A Study of 5-(1,2,4-Triazol-C-yl)tetrazol-1-ols: Combining the Benefits of Different Heterocycles for the Design of Energetic Materials

Electrochemistry-enabled residue-specific modification of peptides and proteins

Selective chemical reactions at precise amino acid residues of peptides and proteins have become an exploding field of research in the last few decades. With the emerging utility of bioconjugated peptides and proteins as drug leads and therapeutic agents, the design of smart protocols to modulate and conjugate biomolecules has become necessary. During this modification, the most important concern of biochemists is to keep intact the structural integrity of the biomolecules. Hence, a soft and selective biocompatible reaction environment is necessary. Electrochemistry, a mild and elegant tunable reaction platform to synthesize complex molecules while avoiding harsh and toxic chemicals, can provide such a reaction condition. However, this strategy is yet to be fully exploited in the field of selective modification of polypeptides. With this possibility, the use of electrochemistry as a reaction toolbox in peptide and protein chemistry is flourishing day by day. Unfortunately, there is no suitable review article summarizing the residue-specific modification of biomolecules. The present review provides a comprehensive summary of the latest manifested electrochemical approaches for the modulation of five redox-active amino acid residues, namely cysteine, tyrosine, tryptophan, histidine and methionine, found in peptides and proteins. The article also highlights the incredible potential of electrochemistry for the regio- as well as chemoselective bioconjugation strategy of biomolecules.

Investigation into a Conformationally Locked (Z)-Azidoxime

High nitrogen compounds find wide use in the development of new propellants and explosives as well as pharmaceutical chemistry as bioisosteres, bacterial stains, and antifungal agents. A class of underexplored high-nitrogen materials includes azidoximes and their 1-hydroxytetrazole isomers. Azidoximes possess an energetic azide group and are quite sensitive to impact, spark, and friction. Therefore, these materials are generated in situ and cyclized under mild acidic conditions to their 1-hydroxytetrazole isomers. Recently, we synthesized a novel 1,2,4-triazine-derived azidoxime; however, upon subjecting this material to established cyclization conditions, no reaction was observed, even after prolonged reaction times with heating. Additional 1,2,4-triazine-derived azidoximes also displayed a similar lack of reactivities. This observation led us to probe the reactivity of these materials with both a DFT investigation and crystallographically based electrostatic potential mapping. In all, the lack of reactivity toward cyclization was found to be due to an inability of 1,2,4-triazine-based azidoximes to isomerize into the reactive (E)-conformation, requiring an activation energy of 26.4 kcal mol-1.

Synthesis and thermal stability of a novel polyfunctional pyridine-based derivative featuring amino, nitro, and guanidine groups

A new pyridine derivative 1-(4-amino-3,5-dinitropyridin-2-yl) guanidine (ADG) was synthesized through nitration and substitution reaction using 4-amino-2-cholopyridine as raw material. Its structure was characterized by spectroscopy, nuclear magnetic resonance, and mass spectrometry. The crystal of ADG is monoclinic, space group Pbca with crystal parameters of a = 7.23 Å, b = 14.56 Å, c = 17.93 Å, α = β= γ= 90 Å, V = 1911 Å3, μ = 0.143 mm−1, and Z = 8. This new molecule exhibits moderate density ( ρ= 1.677 g cm−3), good thermal stability with a decomposition temperature of 217 °C, and moderate detonation property with detonation pressure of 20.83 GPa and detonation velocity of 7.00 km s−1. The Hirshfeld surface analysis revealed a significant contribution of weak interactions to the packing forces for molecules. Electron localization function analysis indicates that there is substantial electron localization in regions of C–H, N–H, and N–O covalent bond sites.

Oxidation of o-dioxime by (diacetoxyiodo)benzene: green and mild access to furoxans

Diversified furoxan derivatives are efficiently obtained through a mild oxidation strategy, which greatly reduces the safety risk.

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.

1,3,4,5-Tetraamino-1,2,4-triazolium Cation: An Energetic Moiety

The amination of 3,4,5-triamino-1,2,4-triazole with O-tosylhydroxylamine yielded the nitrogen-rich 1,3,4,5-tetraamino-1,2,4-triazolium cation as its tosylate salt. Subsequent metathesis reactions produced energetic salts with various energetic anions, including perchlorate, nitrate, nitrotetrazolate, and bistetrazolate diolate. All energetic salts possess relatively high heats of formation, thermal sensitivities, and detonation velocities and pressures. The prepared energetic salts were characterized chemically using single-crystal X-ray crystallography, elemental analysis, and ¹H NMR, ¹³C NMR, and IR spectroscopy and energetically by measuring their thermal, impact, and friction sensitivities. ¹⁵N NMR was carried out on the tosylate salt. Energetic performances were determined by a combined experimental-computational method using calculated heats of formation and experimental crystal densities.

Acidic Stabilization of the Dual-Aromatic Heterocyclic Anions

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second σ-aromaticity in addition to the prototypical π-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional π-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid−base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N5¯, tetrazole anion N4C1H1¯, and 1,2,4-triazole anion N3C2H2¯) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid−base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

Combining 1,2,4-triazole and pyrazole frameworks for new insensitive energetic materials

A new symmetric compound 3,5-bis(3-amine-4H-1,2,4-triazole)-1H-pyrazole and a series of corresponding energetic cationic salts were synthesized.

A Family of Energetic Materials Based on 1,2,4-Oxadiazole and 1,2,5-Oxadiazole Backbones With Low Insensitivity and Good Detonation Performance

Design and synthesis of new compounds with both high detonation performances and good safety properties have always been a formidable task in the field of energetic materials. By introducing -ONO₂ and -NHNO₂ moieties into 1,2,4-oxadiazole- and 1,2,5-oxadiazole-based backbones, a new family of energetic materials, including ammonium 3-nitramino-4-(5-hydroxymethyl-1,2,4-oxadiazol-3-yl)-furazan (4), 3,3′-bis[5-nitroxymethyl-1,2,4-oxadiazol-3-yl]-4,4′-azofuroxan (6), [3-(4-nitroamino-1,2,5-oxadiazol-3-yl)-1,2,4-oxadiazol-5-yl]-methylene nitrate (8), and its energetic ionic salts (10–12), were synthesized and fully characterized. The energetic and physical properties of the materials were investigated through theoretical calculations and experimental determination. The results show that the oxadiazole-based compounds exhibit high enthalpy of formations, good detonation performances, and extraordinary insensitivities. In particular, the hydrazinium salt (11) shows the best energetic properties (11: d = 1.821 g cm⁻³; P = 35.1 GPa, v_D = 8,822 m s⁻¹, IS = 40 J, FS > 360N). The ESP and Hirshfeld surface analysis indicated that a large number of hydrogen bonds as well as π-π stacking interactions within molecules might be the key reason for their low sensitivities and high energy-density levels.

Evolution of Oxidizing Inorganic Metal Salts: Ultrafast Laser Initiation Materials Based on Energetic Cationic Coordination Polymers

An effective and novel design strategy for ultrafast laser-initiating materials has been established on the basis of coordination chemistry for the first time in the present work. In view of the positive effect of Ag ion and perchlorate on laser sensitivity, silver perchlorate as a representative of oxidizing inorganic metal salts was used to construct energetic cationic coordination polymers (ECCPs), which solved the inconvenient situation caused by the difficulty in applying these salts directly in energetic materials because of the unavoidable hygroscopicity and the inhomogeneity of physical mixtures of oxidants and reductants. With the nonenergetic nitrogen-rich ligand 3-amino-1H-1,2,4-triazole-5-carbohydrazide (ATCA), one new laser-sensitive Ag(I)-based ECCP [Ag(ATCA)ClO₄]_n (1) was successfully synthesized with a compact helical structure proved by X-ray single-diffraction crystal data. The physicochemical property evaluation revealed that this Ag-ECCP was not only completely devoid of the undesirable properties of the silver perchlorate and displayed excellent tolerance to moisture and noncorrosive properties to metal shells, but was also endowed with good thermal stability and excellent safety for mechanical stimulation. Moreover, theoretical calculations based on the standard molar enthalpy of formation and the lead plate explosive test as the actual damage experiment have proved that the compound has a superior detonation performance (up to 6800 m s^-1 and 0.511 kcal g^-1) compared to the traditional primary explosives. More importantly, the laser-initiation-experiment-based femtosecond laser-testing system and high-speed photography demonstrated that this ECCP was an energetic material with great potential for application in the safety detonator as an ultrafast photosensitive initiating material for laser direct initiation, whose initiation delay time is as low as 73 ms using only 200 mJ initiation energy.

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.

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.

Synthesis of high-performance insensitive energetic materials based on nitropyrazole and 1,2,4-triazole

A new family of symmetric nitropyrazole and 1,2,4-triazole derivatives and its energetic salts were obtained. The positive effect of ternary hydrogen bonds improve the performances of target compounds.

Theoretical investigations on azole-fused tricyclic 1,2,3,4-tetrazine-2-oxides

Fused compounds, a unique class of large conjugate structures, have emerged as prime candidates over traditional nitrogen-rich mono-ring or poly-ring materials. Meanwhile, compounds containing catenated nitrogen chains have also attracted attention from scientists due to their high heats of formation. On the other hand, the azoxy [–N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N(O)–] moiety has been found to increase density effectively in the molecular structure of compounds. Therefore, combining fused heterocyclic organic skeletons with the azoxy moiety can be regarded as an effective method for increasing the density and heat of formation, which results in substantial increase in detonation properties. Based on the above-mentioned considerations, in this study, a series of new non-hydrogen-containing 5/6/5 fused ring molecules with azoxy moiety structures are designed. Furthermore, their properties as potential high-energy-density materials, including their density, heats of formation, detonation properties, and impact sensitivity, have been extensively evaluated using thermodynamic calculations and density functional theory. Among the investigated compounds, 1,3,8,10-tetranitrodiimidazo[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (B), 1,10-dinitrobis([1,2,3]triazolo)[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (C) and 2,9-dinitrobis([1,2,4]triazolo)[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (D) display remarkable stabilities and are predicted to be high-performance energetic materials due to their high density (>1.94 g cm⁻³), detonation velocity (>9616 m s⁻¹), and detonation pressure (>41.1 GPa). In addition, our design strategy, which combines the azoxy moiety and fused tricyclic skeleton to construct nitrogen-rich molecular structures with high density and positive heat of formation, is a valuable approach for developing novel high-energy-density materials with excellent performance and stability.

In this work, a series of novel non-hydrogen-containing 5/6/5 fused ring molecules with azoxy moiety were designed, some physicochemical or detonation properties of them were calculated.

Asymmetric nitrogen-rich energetic materials resulting from the combination of tetrazolyl, dinitromethyl and (1,2,4-oxadiazol-5-yl)nitroamino groups with furoxan

Three classes of nitrogen-rich energetic compounds were obtained from 3,4-dicyano-furoxan.

Explosives in the Cage: Metal-Organic Frameworks for High-Energy Materials Sensing and Desensitization

An overview of the current status of coordination polymers and metal-organic frameworks (MOFs) pertaining to the field of energetic materials is provided. The explosive applications of MOFs are discussed from two aspects: one for detection of explosives, and the other for explosive desensitization. By virtue of their adjustable pore/cage sizes, high surface area, tunable functional sites, and rich host-guest chemistry, MOFs have emerged as promising candidates for both explosive sensing and desensitization. The challenges and perspectives in these two areas are thoroughly discussed, and the processing methods for practical applications are also discussed briefly.

An interesting 1,4,2,5-dioxadiazine-furazan system: structural modification by incorporating versatile functionalities

An N-trinitroethylamino derivative and amminonium salt based on the 1,4,2,5-dioxadiazene-furazan system are synthesized and characterized. Due to their layer-by-layer structure, these two compounds show good thermal stabilities and low sensitivities.

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.

Theoretical studies on the derivatives of tris([1,2,4]triazolo)[4,3-a:4′,3′-c:4′′,3′′-e][1,3,5]triazine as high energetic compounds

Tris([1,2,4]triazolo)[4,3-a:4′,3′-c:4′′,3′′-e][1,3,5]triazine inherited from triazole and triazine is suggested theoretically as a new framework for insensitive and high-energetic materials.

A DFT study of five-membered nitrogen-containing fused heterocycles for insensitive highly energetic materials

A suitable combination of aromatic rings, hydrogen bonds and layered crystal packing can be applied to obtain new compounds with both power and safety aspects.

N-Trinitroethyl-substituted azoxyfurazan: high detonation performance energetic materials

An N-nitrated trinitroethylamino azoxyfurazan derivative reported here possesses oxygen balance of near zero (+2.5%) and exhibits outstanding properties (Δ_fH_m, 962.1 kJ mol⁻¹; D, 9458 m s⁻¹; P, 41.2 GPa).

A novel cocrystal explosive NTO/TZTN with good comprehensive properties

A novel binary cocrystal explosive of NTO was discovered and it exhibits strong intermolecular hydrogen bond interaction in crystal structure.