Sequential, Electrochemical-Photochemical Synthesis of 1,2,4-Triazolo-[4,3-a]pyrazines.

1,2,4-triazolo-[4,3-a]pyrazine was prepared via a two-step electrochemical, photochemical process. First, a 5-substituted tetrazole is electrochemically coupled to 2,6-dimethoxypyrazine to yield 1,5- and 2,5- disubstituted tetrazoles. Subsequent photochemical excitation of the 2,5-disubstituted tetrazole species using an ultraviolet lamp releases nitrogen gas and produces a short-lived nitrilimine intermediate. Subsequent cyclization of the nitrilimine intermediate yields a 1,2,4-triazolo-[4,3-a]pyrazine backbone. The scope of this reaction was explored using various tetrazoles and pyrazines. Materials produced were identified using chemical analytical techniques and computationally studied for potential application as an insensitive energetic material.

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.

Energetic Salts of a Tri-Annulated Ring System.

Energetic salts of a triazolyl-tetrazinyl-aminotriazine ring system are characterized as energetic materials. Previously known sodium, ammonium, hydrazinium, barium, and triaminoguanidinium salts as well as the parent free acid were synthesized according to literature procedures and fully characterized for the first time as energetic materials. The silver salt was also synthesized and characterized for the first time as an energetic material. Generally, these materials form hydrates that are insensitive to mechanical stimuli; however, in cases in which anhydrous materials can be obtained, high sensitivities are possible.

Exploring Cuneanes as Potential Benzene Isosteres and Energetic Materials: Scope and Mechanistic Investigations into Regioselective Rearrangements from Cubanes.

Cuneane is a strained hydrocarbon that can be accessed via metal-catalyzed isomerization of cubane. The carbon atoms of cuneane define a polyhedron of the C2v point group with six faces─two triangular, two quadrilateral, and two pentagonal. The rigidity, strain, and unique exit vectors of the cuneane skeleton make it a potential scaffold of interest for the synthesis of functional small molecules and materials. However, the limited previous synthetic efforts toward cuneanes have focused on monosubstituted or redundantly substituted systems such as permethylated, perfluorinated, and bis(hydroxymethylated) cuneanes. Such compounds, particularly rotationally symmetric redundantly substituted cuneanes, have limited potential as building blocks for the synthesis of complex molecules. Reliable, predictable, and selective syntheses of polysubstituted cuneanes bearing more complex substitution patterns would facilitate the study of this ring system in myriad applications. Herein, we report the regioselective, AgI-catalyzed isomerization of asymmetrically 1,4-disubstituted cubanes to cuneanes. In-depth DFT calculations provide a charge-controlled regioselectivity model, and direct dynamics simulations indicate that the nonclassical carbocation invoked is short-lived and dynamic effects augment the charge model.

Approach to High-Nitrogen Materials with Dual-Use Properties via Tetraaza-Nazarov Cyclization.

In this report, we describe the application of an electrocyclization toward the synthesis of a high-nitrogen heterocycle. It entails the synthesis of a novel, high-nitrogen, 2-3-disubstituted tetrazolium salt via the tetraaza-Nazarov cyclization (4π electrocyclization) of 3-bromo-1,5-bis(3-nitro-1,2,4-triazole-1H-5-yl)-formazan (BDNF). The cyclization takes place under mild conditions using the oxidant phenyliodine(III) diacetate (PIDA). The proposed electrocyclic mechanism is supported by density functional theory (DFT) calculations and data from previous studies of formazan cyclizations. This is noteworthy because while 4π electrocyclizations with one or two nitrogen atoms have been documented previously, this case represents the first example of generation and cyclization of a conjugated intermediate with four nitrogen atoms. The experimental behavior of electrocyclization is consistent with the predictions of DFT.

Single-Crystal Diffraction, Raman Spectroscopy, and Density Functional Theory of DTO [N-(1,7-Dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide].

A combined experimental and modeling study of energetic compound N-(1,7-dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide [C5H6N8O7, (DTO)] has been performed. We report its crystal structure, solid-phase heat of formation, and its vibrational and electronic structure obtained by single-crystal X-ray diffractometry, Raman spectroscopy, and density functional theory (DFT). DTO exhibits two adjoining six-membered rings, a triazine ring (C3N3) and an oxadiazine ring (C3N2O) ring containing two nitro functional groups and one nitroamino group. DTO crystallizes with four molecules in its unit cell and presents a density of 1.862 kg/m3 at 298 K, in excellent agreement with both DFT calculations performed both at the molecular level using the B3LYP with the 6-311+G** basis set and the solid-state level using the hybrid functional HSE6 optimized with norm-conserving pseudopotentials. The calculated vibrational structure allows for the symmetry assignment of key Raman modes in terms of atomic movements, and the calculated frequency values are in good agreement with experiment. The solid-phase DFT calculations reveal that the N atoms of the triazine ring contribute mostly to the density of states at the Fermi level. In addition, we present and discuss the computed solid-phase heat of formation (215.9 kJ/mol) and molecular electrostatic potential surface of DTO and compare them to complementary materials.

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.

Synthesis and Characterization of the Potential Melt-Castable Explosive 3-(1,2,4-Oxadiazolyl)-5-Nitratomethyl Isoxazole.

The synthesis of 3-(1,2,4-oxadiazolyl)-5-nitratomethyl isoxazole (C6 H4 N4 O5 ), its physical properties, and its theoretical performances are described. This energetic material was found to have a melting point range of 76.6-79.2 °C, and a thermal onset decomposition temperature of 184.5 °C. These thermal features put this material into the standalone melt-castable explosive class. The material was found to have TNT performance, and was found to be insensitive to impact, friction, and electrostatic discharge, despite having a nitric ester functionality. A critical reaction in making this molecule was the desymmetrization of diaminoglyoxime. The optimization of this transformation is described. Previous reports of this desymmetrization were found to be inaccurate, as the desymmetrization reaction produces a co-crystal of mono- and bi-1,2,4-oxadiazole products.

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.

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.

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.

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.

Sensitive Energetics from the N-Amination of 4-Nitro-1,2,3-Triazole.

Energetic N-amino-C-nitro compounds 1-amino-4-nitro-1,2,3-triazole and 2-amino-4-nitro-1,2,3-triazole are characterized for the first time as energetic materials. These compounds were characterized chemically by nuclear magnetic resonance (NMR), Infrared spectroscopy and X-ray crystallography. Compounds were also characterized energetically by differential scanning calorimetry (DSC), impact, and friction and found to possess sensitivities and performances classifying them as primary explosives with PETN-like performance.

Tetrazole Azasydnone (C2 N7 O2 H) And Its Salts: High-Performing Zwitterionic Energetic Materials Containing A Unique Explosophore.

This work reports the first compound containing both a tetrazole and an azasydnone ring, a unique energetic material. Several energetic salts of the tetrazole azasydnone were synthesized and characterized, leading to the creation of new secondary and primary explosives. Molecular structures are confirmed by 1 H and 13 C NMR, IR spectroscopy, and X-ray crystallographic analysis. The high heats of formation, fast detonation velocities, and straight-forward synthesis of energetic azasydnones should capture the attention of future energetics research.

Synthesis of Erythritol Tetranitrate Derivatives: Functional Group Tuning of Explosive Sensitivity.

Understanding the factors that affect explosive sensitivity is paramount to the safe handling and development of new explosives molecules. Erythritol tetranitrate (ETN) is an explosive that recently has attracted significant attention in the explosives community because of its ease of synthesis and physical properties. Herein, we report the synthesis of ETN derivatives using azide, nitramine, and nitrate ester functional groups. Impact, spark, and friction sensitivity measurements, computationally calculated explosive properties, and the crystal structure analysis of the ETN derivatives are reported. Mixing explosive functional groups led to changes in the explosive sensitivity, explosive performance as well as physical properties including melting point and physical state at room temperature. Overall, we have demonstrated that combining functional groups can enable the tuning of explosive and physical properties of a molecule. This tunability can potentially aid in the development of new explosives in which characteristics are varied to meet certain specifications.

Bis(Nitroxymethylisoxazolyl) Furoxan: A Promising Standalone Melt-Castable Explosive.

The synthesis and crystal structure of the heterocyclic explosive bis(nitroxymethylisoxazolyl) furoxan, C10 H6 N6 O10 , are described. In addition, we report its physical properties and theoretical performance. This material was found to exhibit standalone melt-castable explosive properties, with a melting point of 89.8 °C and an onset decomposition temperature of 193.8 °C. Bis(nitroxymethylisoxazolyl) furoxan features an insensitive behavior to impact, friction, and electrostatic discharge, with a calculated detonation pressure about 25 % higher than the state-of-the-art melt-castable explosive TNT.

Heuristics for chemical species identification in dense systems.

A new approach to identify chemical species from molecular dynamics (MD) simulations of reacting materials under extreme temperatures and pressures is presented. The approach is based on bond-distance and vibrational criteria, derived from the examination of atomic behavior during a density functional theory MD simulation of an overdriven shock of the explosive pentaerythritol tetranitrate. For comparison, the trajectory was analyzed using popular bonding criteria commonly used in analysis of reactive MD simulations, including distance, distance-time, and bond-order criteria. Cluster analyses using the new time-dependent bond definition approach presented here and a bond-order approach revealed that species and their corresponding lifetimes were strongly dependent on the chosen approach, indicating significant implications for the development of chemical mechanisms and chemical kinetics models using the results of reactive MD simulations.

Impact of Stereo- and Regiochemistry on Energetic Materials.

The synthesis, physical properties, and calculated performances of six stereo- and regioisomeric cyclobutane nitric ester materials are described. While the calculated performances of these isomers, as expected, were similar, their physical properties were found to be extremely different. By alteration of the stereo- and regiochemistry, complete tunability in the form of low- or high-melting solids, stand-alone melt-castable explosives, melt-castable explosive eutectic compounds, and liquid propellant materials was obtained. This demonstrates that theoretical calculations should not be the main factor in driving the design of new materials and that stereo- and regiochemistry matter in the design of compounds of potential relevance to energetic formulators.