Novel catenated N6 energetic compounds based on substituted 1,2,4-triazoles: synthesis, structures and properties

1-Amino-3,5-dinitro-1,2,4-triazole (ADNT) was prepared using an efficient N-amination process. Three novel catenated N6 energetic derivatives of ADNT, which contain 1,1′-azobis(3,5-dinitro-1,2,4-triazole) (ABDNT), 1,1′-azobis(3-chloro-5-nitro-1,2,4-triazole) (ABCNT) and 1,1′-azobis(3,5-diazido-1,2,4-triazole) (ABDAT), were synthesized from N-amino oxidative-coupling reactions of ADNT. All compounds were fully characterized by 1H and 13C nuclear magnetic resonance spectroscopies, infrared spectroscopy, elemental analysis, mass spectrum, as well as differential scanning calorimetry (DSC). The crystal structure of compound ABCNT was confirmed by single-crystal X-ray diffraction showing an extensive conjugated structure. The densities of energetic derivatives ranged from 1.71 to 1.93 g cm−3, and all compounds have positive heats of formation in the range of 774.8 to 2150.8 kJ mol−1. Based on the measured densities and calculated heats of formation, theoretical performance calculations, including detonation pressures (29.6–42.4 GPa) and detonation velocities (8.22–9.49 km s−1) were carried out using the Gaussian 09 program and Kamlet–Jacobs equations, and they compared favorably with those of TNT and RDX. These properties make them potentially competitive as new high energy-density compounds.


Introduction
Traditional energetic materials including quintessential explosives such as TNT (2,4,6-trinitrotoluene), RDX (1,3,5-trinitro-1,3,5-triazinane) and HMX (1,3,5,7-tetranitro-1,3,5,7tetrazocane) are based on the oldest strategy in the design of energetic materials: the presence of fuel and oxidizer in the same molecule. New strategies in energetic materials' research focus on nitrogen-rich/high positive heat of formation compounds. 1 Nitrogen-rich compounds have therefore received increasing attention as promising candidates for high energydensity materials (HEDM) which might be used as propellants, explosives or especially as gas generators, 2 and the generation of nitrogen gas as a nal product of nitrogen-rich compounds is highly favored for the enhancement of energy and avoiding environmental pollution. 3 Nitrogen-rich compounds based on C/N heteroaromatic rings with high nitrogen content like triazole, tetrazole, triazine and tetrazine are at the forefront of high energy research. 4 Recently, the combination of an azo group with nitrogen-rich heteroaromatic rings has been extensively studied because the azo linkage not only desensitizes but also dramatically increases the heats of formation of high-nitrogen compounds such as 3,3 0azobis (1,2,4-triazole) (1), 5,5 0 -dinitro-3,3 0 -azo-1H-1,2,4-triazole (2), 3,3 0 -azobis-(6-amino-1,2,4,5-tetrazine) (3), 4,4 0 ,6,6 0tetra(azido)azo-1,3,5-triazine (4) and are four representative compounds of this kind (Scheme 1), 5 where the two heteroaromatic rings are connected by a C-N]N-C linkage. However, if the azo group is attached to the nitrogen of the heteroaromatic rings to create a rather long chain of catenated nitrogens (N-N]N-N linkage), such a polynitrogen structure could result in unique properties and features of these energetic compounds.
The N-amination of electron-rich systems, such as imidazoles, pyrazoles, triazoles and tetrazoles, have been reported for several researchers, 7c,8 using the commercially available hydroxylamine-O-sulfonic acid (HOSA) as N-amination agent. However, HOSA aminations unfortunately do not apply to electron-poor systems. The addition of an amino group to energetic compounds could improve the stability, for example amino-nitro compounds such as diaminodinitroethylene (FOX-7) or 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) are low-sensitivity and high-performance explosives that are the standard for insensitive explosives. 7c,9 N-Amino compounds have the further advantage that their heats of formation and other explosive performance increase as a result of the additional N-N bond. Furthermore, new potential nitrogen ligands for high energy-density materials can be designed and synthesized by the N-NH 2 group of N-amino compounds.

Synthesis of N-amination agents
The commercially hydroxylamine-O-sulfonic acid (HOSA) as Namination agent could only be used to the N-amination reaction of electron-rich systems, and the yields of N-amination products were always lower. O-Tosylhydroxylamine (THA) is an efficient N-amination agent that could aminate electron-poor systems and have better yields of targets, but THA is not stable in storage under room temperature condition. 7c,8,10 In our studies, seven new N-amination agents, such as 2-nitrobenzenesulfonyl hydroxylamine (2-NSH), 3-nitrobenzenesulfonyl hydroxylamine (3-NSH), 2,4dinitrobenzenesulfonyl hydroxylamine (DNSH), 2,4-dimethylbenzenesulfonyl hydroxylamine (DMSH), 2,4,6-trimethylbenzenesulfonyl hydroxylamine (MSH), 2,4,6triisopropylbenzenesulfonyl hydroxylamine (TSH) and 2,4,6trinitrophenyl hydroxylamine (PHA), were synthesized using Osubstituted benzenesulfonyl chloride and ethyl acetohydroxamate as starting materials by the reactions of condensation and hydrolysis. The compounds 2-NSH, 3-NSH, DNSH, and DMSH easily decomposed in the solid state at room temperature more than half an hour, but they was stable in the solution at lower temperature for several weeks. According to the results of DSC experiments, the compounds MSH, TSH, and PHA could be stable in storage under ambient condition. The thermal decomposition temperatures were 79.3 C, 107.7 C, and 94.1 C, respectively.
reacted with nucleophiles. The chlorine atoms and nitro groups in compound ABCNT were substituted with azide groups by reaction with sodium azide to produce 1,1 0 -azobis(3,5-diazido-1,2,4-triazole) (ABDAT). The synthetic pathways to ADNT and its all energetic derivatives are depicted in Scheme 4. The structures of all compounds were fully characterized by IR spectroscopy, DSC, 1 H and 13 C NMR spectroscopies and elemental analysis. The data are listed in the Experimental section.

Sensitivities
To evaluate the stabilities of these N,N 0 -diazo-bridged energetic compounds, we studied the impact and friction sensitivities. The sensitivities of all explosives were determined experimentally according to standard BAM methods. 11 As can be seen in Table 1, The impact sensitivities of these energetic derivatives range from 6 J to 20 J. The value of friction sensitivities are from 80 N to 300 N. The compounds ABDNT and ABCNT are considerably less sensitive toward impact and friction sensitivities than RDX and HMX (RDX: IS ¼ 7.4 J, FS ¼ 120 N; HMX: , which suggest that they can serve as promising candidates for safe energetic materials. All the novel catenated N 6 energetic derivatives based on ADNT are less sensitive than the reported high-nitrogen compounds with N,N 0 -diazo-bridged compounds (N 8 and N 10 compounds).

Physiochemical and detonation properties
The thermal stabilities of compounds ABDNT and ABDAT were determined by differential scanning calorimetric (DSC) measurements at a heating rate of 5 C min À1 . The thermal decomposition peak temperatures of ABDNT and ABDAT were 262.4 C and 168.8 C (seen in Table 1), respectively, compared with that of traditional energetic compound RDX (239.2 C). Because of the existence of the four azide groups in one molecular structure, compound ABDAT has lower decomposition temperature (168.8 C). However, the decomposition peak temperatures of ABDNT and ABDAT are much higher than those of hexazene (N 6 ) ligand (140 C) and N 5 + (70 C). 12 pycnometer or single-crystal X-ray diffraction. It is noteworthy that the densities of compounds ABDNT and ABCNT fall in the range designated for new HEDMs (1.8-2.0 g cm À3 ), 13 which are higher than RDX (1.82 g cm À3 ). Moreover, the high densities of ABDNT (1.93 g cm À3 ) and ABCNT (1.92 g cm À3 ) are even comparable with that of HMX (1.90 g cm À3 ).
Heats of formation are one of the important characteristics for energetic compounds and are directly related to the number of nitrogen-nitrogen bonds in molecular structures. The standard enthalpies of formation for these derivatives were calculated by using the Gaussian 09 (Revision A. 02) 14 suite of programs and atomization method based on CBS-4M enthalpies. 15 All of the optimized structures were characterized to be true local energy minima on the potential-energy surface without imaginary frequencies. As shown in Table 1, the solid phase heats of formation for N,N 0 -diazo-bridged energetic derivatives based on ADNT are highly endothermic compounds. All of these compounds exhibit higher positive heats of formation ranging between 774.8 and 2150.8 kJ mol À3 , which are compared with those of RDX (92.6 kJ mol À3 ), HMX (104.8 kJ mol À3 ), and other reported catenated nitrogen-atom compounds (N 8 and N 10 structures in Table 1). Especially, compound ABDAT exhibits the highest heat of formation (2150.8 kJ mol À1 ) among of them because of four azidofunctionalization groups in the same molecular structure.
Based on the calculated values of heats of formation and the experimental values for the densities of these energetic derivatives, the detonation velocities (D) and detonation pressures (P) were calculated using the Kamlet-Jacobs equations. 16 The detonation velocities (D) lie between 8.22 and 9.49 km s À1 (compared with TNT 6.88 km s À1 , and RDX 8.71 km s À1 ). Detonation pressures of these compounds lie in the range between 29.6 to 42.4 GPa (compared with TNT 19.5 GPa,and RDX 33.7 GPa). Compound ABDNT has the highest detonation velocity (9.49 km s À1 ) and detonation pressure (42.4 GPa), which are also higher than other reported N,N 0 -diazo-bridged energetic compounds (N 8 and N 10 structures). Although not all these energetic derivatives perform better than RDX by calculations, they can probably nd use in certain applications in civilian use or in military applications.

X-Ray crystallography
A single crystal of compound ABCNT suitable for crystal structure analysis was obtained by slow evaporation from acetonitrile solution of ABCNT at room temperature. The crystal data and structure renement details of ABCNT are given in Table 2. The crystal structure of ABCNT adopts a planar structure with two almost planar 3-chloro-5-nitro-1,2,4-triazole rings, a planar N 6 chain and an E conguration about the azo bond (Fig. 1). The azo bond adopts a stable E conguration due to lower active energy than the Z conguration. The bond length between the N-atoms of the azo group (N1]N1A) is 1.240 A, shorter than that of compound 5 with the N 4 structure (1.249 A), compound 6 with the N 8 structure (1.250 A) and compound 9 with the N 10 structure (1.243 A), which indicates a stronger delocalization of the nitro p 3 (ref. 4) conjugated bond and azo p-bond along the molecular structure within compound 6. But longer than that of N 4 H 4 (2-tetrazene) 17 with the N 4 structure (1.205 A) and compound 7 with the N 10 structure (1.178 A), respectively. This is probably due to the electron-withdrawing property of the nitro groups.

Caution
Although we experienced no problems during the synthesis of the target compounds, standard safety precautions (leather gloves, face shield and ear plugs) should be used when handling these energetic materials.

General methods
All chemical reagents and solvents (analytical grade) were used as supplied unless otherwise stated. Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000-400 cm À1 . 1 H NMR and 13 C NMR were obtained in DMSO-d 6 on a Bruker AV500 NMR spectrometer. Elemental analyses (C, H and N) were performed on a VARI-El-3 elementary analysis instrument. Mass spectra were acquired using a GCMS-QP 2010 Micromass UK spectrometer. The DSC experiment was performed using a DSC-Q 200 apparatus (TA, USA) under a nitrogen atmosphere at a ow rate of 50 mL min À1 . About 0.5-1.0 mg of the sample was sealed in aluminium pans for DSC. For all energetic materials, the impact sensitivity were determined with a ZBL-B impact sensitivity instrument, the friction sensitivity were determined with a FSKM 10 friction sensitivity instrument. Energetic properties have been calculated with Gaussian 09 program 14 and Kamlet-Jacobs equations 16 using the calculated solid state heat of formation and experimental value of density. These were computed by the atomization method as described in published papers.

X-ray crystallography
Single crystal suitable of compound ABCNT for X-ray measurement was obtained by slow evaporation of acetonitrile solution of ABCNT at room temperature. The crystal structure of ABCNT was determined by a Bruker SMART APEXII CCD X-ray diffractometer and the SHELXTL crystallographic soware package. A single crystal was mounted on a Bruker SMART APEXII CCD Xray diffractometer equipped with graphite-monochromatized Mo-Ka radiation (0.71073 A). Data were collected by the u scan technique. The structure was solved by direct methods using SHELXS-97 program 18 and rened against F 2 by fullmatrix least-squares using SHELXL-97 program. 19 Theoretical study All the quantum computations were performed using the Gaussian 09 (Revision A. 02) suite of programs. 14 The optimized structures were characterized to be true local energy minima on the potential-energy surface without imaginary frequencies. The room-temperature gas-phase enthalpies of all energetic compounds were obtained by the atomization method based on CBS-4M calculated electronic enthalpies using NIST 20 values as standardized values for the standard heats of formation (DH f ).
Oen the standard state of the materials of interest corresponds to the solid phase. Thus, the solid state enthalpies of formation can be determined using the gas-phase enthalpy of formation and enthalpy of sublimation phase transition according to the Hess' law of constant heat summation as the eqn (1). 21 Based on the electrostatic potential of a molecule by quantum mechanical prediction, the heat of sublimation can be represented as the eqn (2). 22 where, (SA) is the molecular surface area for this structure, s Tot 2 is described as an indicator of the variability of the electrostatic potential on the molecular surface, n is interpreted as showing the degree of balance between the positive and negative potentials on the molecular surface and a, b, and c are the tting parameters. We further followed the approach of Politzer to predict the heats of sublimation of energetic materials, and then combined these with eqn (1) and (2) to predict the solid enthalpies of formation. The empirical Kamlet-Jacobs (K-J) equations 16 widely employed to evaluate the energy performance of energetic compounds were used to estimate the detonation velocity (D) and detonation pressure (P) of all target compounds. Empirical Kamlet-Jacobs (K-J) equations can be shown in eqn (3)-(5).
where, D is the predicted detonation velocity (m s À1 ), P is the predicted detonation pressure (GPa), r is the density of explosives (g cm À3 ), N is the moles of detonation gases per gram explosive, M is the average molecular weight of these gases, and Q is the chemical energy of detonation (kJ g À1 ). The densities and the calculated heats of formation were used to compute the D and P values.

Conflicts of interest
There are no conicts to declare.