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

Exploring new frontiers: alliance of pyrazole and thiadiazole in energetic materials

A series of high-performing C-C bonded nitropyrazole and thiadiazole-based energetic materials (compounds 3-7) were synthesized and thoroughly characterized. SC-XRD studies supported the structure of compounds 3, 4, 6, and 7. The synthesized compounds exhibited high densities (≥1.77 g cm-3), compounds 3 and 5 demonstrating admirable detonation properties (VOD = 8300 and 7265 m s-1; DP = 30.31 and 21.25 GPa, respectively), surpassing the present benchmark explosives HNS and TNT and setting new standards for sulfur-based energetic materials. Notably, compound 3 showed an ignition delay of 13 ms in a hot needle test, indicating its potential as an igniter.

Revisiting the sensitive nature of H-FOX: interplay of nitro and hydrazine functionalities to construct an insensitive energetic material

In this study, we successfully synthesized and characterized (2-nitroethene-1,1-diyl)bis(hydrazine) (2) in a single step with high yield. Its dual functionality was evaluated as both a hypergolic fuel with WFNA and a secondary explosive comparable to RDX. Offering a safer alternative to H-FOX, it exhibits a decomposition temperature of 160 °C, density 1.60 g cm-3, detonation velocity 8548 m s-1, and specific impulse 220 s, with excellent insensitivity to impact and friction, making it a promising candidate for hybrid propulsion and energetic materials.

Balancing the Energy and Sensitivity of Primary Explosives: Using Isomers to Prepare Energetic Coordination Compounds

The performance of energetic coordination compounds (ECCs) is influenced by their components and structure. Modifying the chemical structure of the ligands can balance the detonation performance and sensitivity. This study introduced Cu(3-PZCA)2(ClO4)2 (ECCs-1) and Cu(2-IZCA)2(ClO4)2 (ECCs-2), using 3-PZCA and 2-IZCA as ligands. ECCs-2, with a higher symmetry and fewer nitrogen chains, showed the highest thermal decomposition temperature (225 °C). Both ECCs displayed high mechanical sensitivity, with ECCs-2 being slightly less sensitive (IS = 3 J, FS = 8 N). They shared similar detonation properties and ignition capabilities, with ECCs-1 having the highest detonation velocity (7.1 km·s-1) and pressure (23.5 GPa). Initiation tests confirmed their excellent performance and similar DDT. The theoretical decomposition mechanism suggests a free radical reaction, explaining their consistent mechanical sensitivity, ignition, and initiation capabilities. A "SP-DM-DSC-MS-DA" structure-property relationship was established, providing a theoretical foundation for studying Cu(ClO4)2-ECCs and their isomers.

Adjusting the Coordination Configuration by Changing Electrostatic Potential: Introducing N/O/S Heteroatoms Based on the Electronic Effect

Energetic coordination compounds (ECCs) have demonstrated unique advantages in regulating the physicochemical properties of energetic materials through the design of organic ligands. The fundamental approach involves altering the electron cloud density distribution of organic ligands to modify the characteristics of coordination sites and, thus, achieve new coordination configurations. In this study, Mulliken charge distribution and surface electrostatic potential analysis were used to elucidate the effects of pyridinic N, pyrrolic N, oxazolic O, and thiazolic S on the electron cloud density of carbohydrazide groups through the induction effect and conjugate effect. Furthermore, three AgClO4-based ECCs were synthesized based on 1H-imidazole-4-carbohydrazide, oxazole-4-carbohydrazide, and thiazole-4-carbohydrazide. Single-crystal X-ray diffraction analysis revealed that [Ag(IZ-4-CA)ClO4]n has a one-dimensional (1D) chain structure, while Ag2(OZCA)2(ClO4)2 and Ag2(SZCA)2(ClO4)2 exhibit zero-dimensional structures. The 1D structure, with good planarity, results in [Ag(IZ-4-CA)ClO4]n having lower mechanical sensitivity (IS = 21 J, FS = 80 N). The introduction of oxazolic O enhances oxygen balance (OB), leading to a higher predicted detonation velocity and pressure for Ag2(OZCA)2(ClO4)2 (D = 6.4 km s-1, P = 23.6 GPa). Although the introduction of thiazolic S is unfavorable for improving oxygen balance, Ag2(SZCA)2(ClO4)2 exhibits the highest initial decomposition temperature among the three, at 232 °C. Additionally, initiation tests demonstrated that three ECCs can successfully detonate cyclotrimethylenetrinitramine (RDX), indicating good initiation capabilities.

Synthesis of 4-Amino-5-nitro-7H-pyrazolo[3,4-d][1,2,3]triazine-2-oxide and Its Heat-Resistant Derivatives

4-Amino-5-nitro-7H-pyrazolo[3,4-d][1,2,3]triazine-2-oxide (PTO) is synthesized via one step in this study. Subsequently, 4,7-diamino-5-nitro-pyrazolo[3,4-d][1,2,3]triazine-2-oxide (APTO), 4-oxo-5-nitro-7H-pyrazolo[3,4-d][1,2,3]triazine-2-oxide (OPTO), and several heat-resistant salts are synthesized through local structural modifications on PTO. Comparison of thermal stability between PTO and APTO indicates that while the amino group has a negative impact on the thermal stability of APTO, it enhances the detonation performance of APTO and effectively reduces its mechanical sensitivity. Our findings provide a practical new approach for constructing energetic materials with excellent stability.

Skeletal Editing of Energetic Materials: Acid-Catalyzed One-Step Synthesis of Bridged Triazoles as High-Energy-Density Materials via the Nef Reaction

Thermally stable insensitive energetic materials have captivated significant attention from the global research community due to their potential impact. In this study, a series of symmetric and asymmetric nitromethyl-bridged triazole compounds were synthesized from pyrimidine moieties via a skeletal editing approach. Additionally, carbonyl-bridged compounds were synthesized in a single step by using acid-catalyzed Nef reactions from their nitromethyl precursors. Peripheral modifications of pyrimidine resulted in fused energetic moieties. All synthesized compounds were fully characterized by using infrared spectroscopy, high-resolution mass spectrometry, multinuclear magnetic resonance spectroscopy, elemental analysis, and differential scanning calorimetry. Single-crystal X-ray diffraction analysis confirmed the structures of compounds 4 and 10. The newly synthesized moieties exhibit densities ranging from 1.75 to 1.86 g cm-3, detonation velocities between 8044 and 8608 m s-1, and detonation pressures between 23.10 and 30.31 GPa. Notably, compounds 9 and 10 demonstrate exceptional heat resistance, with decomposition temperatures of 315 and 335 °C, respectively. Computational studies, including density functional theory, quantum theory of atoms in molecules, noncovalent interactions, and electrostatic surface potential analysis, account for hydrogen-bonding and noncovalent interactions. This work highlights the potential of skeletal editing in the development of high-performing, thermally stable energetic materials.

Polynitro-1,2,4-triazole Energetic Materials with N-Amino Functionalization

Trinitromethyl and N-amino groups were innovatively incorporated into the framework of 1,2,4-triazole, resulting in 1-amino-5-nitro-3-(trinitromethyl)-1,2,4-triazole (2). Ammonium and hydrazinium salts of 1-amino-5-nitro-3-(dinitromethyl)-1,2,4-triazole were synthesized by acidification, extraction, and neutralization with bases from the potassium salt. All of the newly prepared energetic compounds were comprehensively characterized by using infrared spectroscopy, elemental analysis, nuclear magnetic resonance spectroscopy, and single crystal X-ray diffraction. Compound 2 exhibits favorable properties such as positive oxygen balance (OBCO2 = 5.8%), high density (1.88 g cm-1), good detonation performances (vD = 8937 m s-1, P = 35.5 GPa), and appropriate friction sensitivity (FS = 144 N). The potassium salt 3 demonstrates good thermal decomposition temperature (181 °C) and high density (1.98 g cm-1), while the ammonium salt and hydrazinium salt also display good thermal decomposition temperatures of 183 and 176 °C, respectively. Among these compounds, the ammonium salt exhibits the lowest mechanical sensitivities (FS = 144 N, IS = 6 J).

Solvent Vapor/Gas-Induced Guest Transport and Exchange of a Nonporous Organic Crystal to Construct Smart Host-Guest Energetic Materials

Supramolecular materials with advanced properties constructed by intermolecular interactions have attracted extensive attention in many fields, such as sensing, catalysis, and biomedicine. However, in the field of energetic materials, limited by the tight-packed crystal structure of explosives and the strong intermolecular interaction forces, most supramolecular explosives can only be obtained in organic solution or under extreme external loading (high temperature/high pressure). Given the practical issues such as safety risks, operational difficulties, serious environmental pollution, and large-scale production of the existing technology, a new method of constructing host-guest explosives by solvent vapor/gas induction is proposed. This gas-solid reaction method takes advantage of the metastable properties from the explosives solvate (HNIW/ACN), and cleverly opens a fast channel for gas molecules to enter the explosives cell cavities, which results in the highly efficient preparation of the host-guest explosives (HNIW/CO2 and HNIW/N2O). The embedding of functional gas molecules greatly improves the structural stability and comprehensive performance of the explosive skeleton, and the detonation velocity of HNIW/N2O even reaches 9802 m·s-1, which is higher than that of ε-HNIW (9455 m·s-1). In addition, compared with ε-HNIW, HNIW/CO2 and HNIW/N2O exhibit high energy but low sensitivity, enhanced thermal stability, and combustion properties, which present a potential prospect in the field of energetic materials. The new method effectively overcomes the high-energy barrier of nonporous organic explosives, offering the advantages of simplicity, safety, efficiency, and environmental friendliness. This study provides a valuable pathway for constructing advanced supramolecular energetic materials, which contributes to the enrichment of supramolecular engineering systems.

Understanding the Stability of Highly Nitrated Sensitive Pyrazole Isomers

Two energetic isomers of chemically unstable 3,5-bis(dinitromethyl)-4-nitro-1H-pyrazole (2), namely, 4-methyl-3,5-dinitro-1-(trinitromethyl)-1H-pyrazole (4) and 5-methyl-3,4-dinitro-1-(trinitromethyl)-1H-pyrazole (6), each containing five nitro groups and having the same chemical composition, exhibit major differences in their physiochemical properties. These include density, enthalpy of formation, temperature of decomposition, and sensitivity to impact and friction. Notably, both isomer 4 and isomer 6 demonstrate superior thermal stability compared to isomer 2, making them promising candidates as safer energetic materials.

Single Step Synthesis of gem-Dinitro Methyl-1,2,4-triazole and Its Hydroxylamine Salt: An Alternative to the FOX-7 and Other Benchmark Explosives

gem-Dinitro methyl based high-energy-density material 5-(dinitromethylene)-4,5-dihydro-1H-1,2,4-triazole (2) and its hydroxylamine salt (4) were synthesized for the first time in a single step and characterized. Further, the structure of 2 was confirmed by single-crystal X-ray diffraction (SCXRD) studies. Interestengly, both the compounds show excellent density (> 1.83 g cm-3), detonation velocity (> 8700 m s-1), pressure (> 30 GPa) and are insensitive toward mechanical stimuli such as impact and friction sensitivity. Considering their synthetic fesibility and balanced energetic performance, compounds 2 and 4 show future prospects as potential next-generation energetic materials for the replacenent of many presently used benchmark high energy density materials such as RDX, FOX-7 and highly insensitive H-FOX.

Obtaining superior high-density fused-ring energetic materials via the introduction of carbonyl, o-NH2-NO2 and nitroamino groups

Two carbonyl and o-NH2-NO2-containing energetic materials and their analogues were effectively designed, synthesized and fully characterized with multinuclear NMR, IR and elemental analyses. Their structures were also further confirmed via X-ray diffraction. Among them, compound 7 exhibits good potential for application as a secondary explosive with extremely high density (2.04 g cm-3), good sensitivity (IS > 40 J, FS > 360 N), and excellent calculated detonation performance (Dv = 8943 m s-1, P = 35.0 GPa). Furthermore, a detailed comparative study based on X-ray diffraction, Hirshfeld surfaces and 2D fingerprint plots among compounds 4, 7 and 9 has demonstrated that the density and detonation performance could be effectively improved via introducing a carbonyl group into fused-ring compounds. More importantly, the sensitivity of the resulting energetic materials did not deteriorate. Obviously, this strategy via introducing carbonyl, o-NH2-NO2 and nitroamino groups into fused-ring energetic compounds will help in the design of next-generation high-energy and insensitive fused-ring energetic materials.

Controlling the Energetic Properties of N-Methylene-C-Linked 4-Hydroxy-3,5-dinitropyrazole- and Tetrazole-Based Compounds via a Selective Mono- and Dicationic Salt Formation Strategy

In the quest to synthesize high-performing insensitive high-energy density materials (HEDMs), the main challenge is establishing an equilibrium between energy and stability. For this purpose, we explored 4-hydroxy-3,5-dinitropyrazole- and tetrazole-based energetic scaffolds connected via a N-methylene-C bridge. The hydroxy functionality between nitro groups on the pyrazole ring promotes physical stability via inter- and intramolecular hydrogen bonding and contributes to oxygen balance, supporting better energetic performance. Due to two acidic sites (OH and NH) with different reactivities, a series of monocationic and dicationic salts were synthesized, and their overall performance was compared. All compounds synthesized in this study have high physical stability with impact sensitivity >40 J and friction sensitivity >360 N. Monocationic salts were generally found to have better thermal stability with respect to their corresponding dicationic energetic salts, which showed better energetic performance. The salt formation strategy effectively improved the thermal stability of 2 (Td = 168 °C), where most energetic salts have decomposition temperatures higher than 220 °C. All of the compounds were characterized through IR, multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), and elemental analysis. The structure-property relationship was studied using Hirshfeld surface analysis, noncovalent interaction (NCI) analysis, and electrostatic potential studies.

Regulating the Coordination Environment by Using Isomeric Ligands: Enhancing the Energy and Sensitivity of Energetic Coordination Compounds

Transforming the energy storage structure is an effective approach to achieve a balance between the detonation performance and the sensitivity of energetic compounds, with a goal of high energy and low sensitivity. Building upon previous work, this study employed an isomeric compound 1H-pyrazole-3-carbohydrazide (3-PZCA) as a ligand and creatively designed the energetic coordination compound (ECC) Ag(3-HPZCA)2(ClO4)3 (ECC-1). It is a novel material with a dual structure of ionic salts and coordination compounds, which represents the first report of such a structure in Ag(I)-based ECCs. With its unique structures, ECC-1 exhibits a larger [ClO4-] content, a higher oxygen balance constant (OB = 0%), and superior mechanical sensitivity (IS = 13 J and FS = 40 N). Theoretical calculations indicate that ECC-1 has a higher detonation performance compared to previous work. Furthermore, the explosive experiment testing results demonstrate that it can be ignited by lower-threshold lasers and possesses excellent initiation capability and explosive power, making it suitable not only as a primary explosive but also as a secondary explosive.

A Dihydrazone as a Remarkably Nitrogen-Rich Thermostable and Insensitive Energetic Material

Hydrogen bonds (H-bonds) in energetic compounds have a very pronounced effect on physicochemical properties such as density, thermal stability, sensitivity, and solubility. Now a strategy to synthesize nitrogen-rich energetic materials with overall good properties, which stem from the synergetic effects of inter- or intramolecular H-bonds, is reported. 1,2-Dihydrazono-1,2-di(1H-tetrazol-5-5-yl)ethane (4), a new thermostable and insensitive material, is obtained from the reaction of dioxime (2) with hydrazine hydrate. The exchange of the oxime (NOH) with the hydrazone (NNH2) functionality results in the reduced acidic character and low solubility in water, which make it remarkably suitable for practical use. While the detonation velocity of 4 is comparable with RDX, it has an advantage of high nitrogen content (76%) and high thermal stability (275 °C) and is insensitive toward external stimuli.

Thermal stability of azole-rich energetic compounds: their structure, density, enthalpy of formation and energetic properties

Energetic compounds, as a type of special material, are widely used in the fields of national defense, aerospace and exploration. Their research and production have received growing attention. Thermal stability is a crucial factor for the safety of energetic materials. Azole-rich energetic compounds have emerged as a research hotspot in recent years owing to their excellent properties. Due to the aromaticity of unsaturated azoles, many azole-rich energetic compounds have significant thermal stability, which is one of the properties that researchers focus on. This review presents a comprehensive summary of the physicochemical and energetic properties of various energetic materials, highlighting the relationship between thermal stability and the structural, physicochemical, and energetic properties of azole-rich energetic compounds. To improve the thermal stability of compounds, five aspects can be considered, including functional group modification, bridging, preparation of energetic salts, energetic metal-organic frameworks (EMOFs) and co-crystals. It was demonstrated that increasing the strength and number of hydrogen bonds of azoles and expanding the π-π stacking area are the key factors to improve thermal stability, which provides a valuable way to develop energetic materials with higher energy and thermal stability.

Triazole-induced planarization of a twisted tetrazole-based molecule towards energetic materials with improved thermostability and insensitivity

Pursuing the structural planarization of energetic materials is an efficient method for achieving improved performance. Although many planar energetic molecules have been prepared so far, the innovation of advanced planar explosives still relies on the scientific intuition, experience and trial-and-error of researchers. Now, a triazole-induced planarization strategy is proposed based on the regulation of aromaticity, charge distribution, and hydrogen bonds. The incorporation of a triazole ring into the non-planar molecule 5-amino-1-nitriminotetrazole (VII) results in a planar energetic material named N-[5-amino-1-(1H-tetrazol-5-yl)-1H-1,2,4-triazol-3-yl]nitramide (3). Compared with VII (T_d = 85 °C; IS < 0.25 J; FS < 5 N), 3 shows remarkably improved thermal stability (T_d = 145 °C) and reduced sensitivities (IS = 20 J; FS > 360 N). The variation of thermal stability and mechanical sensitivity from VII to 3 reflects the effectiveness and superiority of the planarization strategy. Benefiting from the properties of 3, its energetic salt 5 exhibits excellent overall performance (D_v = 9342 m s^-1; P = 31.6 GPa; T_d = 201 °C; IS = 20 J; FS = 360 N), which is comparable to that of HMX. Moreover, the triazole-induced planarization strategy may serve as a guide for exploring advanced energetic materials.