Synthesis and Energetic Characterization of Borane-Amines on High-Nitrogen Heterocycles.

Borane-amines have garnered attention over the last several decades in a variety of applications, ranging from hydrogen storage materials to hypergolic fuel systems. An investigation into the synthesis of borane-amines with high-nitrogen content heterocycles was undertaken in this work. Borane-amines were formed by the reaction of BH3·Me2S in tetrahydrofuran (THF) with the requisite nitrogen-containing heterocycle and isolated by placing the crude reaction mixture in hexanes to precipitate the product. X-ray crystallography, thermogravimetric analysis (TGA), high resolution mass spectroscopy (HRMS), 1H NMR, 13C NMR, and 11B NMR were utilized for product characterization, while impact and friction sensitivity testing were conducted to identify sensitivity in the synthesized compounds. Most isolated borane-amines, except one, were found to decompose in the atmosphere and were more sensitive to mechanical stimuli than their starting materials; however, all synthesized compounds were found to be hypergolic in the presence of white fuming nitric acid (WFNA).

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.

Nitrodiazene oxides: a unique nitrogen- and oxygen-containing functional group.

Nitrogen-/oxygen-containing functional groups (N/O groups) may be found in a wide variety of areas such as agriculture, drug design and energetic materials. Exploring the chemistry and synthesis of N/O groups is desirable as compounds containing their functionality may prove to be invaluable in a variety of fields. A unique N/O functional group which may offer additional insight into the design of high-heteroatom content systems is the 2-nitrodiazene-1-N-oxide group (NDO group). While unique on their own, NDOs combine the well-known azoxy (N(O)=N) and nitro (-NO2) groups into a single unique N/O functional group. Although NDOs may offer superior densities and enthalpies of formations relative to their nitro counterparts, NDOs have been significantly less investigated than their nitro-bearing counterparts. This work will discuss NDOs in chemical literature from their initial discovery to modern synthesis techniques, energetic properties and chemical stability.

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.

A Laboratory Preparation of High-Purity Calcium Cyanamide.

Calcium cyanamide is an important fertilizer and a chemical precursor. However, its large scale synthesis is extremely energy intensive via the historical Frank-Caro synthesis of fixing atmospheric nitrogen with calcium carbide at an elevated temperature, and the product material contains a large number of impurities. In this work, we prepare calcium cyanamide in a purity higher than that achievable by the Frank-Caro route in a convenient laboratory method.

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.

Electrochemical Synthesis of High-Nitrogen Materials and Energetic Materials.

Electrochemical synthesis is a valuable method for the preparation of molecules. It is innately eco-friendly, as potentially hazardous oxidation and reduction agents are replaced with electrochemical potentials. Electrochemistry is commonly applied globally in the synthesis of numerous chemicals, but the energetic materials field lags in this regard. In this review, we endeavor to cover the entire history of synthetic electrochemistry for the preparation of energetic materials and detail the electrochemical transformations of high-nitrogen materials that are relevant for the preparation of new energetic molecules. We hope this review serves as a starting point to inform those involved in synthetic energetic materials chemistry, and those interested in other applications of high-nitrogen molecules, about the environmentally friendly electrochemical methods available for such compounds.

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.

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.

Synthesis of 5-Nitrotetrazolates by the Direct Oxidation of 5-Aminotetrazole in a Single-Pot Synthesis without Isolation of Explosive Intermediates.

A readily available oxidizer, potassium superoxide can be used for the oxidation of 5-amino-1H-tetrazole (5-AT) to 5-nitrotetrazole (5-NT) in high yields in a single-pot synthesis. This strategy reduces the synthesis of this important energetic material down to a single step and eliminates highly sensitive diazonium and copper salt primary explosive intermediates. Use of dimethyl sulfoxide along with molecular sieves and 18-crown-6 as phase transfer catalyst promoted faster reaction rates and greater yield. The overall yield of this process is 48-53 % and the resultant aqueous solution of product was effectively used for the preparation of nitrotetrazole-containing primary explosive DBX-1. Unlike current methods of nitrotetrazole preparation, this entirely solution-based route results in a final solution of sodium nitrotetrazolate, without the need to handle energetic intermediates or products, making it the safest method of nitrotetrazole preparation reported to date.

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.

Nitroacetonitrile as a versatile precursor in energetic materials synthesis.

Nitroacetonitrile is the simplest α-nitronitrile; it possesses a single central carbon attached to two strong electronegative, electron-withdrawing groups allowing extensive chemistry through the active methylene center. Free nitroacetonitrile has purification and stability issues, however stable salts of nitroacetonitrile possess the same reactivity as the free acid and are much more stable. Nitroacetonitrile serves as a versatile synthetic precursor in the formation of heterocyclic and polyfunctional aliphatic products and can allow for straightforward conversion to amino, acyl, and other functional groups. A main advantage of using nitroacetonitrile in the formation of heterocyclic-based energetics is its ability to add vicinal amino and nitro moieties onto fused ring structures, a common structural motif in insensitive energetic materials. In this minireview we discuss the preparation of nitroacetonitrile and its stable salts, as well as discuss the range of energetic materials this versatile precursor has found use in.

An Energetic Triazolo-1,2,4-Triazine and its N-Oxide.

The reaction of 3-amino-5-nitro-1,2,4-triazole with nitrous acid produces the corresponding diazonium salt. When the diazonium salt is treated with nitroacetonitrile, a subsequent condensation and cyclization reaction occurres to produced 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine (DPX-26). X-ray crystallographic analysis shows that the DPX-26 has a density of 1.86 g cm-3 , while it is calculated to have a heat of formation of 398.3 kJ mol-1 . DPX-26 is predicted to approach the explosive performance of RDX but displays significantly better safety properties. Oxidation of DPX-26 using hypofluorous acid produces 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine 4-oxide (DPX-27), which is also predicted to be a high-performance material with enhanced safety properties.

Detection of high-energy compounds using photoluminescent silicon nanocrystal paper based sensors.

Luminescent silicon nanocrystals (Si-NCs) surface functionalized with dodecyl groups were exposed to solutions of nitroaromatic compounds including nitrobenzene, nitrotoluene, and dinitrotoluene. It was found that Si-NC luminescence was quenched upon exposure to nitroaromatics via an electron transfer mechanism as indicated by Stern-Volmer analysis. This quenching was exploited and a straightforward paper-based Si-NC sensor was developed. This paper motif was found to be sensitive to solution, vapor, and solid phase nitroaromatics, as well as solution borne RDX and PETN.

Synthesis of 5-aminotetrazole-1N-oxide and its azo derivative: a key step in the development of new energetic materials.

1-Hydroxy-5-aminotetrazole (1), which is a long-desired starting material for the synthesis of hundreds of new energetic materials, was synthesized for the first time by the reaction of aqueous hydroxylamine with cyanogen azide. The use of this unique precursor was demonstrated by the preparation of several energetic compounds with equal or higher performance than that of commonly used explosives, such as hexogen (RDX). The prepared compounds, including energetic salts of 1-hydroxy-5-aminotetrazole (hydroxylammonium (2, two polymorphs) and ammonium (3)), azo-coupled derivatives (potassium (5), hydroxylammonium (6), ammonium (7), and hydrazinium 5,5'-azo-bis(1-N-oxidotetrazolate (8, two polymorphs)), as well as neutral compounds 5,5'-azo-bis(1-oxidotetrazole) (4) and 5,5'-bis(1-oxidotetrazole)hydrazine (9), were intensively characterized by low-temperature X-ray diffraction, IR, Raman, and multinuclear NMR spectroscopy, elemental analysis, and DSC. The calculated energetic performance, by using the EXPLO5 code, based on the calculated (CBS-4M) heats of formation and X-ray densities confirm the high energetic performance of tetrazole-N-oxides as energetic materials. Last but not least, their sensitivity towards impact, friction, and electrostatic discharge were explored. 5,5'-Azo-bis(1-N-oxidotetrazole) deflagrates close to the DDT (deflagration-to-detonation transition) faster than all compounds that have been investigated in our research group to date.