1,3,4‐Oxadiazole Bridges: A Strategy to Improve Energetics at the Molecular Level

Covalent Organic Frameworks-Based Fluorescence Sensor Array and QSAR Study for Identification of Energetic Heterocyclic Compounds

The accurate identification of energetic heterocyclic compounds (EHCs) is of great significance in munition assessment, environmental monitoring, and biosafety but remains largely underexplored. Herein, a covalent organic frameworks-based fluorescence sensor array (COFx sensor array) for efficient screening of EHCs is reported. The topologies of the COFs were rationally designed by modulating the pore sizes and linkage strategies to achieve the simplified sensor array. Eighteen EHC representatives, including single-, dual-, and three-ring EHCs with multivariate substructures, were successfully discriminated ranging from 10 μM to 1 mM. The sensor array showed robust selectivity against a wide range of interferences. The quantitative structure-activity relationship (QSAR) analysis has been conducted for the mechanistic study of the sensor array. Three multiple linear regression models have been established using molecular descriptors to evaluate and predict Stern-Volmer coefficient values, achieving explicit correlation between EHC structures and the signal outputs of the sensor array. Five molecular descriptors are retained to reveal the governing factors of the sensor array resolution. The QSAR analysis facilitates the design and development of the COFx sensor array, offering a new approach for customized multivariate analysis.

Intramolecular Hydrogen Bonds Assisted Construction of Planar Tricyclic Structures for Insensitive and Highly Thermostable Energetic Materials

Safety is fundamental for the practical development and application of energetic materials. Three tricyclic energetic compounds, namely, 1,3-di(1H-tetrazol-5-yl)-1H-1,2,4-triazol-5-amine (ATDT), 5'-nitro-3-(1H-tetrazol-5-yl)-2'H-[1,3'-bi(1,2,4-triazol)]-5-amine (ATNT), and 1-(3,4-dinitro-1H-pyrazol-5-yl)-3-(1H-tetrazol-5-yl)-1H-1,2,4-triazol-5-amine (ATDNP), were effectively synthesized through a simple two-step synthetic route. The introduction of intramolecular hydrogen bonds resulted in excellent molecular planarity for the three new compounds. Additionally, they exhibit regular crystal packing, leading to numerous intermolecular hydrogen bonds and π-π interactions. Benefiting from planar tricyclic structural features, ATDT, ATNT, and ATDNP are insensitive (IS > 60 J, FS = 360 N) when exposed to external stimuli. Furthermore, ATNT (Td = 361.1 °C) and ATDNP (Td = 317.0 °C) exhibit high decomposition temperatures and satisfying detonation performance. The intermolecular hydrogen bonding that produced this planar tricyclic molecular structure serves as a model for the creation of innovative multiple heterocycle energetic materials with excellent stability.

Combining the advantages of 1,3,4-oxadiazole and tetrazole enables achieving high-energy insensitive materials

Combining the advantages of energetic heterocycles to achieve high-energy insensitive explosives is a significant challenge. Herein, based on high-energy tetrazole rings and highly stable 1,3,4-oxadiazole rings, a series of novel nitrogen rich energetic compounds 5-9 were successfully constructed. The related compounds were fully characterized by EA, FT-IR, NMR, DSC, and MS, and compounds 6-9 were further confirmed by X-ray single crystal diffraction. Among them, the energetic ion salts 6-8 show high thermal stability (Tdec > 250 °C) and low mechanical sensitivity (IS > 40 J, FS > 360 N), as well as good energy properties (7552-8050 m s-1, 19.4-23.3 GPa). In particular, the azo compound 9 exhibits competent comprehensive performances (Tdec = 226.2 °C, D = 8502 m s-1, P = 28.9 GPa, IS = 32 J, FS = 320 N). These results suggest that the strategy of integrating tetrazole and 1,3,4-oxadiazole and employing an azo structure as a bridging unit are effective approaches to construct high-energy insensitive materials.

A multi-fused heat-resistant energetic compound constructed by hydrogen bonds

The design of heat-resistant energetic compounds generally employs symmetry, planarity, and multi-hydrogen bonds to obtain compounds with high density, good thermal stability, and low sensitivity. In this paper, a heat-resistant hydrazine-bridged compound, 6,6'-(hydrazine-1,2-diyl)bis(5-nitropyrimidine-2,4-diamine) (PHP), was designed and synthesized with the strategy of multi-fused conjugated structure constructed by hydrogen bonds. The compound featured high symmetry, high planarity, and strong conjugation with good thermal stability (364 °C). This strategy provides a basis for the design of heat-resistant energetic compounds.

An Efficient One-Step Reaction for the Preparation of Advanced Fused Bistetrazole-Based Primary Explosives

The first example of [5,6,5]-tricyclic bistetrazole-fused energetic materials has been obtained through a one-step reaction from commercial and inexpensive 4,6-dichloro-5-nitropyrimidine. This one-step reaction including nucleophilic substitution, nucleophilic addition, cyclization, and electron transfer is rarely reported, and the reaction mechanism and scope is well investigated. Among target compounds, organic salts exhibit higher detonation velocities (D: 8898-9077 m s^-1) and lower sensitivities (IS: 16-20 J) than traditional high energy explosive RDX (D = 8795 m s^-1; IS = 7.5 J). In addition, the potassium salt of 5-azido-10-nitro-bis(tetrazolo)[1,5-c:5',1'-f]pyrimidin (DTAT-K) possesses excellent priming ability, comparable to traditional primary explosive Pb(N₃)₂, and ultralow minimum primary charge (MPC = 10 mg), which is the lowest MPC among the reported potassium-based primary explosives. The simple synthesis route, free of heavy metal and expensive raw materials, makes it promising to quickly realize this material in large-scale industrial production as a green primary explosive. This work accelerates the upgrade of green primary explosives and enriches future prospects for the design of energetic materials.

Trinitromethyl Energetic Groups Enhance High Heats of Detonation

The introduction of groups with high enthalpies of formation can effectively improve the detonation performance of the compounds. A series of novel energetic compounds (10-13) with high enthalpies of formation, high density, and high nitrogen-oxygen content were designed and synthesized by combining gem-polynitromethyl, 1,2,4-oxadiazole, furoxan, and azo groups. All the new compounds were thoroughly characterized by IR, NMR, elemental analysis, and differential scanning calorimetry. Compounds 10 and 11 were also further characterized with single-crystal X-ray diffraction. Compound 11 has high density (1.93 g cm^-3), high enthalpy of formation (993.5 kJ mol^-1), high detonation velocity (9411 m s^-1), and high heat of detonation (6889 kJ kg^-1) and is a potentially excellent secondary explosive.

Energetic [1,2,5]oxadiazolo [2,3-a]pyrimidin-8-ium Perchlorates: Synthesis and Characterization

A convenient method to access the above perchlorates has been developed, based on the cyclocondensation of 3-aminofurazans with 1,3-diketones in the presence of HClO₄. All compounds were fully characterized by multinuclear NMR spectroscopy and X-ray crystal structure determinations. Initial safety testing (impact and friction sensitivity) and thermal stability measurements (DSC/DTA) were also carried out. Energetic performance was calculated by using the PILEM code based on calculated enthalpies of formation and experimental densities at r.t. These salts exhibit excellent burn rates and combustion behavior and are promising ingredients for energetic materials.

Toward Advanced High-Performance Insensitive FOX-7-like Energetic Materials via Positional Isomerization

For an energetic molecule with a definite elemental composition, the substituent type and position are the most important factors to influence its detonation performance and mechanical sensitivities. In this work, two pairs of FOX-7-like energetic isomers based on (2 and HTz-FOX; 5 and 6) were synthesized and characterized. Through positional isomerization, advanced high-performance insensitive explosives were obtained. Compounds 2 and 5 with an amino group adjacent to the electron-withdrawing side of the ethene bridge show both higher thermal stability and lower mechanical sensitivities (2: Td = 258 °C, impact sensitivity (IS) = 25 J, and friction sensitivity (FS) = 300 N; 5: Td = 264 °C, IS = 30 J, and FS = 320 N). In addition, 2 shows ultrahigh detonation performance (Dv = 9224 m s-1 and P = 31.1 GPa). These promising physicochemical properties are comparable to those of HMX (Dv = 9193 m s-1, P = 37.8 GPa, Td = 275 °C, IS = 7.4 J, and FS = 120 N), which suggests that 2 may be a promising energetic material in future applications.

Assessing p-tolyloxy-1,3,4-oxadiazole acetamides as lipoxygenase inhibitors assisted by in vitro and in silico studies

The underlying correlation between the inflammation, innate immunity and cancer is extensively familiar and linked through a process mediated by three enzymes; cyclooxygenase (COX), lipoxygenase (LOX) and cytochrome P450 (CYP₄₅₀). The ever increase in the reported side effects of the antiinflammatory drugs against the targeted enzymes and the resistance developed afterwards compels the researchers to synthesize new effective molecules with safer profile. On the basis of these facts, our ongoing research on 1,3,4-oxadiazole derivatives deals with the synthesis of a new series of N-alkyl/aralky/aryl derivatives of 5-((p-tolyloxymethyl)-4H-1,3,4-oxadiazole-2-ylthio)acetamide (6a-o) which were developed by the sequential conversion of p-tolyloxyacetic acid (a) into ester (1) hydrazide (2) and 5-(p-tolyloxymethyl)-4H-1,3,4-oxadiazole-2-thiol (3). The designed compounds (6a-o) were acquired by the reaction of 1,3,4-oxadiazole (3) with numerous electrophiles (5a-o) in KOH. The synthesized analogues (6a-o) were characterized by FTIR, ¹H-, ¹³C NMR spectroscopy, EI-MS and HR-EI-MS spectrometry, and were further assessed for their inhibitory potential against the soybean 15-LOX enzyme. The results showed excellent inhibitory potential of the compounds against the said enzyme, specifically 6o, 6b, 6n and 6e with inhibitory values (IC₅₀ ± SEM) of 21.5 ± 0.76, 24.3 ± 0.45, 29.1 ± 0.65 and 31.3 ± 0.78 µM, respectively. These compounds displayed < 55 % blood mononuclear cells (MNCs) cellular viability as measured by MTT assay at 0.25 mM concentration. Other compounds demonstrated moderate inhibitory activities with IC₅₀ values in the range of 33.2 ± 0.78 to 96.3 ± 0.73 µM and exhibited little cellular viability against MNCs except 6i, 6j, 6 m and 6 k that showed 61-79 % cellular viability. It was observed that most of the compounds (6o, 6b, 6n, 6e) were found more toxic towards MNCs at studied concentration of 0.25 mM. SAR studies revealed that the positions and nature of substituents accompanying phenyl ring have great influence on 15-LOX inhibitory activity. In the most active compound 6o, the amino acids Asp768 and Val126 were involved in hydrogen bonding, Thr529 was linked with π-anion interaction and π-sulphur interaction was displayed with Tyr525 and two π-alkyl interactions were formed with the benzene ring and amino acid residues Pro530 and Arg533. The in silico pharmacokinetics profiles and density functional theory calculations of the compounds further supported the in vitro findings. Further work on the synthesis of more oxadiazole derivatives is in progress in search for potential 'leads' for the drug discovery as LOX inhibitors.

From Nitro- to Heterocycle-Functionalized 1,2,4-Triazol-3-one Derivatives: Achieving High-Performance Insensitive Energetic Compounds

5-Nitro-1,2,4-triazol-3-one, a nitro-functionalized 1,2,4-triazol-3-one (TO) derivative, shows excellent energetic properties and promising application potential. However, the use of the TO skeleton as an energetic material is still largely underexplored both theoretically and practically. We report here a mild and efficient method for obtaining the TO skeleton via a reaction of aminocarbohydrazide with BrCN. Two energetic compounds (2 and 5) were synthesized and fully characterized by ¹⁵N nuclear magnetic resonance, two-dimensional ¹H-¹⁵N heteronuclear multiple-bond correlation, and single-crystal X-ray diffraction. The reaction mechanism was also studied with the aid of quantum calculations. Compound 2 shows promising properties as a high-performance insensitive energetic material.

Polynitro-Functionalized Azopyrazole with High Performance and Low Sensitivity as Novel Energetic Materials

The development of energetic materials is still facing a huge challenge because the relationship between energy and sensitivity is usually contradictory: high energy is always accompanied with low sensitivity. Here, a high-energy, low-sensitivity energetic polynitro-functionalized azopyrazole (TNAP) and its energetic salts have been synthesized. The structural characterization of these compounds was analyzed by elemental analysis, ¹H and ¹³C NMR spectroscopies, and infrared spectroscopy. The single-crystal structure of compounds K₂TNAP, TNAP, 5, and 6 was obtained by X-ray diffraction, and K₂TNAP is a novel energetic metal-organic framework. The calculated detonation properties of TNAP (9040 m s^-1 and 36.0 GPa) are superior to that of RDX (8796 m s^-1 and 33.6 GPa). In addition, TNAP also has lower mechanical sensitivity (IS > 40 J, FS = 244 N) and higher decomposition temperature (T_d = 221 °C) than RDX (IS = 7.4 J, FS = 120 N, and T_d = 204 °C). These experimental results suggest that TNAP may become a new candidate for secondary explosives.

Synthesis, structure and properties of a high-energy metal–organic framework fuel [Cu(MTZ)2(CTB)2]n

In this study, a novel high-energy metal–organic framework (MOF, [Cu(MTZ)2(CTB)2]n) was constructed based on the nitrogen-rich cyanotetrazolylborohydride (CTB) and 1-methyltriazole (MTZ) ligands, with Cu2+ as the autocatalytic metal centers.

Energetic isomers of bridged oxadiazole nitramines: the effect of asymmetric heterocyclics on stability and energetic properties

Energetic isomers often exhibit different properties. To understand the effect of arrangement and connection of isomers on energetic properties and sensitivity, in this study, we designed and synthesized a series of oxadiazole nitramine compounds including N-(5-(5-(nitramino)-1,3,4-oxadiazol-2-yl)-1,2,4-oxadiazol-3-yl)nitramide (NOON) and its ionic derivatives. NOON exhibits comparable performance (D = 8888 m s^-1, P = 34.1 GPa) to highly explosive RDX. A comparative study of detonation properties, sensitivity, and thermal stability of the three oxadiazole nitramine isomers (NOON, ICM-101, and DNBO) is carried out. The results show that due to the proton transformation, strong intramolecular hydrogen bonding interaction, and formation of six-membered ring conformation, the 2-nitramino-1,3,4-oxadiazole building block exhibits better detonation properties and higher thermal stability than its isomer 2-nitramino-1,2,4-oxadiazole.

Construction of Coplanar Bicyclic Backbones for 1,2,4-Triazole-1,2,4-Oxadiazole-Derived Energetic Materials

Combining different nitrogen-rich heterocycles into a molecule can fine-tune its energetic performance and physical properties as well as its safety for use in energetic materials. Here, 1,2,4-oxadiazole was incorporated into 1,2,4-triazole to construct new energetic backbones. 3-(5-Amino-1H-1,2,4-triazol-3-yl)-1,2,4-oxadiazol-5-amine (5) was designed and synthesized. Nitramino-functionalized N-(5-(5-amino-1,2,4-oxadiazol-3-yl)-3H-1,2,4-triazol-3-yl)nitramide (6) and N-(5-(5-(nitramino)-1,2,4-oxadiazol-3-yl)-3H-1,2,4-triazol-3-yl)nitramide (7) were also obtained, and two series of corresponding nitrogen-rich salts were prepared, leading to the creation of new energetic compounds. All derivatives were fully characterized, and five of them were further confirmed by X-ray diffraction. The theoretical calculations, energetic performance, safety, and the main decomposition gaseous products of 1,2,4-triazole-1,2,4-oxadiazole-derived energetic materials were studied. Compound 7 and its dihydroxylammonium salt (7 c) exhibited prominent detonation performance comparable to that of RDX while possessing satisfying thermal stabilities and mechanical sensitivities.

Effect of multiple oxadiazole rings with nitro and nitramino functionalities on energetic properties: computational analysis of the structure–property relationship

The oxadiazole ring is an essential nitrogen-containing heterocycle and a valuable building block that is used in developing numerous functionalized energetic compounds.