Tetrazole Azasydnone (C 2 N 7 O 2 H) And Its Salts: High‐Performing Zwitterionic Energetic Materials Containing A Unique Explosophore

Zwitterionic Energetic Materials: Synthesis, Structural Diversity and Energetic Properties

Zwitterionic compounds are an emergent class of energetic materials and have gained synthetic interest of many in the recent years. Due to their better packing efficiencies and strong inter/intramolecular electrostatic interactions, they often ensue superior energetic properties than their salt analogues. A systematic review from the perspective of design, synthesis, and physicochemical properties evaluation of the zwitterionic energetic materials is presented. Depending on the parent ring(s) used for the synthesis and the type of moieties bearing positive and negative charges, different classes of energetic materials, such as primary explosives, secondary explosives, heat resistant explosives, oxidizers, etc., may result. The properties of some of the energetic zwitterionic compounds are also compared with analogous energetic salts. This review will encourage readers to explore the possibility of designing new zwitterionic energetic materials.

Expanding the Limits of Organic Energetic Materials: High-Performance Alliance of 1,3,4-Thiadiazole and Furazan Scaffolds

A majority of known and newly synthesized energetic materials comprise polynitrogen or nitrogen-oxygen heterocycles with various explosophores. However, available structural combinations of these organic scaffolds are finite and are about to reach their limits. Herein, we present the design and synthesis of a series of sulfur-containing polyazole structures comprising 1,3,4-thiadiazole and furazan rings linked by C-C bonds and enriched with energetic nitro and azo functionalities. In terms of detonation performance, all synthesized 1,3,4-thiadiazole-furazan assemblies (D = 7.7-7.9 km s-1; P = 26-28 GPa) lie between the powerful explosive TATB (D = 8.0 km s-1; P = 31 GPa) and melt-cast material TNT (D = 6.9 km s-1; P = 23 GPa). In the synthesized series, azo-bridged derivative 5 seems to be most practically interesting, as it combines a relatively high energetic performance (D = 7.9 km s-1; P = 28 GPa), a very high thermal stability (271 °C), and insensitivity to friction. By these functional properties, 5 outperforms the benchmark heat-resistant explosive hexanitrostilbene (HNS). To the best of our knowledge, this is the first example of an energetic alliance of furazan and 1,3,4-thiadiazole scaffolds and a rare case of sulfur-containing high-energy materials, which can certainly be considered as an evolutionary step in energetic materials science.

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.

Toward High-Energy and Low-Sensitivity Energetic Materials Based on a Fused [5,5,5,6]-Tetracyclic Backbone

A route for fused [5,5,5,6]-tetracyclic energetic compounds based on the facile cyclization reaction has been explored. Fused [5,5,5,6]-tetracyclic compound 4 shows a high measured density (1.924 g cm^-3), a low sensitivity (IS = 10 J, and FS = 144 N), and an excellent detonation velocity (9241 m s^-1), which are much better than those of RDX. The results indicate that compound 4 is a potential candidate as a secondary explosive and provide new insight into the construction of fused polycyclic heterocycles.

Combination of Energetic Tetrazole and Triazole: Promising Materials with Exceptional Stability and Low Mechanical Sensitivity as Propellants and Gas Generators

An innovative synthesis of 5-((1H-tetrazol-5-yl)methyl)-4H-1,2,4-triazole-3,4-diamine (TMT) based on triazole and tetrazole frameworks bearing double amino groups was reported. It is worth mentioning that TMT is insensitive to impact and friction (IS > 40 J, FS > 360 N), thus enabling it to have an exceptional thermal decomposition behavior that is superior to RDX and TNT. Meanwhile, it also has relatively high energetic performance (D_v = 8.417 km/s). A series of energy-containing salts TMT-1-8 were also investigated for their potential applications. Except for TMT-4 and TMT-7, the remaining nitrogen-rich salts have initial decomposition temperatures above 200 °C. Furthermore, the salts with positive heat generation all have extraordinary gas production, especially for TMT-1 (V_o = 840.5 dm³/kg), TMT-2 (V_o = 803.9 dm³/kg), and TMT-7 (V_o = 844.3 dm³/kg). The low mechanical sensitivities of the TMT series were discovered, and a majority of them have impact sensitivities exceeding 40 J with friction sensitivities exceeding 360 N which are superior to TNT (IS = 15 J, FS = 353 N). The intermolecular and intramolecular interactions of the crystals TMT-1-3 were explored by Hirshfeld surfaces, 2D fingerprint plots, noncovalent interaction (NCI) analysis, and electrostatic potential surface analysis to understand the physicochemical property changes in relation to the structure. Consequently, this novel tri/tetrazole and polyamine system as a promising material provides the impetus for the development of gas generators and propellants.

N-Functionalization of 5-Aminotetrazoles: Balancing Energetic Performance and Molecular Stability by Introducing ADNP

5-aminotetrazole is one of the most marked high-nitrogen tetrazole compounds. However, the structural modification of 5-aminotetrazole with nitro groups often leads to dramatically decreased molecular stability, while the N-bridging functionalization does not efficiently improve the density and performance. In this paper, we report on a straightforward approach for improving the density of 5-aminotetrazole by introducing 4-amino-3,5-dinitropyrazole. The following experimental and calculated properties show that nitropyrazole functionalization competes well with energetic performance and mechanic sensitivity. All compounds were thoroughly characterized using IR and NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Two energetic compounds (DMPT-1 and DMPT-2) were further confirmed by implementing single-crystal X-ray diffraction studies. Compound DMPT-1 featured a high crystal density of 1.806 g cm-3, excellent detonation velocity (vD = 8610 m s-1), detonation pressure (P = 30.2 GPa), and impact sensitivity of 30 J.

(2-Vinyltetrazolyl)furoxans as New Potential Energetic Monomers

A regioselective approach toward the synthesis of a set of new (2-vinyltetrazolyl)furoxans as potential energetic monomers has been realized. All target energetic materials were thoroughly characterized by spectral and analytical methods. Moreover, crystal structures of two representative heterocyclic systems were studied by single-crystal X-ray diffraction. Prepared high-energy substances have high combined nitrogen-oxygen content (63-71 %), high enthalpies of formation and good detonation parameters (D: 6.7-7.8 km s-1; P: 18-28 GPa). Mechanical sensitivities of the synthesized vinyltetrazoles range these explosives from highly sensitive to completely insensitive. Using calculations of molecular electrostatic potentials (ESP), structural factors influencing the impact sensitivity were revealed. Overall, newly synthesized (2-vinyltetrazolyl)furoxans are of interest as promising energetic monomers due to the presence of the vinyl moiety and explosophoric heterocyclic combination, while their performance exceeds that of benchmark explosive TNT.

Series of Azido and Fused-Tetrazole Explosives: Combining Good Thermal Stability and Low Sensitivity

The introduction of azido groups into the energetic skeleton has the advantages of increasing the energy level. In this work, a series of azido compounds with good stability and low sensitivity as well as tetrazole-fused compounds based on energetic salts are synthesized. The detonation pressures and velocities of these new compounds fall in the ranges of 18.9-27.3 GPa and 7153-8450 m s^-1, respectively. The detonation velocity of the tetrazole-fused compounds based on the potassium salts 3, 6, and 7 are 7810, 7153, and 7989 m s^-1, respectively. Also, their decomposition temperatures (244, 237, and 240 °C, respectively) are higher than that of traditional explosive RDX (204 °C). Notably, two representative compounds 2 and 5 possess higher decomposition temperature (2: 196 °C and 5: 178 °C) and overall detonation properties (2: D = 8129 m s^-1 and P = 26.6 GPa and 5: D = 8336 m s^-1 and P = 27.3 GPa) as well as relativity lower sensitivities (2: IS = 12 J and FS = 240 N and 5: IS = 10 J and FS = 144 N) than that of primary explosive 2-diazo-4,6-dinitrophenol (T_d = 157 °C, D = 6900 m s^-1, P = 24.7 GPa, IS = 1 J, and FS = 24.7 N). Moreover, the initiation capacity of compounds 2 and 5 was also assessed through the initiation tests. The results indicate that the two compounds could be a promising environmentally friendly primary explosive.

Renaissance of dinitroazetidine: novel hybrid energetic boosters and oxidizers

Nitrogen-oxygen organic materials constitute an important family of multipurpose high-energy materials. However, the preparation of energetic boosters and oxidizers for various civil and space technologies remains a challenging task and such materials usually require special precautions and fine tunability of their functional properties. To find a balance between energy and safety while retaining the oxidizing ability of target energetic materials, novel hybrid organic compounds comprising furoxan and 3,3-dinitroazetidine scaffolds enriched with additional nitro groups were synthesized. The prepared 3-(3,3-dinitroazetidinoyl)-4-nitrofuroxan and 3,3-dinitro-1-(2,2,2-trinitroethyl)azetidine have high nitrogen-oxygen contents (75-79%), positive oxygen balance to CO (up to +10.3%) and good experimental densities (1.75-1.80 g cm^-3). A combination of superior detonation performance (D = 8.3-8.5 km s^-1 and P = 32-33 GPa) and moderate mechanical sensitivity enables the application potential of these energetic materials as booster explosives or oxidizers. Additionally, their functional properties remain essentially competitive with other oxygen-rich energetic materials (pentaerythritol tetranitrate, ammonium dinitramide, and tetranitratoethane). Hirshfeld surface calculations supported by energy framework plots were also performed to better understand the relationship between the molecular structure and stability/sensitivity. This work unveils novel directions in the construction of balanced energetic boosters and oxidizers for various applications.

1-(Azidomethyl)-5H-Tetrazole: A Powerful New Ligand for Highly Energetic Coordination Compounds

Highly energetic 1-(azidomethyl)-5H-tetrazole (AzMT, 3) has been synthesized and characterized. This completes the series of 1-(azidoalkyl)-5H-tetrazoles represented by 1-(azidoethyl)-5H-tetrazole (AET) and 1-(azidopropyl)-5H-tetrazole (APT). AzMT was thoroughly analyzed by single-crystal X-ray diffraction experiments, elemental analysis, IR spectroscopy and multinuclear (1 H, 13 C, 14 N, 15 N) NMR measurements. Several energetic coordination compounds (ECCs) of 3d metals (Mn, Fe, Cu, Zn) and silver in combination with anions such as (per)chlorate, mono- and dihydroxy-trinitrophenolate were prepared, giving insight into the coordination behavior of AzMT as a ligand. The synthesized ECCs were also analyzed by X-ray diffraction experiments, elemental analysis, and IR spectroscopy. Differential thermal analysis for all compounds was conducted, and the sensitivity towards external stimuli (impact, friction, and ESD) was measured. Due to the high enthalpy of formation of AzMT (+654.5 kJ mol-1 ), some of the resulting coordination compounds are extremely sensitive, yet are able to undergo deflagration-to-detonation transition (DDT) and initiate pentaerythritol tetranitrate (PETN). Therefore, they are to be ranked as primary explosives.

Tuning the Properties of 5‐Azido and 5‐Nitramino‐tetrazoles by Diverse Functionalization – General Concepts for Future Energetic Materials

5‐Azido and 5‐nitraminotetrazole backbones are established heterocyclic motifs in the research field of energetic materials synthesis. Despite the high energy content of the compounds, the problem with many derivatives is that their sensitivities are far too high. Functionalization of one of the ring nitrogen atoms is the aim of this study to adjust the sensitivity by inserting nitratoethyl, azidoethyl and methyl groups. In this context, derivatives of 2‐(2‐azidoethyl)‐5‐nitraminotetrazoles (2, 2  a–2  d), as well as 1‐nitrato and 1‐azidoethyl substituted 5‐azidotetrazole (7 and 10) and the methylation products of 5‐azidotetrazole (5‐azido‐1‐methyl‐tetrazole, 11 and 5‐azido‐2‐methyl‐tetrazole, 12) were prepared. The obtained nitrogen‐rich compounds were extensively characterized through multinuclear NMR spectroscopy and IR spectroscopy. The structural confinement was checked by X‐ray diffraction experiments. The pure samples (verified by elemental analysis) were investigated regarding their behavior toward friction, impact (BAM methods) and electrostatic discharge as well as heating (DTA and DSC). For all metal‐free compounds the detonation properties were computed with the EXPLO5 code using their density and heat of formation, calculated based on CBS‐4 M level of theory.

Nitrogen-rich ion salts of 1-hydroxytetrazole-5-hydrazide: a new series of energetic compounds that combine good stability and high energy performance

High-efficiency explosives that combine high stability and excellent energy performance are one of the key directions of energetic materials research. In this study, a novel monocyclic hydroxytetrazole derivative (3) with high stability was prepared, and a series of insensitive energetic ionic salts were derived from it. Benefiting from their outstanding performance in terms of density, 3D hydrogen bonding and π-electron interactions, these salts are excellent in both detonation performance (D = 8709 to 9314 m s^-1 and P = 29.9 to 35.6 GPa) and thermal stability (T_d = 193.0-232.2 °C). The hydrazine salt (2) exhibits high detonation properties (D = 9314 m s^-1 and P = 35.6 GPa), due to its high density (ρ = 1.71 g cm^-3) and high heat of formation (Δ_fH = 563.2 kJ mol^-1 = 3.19 kJ g^-1). In addition, the high thermal stability (T_d = 232.0 °C) and low mechanical sensitivity (IS = 30 J and FS = 360 N) of 2 are also unmatched by HMX and TKX-50. These improved properties demonstrate the great promise of 2 as an insensitive high-energy explosive.

4,4'-Dinitrimino-5,5'-diamino-3,3'-azo-bis-1,2,4-triazole: A High-Performing Zwitterionic Energetic Material

Mixed acid nitration of electrochemically generated 4,4',5,5'-tetraamino-3,3'-azo-bis-1,2,4-triazole (TAABT) generated the novel energetic material 4,4'-dinitrimino-5,5'-diamino-3,3'-azo-bis-1,2,4-triazole (DNDAABT). Various energetic salts of DNDAABT were also prepared and characterized to confirm their structures and determine their explosive sensitivities and performances. The free acid of DNDAABT exists as a zwitterionic molecule that leads to a high-density material with predicted detonation parameters comparable to those of TKX-50 (bis(hydroxylammonium) 5,5'-bis(tetrazolate-1 N-oxide). Due to the insensitive nature of TAABT, it was predicted that DNDAABT would demonstrate remarkably low sensitivities for a primary N-nitramine. However, it was found that DNDAABT and all salts produced have primary explosive sensitivities, albeit with relatively high thermal stabilities for primary N-nitramines.

Nitrogen-rich metal-free salts: a new look at the 5-(trinitromethyl)tetrazolate anion as an energetic moiety

A series of energetic nitrogen-rich salts comprised of a 5-(trinitromethyl)tetrazolate anion and high-nitrogen cations was synthesized by simple and efficient chemical routes from readily available commercial reagents. These energetic materials were fully characterized by IR and multinuclear NMR (¹H, ¹³C, ¹⁴N) spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Additionally, the structure of an energetic salt containing the 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazolium cation was confirmed by single-crystal X-ray diffraction. The synthesized compounds exhibit decent experimental densities (1.648-1.845 g cm^-3) and positive enthalpies of formation (up to 725.5 kJ mol^-1) and, as a result, superior detonation performance (detonation velocities 8.2-9.2 km s^-1 and detonation pressures 28.5-37.8 GPa), which is comparable to or even exceeding those of commonly used booster explosive PETN. On the other hand, high mechanical sensitivity of several novel 5-(trinitromethyl)tetrazolate salts along with their high combined nitrogen-oxygen content (>81%) and excellent detonation performance render them environmentally friendly alternatives to lead-based primary explosives.

Recent Advances in the Synthesis and Biomedical Applications of Heterocyclic NO-Donors

Nitric oxide (NO) is a key signaling molecule that acts in various physiological processes such as cellular metabolism, vasodilation and transmission of nerve impulses. A wide number of vascular diseases as well as various immune and neurodegenerative disorders were found to be directly associated with a disruption of NO production in living organisms. These issues justify a constant search of novel NO-donors with improved pharmacokinetic profiles and prolonged action. In a series of known structural classes capable of NO release, heterocyclic NO-donors are of special importance due to their increased hydrolytic stability and low toxicity. It is no wonder that synthetic and biochemical investigations of heterocyclic NO-donors have emerged significantly in recent years. In this review, we summarized recent advances in the synthesis, reactivity and biomedical applications of promising heterocyclic NO-donors (furoxans, sydnone imines, pyridazine dioxides, azasydnones). The synthetic potential of each heterocyclic system along with biochemical mechanisms of action are emphasized.

Nitratoethyl-5H-tetrazoles: improving the oxygen balance through application of organic nitrates in energetic coordination compounds

1- and 2-Nitratoethyl-5H-tetrazole (1-NET and 2-NET) were prepared through nitration of the respective alkyl alcohol using 100% nitric acid. A mixture of 1- and 2-hydroxyethyl-5H-tetrazole was obtained after alkylation of 1,5H-tetrazole. Also, a one-pot synthesis of 1-hydroxyethyl-5H-tetrazole was developed enabling the selective preparation of 1-NET. Both organic nitrates were characterized by ¹H, ¹³C, and ¹H-¹⁵N HMBC NMR spectroscopy. In addition, calculations using the Hirshfeld method and the EXPLO5 code were performed. Principally, 20 energetic coordination compounds involving the d-metals Mn, Cu, Zn, and Ag, each structurally characterized by low temperature single crystal X-ray diffraction, were prepared based on 1-NET and 2-NET. Of these complexes, 18 were obtained as pure bulk materials, and therefore, characterized for impact, friction, and ball drop impact sensitivity, as well as electrostatic discharge and thermal stability using differential thermal analysis. Hot plate and hot needle tests were performed mostly showing strong deflagrations making the complexes candidates for green combustion catalysts. Furthermore, successful PETN initiation experiments were carried out for several complexes and all ECCs were investigated by laser ignition experiments.

Heterocyclic Nitrilimines and Their Use in the Synthesis of Complex High-Nitrogen Materials

We show the ability of a nitrilimine prepared from 3-amino-5-nitro-1,2,4-triazole to undergo various cyclization and rearrangement reactions, giving a beautiful diversity of nitrogen-rich heterocyclic products. This chemistry includes the first cyclization of a nitrilimine with a diazonium species, giving a tetrazole, a previously unknown transformation, as well as leading to the creation of several new energetic materials with backbones not available by traditional techniques. New materials prepared were characterized both chemically (multinuclear NMR, IR, mass spectrometry, and elemental analysis) and energetically, with sensitivities and performances reported.

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.

Tailoring the properties of 3d transition metal complexes with different N-cycloalkyl-substituted tetrazoles

N-Cycloalkyl-substituted tetrazoles were coordinated with different late 3d transition metal(ii) salts to give moderately sensitive energetic coordination compounds.

3-Methyl-1,2,3-triazolium-1N-dinitromethylylide and the strategy of zwitterionic dinitromethyl groups in energetic materials design

3-Methyl-1,2,3-triazolium-1N-dinitromethylylide, an exemplary zwitterionic energetic molecule, is the first fully-studied energetic material making use of the zwitterionic dinitromethyl functional group. This compound has impact and friction sensitivities of 8 J and 144–160 N respectively with a detonation velocity of 8162 m s⁻¹.

Fully characterized energetic material containing a unique zwitterionic dinitromethyl functional group.

Methyl sydnone imine and its energetic salts

First investigation of this new energetic cation, which may greatly expand the range of possible energetic salts available.

Tailoring Energetic Sensitivity and Classification through Regioisomerism

The azo-coupling of 1- and 2-amino-4-nitro-1,2,3-triazole yielded two new energetic compounds whose detonation properties compete with that of HMX. Though the calculated performances are impressive, the regioisomers have differing sensitivities and detonation behavior. One has sensitivities similar to a very sensitive primary explosive, while the other has sensitivities more comparable to a sensitive secondary explosive. This serves as an example of the ability to tailor the sensitivities and end use of energetic compounds via regioisomerization.