N-oxide 1,2,4,5-tetrazine-based high-performance energetic materials

Molecular engineering of 3-arylated tetrazo[1,2-b]indazoles: divergent synthesis and structure-property relationships

The synthetic scope of 3-arylated tetrazo[1,2-b]indazoles is reported based on a Pd-catalyzed Liebeskind-Srogl cross-coupling reaction followed by an N-cyclisation process. The reactivity of the nitrogen atoms was used to further diversify these N-rich polyaromatic tetrazo[1,2-b]indazoles in a panel of reactions (protonation, selective oxidation, metallations). Selective ortho-C-H activation/functionalization on the heterocycle was also demonstrated with three transition metals (TM = Pd, Ir and Rh). The effects of all these molecular engineering strategies, particularly the N-modifications, on the optical and redox properties of the 3-arylated tetrazoindazoles were studied experimentally and theoretically. This study highlights the diversity of molecular structures and electronic properties offered by the tetrazo[1,2-b]indazole platform.

Substituted tetrazoles with N-oxide moiety: critical assessment of thermochemical properties

Modeling of the structure of molecules and simulation of crystal structure followed by the calculation of the enthalpies of formation for 21 salts of three high-energy tetrazole 1N-oxides: 5-nitro-1-hydroxy-1H-tetrazole 1a-1g, 5-trinitromethyl-1-hydroxy-1H-tetrazole 2a-2g and 6-amino-3-(1-hydroxy-1H-tetrazol-5-yl)-1,2,4,5-tetrazine 1,5-dioxide 3a-3g was performed. The methods of quantum chemistry and the method of atom-atom potentials were used. Structural search for optimal crystal packings was carried out in 11 most common space symmetry groups. The enthalpies of formation were obtained and analyzed using two different approaches: VBT and MICCM methods, which allowed to evaluate the quality of these calculation methods. In addition, the results obtained indicate high values of thermochemical characteristics for some of the considered compounds, which have a positive effect on their explosive properties and unveil their future application potential.

An Alliance of Polynitrogen Heterocycles: Novel Energetic Tetrazinedioxide-Hydroxytetrazole-Based Materials

Energetic materials constitute one of the most important subtypes of functional materials used for various applications. A promising approach for the construction of novel thermally stable high-energy materials is based on an assembly of polynitrogen biheterocyclic scaffolds. Herein, we report on the design and synthesis of a new series of high-nitrogen energetic salts comprising the C-C linked 6-aminotetrazinedioxide and hydroxytetrazole frameworks. Synthesized materials were thoroughly characterized by IR and multinuclear NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction and differential scanning calorimetry. As a result of a vast amount of the formed intra- and intermolecular hydrogen bonds, prepared ammonium and amino-1,2,4-triazolium salts are thermally stable and have good densities of 1.75–1.78 g·cm⁻³. All synthesized compounds show high detonation performance, reaching that of benchmark RDX. At the same time, as compared to RDX, investigated salts are less friction sensitive due to the formed net of hydrogen bonds. Overall, reported functional materials represent a novel perspective subclass of secondary explosives and unveil further opportunities for an assembly of biheterocyclic next-generation energetic materials.

Synthesis of heterocyclic (triazole, furoxan, furazan) fused pyridazine di-N-oxides via hypervalent iodine oxidation

Use of a mild PIDA oxidation strategy to access heterocyclic (triazole, furoxan, furazan) fused pyridazine di-N-oxides.

Construction of Bicyclic 1,2,3-Triazine N-Oxides from Aminocyanides

Using a facile and cost-effective method, nine bicyclic 1,2,3-triazine 2-oxides were synthesized from o-aminocyanide substrates through an unusual nitration cyclization. The reaction mechanism was studied experimentally and theoretically. Moreover, nine 1,2,3-triazine 3-oxides were also obtained in good yields.

Boosting intermolecular interactions of fused cyclic explosives: the way to thermostable and insensitive energetic materials with high density

Improving the packing efficiency of explosives by strong intermolecular interactions can acquire high density while avoiding the expense of stability.

[(3-Nitro-1H-1,2,4-triazol-1-yl)-NNO-azoxy]furazans: energetic materials containing an N(O)N–N fragment

The strategy for the synthesis of substituted [(3-nitro-1H-1,2,4-triazol-1-yl)-NNO-azoxy]furazans 4–7, in which the distal nitrogen of the azoxy group is bonded to the nitrogen atom of the azole ring, includes, firstly, the reaction of 1-amino-3-nitro-1H-1,2,4-triazole with 2,2,2-trifluoro-N-(4-nitrosofurazan-3-yl)acetamide in the presence of dibromisocyanuric acid followed by removing of the trifluoroacetyl protecting group to afford aminofurazan (4). Transformation of the amino group in the latter made it possible to synthesize the corresponding nitro (5), azo (6), and methylene dinitramine (7) substituted furazans. The compounds synthesized are thermally stable (decomposition onset temperatures 147–228 °C), exhibit acceptable densities (1.77–1.80 g cm⁻³) and optimal oxygen balance (the oxidizer excess coefficients α = 0.42–0.71). Their standard enthalpies of formation (576–747 kcal kg⁻¹) were determined experimentally by combustion calorimetry and these compounds have been estimated as potential components of solid composite propellants. In terms of the specific impulse level, model solid composite propellant formulations based on nitro and methylene dinitramine substituted furazans 5 and 7 outperform similar formulations based on CL-20 by 1–4 s, and formulations based on HMX and RDX by 5–8 s.

A novel class of energetic compounds with a N(O) N–N fragment, [(3-nitro-1H-1,2,4-triazol-1-yl)-NNO-azoxy]furazans, which exhibit good thermal stability and high experimental enthalpies of formation are estimated as possible components of solid composite propellants.

Adjacent N→O and C–NH2 groups — a highly efficient amphoteric structure for energetic materials resulting from tautomerization proved by crystal engineering

Adjacent N→O and C–NH₂ groups (O←NC–NH₂) were found to be a highly efficient and fairly balanced amphoteric energetic structure for energetic materials by crystal engineering.

Mono-N-oxidation of heterocycle-fused pyrimidines

Mild oxidation reactions of nitrogen-rich heterocyclic rings lead to the formation of energetic compounds with the mono-N-oxide moiety which show good thermal stabilities and detonation performances.

A series of guanidine salts of 3,6-bis-nitroguanyl-1,2,4,5-tetrazine: green nitrogen-rich gas-generating agent

Nitrogen-rich energetic materials (EMs) have been widely studied because of their high thermal stability, insensitivity, excellent detonation performance and non-toxic characteristics. In particular, these compounds are well applied as gas-generating agents (GGAs). As a nitrogen-rich heterocyclic framework, 1,2,4,5-tetrazine derivatives have shown great potential in the design of GGAs. The guanidine salts of 3,6-bis-nitroguanyl-1,2,4,5-tetrazine (DNGTz), guanidine (G2DNGTz) (1), aminoguanidine (AG2DNGTz) (2), diaminoguanidine (DAG2DNGTz) (3), and triaminoguanidine (TAG2DNGTz) (4) have been synthesized and characterized by elemental analysis and FT-IR. The crystal structures of 1 and 2 were obtained by X-ray single crystal diffraction. Crystal analysis shows that 1 and 2 arrange through zigzag-chain-like assembly and face-to-face geometries, which is helpful in decreasing mechanical sensitivity. The thermal stability and thermal decomposition kinetics of these four salts were studied by Differential Scanning Calorimetry (DSC). Furthermore, the thermogravimetry-Fourier transform infrared-mass spectrometry (TG-FTIR-MS) analysis of thermal decomposition products reveals that the main decomposition gaseous products are H2O, N2O, CO2, NO, N2 and NH3. Then, the cytotoxicity of the four salts was tested by MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) method, and it was found that salts 1-4 show slight cytotoxicity in mouse fibroblasts (L929), at a concentration of 0.125 mg ml-1. The insensitivity, low toxicity, and production of clean gases without solid residue after burning of salt 1 indicate that it can be used as a potential green energetic material for GGAs.

Derivatives of 3,6-Bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine: Excellent Energetic Properties with Lower Sensitivities

To find a balance between energy and safety, a series of compounds based on azo-, azoxy-, 1,4,2,5-dioxadiazene-, and 3,6-diamino-1,2,4,5-tetrazine-bridged bis(aminofurazan) were designed and synthesized. These compounds were analyzed by nitro group charges (Q_nitro) and bond dissociation energy (BDE) calculations, which are related to sensitivity and stability. Based on the calculated results, derivatives of 3,6-bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine have the largest values for -Q_nitro and BDE of all of the bis(aminofurazan) compounds. This shows that compounds based on 3,6-bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine have the lowest sensitivities and best stabilities, which has been substantiated by experiments. Additionally, their explosive properties remain essentially competitive with compounds based on azo-, azoxy-, and 1,4,2,5-dioxadiazene-bridged bis(aminofurazan). Hirshfeld surface calculations were also performed to better understand the relationship between the molecular structure and stability/sensitivity. This work highlights the value of 3,6-diamino-1,2,4,5-tetrazine as a linker to achieve good balance between safety and energy.

Nitroacetonitrile as a versatile precursor in energetic materials synthesis

Nitroacetonitrile is the simplest α-nitronitrile; it possesses a single central carbon attached to two strong electronegative, electron-withdrawing groups allowing extensive chemistry through the active methylene center. Free nitroacetonitrile has purification and stability issues, however stable salts of nitroacetonitrile possess the same reactivity as the free acid and are much more stable. Nitroacetonitrile serves as a versatile synthetic precursor in the formation of heterocyclic and polyfunctional aliphatic products and can allow for straightforward conversion to amino, acyl, and other functional groups. A main advantage of using nitroacetonitrile in the formation of heterocyclic-based energetics is its ability to add vicinal amino and nitro moieties onto fused ring structures, a common structural motif in insensitive energetic materials. In this minireview we discuss the preparation of nitroacetonitrile and its stable salts, as well as discuss the range of energetic materials this versatile precursor has found use in.

Nitroacetonitrile is a useful synthetic precursor capable of participating in a wide range of reactions and enables the simple synthesis of annulated heterocyclic systems which are rapidly becoming promising new energetic materials.

Combinations of furoxan and 1,2,4-oxadiazole for the generation of high performance energetic materials

Several energetic materials, which are composed of furoxan and 1,2,4-oxadiazole backbones, were synthesized by nitrating 3,3'-bis(5-amino-1,2,4-oxadiazol-3-yl)-4,4'-azofuroxan (2) under 100 wt% HNO3 or 100 wt% HNO3/Ac2O followed by a cation metathesis. All synthesized compounds were fully characterized by multinuclear NMR spectroscopy, IR spectroscopy, and elemental analysis, while 3,3'-bis(1,2,4-oxadiazol-5(4H)-one-3-yl)-4,4'-azofuroxan (3) and diammonium 3,3'-bis(5-nitramino-1,2,4-oxadiazole-3-yl)-4,4'-azofuroxan (4a) were confirmed by single crystal X-ray diffraction. The physicochemical and energetic properties of these compounds including density, thermal stability and sensitivity were investigated. Compounds 3 and 4 have high densities (3: 1.90 g cm-3, 4: 1.92 g cm-3), which are comparable to that of HMX (1.91 g cm-3). All energetic compounds show relatively high calculated heat of formation in the range from 504.79 kJ mol-1 to 1405.62 kJ mol-1. Their detonation properties were evaluated by EXPLO5 code using the measured density and calculated heat of formation. Among them, compounds 3 and 4 have good detonation performance (3: D = 8891 m s-1, P = 34.7 GPa, 4: D = 9505 m s-1, P = 41.3 GPa) and acceptable sensitivities (3: IS = 10 J, 4: IS = 4 J), which indicate their potential applications as high-performance energetic materials.

New Strategy for Enhancing Energetic Properties by Regulating Trifuroxan Configuration: 3,4-Bis(3-nitrofuroxan-4-yl)furoxan

It is of current development to construct high-performance energetic compounds by aggregation of energetic groups with dense arrangement. In this study, a hydrogen-free high-density energetic 3,4-bis(3-nitrofuroxan-4-yl)furoxan (BNTFO-I) was designed and synthesized in a simple, and straightforward manner. Its isomer, 3,4-bis(4-nitrofuroxan-3-yl)furoxan (BNTFO-IV), was also obtained by isomerization. The structures of BNTFO-I and BNTFO-IV were confirmed by single-crystal X-ray analysis for the first time. Surprisingly, BNTFO-I has a remarkable calculated crystal density of 1983 g cm-3 at 296 K, which is distinctly higher than BNTFO-IV (1.936 g cm-3, 296 K), and ranks highest among azole-based CNO compounds yet reported. It is noteworthy that BNTFO-I exhibits excellent calculated detonation properties (vD, 9867 m s-1, P, 45.0 GPa). The interesting configuration differences of BNTFO-I and BNTFO-IV provide insight into the design of new advanced energetic materials.

Stability and performance of gun propellants incorporating 3,6-dihydrazino-s-tetrazine and 5-aminotetrazolium nitrate

The addition of either 3,6-dihydrazino-s-tetrazine (DHT) or 5-aminotetrazolium nitrate (HAT-NO₃) to nitrocellulose-based propellants were investigated. At 25% (m/m) concentration, DHT and HAT-NO3 had significant impact on the burning rate of the propellant, up to 80% higher than that of the reference propellant. DHT was found to have very poor compatibility with nitrocellulose and the nitrated esters used in the formulation despite the presence of stabilizer. This lead to a rapid autocatalytic decomposition reaction resulting in a deflagration. HAT-NO₃ also had poor compatibility with the same materials. On the contrary, non-ionic tetrazoles were found to be fully compatible with nitrocellulose and nitrated esters based propellants. Most nitrogen-rich energetic molecules have been studied for their explosive characteristics. This study shed light on the potential use of these materials as burning rate modifiers for gun propellant applications, for which very little is known. Moreover, it investigates the stability of the formulations incorporating nitrogen-rich molecules, as a means of assessing the safe use of these novel propellants.

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.

Fused rings with N-oxide and –NH2: good combination for high density and low sensitivity energetic materials

Energetic materials with N-oxide and amino groups suggest a promising alternative for the design of high-energy materials with low sensitivity.

Sub-500 fs electronically nonadiabatic chemical dynamics of energetic molecules from the S1 excited state: Ab initio multiple spawning study

Energetic materials store a large amount of chemical energy. Different ignition processes, including laser ignition and shock or compression wave, initiate the energy release process by first promoting energetic molecules to the electronically excited states. This is why a full understanding of initial steps of the chemical dynamics of energetic molecules from the excited electronic states is highly desirable. In general, conical intersection (CI), which is the crossing point of multidimensional electronic potential energy surfaces, is well established as a controlling factor in the initial steps of chemical dynamics of energetic molecules following their electronic excitations. In this article, we have presented different aspects of the ultrafast unimolecular relaxation dynamics of energetic molecules through CIs. For this task, we have employed ab initio multiple spawning (AIMS) simulation using the complete active space self-consistent field (CASSCF) electronic wavefunction and frozen Gaussian-based nuclear wavefunction. The AIMS simulation results collectively reveal that the ultrafast relaxation step of the best energetic molecules (which are known to exhibit very good detonation properties) is completed in less than 500 fs. Many, however, exhibit sub-50 fs dynamics. For example, nitro-containing molecules (including C-NO₂, N-NO₂, and O-NO₂ active moieties) relax back to the ground state in approximately 40 fs through similar (S₁/S₀)_CI conical intersections. The N₃-based energetic molecule undergoes the N₂ elimination process in 40 fs through the (S₁/S₀)_CI conical intersection. Nitramine-Fe complexes exhibit sub-50 fs Fe-O and N-O bond dissociation through the respective (S₁/S₀)_CI conical intersection. On the other hand, tetrazine-N-oxides, which are known to exhibit better detonation properties than tetrazines, undergo internal conversion in a 400-fs time scale, while the relaxation time of tetrazine is very long (about 100 ns). Many other characteristics of sub-500 fs nonadiabatic decay of energetic molecules are discussed. In the end, many unresolved issues associated with the ultrafast nonadiabatic chemical dynamics of energetic molecules are presented.

5,6-Di(2-fluoro-2,2-dinitroethoxy)furazano[3,4-b]pyrazine: a high performance melt-cast energetic material and its polycrystalline properties

A new high-energetic and insensitive melt-cast material was synthesized, and its polycrystalline properties were characterized and theoretically investigated.

Energetic compounds featuring bi(1,3,4-oxadiazole): a new family of insensitive energetic materials

Energetic compounds featuring bi(1,3,4-oxadiazole) with good detonation performances and acceptable sensitivities were synthesized.

Compounds based on 3-amino-4-(5-methyl-1,2,4-oxadiazol-3-yl)furazan as insensitive energetic materials

Insensitivity: Derivatives based on 3-amino-4-(5-methyl-1,2,4-oxadiazol-3-yl)furazan were synthesized and characterized. Most of these compounds are thermally stable and insensitive which make them competitive replacement as insensitive energetic materials.

Energetic Chromophores: Low-Energy Laser Initiation in Explosive Fe(II) Tetrazine Complexes

The synthesis and characterization of air stable Fe(II) coordination complexes with tetrazine and triazolo-tetrazine ligands and perchlorate counteranions have been achieved. Time-dependent density functional theory (TD-DFT) was used to model the structural, electrochemical, and optical properties of these materials. These compounds are secondary explosives that can be initiated with Nd:YAG laser light at lower energy thresholds than those of PETN. Furthermore, these Fe(II) tetrazine complexes have significantly lower sensitivity than PETN toward mechanical stimuli such as impact and friction. The lower threshold for laser initiation was achieved by altering the electronic properties of the ligand scaffold to tune the metal ligand charge transfer (MLCT) bands of these materials from the visible into the near-infrared region of the electromagnetic spectrum. Unprecedented decrease in both the laser initiation threshold and the mechanical sensitivity makes these materials the first explosives that are both safer to handle and easier to initiate than PETN with NIR lasers.

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.