Pushing the Limit of Nitro Groups on a Pyrazole Ring with Energy-Stability Balance

A Promising Strategy toward the Development of C-C- and C-N-Linked Tricyclic Tetrazole Energetic Materials with High Energy Density

Tetrazole-based energetic materials have received extensive attention from researchers due to their high heats of formation and high nitrogen contents. In this work, a novel tricyclic tetrazole compound linked by C-C and C-N bonds was synthesized for the first time, and a series of energetic salts (1-4) were prepared. In addition, the energetic compound 2,2'-bis(dinitromethyl)-2H,2'H-5,2'-5,5'-tertetrazole (DNTBT) was synthesized by assembling dinitromethyl with tricyclic tetrazole. Among them, six compounds were further determined by single-crystal X-ray diffraction analysis. And theoretical calculations demonstrate that all compounds exhibit high positive heat of formation. The heats of formation of all target compounds are much greater than those of RDX, HMX, and CL-20. The computational results indicate that DNTBT (D = 9580 m·s-1, P = 39.2 GPa) and its ionic salts 6 (D = 9038 m·s-1, P = 34.4 GPa), 7 (D = 9178 m·s-1, P = 35.2 GPa), and 8 (D = 9297 m·s-1, P = 37.7 GPa) exhibit superior detonation performance compared to HMX (D = 8795 m·s-1, P = 34.9 GPa) and are comparable to RDX (D = 9144 m·s-1, P = 39.2 GPa). This work provides guidance for future researchers in developing high-energy-density materials.

Theoretical study on highly nitrated sensitive pyrazole isomers in the presence of external electric field

CONTEXT

This article mainly studies three isomers of C5H3N7O10, namely 5-methyl-3,4-dinitro-1- (trinitromethyl) -1H pyrazole (1), 4-methyl-3,5-dinitro-1- (trinitromethyl) -1H pyrazole (2), and 3,5-bis (dinitromethyl) -4-nitro-1H-pyrazole (3). These three substances are excellent candidates for energetic materials, but their properties under external electric fields (EEF) have not been studied. Therefore, this article studied the properties of three isomers under EEF using density functional theory (DFT), and conducted statistical analysis on the obtained data, including the molecular structure, frontier molecular orbitals, surface electrostatic potential, and nitrate charge of the three isomers. The results showed that applying EEF to the trigger bonds of 1 and 2 increased bond length, leading to a decrease in material stability. The change in bond length induced by 3 was relatively stable, and the results obtained from calculating the nitro charge were consistent with the bond length results. When an EEF is applied to three substances, the polarization degree of the molecules of the three substances increases. It is worth mentioning that the polarization degree of the molecules under the influence of a negative EEF is greater than that of a positive EEF.

METHODS

Using density functional theory, the B3LYP/6-311 + G (d, p) method was employed for structural optimization. After optimizing convergence, ensure that there are no imaginary frequencies to obtain a stable structure. Wave function analysis was performed using Multiwfn 3.8 and VMD 1.9.3. The EEF strength ranged from - 0.02 a.u. to 0.02 a.u., with a growth gradient of 0.005 a.u.

Tailoring the Energetic Properties of Pyrazole Hybrids through Functionalization with Dinitromethyl and N-Hydroxytetrazole

The functionalization of pyrazole-based compounds with dinitromethyl and N-hydroxytetrazole groups resulted in enhanced energetic properties. Two key compounds, 5-(dinitromethyl)-3,4-dinitro-1H-pyrazole (5) and 5-(3,4-dinitro-1H-pyrazol-5-yl)-1H-tetrazol-1-ol (7), along with their salts, were synthesized and evaluated for their energetic properties. Notably, the bishydroxylammonium salts 5b (Dv: 8778 m·s-1; P: 33.1 GPa) and 7b (Dv: 8988 m·s-1; P: 34.6 GPa) demonstrated an optimal balance between high detonation performance, good thermal stability, and low sensitivity, outperforming RDX, and positioning them as promising candidates for advanced secondary energetic materials.

A Method for the Preparation of Fused Dinitromethyl High-Energy-Density Materials

This work investigates a simple synthetic method for producing fused dinitromethyl energetic compounds. Fused compound 2 has a high detonation velocity (9358 m s-1) close to that of the well-known high-energy-density explosive CL-20 (9455 m s-1), an extremely high density (1.97 cm-1), and acceptable sensitivity (IS = 8 J). The good thermal stability of 2 (Td = 181 °C) is rarely reported in the field of dinitromethyl-based high explosives. The results show that compound 2 is a promising candidate for high explosives and provide new insight into the synthesis of dinitromethyl compounds.

High-Energetic Salts and Metal Complexes: Comprehensive Overview with a Focus on Use in Homemade Explosives (HME)

Metal-containing compounds form a large and rapidly expanding group of high-energy materials. Many compounds in this class attract the attention of non-professionals, who may attempt the illegal production of explosives. Several of these substances have been commercially available and pose significant danger if used by terrorists or for criminal purposes. Others are experimental compounds, kinds of curiosities, often created by pyrotechnics enthusiasts, which can present serious risks to both the creators and their immediate surroundings. The internet hosts a vast amount of information, including recipes and discussions on forums, private websites, social media, and more. This paper aims to review the variety of metal-containing explosives and discuss their appeal and potential accessibility to unauthorized individuals.

Construction of Nitrogen-Rich Energetic Isomers Containing Multiple Hydrogen Bond Networks

Nitrogen-rich energetic materials have been the focus of a few studies on their isomers. Novel nitrogen-rich energetic compounds TZ, DTZ, and NTZ were synthesized through simple steps. The hydrogen bond networks significantly enhanced their properties (TZ, Td = 290 °C and Dv = 8370 m s-1; DTZ, Td = 282 °C and Dv = 8392 m s-1; and NTZ, Td = 272 °C and Dv = 8762 m s-1), which are superior to their isomers. This realized a balance between the energy and stability of polycyclic tetrazoles, providing insights for high-performance energetic materials.

2,2'-Bisdinitromethyl-5,5'-bistetrazole: A High-Performance, Multi-Nitro Energetic Material with Excellent Oxygen Balance

Bistetrazoles are highly sought after for developing innovative high-energy density materials. The 1,1'-substituted bistetrazoles, exemplified by TKX-50, have outstanding performance. However, the research of high-perfomance 2,2'-substituted bistetrazoles remains limited. In this work, dinitromethyl groups were introduced into bistetrazole structures as 2,2'-substituted bistetrazoles (BDBTZ), which was extensively characterized through NMR, thermal analysis, and single crystal X-ray diffraction, exhibiting excellent oxygen balance, moderate sensitivity, acceptable thermal stability, high crystal density, and excellent detonation performance.

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.

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.

Application and prospects of EMOFs in the fields of explosives and propellants

Energetic Metal-Organic Framework (EMOF) compounds have gained significant attention in recent years as a hot research topic in the fields of explosives and propellants. This article provides an overview of the latest research progress of EMOFs in various areas, including heat-resistant explosives, burning rate catalysts and initiating explosives. It discusses the recent development trends of high-energy EMOFs, such as high-dimensional and solvent-free structural design, simplified and scalable synthesis conditions, environmentally friendly manufacturing processes with tunable structures, high-energy, low-sensitivity and multifunctional target products. The challenges and issues faced by EMOFs in heat-resistant explosives, burning rate catalysts and initiating explosives are presented. Furthermore, the key research directions for future applications of EMOFs in the fields of explosives and propellants are discussed, including solvent-free high-dimensional EMOFs design and synthesis, precise modulation of EMOFs molecular composition and pore structure, improvement of accurate prediction methods for physicochemical properties of high-energy EMOFs, low-cost large-scale production and development of multifunctional composite EMOFs as energetic materials, exploration of influencing factors, and comprehensive study on the application of novel and high-performance multifunctional EMOFs.

Design of N-N ylide bond-based high energy density materials: a theoretical survey

The generally encountered contradiction between large energy content and stability poses great difficulty in designing nitrogen-rich high-energy-density materials. Although N-N ylide bonds have been classified as the fourth type of homonuclear N-N bonds (besides >N-N<, -N[double bond, length as m-dash]N-, and N[triple bond, length as m-dash]N), accessible energetic molecules with N-N ylide bonds have rarely been explored. In this study, 225 molecules with six types of novel structures containing N-N ylide bonds were designed using density functional theory and CBS-QB3 methods. To guide future synthesis, the effects of substitution on the thermal stability, detonation velocity, and detonation pressure of the structures were evaluated under the premise that the N-N ylide skeleton remains stable. The calculations show that the bond dissociation energy values of the N-N ylide bonds of the designed 225 structures were in the range of 61.21-437.52 kJ mol-1, except for N-1NNH2. Many of the designed structures with N-N ylide bonds exhibit high detonation properties, which are superior to those of traditional energetic compounds. This study convincingly demonstrates the feasibility of the design strategy of introducing an N-N ylide bond to develop new types of energetic materials.

Bis(dinitropyrazolyl)methanes spruced up with hydroxyl groups: high performance energetic salts with reduced sensitivity

With an aim to improve the overall physical stability of high-performing 3,5-dinitro-functionalised bispyrazolymethanes, a hydroxyl functionality was introduced at the fourth position to obtain 1,1'-methylenebis(3,5-dinitro-1H-pyrazol-4-ol) and its energetic salts. Superior oxygen balance and energy in comparison to the amino substituent at the 4th position and enhanced sensitivity with respect to the nitro and azido substituents helped in unlocking the potential of less explored N-alkylated-4-hydroxy-3,5-dinitropyrazoles. Fine-tuning of properties via dicationic salt formation, which is not feasible in any other reported symmetrically connected pyrazole-based energetic materials, resulted in improved physical and thermal stabilities, as well as energetic performance. Hirshfeld surface analysis, electrostatic potential analysis, the study of aromaticity and weakest Mayer-bond order analysis helped further in studying the structure-property relationship of the synthesized compounds with respect to different reported methylene-bridged symmetrical compounds.

Pushing the limits of the heat of detonation via the construction of polynitro bipyrazole

The trinitromethyl group is a highly oxidized group that is found as an active functionality in many high-energy-density materials. The most frequently used previous synthetic method for the introduction of the trinitromethyl group is the nitration of heterocyclic compounds containing an acetonyl/ethyl acetate/chloroxime group. Now a novel strategy for constructing a trinitromethyl group (5) via nitration of an ethylene bridged compound, dipyrazolo[1,5-a:5',1'-c]pyrazine (2), is reported. In addition, the other two nitrated products (3 and 4) were obtained under different nitrating conditions. Compound 5 has excellent detonation performance (Dv = 9047 m s-1, P = 35.6 GPa), and a low mechanical sensitivity (IS = 10 J, FS = 216 N), with an especially attractive heat of detonation of 6921 kJ kg-1, which significantly exceeds that of the state-of-the-art explosive CL-20 (Q: 6162 kJ kg-1).

Manipulating nitration and stabilization to achieve high energy

Nitro groups have played a central and decisive role in the development of the most powerful known energetic materials. Highly nitrated compounds are potential oxidizing agents, which could replace the environmentally hazardous used materials such as ammonium perchlorate. The scarcity of azole compounds with a large number of nitro groups is likely due to their inherent thermal instability and the limited number of ring sites available for bond formation. Now, the formation of the first azole molecule bonded to seven nitro groups, 4-nitro-3,5-bis(trinitromethyl)-1H-pyrazole (4), by the stepwise nitration of 3,5-dimethyl-1H-pyrazole is reported. Compound 4 exhibits exceptional physicochemical properties with a positive oxygen balance (OBCO2 = 13.62%) and an extremely high calculated density (2.04 g cm-3 at 100 K). This is impressively high for a C, H, N, O compound. This work is a giant step forward to highly nitrated and dense azoles and will accelerate further exploration in this challenging field.

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.

2,2'-Azobis(1,5'-bitetrazole) with a N10 Chain and 1,5'-Bitetrazolate-2N-oxides: Construction of Highly Energetic Nitrogen-Rich Materials Based on C-N-Linked Tetrazoles

A series of high-nitrogen compounds, including a unique molecule 2,2'-azobis(1,5'-bitetrazole) with a branched N10 chain and 1,5'-bitetrazolate-2N-oxides, were synthesized successfully based on C-N-linked 1,5'-bistetrazoles using azo coupling of N-amine bonds and N-oxide introduction strategies. All compounds were characterized by NMR spectroscopy, IR spectroscopy, elemental analysis, and differential scanning calorimetry, in which the structures of five compounds were further determined by single-crystal X-ray diffraction analysis (2, T-N10B, 3a, 3b, and THX). The nitrogen contents of these five compounds range from 63.62 (THX) to 83.43% (T-N10B), which are much higher than that of CL-20 (38.34%). The heat of formation for the prepared compounds was calculated by using the Gaussian 09 program, with T-N10B having the highest value of 5.13 kJ g-1, about 6 times higher than that of CL-20 (0.83 kJ g-1). The calculated detonation performances by EXPLO5 v6.05.04 show that THX has excellent detonation performance (D = 9581 m s-1, P = 35.93 GPa) and a remarkable specific impulse (Isp = 284.9 s).

Polynitro-functionalized 4-phenyl-1H-pyrazoles as heat-resistant explosives

A new class of heat-resistant explosives was synthesized by coupling N-methyl-3,5-dinitropyrazole with polynitrobenzene moieties through carbon-carbon bonds. Simple Pd(0)-based Suzuki cross-coupling reactions between N-methyl-4-bromo-3,5-dinitropyrazole and 4-chloro/3-hydroxy-phenylboronic acid followed by nitration, amination and oxidation lead to the formation of C-C connected penta-nitro energetic derivatives 6 and 10. Various other energetic derivatives, such as amino (5), azido (7), nitramino (8) and energetic salts (11-14), were also explored to fine-tune their properties. All the compounds were thoroughly characterized using IR, NMR [1H, 13C{1H}], differential scanning calorimetry (DSC), elemental analysis, and HRMS studies. Compounds 5, 10 and 13 were further characterized through 15N NMR, and the crystal structures of 6 and 14 were confirmed through single-crystal X-ray diffraction studies. The physicochemical and energetic properties of all the energetic compounds were explored. Most of the synthesized compounds demonstrated high thermal stability (decomposition temperature Tdec > 250 °C), among which compounds 5 and 6 showed excellent thermal stability, having decomposition temperatures above 300 °C. The excellent thermal stability, acceptable sensitivity and good energetic properties of compounds 5, 6, 10 and 13 make them promising heat-resistant explosives. Furthermore, these compounds were found to be more thermally stable than the known N-methyl-3,5-dinitropyrazole-based and C-N coupled 3,4,5-trinitrobenzene-azole-based energetic compounds.

Lithium-Promoted Formation of (M = N2H5+ or NH3OH+ and = Anion of 1-Hydroxytetrazole-5-hydrazide)-Type "Quaternary" Complexes with Nitrogen-Rich Characteristics: Construction of Novel Insensitive Energetic Materials

Lithium-based nitrogen-rich complexes are important research objects in the field of high-energy materials. However, the weak coordination abilities of lithium ions relative to those of other metal ions with greater atomic numbers have hindered their applications in the field of nitrogen-rich complexes. Herein, we successfully prepared novel lithium-based nitrogen-rich complexes (N₂H₅-2AZTO-Li and NH₃OH-2AZTO-Li) by exploiting the structural properties of 1-hydroxytetrazolium-5-hydrazine (HAZTO). Both N₂H₅-2AZTO-Li and NH₃OH-2AZTO-Li were found to exhibit physicochemical parameters (including the density, stability, and energetic properties) that were intermediate between those of the simple ionic compounds (3 and 4) and the complexes (5) that formed them, enabling a favorable balance between high energy, high stability, and environmental friendliness (for N₂H₅-2AZTO-Li: detonation velocity (D) = 9005 m s^-1, detonation pressure (P) = 35.5 GPa, decomposition temperature (T_dec) = 238.1 °C, impact sensitivity (IS) = 24 J, friction sensitivity (FS) = 210 N, and detonation product (DP) (CO) < 2%; for NH₃OH-2AZTO-Li: D = 9028 m s^-1, P = 35.7 GPa, T_dec = 211.2 °C, IS = 20 J, FS = 180 N, and DP (CO) < 2%). This study transcends the conventional structural forms of nitrogen-rich complexes, opening new horizons for the design of novel insensitive energetic materials.

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.

Energetic Salts of Sensitive N,N'-(3,5-Dinitropyrazine-2,6-diyl)dinitramide Stabilized through Three-Dimensional Intermolecular Interactions

N-nitration of 2,6-diamino-3,5-dinitropyrazine (ANPZ) leads to a sensitive energetic compound N,N'-(3,5-dinitropyrazine-2,6-diyl)dinitramide. This nitro(nitroamino) compound was stabilized by synthesizing energetic salts, dipotassium (3,5-dinitropyrazine-2,6-diyl)bis(nitroamide) (3) and diammonium (3,5-dinitropyrazine-2,6-diyl)bis(nitroamide) (4). Compounds 3 and 4 are fully characterized by single-crystal X-ray diffraction. Compound 3 exhibits a three-dimensional energetic metal-organic framework (3D EMOF) structure and an outstanding overall performance by combining high experimental density (2.10 g cm^-3), good thermal stability (T_d(onset) = 220 °C), and good calculated performance of detonation (D = 8300 m s^-1, P = 29.9 GPa). Compound 4 has acceptable thermal stability (155 °C), moderate experimental density (1.73 g cm^-3), and good calculated performance of detonation (D = 8624 m s^-1, P = 30.8 GPa). The sensitivities of compounds 3 and 4 toward impact and friction were determined following standard methods (BAM). The energetic character of compounds 3 and 4 was determined using red-hot needle and heated plate tests. The results highlight a 3D EMOF (3) based on a six-membered heterocycle as a potential energetic material.

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.

The Structure of Biologically Active Functionalized Azoles: NMR Spectroscopy and Quantum Chemistry

This review summarizes the data on the stereochemical structure of functionalized azoles (pyrazoles, imidazoles, triazoles, thiazoles, and benzazoles) and related compounds obtained by multipulse and multinuclear 1H, 13C, 15N NMR spectroscopy and quantum chemistry. The stereochemistry of functionalized azoles is a challenging topic of theoretical research, as the correct interpretation of their chemical behavior and biological activity depends on understanding the factors that determine the stereochemical features and relative stability of their tautomers. NMR spectroscopy, in combination with quantum chemical calculations, is the most convenient and reliable approach to the evaluation of the stereochemical behavior of, in particular, nitrogen-containing heteroaromatic and heterocyclic compounds. Over the last decade, 15N NMR spectroscopy has become almost an express method for the determination of the structure of nitrogen-containing heterocycles.