New Promises from an Old Friend: Iodine-Rich Compounds as Prospective Energetic Biocidal Agents.

For a very long time, frequent occurrences of biocrises have wreaked havoc on human beings, animals, and the environment. As a result, it is necessary to develop biocidal agents to destroy or neutralize active agents by releasing large amounts of strong biocides which are obtained upon detonation. Iodine is an efficient biocidal agent for bacteria, fungi, yeasts, viruses, spores, and protozoan parasites, and it is the sole element in the periodic table that can destroy microbes without contaminating the environment. Based on chemical biology, the mechanism of iodine as a bactericide may arise from oxidation and iodination reactions of cellular proteins and nucleic acids. However, because of the high vapor pressure causing elemental iodine to sublime readily at room temperature, it is inconvenient to use this material in its normal solid state directly as a biocidal agent under ambient conditions. Iodine-rich compounds where iodine is firmly bonded in molecules as a C-I or I-O moiety have been observed to be among the most promising energetic biocidal compounds. Gaseous products comprised of large amounts of iodine or iodine-containing components as strong biocides are released in the decomposition or explosion of iodine-rich compounds. Because of the detonation pressure, the iodine species are distributed over a large area greatly improving the efficacy of the system and requiring considerably less effort compared to traditional biocidal methods. The commercially available tetraiodomethane and tetraiodoethene, which possess superb iodine content also have the disadvantages of volatility, light sensitivity, and chemically reactivity, and therefore, are not suitable for use directly as biocidal agents. It is absolutely critical to synthesize new iodine-rich compounds with good thermal and chemical stabilities.In this Account, we describe our strategies for the syntheses of energetic iodine-rich compounds while maintaining the maximum iodine content with concomitant stability and routes for the synthesis of oxygen-containing iodine-rich compounds to improve the oxygen balance and achieve both high-energy and high-iodine content. In the other work, which involves cocrystals, iodine-containing polymers were also summarized. It is hoped that this Account will provide guidelines for the design and syntheses of new iodine-rich compounds and a route for the development of inexpensive, more efficient, and safer iodine-rich antibiological warfare agents of the future.

From Nitro- to Heterocycle-Functionalized 1,2,4-Triazol-3-one Derivatives: Achieving High-Performance Insensitive Energetic Compounds.

5-Nitro-1,2,4-triazol-3-one, a nitro-functionalized 1,2,4-triazol-3-one (TO) derivative, shows excellent energetic properties and promising application potential. However, the use of the TO skeleton as an energetic material is still largely underexplored both theoretically and practically. We report here a mild and efficient method for obtaining the TO skeleton via a reaction of aminocarbohydrazide with BrCN. Two energetic compounds (2 and 5) were synthesized and fully characterized by 15N nuclear magnetic resonance, two-dimensional 1H-15N heteronuclear multiple-bond correlation, and single-crystal X-ray diffraction. The reaction mechanism was also studied with the aid of quantum calculations. Compound 2 shows promising properties as a high-performance insensitive energetic material.

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 (Td(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.

One Step Closer to an Ideal Insensitive Energetic Molecule: 3,5-Diamino-6-hydroxy-2-oxide-4-nitropyrimidone and its Derivatives.

Reaching the goal of developing an insensitive high-energy molecule (IHEM) is a major challenge. In this study, 3,5-diamino-6-hydroxy-2-oxide-4-nitropyrimidone (IHEM-1) was synthesized in one step from 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide hydrate (ICM-102 hydrate). The density of compound IHEM-1 is 1.95 g cm-3 with a decomposition temperature of 271 °C. Its detonation velocity and pressure are 8660 m s-1 and 33.64 GPa, respectively, which are far superior to the detonation performance of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), while its sensitivity is identical with that of TATB. In addition, four derivatives (1a, chloride; 1b, nitrate; 1c, perchlorate; and 1d, dinitramide) were prepared on the basis of the weak base site (N-O group) and show excellent energetic properties. By combining a series of advantages, including simple preparation, high yield, high density, very low solubility in aqueous solution, high thermostability, insensitivity, and excellent detonation performance, IHEM-1 approaches an ideal insensitive high-energy molecule. Compounds 1b-1d are also competitive as new high-energy-density materials.

Selective Synthesis of Bis(3-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)-4,4'-azo- and -azoxyfurazan Derivatives.

In this paper, we report the synthesis of two new derivatives, bis(3-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)-4,4'-azo- and -azoxyfurazans by selective oxidation of 4-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)-1,2,5-oxadiazol-3-amine. Ammonium salts of these derivatives were prepared, and all of them were fully characterized by multinuclear NMR, FTIR spectroscopy, elemental analysis, differential scanning calorimetry (DSC), and single-crystal X-ray diffraction. All of the new compounds have high measured crystal densities, and the energetic properties have been investigated.

Energetic Tricyclic Polynitropyrazole and Its Salts: Proton-Locking Effect of Guanidium Cations.

An axisymmetric polynitro-pyrazole molecule, 3,5-di(3,5-dinitropyrazol-4-yl)]-4-nitro-1H-pyrazole (5), and its salts (6-12) were prepared and fully characterized. These compounds not only show promising energetic properties but also show a unique tautomeric switch via combining different cations with the axisymmetric compound (5). Its salts (6-9) remain axisymmetric when the cations are potassium, ammonium, or amino-1,2,4-triazolium. However, when the cations are guanidiums, the salts (10-12) dramatically become asymmetric owing to the fixed proton. The introduction of guanidium cations breaks the tautomeric equilibrium by blocking the prototropic transformations and results in the switch-off effect to tautomerism. The structural constraints of 1H NMR and 13C NMR spectra provide strong evidence for the unusual structural constraint phenomenon. These stabilized asymmetric tautomers are very important from the point of molecular recognition, and this research may promote further developments in synthetic and isolation methodologies for novel bioactive pyrazole-based compounds.

Response to "What Shall We Do with Computed Detonation Performance? Comment on '1,3,4-Oxadiazole Bridges: A Strategy to Improve Energetics at the Molecular Level'".

The values obtained for detonation performance are a function of the computational methods utilized. Since there are many such methods, the literature may contain a range of values for a single compound.

1,3,4-Oxadiazole Bridges: A Strategy to Improve Energetics at the Molecular Level.

Many energetic materials synthesized to date have limited applications because of low thermal and/or mechanical stability. This limitation can be overcome by introducing structural modifications such as a bridging group. In this study, a series of 1,3,4-oxadiazole-bridged furazans was prepared. Their structures were confirmed by 1 H and 13 C NMR, infrared, elemental, and X-ray crystallographic analyses. The thermal stability, friction sensitivity, impact sensitivity, detonation velocity, and detonation pressure were evaluated. The hydroxylammonium salt 8 has an excellent detonation performance (D=9101 m s-1 , P=37.9 GPa) and insensitive properties (IS=17.4 J, FS=330 N), which show its great potential as a high-performance insensitive explosive. Using quantum computation and crystal structure analysis, the effect of the introduction of the 1,3,4-oxadiazole moiety on molecular reactivity and the difference between the sensitivities and thermal stabilities of mono- and bis-1,3,4-oxadiazole bridges are considered. The synthetic method for introducing 1,3,4-oxadiazole and the systematic study of 1,3,4-oxadiazole-bridged compounds provide a theoretical basis for future energetics design.

Enforced Planar FOX-7-like Molecules: A Strategy for Thermally Stable and Insensitive π-Conjugated Energetic Materials.

Exploring new energetic derivatives of 1,1-diamino-2,2-dinitroethylene (FOX-7) is still a key aspect in the field of energetic materials. However, so far most of the attention has been focused on modification of FOX-7 via different reaction strategies. Now we report the design of three new FOX-7-like compounds (3-5) where one nitro group in FOX-7 is replaced by a nitrogen-rich heterocyclic ring. Each of them is characterized by single-crystal X-ray crystallography. Electronic structures are studied through computational methods in comparison with FOX-7. In addition, the chemical reactivity of 3 was also investigated. Its hydroxylammonium (7), hydrazinium (8), and ammonium (9) salts were prepared, and the nitrate product (10) was also isolated. Compound 10 has a C-N bond length of 1.577 Å that is one of the longest values found for the C-NO2 bond. It was found that the incorporation of a tetrazole or triazole ring into the backbone of a conjugated nitroenamine does lead to a planar structure, which not only enhances the thermal stability but also improves the sensitivity of the product.

Nitrogen-Rich Tetrazolo[1,5-b]pyridazine: Promising Building Block for Advanced Energetic Materials.

Two metal-free explosives, tetrazolo[1,5-b]pyridazine-containing molecules [6-azido-8-nitrotetrazolo[1,5-b]pyridazine-7-amine (3at) and 8-nitrotetrazolo[1,5-b]pyridazine-6,7-diamine (6)], were obtained via straightforward two-step synthetic routes from commercially available reagents. Compound 3at displays an excellent detonation performance (Dv = 8746 m s-1 and P = 31.5 GPa) that is superior to commercial primary explosives such as lead azide and diazodinitrophenol (DDNP). Compound 6 has superior thermal stability, remarkable insensitivity, and good detonation performance, strongly suggesting it as an acceptable secondary explosive. The initiating ability of compound 3at has been tested by detonating 500 mg of RDX with a surprisingly low minimum primary charge of 40 mg. The extraordinary initiating power surpasses conventional primary explosives, such as commercial DDNP (70 mg) and reported 6-nitro-7-azido-pyrazol[3,4-d][1,2,3]triazine-2-oxide (ICM-103) (60 mg). The outstanding detonation power of 3at contributes to its future prospects as a promising green primary explosive. In addition, the environmentally benign methodology for the synthesis of 3at effectively shortens the time from laboratory-scale research to practical applications.

A Duo and a Trio of Triazoles as Very Thermostable and Insensitive Energetic Materials.

The triazole moiety with a high heat of formation and a high nitrogen content has been investigated for decades in combination with other nitrogen-rich heterocyclic rings in the field of energetic materials. A novel strategy for the construction of both thermally stable and mechanically insensitive energetic materials using a multi-aminotriazole system is now described. Using this methodology, two series of energetic materials were created on the basis of a duo of triazoles, 5-amino-3-(3,4-diamino-1,2,4-triazol-5-yl)-1H-1,2,4-triazole (TT), and a trio of triazoles, 4,5-di(3,4-diamino-1,2,4-triazol-5-yl)-2H-1,2,3-triazole (TTT). Their nitrogen-rich salts were also synthesized. Compound TT exhibits an excellent onset decomposition temperature (Td = 341 °C), which is superior to that of the conventional heat-resistant explosive hexanitrostilbene (HNS) (Td = 318 °C). The nitrogen-rich salt 4,5-di(3,4-diamino-1,2,4-triazol-5-yl)-2H-1,2,3-triazolium 3,4,5-trinitropyrazol-1-ide (TTT-1) exhibits both remarkable detonation properties and low sensitivities (Dv = 8715 m s-1; P = 32.6 GPa; IS > 40 J; FS > 360 N), which are superior to those of the traditional explosive LLM-105 (Dv = 8639 m s-1; P = 31.7 GPa; IS = 20 J; FS = 360 N). Therefore, this methodology of building a multi-aminotriazole system could effectively assist in the design of thermally stable and mechanically insensitive energetic materials in future exploration.

Bis(3-nitro-1-(trinitromethyl)-1H-1,2,4-triazol-5-yl)methanone: An Applicable and Very Dense Green Oxidizer.

Ammonium perchlorate (AP) is most often used as a practical solid rocket propellant because of its excellent performance. However, AP has many shortcomings, including instability, high negative enthalpy of formation, and claimed health and environmental issues resulting from its combustion products. The pursuit of highly dense, high-performance, and environmentally friendly oxidizers as solid propellants has long attracted scientists around the world. In this work, bis(3-nitro-1-(trinitromethyl)-1H-1,2,4-triazol-5-yl)methanone (3) was obtained from bis(3-nitro-1H-1,2,4-triazol-5-yl)methane (1) with chloroacetone followed by nitration. The structure of 3 was confirmed by elemental analysis and single-crystal X-ray diffraction. By introducing the carbonyl moiety, the density of 3 was increased to 1.945 g/cm3 and the decomposition temperature increased to 164 °C. Compound 3 is a green energetic oxidizer that has a positive oxygen balance (+8.7%), a high specific impulse (218 s), and an acceptable sensitivity (9 J, 240 N), making it a practical replacement for AP in solid rocket propellant formulations.

Intermolecular Weak Hydrogen Bonding (Het-H-N/O): an Effective Strategy for the Synthesis of Monosubstituted 1,2,4,5-Tetrazine-Based Energetic Materials with Excellent Sensitivity.

A series of monosubstituted 1,2,4,5-tetrazine-based energetic materials was effectively synthesized and fully characterized with IR, multinuclear nuclear magnetic resonance (NMR), and elemental analyses. Heats of formation and detonation performances were determined using Gaussian 03 and EXPLO5 v6.01 programs, which show that 5 and 9 as secondary explosives have detonation velocities superior to the current secondary-explosive benchmark, triaminotrinitrobenzene (TATB). Importantly, compounds 2, 5, and 9 were first characterized with single-crystal X-ray diffraction and Hirshfeld surface calculations, and some intermolecular weak hydrogen bonds (Het-H-N/O) among these compounds illustrate the relationship between these weak interactions and excellent sensitivity of energetic materials. This design method for next-generation energetic materials by incorporating intermolecular weak hydrogen bonds may be of future importance.

Conjugated Energetic Salts Based on Fused Rings: Insensitive and Highly Dense Materials.

Nitroamino-functionalized 1,2,4-triazolo[4,3- b][1,2,4,5]tetrazine (1), when combined with intermolecular hydrogen bonds (HBs) and strong noncovalent interactions between layers, results, for example, in an interlayer distance of 2.9 Å for dihydroxylammonium 3,6-dinitramino-1,2,4-triazolo[4,3- b][1,2,4,5]tetrazine (2c) with a packing coefficient of 0.805. For dihydroxylammonium 6,6'-dinitramino-3,3'-azo-1,2,4-triazolo[4,3- b][1,2,4,5]tetrazine (3b), two fused rings are linked by an azo group, which expands the conjugated system resulting in an even shorter interlayer distance of 2.7 Å and a higher packing coefficient of 0.807. These values appear to be the shortest interlayer distances and the highest packing coefficients reported for tetrazine energetic materials. With high packing coefficients, both possess high densities of 1.92 g cm-3 and 1.99 g cm-3 at 293 K, respectively. Compared with its precursor, the hydroxylammonium moiety serves as a buffer chain (H-N-O-H), connecting the anion and cation through hydrogen bonds, giving rise to more favorable stacking, and resulting in higher density and lower sensitivity. The sensitivities of all the hydroxylammonium salts are lower than that of their neutral precursors, such as compound 2 (3 J, >5 N) and compound 2c (25 J, 360 N). The detonation properties of 2c (detonation velocity vD = 9712 m s-1 and detonation pressure P = 43 GPa) and 3b (vD = 10233 m s-1; P = 49 GPa) exceed those of present high explosive benchmarks, such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexanitrohexaazaisowurzitane (CL-20). The molecular structures of several of these new energetic materials are confirmed by single-crystal X-ray diffraction measurements. Using calculated and experimental results, the fused ring with a planar large π-conjugated system results in a compromise between desirable stabilities and high detonation properties, thus enhancing future utilization in the design of energetic materials.

Efficient Construction of Energetic Materials via Nonmetallic Catalytic Carbon-Carbon Cleavage/Oxime-Release-Coupling Reactions.

The exploitation of C-C activation to facilitate chemical reactions is well-known in organic chemistry. Traditional strategies in homogeneous media rely upon catalyst-activated or metal-mediated C-C bonds leading to the design of new processes for applications in organic chemistry. However, activation of a C-C bond, compared with C-H bond activation, is a more challenging process and an underdeveloped area because thermodynamics does not favor insertion into a C-C bond in solution. Carbon-carbon bond cleavage through loss of an oxime moiety has not been reported. In this paper, a new observation of self-coupling via C-C bond cleavage with concomitant loss of oxime in the absence of metals (either metal-complex mediation or catalysis) results in dihydroxylammonium 5,5-bistetrazole-1,10-diolate (TKX-50) as well as N, N'-([3,3'-bi(1,2,4-oxadiazole)]-5,5'-diyl)dinitramine, a potential candidate for a new generation of energetic materials.

Synthesis and Characterization of 4-(1,2,4-Triazole-5-yl)furazan Derivatives as High-Performance Insensitive Energetic Materials.

3-Nitro-4-(5-nitro-1,2,4-triazol-3-yl)furazan (2), N,N'-bis(trinitroethyl)-3,5'-diamino-4-(1,2,4-triazol-3-yl)furazan (3), N,N'-bis(trinitroethyl)-3,5'-dinitramino-4-(1,2,4-triazol-3-yl)furazan (4) and eighteen nitrogen-rich salts (5 a, 5 b, 5 d-5 i, 5 g-1, 6 a-6 i) were designed and synthesized. These 4-(1,2,4-triazole-5-yl)furazan derivatives were fully characterized by IR and NMR spectra, elemental analysis, and differential scanning calorimetry (DSC). The solid-state structures of 2, 5 d, 5 e, 5 h, 5 g-1, 6 g, and 6 i were confirmed via single crystal X-ray analysis. Detonation performance (detonation velocities and pressures) of these energetic compounds was evaluated and the impact and friction sensitivities were measured using standard BAM technology. Some of the compounds, for example, 2 (D: 9152 m s-1 , P=37.1 GPa) and 4 (D: 9355 m s-1 , P=40.1 GPa) exhibit excellent detonation performance, which are comparable to the highly explosive benchmarks such as RDX (D: 8795 m s-1 , P=34.9 GPa) and HMX (D: 9144 m s-1 , P=39.2 GPa).

Multipurpose Energetic Materials by Shuffling Nitro Groups on a 3,3'-Bipyrazole Moiety.

A family of 3,3'-bipyrazole-based energetic compounds having C-NO2 /N-NO2 functionalities was synthesized by using various nitrating conditions. These nitro derivatives of bipyrazole are significantly more dense and energetic compared to the corresponding nitropyrazole analogues while maintaining the desired thermal stability and sensitivity. Depending on the number and nature of energetic nitro groups (C-NO2 /N-NO2 ), different classes of energetic materials, such as green primary explosives, high-performance secondary explosives and heat-resistant explosives, were obtained. All the compounds were thoroughly characterized by IR, NMR [1 H, 13 C{1 H}, 15 N], elemental analysis, and differential scanning calorimetry (DSC). Four were also structurally characterized with single-crystal X-ray diffraction studies. Heats of formation and detonation performance were calculated using Gaussian 03 and EXPLO5 v6.01 programs, respectively.

Control of Biohazards: A High Performance Energetic Polycyclized Iodine-Containing Biocide.

Biohazards and chemical hazards as well as radioactive hazards have always been a threat to human health. The search for solutions to these problems is an ongoing worldwide effort. In order to control biohazards by chemical methods, a synthetically useful fused tricyclic iodine-rich compound, 2,6-diiodo-3,5-dinitro-4,9-dihydrodipyrazolo [1,5- a:5',1'- d][1,3,5]triazine (5), with good detonation performance was synthesized, characterized, and its properties determined. This compound which acts as an agent defeat weapon has been shown to destroy certain microorganisms effectively by releasing iodine after undergoing decomposition or combustion. The small iodine residues remaining will not be deleterious to human life after 1 month.

Balancing Excellent Performance and High Thermal Stability in a Dinitropyrazole Fused 1,2,3,4-Tetrazine.

The key to successfully designing high-performance and insensitive energetic compounds for practical applications is through adjusting the molecular organization including both fuel and oxidizer. Now a superior hydrogen-free 5/6/5 fused ring energetic material, 1,2,9,10-tetranitrodipyrazolo[1,5-d:5',1'-f][1,2,3,4]tetrazine (6) obtained from 4,4',5,5'-tetranitro-2H,2'H-3,3'-bipyrazole (4) by N-amination and N-azo coupling reactions is described. The structures of 5 and 6 were confirmed by single crystal X-ray diffraction measurements. Compound 6 has a remarkable room temperature experimental density of 1.955 g cm-3 and shows excellent detonation performance. In addition, it has a high decomposition temperature of 233 °C. These fascinating properties, which are comparable to those of CL-20, make it very attractive in high performance applications.

Pushing the Limits of Oxygen Balance in 1,3,4-Oxadiazoles.

Gem-trinitromethyl groups were introduced into a 1,3,4-oxadiazole ring to give the first example of a bifunctionalized single five-membered ring with six nitro groups. 2,5-Bis(trinitromethyl)-1,3,4-oxadiazole (12) has a high calculated crystal density of 2.007 g cm-3 at 150 K (1.941 g cm-3 at 293 K) and a very high positive oxygen balance (39.12%), which makes it a strong candidate as a high energy dense oxidizer. The dihydroxylammonium and dihydrazinium salts of bis(trinitromethyl)-1,3,4-oxadiazole (5 and 6) exhibit excellent calculated detonation properties (5, vD = 9266 m s-1, P = 38.9 GPa; 6, vD = 8900 m s-1, P = 36.3 GPa) and acceptable impact sensitivities (5 20 J, 6 19 J), which are superior to those of RDX (7.4 J) and HMX (7.4 J). Such attractive features support the application potential of the gem-polynitromethyl group in the design of advanced energetic materials. Surprisingly, 2,5-bis(trinitromethyl)-1,3,4-oxadiazole (12) is more thermally stable and less sensitive than its bis(dinitromethyl) analogue, 8.