structures, densities, and heats of formation of conformational isomers of 3,4-bis(1H-5-tetrazolyl)furoxan (H₂BTF) were performed based on density functional theory (DFT) calculations. Two stable planar conformational isomers, the face-to-back and the back-to-face conformers, and one stable slightly twisted conformer, the back-to-back conformer, were predicted for H₂BTF at the M06-2X/6-311 + G(d,p) level of theory. The face-to-back conformer was calculated to be the most stable conformational isomer on the potential energy surface. No stable face-to-face conformer, whether planar or tilted, was identified in the calculations. The Vienna Ab Initio Simulation Package (VASP) was used in combination with molecular dynamics simulation to explore the stable crystal forms and the densities of the stable conformational isomers. Two of them exhibited high densities: the face-to-back conformer with P2₁ symmetry (2.01 g/cm³) and the back-to-back conformer with Pna2₁ symmetry (2.05 g/cm³). Their heats of formation were also predicted to be high when calculated at the same DFT level. The detonation pressures and velocities of these polymorphs, as calculated using the EXPLO5 program, are well above those of many advanced high energy density materials, pointing to the potential use of these conformers as novel explosives with good detonation performance. Also, IR spectra are shown to be able to distinguish these denser polymorphs of H₂BTF. This study suggests that it could be worth investigating whether denser polymorphs of H₂BTF can be grown.

Design and properties of a new family of bridged bis(nitraminotetrazoles) as promising energetic materials

A new family of bridged bis(nitraminotetrazoles) on the basis of the combination of bistetrazoles and the energetic nitramino as well as various linkage groups was designed, and their properties were investigated in detail. Their good performance makes them promising candidates for new environmentally friendly energetic materials.

5-(4-Azidofurazan-3-yl)-1-hydroxytetrazole and its derivatives: from green primary to secondary explosives

These new high-nitrogen-content azidofurazan compounds show potential for use as next-generation green primary and secondary explosives.

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.

Improving the density and properties of nitrogen-rich scaffolds by the introduction of a C–NO2 group

5,5′-(Nitromethylene)bis(1H-tetrazole) and 5,5′-(2-(nitromethyl)-2H-1,2,3-triazole-4,5-diyl)bis(1H-tetrazole) were synthesized by introducing a C–NO₂ group to increase the density and detonation performance.

Prospective Symbiosis of Green Chemistry and Energetic Materials

A global increase in environmental pollution demands the development of new "cleaner" chemical processes. Among urgent improvements, the replacement of traditional hydrocarbon-derived toxic organic solvents with neoteric solvents less harmful for the environment is one of the most vital issues. As a result of the favorable combination of their unique properties, ionic liquids (ILs), dense gases, and supercritical fluids (SCFs) have gained considerable attention as suitable green chemistry media for the preparation and modification of important chemical compounds and materials. In particular, they have a significant potential in a specific and very important area of research associated with the manufacture and processing of high-energy materials (HEMs). These large-scale manufacturing processes, in which hazardous chemicals and extreme conditions are used, produce a huge amount of hard-to-dispose-of waste. Furthermore, they are risky to staff, and any improvements that would reduce the fire and explosion risks of the corresponding processes are highly desirable. In this Review, useful applications of almost nonflammable ILs, dense gases, and SCFs (first of all, CO₂ ) for nitration and other reactions used for manufacturing HEMs are considered. Recent advances in the field of energetic (oxygen-balanced and hypergolic) ILs are summarized. Significant attention is paid to the SCF-based micronization techniques, which improve the energetic performance of HEMs through an efficient control of the morphology and particle size distribution of the HEM fine particles, and to useful applications of SCFs in HEM processing that makes them less hazardous.

Nitrogen-Rich Tetranuclear Metal Complex as a New Structural Motif for Energetic Materials

For designing energetic materials (EMs), the most challenging issue is to achieve a balance between energetic performance and reliable stability. In this work, we employed an efficient and convenient method to synthesize a new class of EMs: nitrogen-rich tetranuclear metal complexes [M(Hdtim)(H₂O)₂]₄ (M = Zn 1, Mn 2; H₃dtim = 1H-imidazol-4,5-tetrazole) with the N content of >46%. The structural analyses illustrate that isomorphous compounds 1 and 2 feature isolated hollow ellipsoid tetranuclear units, which are linked by both π-π interactions and hydrogen-bonding interactions to give a 3D supramolecular architecture. Compounds 1 and 2 exhibit prominent energetic characteristics: excellent detonation performances and reliable thermal, impact, and friction stabilities. Being nitrogen-rich tetrazolate compounds, the enthalpies of combustion of 1 (-11.570 kJ g^-1) and 2 (-12.186 kJ g^-1) are higher than those of classical EMs, RDX and HMX, and they possess high positive heats of formation. Sensitivity tests demonstrate that 1 and 2 are insensitive to external mechanical action. Excellent energetic performances and low sensitivities promote 1 and 2 to serve as a new class of promising EMs with a desirable level of safety.

Comparative Study of Various Pyrazole-based Anions: A Promising Family of Ionic Derivatives as Insensitive Energetic Materials

In the design of advanced energetic materials, high-density explosophores play a pivotal role because of their remarkable enhancement of both density and molecular stability. Using diversified functionalization strategies, a comparative study involving various nitropyrazole anions shows that these are crucially important in determining performance and stability. A promising family of pyrazole-based energetic ionic derivatives were synthesized and characterized by NMR and IR spectroscopies, and elemental analysis. Among them, 7, 8, 11-13 exhibit favorable overall performance as energetic materials.

Resolving synthetic challenges faced in the syntheses of asymmetric N,N′-ethylene-bridged energetic compounds

Synthetic challenges faced during the syntheses of asymmetric N,N′-ethylene-bridged energetic compounds due to the differences in the reactivity and stability of various types of energetic rings are addressed.

Structures and properties of energetic cations in energetic salts

Energetic cations play important roles in energetic performance, which may attribute to the diverse backbones and substituted groups of energetic cations. This review emphasizes the roles of backbones and substituted groups in the energetic performance of energetic salts.

Exploiting the energetic potential of 1,2,4-oxadiazole derivatives: combining the benefits of a 1,2,4-oxadiazole framework with various energetic functionalities

A series of 1,2,4-oxadiazole derived energetic compounds were synthesized and structurally characterized, some of which have the potential as melt-cast explosives or secondary explosives.

Crystal structure transformation and step-by-step thermal decomposition behavior of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate

The crystal structure transformation and step-by-step thermal decomposition behavior of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) under thermal stimulation were studied.

Carbonyl-bridged energetic materials: biomimetic synthesis, organic catalytic synthesis, and energetic performances

In order to obtain high-performance energetic materials, in this work, carbonyl groups (C[double bond, length as m-dash]O) have been newly introduced as sole bridging groups in the field of energetic materials. To this end, two tailored green methods for the synthesis of carbonyl-bridged energetic compounds have been developed for the first time. One is a biomimetic synthesis, in which the conversion route of heme to biliverdin has been used to obtain metal-containing energetic compounds. The other one is an organocatalysis, in which guanidinium serves as an energetic catalyst to afford other energetic compounds. Experimental studies and theoretical calculations have shown that carbonyl-bridged energetic compounds exhibit excellent energetic properties, which is promising for the carbonyl group as a new important and effective linker in energetic materials.

Dancing with Energetic Nitrogen Atoms: Versatile N-Functionalization Strategies for N-Heterocyclic Frameworks in High Energy Density Materials

Nitrogen-rich heterocycles represent a unique class of energetic frameworks featuring high heats of formation and high nitrogen content, which have generated considerable research interest in the field of high energy density materials (HEDMs). Although traditional C-functionalization methodology of aromatic hydrocarbons has been fully established, studies on N-functionalization strategies of nitrogen-containing heterocycles still have great potential to be exploited by virtue of forming diverse N-X bonds (X = C, N, O, B, halogen, etc.), which are capable of regulating energy performance and the stability of the resulting energetic compounds. In this sense, versatile N-functionalization of N-heterocyclic frameworks offers a flexible strategy to meet the requirements of developing new-generation HEDMs. In this Account, the role of strategic N-functionalization in designing new energetic frameworks, including the formation of N-C, N-N, N-O, N-B and N-halogen bonds, is emphasized. In the family of N-functionalized HEDMs, energetic derivatives, by virtue of forming N-C bonds, are the most widely used type due to the good nucleophilic capacity of most heterocyclic backbones. Although introduction of carbon tends to decrease energetic performance, significant improvement in material sensitivity makes this strategy attractive for safety concerns. More importantly, most "explosophores" can be readily introduced into the N-C linkage, thus providing a promising route to various HEDMs. Formation of additional N-N bonds typically gives rise to higher heats of formation, implying the potential enhancement in detonation performance. In many cases, the increased hydrogen bonding interactions within N-N functionalized heterocycles also improve thermal stability accordingly. Introduction of a single N,N'-azo bridge into several azole moieties leads to an extended nitrogen chain, demonstrating a new strategy for designing high-nitrogen compounds. The strategy of N-O functionalization has become an increasingly efficient tool for exploring new HEDMs with both high energy and low sensitivity. As a highly dense building block, introduction of oxygen not only improves density significantly but also gives rise to a better oxygen balance. Furthermore, the N-O functionalized strategy is highly suitable for a broad variety of N-heterocycles including five-membered azoles and six-membered azines. Newly explored N-halogen and N-B functionalization strategies have endowed the resulting HEDMs with some new energetic characteristics. Typical examples include the N-halogenated fused triazole and FOX-7 as potential hypergolic oxidizers with very short ignition delay times. In addition, some exploratory studies of N-B functionalized heterocycles have expanded energetic applications as hypergolic ionic liquids, green pyrotechnic colorants, and high-oxygen carriers. Overall, flexible N-functionalization methodologies involving different N-X bond formation have not only provided an efficient approach to diverse energetic ingredients but also expanded the application scope of energetic materials. Discussion and perspectives of N-functionalized protocols are given to summarize possible structure-property correlations, thus providing efficient guidelines for future design of new HEDMs.

Nitrogen-rich energetic salts of 1H,1′H-5,5′-bistetrazole-1,1′-diolate: synthesis, characterization, and thermal behaviors

A series of nitrogen-rich heterocyclic 1H,1′H-5,5′-bistetrazole-1,1′-diolate salts with excellent thermal stabilities, impact sensitivities and promising detonation performances was synthesized.

Studies on the synthesis and properties of polynitro compounds based on esteryl backbones

Four polynitro esters were derived from 2,2,2-trinitroethanol with multi-nitrobenzoic acids by transesterification.

Efficient synthesis of 4-amino-2,6-dichloropyridine and its derivatives

A facile synthetic route to an important intermediate 4-amino-2,6-dichloropyridine was developed. Oxidation of 2,6-dichloropyridine as a starting material gave pyridine