Young‐Hyuk Joo

High-Density Energetic Mono- or Bis(Oxy)-5-Nitroiminotetrazoles†

In the last decade, the investigation of energetic tetrazoles— cyanotetrazole, aminotetrazole, azidotetrazole, nitrotetrazole, and nitroiminotetrazole—has led to major developments in the area of high-energetic materials by many research groups. Several exciting classes of 5-substituted tetrazole moieties are introduced in the literature. Tetrazoles can be protonated to form tetrazolium salts or deprotonated to give tetrazolates. While deprotonation increases the thermal stability of tetrazoles, protonation can lower the decomposition temperature. These energetic materials based on high nitrogen content are derived from their high heats of formation due to the large number of N N and C N bonds. The combination of a tetrazole ring with energetic substituent groups containing oxygen atoms, such as nitro groups, nitrate esters, or nitramine, is of interest leading to excellent oxidizers. Current research issues in the field of high energetic materials include increasing oxygen content, which may result in the replacement of ammonium perchlorate in an effort to decrease pollution. Organic 5-nitrotetrazole derivatives, especially for synthesis of the highly energetic 1-methyl-5-nitrotetrazole, were synthesized in good yields. These were calculated to be endothermic with heats of formation of 2.16 kJg , which was assessed by means of standard tests and quantum chemical calculations. In addition, the highly energetic, nitrogen-rich 1-methyl-5-nitroiminotetrazole and its salts can easily be obtained by nitration of aminotetrazole followed by metathesis reactions using silver 1-methyl-5-nitroiminotetrazolate and the guanidinium family of chlorides in aqueous solution with high yields. Although 5-nitroiminotetrazole derivatives and their salts are energetic materials with high nitrogen content, they show good stabilities towards friction and impact, and good thermal stability. The development of new energetic compounds with similar attractive properties exhibit significant promise in optimizing environmentally benign replacements for toxic materials. In the past few years, the most convenient route to 1substituted 5-aminotetrazole is the addition of amine or hydrazine to cyanogen azide, which was found to be an efficient reagent for the synthesis of readily purified 1substituted 5-aminotetrazoles from primary amines under non-catalytic mild conditions. Nitration of these aminotetrazoles using 100% nitric acid has been shown to form mono-, di-, or trisubstituted nitroiminotetrazole derivatives. With these features in mind, our group became interested in examining the analogous chemistry using alkoxy amine derivatives. Here we describe the work leading to a series of nitroiminotetrazole derivatives of oxy nitroiminotetrazoles and their salts, with potentially significant physical and energetic properties. The aim of our study was to elucidate the structures in the crystalline state using X-ray diffraction analysis and to find new, potent oxygenand nitrogen-rich tetrazoles. Alkoxy 5-nitroiminotetrazolates may be of interest as a new class of ionic energetic materials, which have good thermal stabilities, high densities, good oxygen balance, and high heats of formation and which are realizable in high yields though straightforward routes. The synthesis of 1-methoxy-5-aminotetrazole (2) was achieved from the reaction of cyanogen azide with methoxy amine (obtained by neutralization of 1 with sodium hydroxide) (Scheme 1). At ambient temperature, nitration of amino-

Energetic Nitrogen-Rich Derivatives of 1,5-Diaminotetrazole†

these high-energy-density materials (HEDM). Surprisingly, substitution of the heterocyclic tetrazole ring with amino groups is one of the simplest methods to enhance thermal stability, even though 1 contains 84.0% nitrogen. Nearly 80 years ago 1 was prepared by treatment of thiosemicarbazide with lead(II) oxide and sodium azide. In 1984, further investigation into its synthesis and properties gave 1 in 59% yield. Later, 1 was synthesized by using aminoguanidinium chloride and HNO2. [5] The reaction mixture was carefully brought to pH 8 with sodium carbonate in order to deprotonate the amino-substituted azido guanyl chloride intermediate, which cyclized to form 1 in 58% yield. However, a further report that appeared in the same year recommended special caution in the synthesis of 1. This stated that 1 was pure following ethanol extraction; however, during extraction by ethanol a very shock sensitive alkali metal salt of tetrazolyl azide often was observed as a byproduct, produced by double diazotization of diaminoguanidine with HNO2. In our continuing interest in the development of energetic materials, we have now synthesized derivatives of 1,5diaminotetrazole in situ by reaction of cyanogen azide with monosubstituted hydrazine derivatives (Scheme 2).

3,4,5-Trinitropyrazole-Based Energetic Salts

High-density energetic salts that are comprised of nitrogen-rich cations and the 3,4,5-trinitropyrazolate anion were synthesized in high yield by neutralization or metathesis reactions. The resulting salts were fully characterized by (1)H, (13)C NMR, and IR spectroscopy; differential scanning calorimetry; and elemental analysis. Additionally, the structures of the 3,5-diaminotriazolium and triaminoguanidinium 3,4,5-trinitropyrazolates were confirmed by single-crystal X-ray diffraction. Based on the measured densities and calculated heats of formation, the detonation performances (pressure: 23.74-31.89 GPa; velocity: 7586-8543 ms(-1); Cheetah 5.0) of the 3,4,5-trinitropyrazolate salts are comparable with 1,3,5-triamino-2,4,6-trinitrobenzene (TATB; 31.15 GPa and 8114 ms(-1)). Impact sensitivities were determined to be no less than 35 J by hammer tests, which places these salts in the insensitive class.

Ionic liquids as hypergolic fuels.

In propellant systems, fuels of choice continue to be hydrazine and its derivatives, even though they comprise a class of acutely carcinogenic and toxic substances which exhibit rather high vapor pressures and require expensive handling procedures and costly safety precautions. Only recently (2008), ionic liquids (salts with melting points less than 100 °C) with the dicyanamide anion were shown to exhibit hypergolic properties (instantaneous ignition when contacted with oxidizers (100 % nitric acid, WFNA)). Such liquids tend to have low volatilities, and high thermal and chemical stabilities, and often exhibit long liquid ranges which could allow utilization of these substances as bipropellant fuels over a variety of conditions. A new family of dicyanoborates is presented, which can be synthesized in water, with substituted N-acyclic, N-cyclic, and azolium cations has met nearly all of the desired important criteria needed for well-performing fuels.

Hypergolic Ionic Liquids with the 2,2‐Dialkyltriazanium Cation

No flame, no gain: A hypergolic mixture is composed of stable species that readily react/ignite on molecular contact. Both the anion and the cation in an ionic liquid play prominent roles in determining hypergolic properties as well as ignition delay times. With the 2,2-dialkyltriazanium cation, salts with nitrate, chloride, nitrocyanamide, and dicyanamide anions are hypergolic.

Energetic nitrogen-rich salts and ionic liquids: 5-aminotetrazole (AT) as a weak acid

5-Aminotetrazole (AT) behaves as a weak acid and can be used to obtain nitrogen-rich energetic salts and ionic liquids. These AT salts have been characterized by IR, NMR, elemental analysis, thermal stability, phase behavior, and density. The salt 2 contains 82% nitrogen, and 7 has the highest nitrogen content (68%) of any known room temperature ionic liquid. Compound 5 crystallizes in the chiral orthorhombic system P2(1)2(1)2(1). Based on their calculated heats of formation, detonation properties, and thermal and hydrolytic stabilities, these stable AT compounds hold promise for energetic applications.

Inorganic or organic azide-containing hypergolic ionic liquids.

Recently extensive research has focused on replacing toxic hydrazine, monomethylhydrazine, and unsymmetrical dimethylhydrazine as liquid propellant fuels. 2-Azido-N,N-dimethylethylamine (1) is a good candidate to replace hydrazine derivatives in certain hypergolic fuel applications. Energetic ionic liquids that contain the 2-azido-N,N,N-trimethylethylammonium cation with nitrocyanamide, dicyanamide, dinitramide, or azide anion have been successfully synthesized in good yields by metathesis reactions. Ionic liquids have received considerable attention as energetic materials. The replacement of hydrazine with tertiary ammonium salts is especially attractive since many ionic liquids are models for green chemistry. In this work, new azide-functionalized ionic liquids are demonstrated to exhibit hypergolic activity with such oxidizers as 100% nitric acid or nitrogen tetraoxide (NTO).

Hypergolic N,N-Dimethylhydrazinium Ionic Liquids

N,N-Dimethylhydrazinium dicyanamide and nitrocyanamide ionic liquids (ILs) were prepared by quaterization of N,N-dimethylhydrazine with alkyl halides followed by metathesis reactions with silver dicyanamide or silver nitrocyanamide. The key physicochemical properties, such as melting point and decomposition temperatures, density, viscosity, heat of formation, detonation pressure and velocity, and specific impulse were measured/calculated. The impact of anions and alkyl-substituted cations on these properties is demonstrated. Droplet tests with white-fuming nitric acid (WFNA) as an oxidizer were utilized to show that the 14 new N,N-dimethylhydrazinium salts are hypergolic with ignition delay (ID) times ranging from 22 to 1642 ms, thereby suggesting that some may have potential as bipropellants.

1-substituted 5-aminotetrazoles: syntheses from CNN3 with primary amines.

1-Substituted 5-aminotetrazoles were prepared in situ by an excellent reaction of cyanogen azide and primary amines to generate an imidoyl azide as an intermediate in acetonitrile/water. After cyclization, the intermediate gave 1-substituted aminotetrazole in good yield. This protocol also was utilized in the syntheses of bis- and tris(1-substituted 5-aminotetrazole) derivatives.

Carbonyl and oxalyl bridged bis(1,5-diaminotetrazole)-based energetic salts.

High density energetic salts containing nitrogen rich cations and carbonyl- or oxalylbis(diamino-tetrazole) anions, which were obtained from cyanogen azide and hydrazine, were readily synthesized. In every case, a new family of energetic salts 3-14 were characterized by vibrational spectroscopy, multinuclear ((1)H, (13)C, (15)N) NMR, elemental analyses, density, differential scanning calorimetry and impact sensitivity. Compound 12 was structured by single crystal X-ray diffraction. The densities of 3-14, determined by gas pycnometer, range between 1.500 and 1.676 g cm(-3). The heats of formation and detonation properties for these stable salts were calculated by using Gaussian 03 and Cheetah 5.0, respectively.