4,5-Dicyano-1,2,3-Triazole—A Promising Precursor for a New Family of Energetic Compounds and Its Nitrogen-Rich Derivatives: Synthesis and Crystal Structures

The nitrogen-rich compounds and intermediates with structure of monocyclic, bicyclic, and fused rings based on 1,2,3-triazole were synthesized and prepared by using a promising precursor named 4,5-dicyano-1,2,3-triazole, which was obtained by the cyclization reaction of diaminomaleonitrile. Their structure and configurational integrity were assessed by Fourier transform-infrared spectroscopy (FT-IR), mass spectrometry (MS), and elemental analysis (EA). Additionally, fourteen compounds were further confirmed by X-ray single crystal diffraction. Meanwhile, the physical properties of four selected compounds (3·H2O, 6·H2O, 10·H2O, and 16) including thermal stability, detonation parameters, and sensitivity were also estimated. All these compounds could be considered to construct more abundant 1,2,3-triazole-based neutral energetic molecules, salts, and complex compounds, which need to continue study in the future in the field of energetic materials.

In the present work, diaminomaleonitrile was used as the starting material to prepare 4,5-dicyano-1,2,3-triazole. This compound acted as a precursor, from which a proposed series of nitrogen-rich 1,2,3-triazole-based derivatives was synthesized. All these compounds were fully characterized by Fourier transform-infrared spectroscopy (FT-IR), mass spectrometry (MS), and elemental analysis (EA). Additionally, many of them were further confirmed by single-crystal X-ray diffraction. The thermal decomposition behavior was further investigated using differential scanning calorimetry (DSC) under nitrogen atmosphere. The energetic properties and mechanical sensitivity were also evaluated to better understand the structure-property relationships.
(2) Manganese dioxide can pollute the environment beyond redemption. (3) The dropping speed of potassium permanganate solution should be controlled during the operation; post-treatment has four steps that include filtration, concentration, acidification, and decolorization. (4) Low yield: 68-75%. Surprisingly, a new synthetic method of 7 was found in the present work. Compared with the literature, the merits of the new method are as follows: (1) Sodium hydroxide presents the lowest explosive risk. (2) Besides aim product, the sodium chloride is formed. Therefore, products have no manganese pollution.
(3) The experimental procedure is one-step feeding; post-treatment has only two steps just involving acidification and filtration. (4) High yield: 91%. According to the conclusion reported in the literature [28,29], the synthetic method of compound 7 was almost described in Scheme 4. The method in reference has the following disadvantages: (1) Potassium permanganate has a potential explosive risk that concentrated sulfuric acid reacts with KMnO4 to give Mn2O7, which could be explosive.
The molecular structures and crystal packing diagrams of crystals 11-16 were plotted in Figure 3a-f. From the molecular structures, we can see that all compounds contain no water except for compounds 13·H 2 O and 15·H 2 O. Additionally, they belong to different crystalline systems with orthorhombic (11), monoclinic (12, 13, 14, and 15), and triclinic (16), respectively. Obviously, all crystals possessed a plane-layered stacked structure formed by several hydrogen bonds and π-π interactions observed from crystal packing pictures. Notably, the two novel 1,2,3-triazole-fused heterocyclic compounds 10·H 2 O and 16 had a perfect coplanar geometry, which could be further studied to obtain more 1,2,3-triazolefused derivatives with good properties, including thermal stability, energetic performance, and low sensitivity. The molecular structures and crystal packing diagrams of crystals 11-16 were plotted in Figure 3a-f. From the molecular structures, we can see that all compounds contain no water except for compounds 13·H2O and 15·H2O. Additionally, they belong to different crystalline systems with orthorhombic (11), monoclinic (12, 13, 14, and 15), and triclinic (16), respectively. Obviously, all crystals possessed a plane-layered stacked structure formed by several hydrogen bonds and π-π interactions observed from crystal packing pictures. Notably, the two novel 1,2,3-triazole-fused heterocyclic compounds 10·H2O and 16 had a perfect coplanar geometry, which could be further studied to obtain more 1,2,3triazole-fused derivatives with good properties, including thermal stability, energetic performance, and low sensitivity. The molecular structures and crystal packing diagrams of crystals 11-16 were plotted in Figure 3a-f. From the molecular structures, we can see that all compounds contain no water except for compounds 13·H2O and 15·H2O. Additionally, they belong to different crystalline systems with orthorhombic (11), monoclinic (12, 13, 14, and 15), and triclinic (16), respectively. Obviously, all crystals possessed a plane-layered stacked structure formed by several hydrogen bonds and π-π interactions observed from crystal packing pictures. Notably, the two novel 1,2,3-triazole-fused heterocyclic compounds 10·H2O and 16 had a perfect coplanar geometry, which could be further studied to obtain more 1,2,3triazole-fused derivatives with good properties, including thermal stability, energetic performance, and low sensitivity.

Thermal Behavior
The thermal stabilities of the newly prepared compounds were evaluated using DSC at a heating rate of 10 • C·min −1 under a nitrogen atmosphere. The DSC curves were shown in Figure 4 with the main exothermic decomposition peak temperature (T d ), and decomposition temperature values are listed in Table 1 . Compounds 7, 8, 10, 11, 14, and 15 only possessed an endothermic process at the experimental range of 40-500 • C. The exothermic decomposition temperatures of other compounds were in the range of 256 • C (16) and 421 • C (13). The sodium salt S4 revealed a multi-stage decomposition progress.
As can be seen, most 1,2,3-triazole-based molecules were thermally stable beyond 250 • C ( Figure 4) and met the demand of thermal stability for energetic compounds in practical application.

Thermal Behavior
The thermal stabilities of the newly prepared compounds were evaluated using DSC at a heating rate of 10 °C·min −1 under a nitrogen atmosphere. The DSC curves were shown in Figure 4 with the main exothermic decomposition peak temperature (Td), and decomposition temperature values are listed in Table 1 . Compounds 7, 8, 10, 11, 14, and 15 only possessed an endothermic process at the experimental range of 40-500 °C. The exothermic decomposition temperatures of other compounds were in the range of 256 °C (16) and 421 °C (13). The sodium salt S4 revealed a multi-stage decomposition progress. As can be seen, most 1,2,3-triazole-based molecules were thermally stable beyond 250 °C ( Figure 4) and met the demand of thermal stability for energetic compounds in practical application.

Energetic Properties and Safety
Compounds 3·H 2 O, 6·2H 2 O, 10·H 2 O, and 16 were selected to explore the detonation performance and sensitivity to impact and friction. The crucial parameters were summarized in Table 1.
Enthalpy of formation is one of the most important parameters to obtain the detonation velocity and pressure for energetic compounds. Thus, the heats of formation for 3·H 2 O, 6·2H 2 O, 10·H 2 O, and 16 were obtained via the atom equivalent scheme by converting quantum mechanical energies of atoms to the enthalpy of formation of the molecule in the gas phase, Equation (1) [30]. The enthalpy of sublimation either vaporization can be predicted through the calculation of molecular electrostatic potentials [31,32], the equation represented as Equation (2). Then, the Hess' law of constant heat summation was employed to determine enthalpies of formation of the condensed-phase shown in Equation (3) [33]. The geometries of the four compounds were optimized by using the hybrid DFT-B3LYP functional with the 6-311++g(d,p) basis set. All the ab initio calculations of this work were performed via the commercial Gaussian 09 suite of programs [34]. As shown in Table 1, crystals 3·H 2 O, 6·2H 2 O, 10·H 2 O, and 16 exhibited positive enthalpies of formation of 245.0, 204.5, 92.1, and 154.3 kJ·mol −1 , respectively. Compound 3·H 2 O had the highest Enthalpy of formation among them, which could be attribute to the introduction of the energetic nitro group.
In Equation (2), SA is the molecular surface area of the 0.001 electron/bohr 3 isosurface of the electron density of the molecule, σ 2 tot is described as an indicator of the variability of the electrostatic potential on the molecular surface, and υ is interpreted as showing the degree of balance between the positive and negative potentials on the molecular surface where a, b, and c are fitting parameters.
Based on the experimental crystal density values and calculated enthalpies of formation, the detonation properties of the four compounds were estimated by using EXPLO5 (v6.04) program [35]. The energetic parameters were listed in Table 1. Results indicated that the four molecules had moderate detonation parameters, with detonation velocities in the range of 6.9 km·s −1 and 7.9 km·s −1 and detonation pressures between 16.6 GPa and 23.9 GPa, respectively. Compare the detonation parameters of compounds 10·H 2 O and 16, it can be easily observed that the existence of methyl group greatly reduces the density and then decreases the detonation properties of the compound 16.
The impact and friction sensitivity were tested by employing a standard BAM fall hammer and a BAM friction tester, respectively. All compounds exhibited low mechanical sensitivities (IS: > 40 J; FS: > 360 N). Combined with thermal stability and detonation performance, compound 6·2H 2 O possessed the most balanced comprehensive properties (d = 1.659 g·cm −3 , P = 23.9 GPa, D = 7.9 km·s −1 , T d = 262 • C, IS > 40 J, FS > 360 N). Therefore, all these 1,2,3-triazole-based derivatives could be used as precursors to prepare more energetic materials with excellent detonation performance.

General Information
All chemical reagents and solvents of analytical grade were obtained from commercially sources. Elemental analyses were performed on a Flash EA 1112 fully automatic trace element analyzer (Waltham, MA, USA). The FT-IR spectra were recorder as KBr pellets on a Bruker Equinox 55 (Bruker, Germany). Mass spectra were recorded on an Agilent 500-MS (Palo Alto, CA, USA). The single-crystal X-ray diffraction analysis was carried out by on Bruker CCD area-detector diffractometer (Bruker, Germany). The single-crystal X-ray diffraction data were collected at 293-298 K using a Bruker CCD area detector diffractometer equipped with a graphite-monochromatized Mo/Kα radiation (λ = 0.71073 Å) using phi and omega scans. Data collection and initial unit cell refinement were performed with Bruker SMART (Bruker, Germany). Data reduction were performed by using Bruker SHELXTL (Bruker, Germany). Using Olex2 [36], the structures were further solved and refined with the aid of the programs using direct methods and full-matrix least-squares method on F2 by SHELXS-97 and SHELXL-97 [37]. The full-matrix least-squares refinement on F2 involved atomic coordinates and anisotropic thermal parameters for all non-H atoms. The H atoms were included using a riding model. The non-H atoms were refined anisotropically. Differential scanning calorimetry (DSC) was carried out on a model Pyris-1 differential scanning calorimeter under nitrogen atmosphere. Impact and friction sensitivities were performed on a BAM fall hammer BFH-10 and a BAM friction apparatus FSKM-10, respectively.

Conclusions
In summary, we have shown a study on the 1,2,3-triazole-based compounds and designed five routes to synthesize fifteen compounds starting from commercially available diaminomaleonitrile. The operation of experiments is simple, convenient, and sustainable. All prepared compounds were characterized by EA, FT-IR, and MS spectra. Additionally, the crystal structure of fourteen compounds were cultivated and determined with single crystal X-ray crystallography technology. Among them, nine compounds have high nitrogen content (compound 1, N% = 51%; 2, N% = 64%; 3, N% = 50%; 4, N% = 62%; 6, N% = 54%; 9, N% = 53%; 10, N% = 46%; 15, N% = 49%; 16, N% = 42%), of which as high-nitrogen content molecules has a potential possibility to be used in pyrotechnic fire extinguishers as a coolant and reductant, which can produce large quantities of nitrogen when combusted. The DSC results indicated that these compounds had good thermal stabilities with the exothermic decomposition temperatures between 256 • C (16) to 421 • C (13), except for the six compounds 7, 8, 10, 11, 14, and 15. Four compounds 3·H 2 O, 6·2H 2 O, 10·H 2 O, and 16 were selected to explore the detonation performance and sensitivity. We found that the four compounds exhibited a moderate detonation performance, with detonation velocities in the range of 6.9 km·s −1 and 7.9 km·s −1 and detonation pressures between 16.6 GPa and 23.9 GPa, respectively. As expected, they are also insensitive to impact and friction (IS: > 40 J; FS: > 360 N). Consequently, these compounds have the