There are currently ongoing efforts to synthesize novel energetic materials with improved performance and decreased sensitivity [1,2,3,4,5,6]. However, these requirements are quite often mutually exclusive, such that the desired combination of high detonation performance with maximum possible chemical stability is a major challenge. Over the last few decades, several strategies for the design of energetic materials that have a high energy content in conjunction with good thermal and mechanical stabilities have been developed [7,8], one of which is to replace benzene rings with nitrogen-containing heterocycles as fundamental components of molecular structures [2,9]. Compared with the benzene ring, nitrogen-containing heterocycles typically possess a greater enthalpy of formation as well as a higher density and more positive oxygen balance [10,11], all of which are desirable characteristics for new energetic compounds. In recent years, the application of azine as a new energetic skeleton has attracted considerable interest, and many polynitro-azine derivatives have been designed and synthesized . However, due to the electron deficient character of the azine ring, it is difficult to perform electrophilic aromatic substitution on the ring, so the introduction of nitro groups into azine rings remains a challenging task in practice.
Nitro-based moieties represent the most common energetic functional group, and contribute markedly to the overall performance of an energetic compound by enhancing both the oxygen balance (OB) and density [13,14,15,16,17]. Among these groups, the trinitromethyl group has the highest oxygen content, and its introduction to a molecule is essentially equivalent to adding two nitro groups (since one of the nitro groups is necessary for the complete oxidation of the carbon atom in the methyl group). The incorporation of trinitromethyl moieties into an azine ring is thus a potential means of increasing the OB so as to improve the detonation performance, and some trinitromethylazines have been synthesized using various methods. As an example, 2,4,6-tris(trinitromethyl)-1,3,5-triazine was obtained by the destructive nitration of the carboxyl groups to trinitromethyl entities in 2,4,6-tris(dicarboxymethylene)-1,3,5-triazine by action of concentrated HNO3 . This compound is an oxidant and has a high density (2.00 g/cm3), but it is unstable in the air and very easy to hydrolyze. Trinitromethyl-substituted pyrazine was also obtained as an unexpected product by the treatment of 2-alkoxy-3-methylpyrazine with an HNO3/H2SO4 mixture . The attachment of trinitromethyl groups with a pyridine ring has also been attempted by different techniques. Gakh and Khutoretskii reported the synthesis of 2-trinitromethylpyridine by the reaction of N-fluoropyridinium fluoride with trinitromethane . Pyridine with methyl groups in the 2, 3, and 4 positions behaved in the same way and the corresponding trinitromethyl-substituted methylpyridines were obtained. Subsequently, they also studied the behavior of this series of trinitromethylpyridines upon electron impact . Another synthesis method for 2-trinitromethylpyridine was reported in 1993 by the ozone-mediated C-nitration of methylpyridines with nitrogen dioxide . More recently, Katritzky et al. obtained 5-methyl-2-trinitromethylpyridine on the nitration of 2,5-lutidine with nitric acid in trifluoroacetic anhydride . However, these methods shared an obvious disadvantage in that the yields were rather low (below 20%).
In contrast, the synthesis route of 2-trinitromethylpyridine through the reaction of N2O4 with pyridine-2-carboxaldoxime, which was developed at the N.D. Zelinsky Institute of Organic Chemistry, appeared to be a favorable method for high yield (more than 60%) . In fact, a series of trinitromethyl-substituted arenes and hetarenes have been prepared through the interaction of N2O4 with corresponding carboxaldehyde oximes [25,26]. Although tris(trinitromethyl)benzene has been synthesized by the reaction , to the best of our knowledge, heterocyclic compounds with more than one trinitromethyl group synthesized in this way have not yet been reported. Hence, introducing more trinitromethyl groups into the pyridine ring becomes an attractive but challenging objective, as it requires overcoming electronic and stereo-hindrance effects (Figure 1). At the same time, in consideration of the well-known instability of the trinitromethyl moiety at higher temperatures, the ionization of trinitromethyl-substituted pyridines is also worth exploring, as ionic compounds usually display better stabilities than their nonionic precursors . An interesting denitration reaction of trinitromethyl groups into dinitromethyl groups in heterocycle substrates has been studied. For example, The denitration of 2-trinitromethylpyridine with potassium iodide in methanol allowed for the obtainment of the potassium salt of 2-(dinitromethyl)pyridine . The reaction of mono(trinitromethyl)-1,3,5-triazines with potassium iodide in methanol resulted in dinitromethyl-1,3,5-triazine potassium salts . Additionally, the conversion of trinitromethyl groups attached to imidazoles, triazoles, and tetrazoles into the corresponding dinitromethylide salts following treatment with hydrazine has been reported [4,29,30]. Based on these successful examples, the trinitromethyl-substituted pyridines in our research were subject to attempted denitration using hydrazine.
In our continuing search for new high-performance polynitro energetic materials, we are interested in developing novel trinitromethyl-substituted heterocyclic compounds. Thus, in the present study, we attempted to introduce more trinitromethyl groups onto the pyridine ring to increase energetic performance. These trinitromethyl-substituted pyridines were subsequently converted into dinitromethylide hydrazinium salts. Each of these compounds was fully characterized and their energetic properties were determined either empirically or theoretically.
2. Results and Discussion
As was stated in the introduction, 2-trinitromethylpyridine (1) was synthesized through the reaction of N2O4 with pyridine-2-carboxaldoxime (Scheme 1). Based on a method reported in literature, a simpler procedure was employed. N2O4 was added into a solution of pyridine-2-carboxaldoxime at room temperature and the purification of products was carried out by column chromatography in a Biotage Isolera One apparatus. The reaction mechanism has been given in the literature earlier (Scheme 2) [25,31]. The preparation of bis(trinitromethyl)-substituted pyridine was somewhat more challenging. Firstly, compared with the preparation of 1, the reaction of pyridine-2,6-dicarboxaldehyde dioxime with N2O4 required a higher reaction temperature (60 °C rather than 40 °C). In addition, X-ray diffraction (XRD) data indicated that the reaction yielded two main products: the desired 2,6-bis(trinitromethyl)pyridine (2) and the byproduct 2-cyano-6-(trinitromethyl)pyridine (3). It was found that increasing the N2O4 concentration (within the range of 0.5‒2.5 mL N2O4/mmol pyridine-2,6-dicarboxaldehyde dioxime) increased the relative proportion of 2 in the product mixture. The formation of the by-product 3 is also explained based on literature reports (Scheme 3) . Firstly, the intermediate nitrolic acid underwent the loss of a molecule of nitrous acid and the nitrile oxide was obtained. In the second step, nitrogen monoxide removed an oxygen atom with the formation of nitrile and nitrogen dioxide.
Following the preparation of the above compounds, the ionization of 1 and 2 was attempted. Based on reports, the analogous denitration of trinitromethyl-substituted pyridines using hydrazine was examined in our research. The trinitromethyl-substituted pyridine 1 was first reacted with 80% aqueous hydrazine, and the light yellow hydrazinium salt of 2-dinitromethylpyridine (4) was obtained in high yield (Scheme 4). Subsequently, 2,6-bis(trinitromethyl)pyridine was also successfully converted into the dihydrazinium salt of 2,6-bis(dinitromethyl)pyridine (5). To the best of our knowledge, this is the first report of the preparation of a dihydrazinium salt in this manner.
All the compounds prepared in this work were stable in air and could be stored for extended periods of time. Their structures were confirmed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), and elemental analysis. Compounds 1, 2, 3, 4 and 5 were also characterized by X-ray crystallography. Selected data and X-ray structural parameters are provided in the Extended Supplemental Information.
2.2. X-ray Crystallography
Single crystals of 1, 2 and 3 suitable for single-crystal XRD were obtained by the slow evaporation of dichloromethane and n-hexane solutions of these compounds at room temperature, whereas crystals of 4 and 5 were obtained from methanol and water, respectively. All crystals were stable at room temperature and were found not to be hygroscopic. Their molecular structures are shown in Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 and the crystallographic data are summarized in Table 1.
Compound 1 crystallizes in the monoclinic space group P21/c with four molecules per unit cell and has a calculated density of 1.692 g·cm−3 at 153 K (Figure 2). The bond lengths and bond angles in the pyridine ring of this compound are in the typical ranges , although the C6–N2, C6–N3, and C6–N4 bond lengths in the trinitromethyl group are 1.528, 1.538, and 1.524 Å, respectively, all of which are longer than a standard C–N single bond (1.460 Å) . The C6 atom is in the plane of the pyridine ring and is associated with the torsion angles C1–N1–C5–C6 = −179.47° and C3–C4–C5–C6 = −179.68°. The pyridine ring and three nitro groups are attached to C6 in a tetragonal configuration.
Compound 2 crystallizes in the triclinic space group P with eight molecules per unit cell and a calculated density of 1.832 g·cm−3 at 105 K (Figure 3). Compared to 1, its density is significantly increased because of the introduction of the second trinitromethyl group. Similar to 1, the C–N bond lengths in the trinitromethyl groups are longer than normal C–N single bonds. The large volume of the two trinitromethyl groups makes the region beside the pyridine N atom quite crowded and they stretch in an optimal conformation, trying to decrease the repulsive strain. The N1, N2, N6 and N7 atoms are distributed in an approximately symmetric manner on both sides of the pyridine ring. The N5 atom and the pyridine ring are almost coplanar, as can be seen from the N4–C6–C7–N5 torsion angle (−3.65°), while the N3 is slightly twisted out of this plane based on the N4–C2–C1–N3 torsion angle (−15.7°). However, it is evident that a repulsive strain is still present, as demonstrated by a comparison of bond angles; the N4–C2–C1 and N4–C6–C7 bond angles are 114.7° and 113.8°, respectively, while the corresponding bond angle in compound 1 is 111.9°.
Compound 4 crystallizes in a monoclinic crystal system with space group P21/n and a density of 1.617 g·cm−3 at 153 K (Figure 5). The C–N and C–C bond lengths of the pyridine ring are in the expected ranges. Compared to 1, the C1–C2 bond length (1.484 Å) is slightly shorter. In addition, the C1–N1 and C1–N2 bond lengths of the dinitromethyl group are 1.362 Å and 1.391 Å, respectively, and thus are both much shorter than the analogous bonds in 1 and normal C–N single bonds. This can be explained by the change in the electron density distribution after the removal of a nitro substituent of the trinitromethyl group. Compared with the earlier reported crystal structures of several zwitterionic dinitromethylazine derivatives in which the protonation occurred at one of the nitrogens of the azine ring [35,36], the only nitrogen atom in the pyridine ring was not protonated, while the N2H5+ cation was formed, which was the same situation with the analogical reactions in earlier literature [4,29,30]. Interestingly, the remaining atoms of the dinitromethyl group are inclined to form a plane with the torsion angles O4–N2–C1–N1 = 171.24° and O2–N1–C1–N2 = 3.7°. The two planes defined by the dinitromethyl group and the pyridine ring are almost perpendicular (N3–C2–C1–N2 = 94.53° and C3–C2–C1–N1 = 95.42°).
Compound 5 crystallizes in an orthorhombic crystal systemspace group Pbcn with a calculated density of 1.782 g·cm−3 at 153 K and four molecules in a unit cell (Figure 6). Disorder of hydrazinium cations was observed in the crystal. Similar to compound 4, the bonds connecting the pyridine and dinitromethyl groups as well as the C–NO2 bonds become shorter than its parent compound 2 and the protonation sites are also located in the nitrogen atoms of hydrazine. However, the remaining atoms of each dinitromethyl group in compound 5 are not inclined to be coplanar with the torsion angles (O1–N1–C1–N2 and O8–N5–C7–N5 = −160.70°; O3–N2–C1–N1 and O6–N4–C7–N5 = 9.07°), which is different from compound 4.
2.3. Physicochemical and Energetic Properties
The physicochemical and energetic properties of each of the compounds are summarized in Table 2.
In this work, the thermal stabilities of all compounds were determined by differential scanning calorimetry (DSC) measurements scanning at 5 °C·min−1 under dry, oxygen-free nitrogen over the temperature range 25–500 °C. The melting points of compounds 1‒3 were in the range 68–96 °C, while their decomposition temperatures ranged from 101 to 114 °C. Both 1 and 3 were found to have lower melting points than TNT. These two compounds were also observed to have very similar melting points, but the latter had a significantly lower decomposition point, demonstrating that the introduction of a cyano group has negative effects on the thermal behavior of 1. The introduction of the second trinitromethyl in the case of 2 significantly increases the melting temperature, but its thermal stability becomes worse with a decomposition temperature of 101 °C. The relatively low decomposition points of 1, 2 and 3 can be attributed to the well-known instability of the trinitromethyl group at higher temperatures. However, when the trinitromethyl group underwent the loss of a nitro group and was converted to a dinitromethylide salt, salts 4 and 5 displayed much better thermal stability than their parent compounds, with decomposition temperatures of 181 and 125 °C, respectively.
The sensitivities of these compounds towards impact (IS) and friction (FS) were determined according to the standard BAM methods . Compound 1 (IS = 16 J, FS > 360 N) has impact and friction sensitivities similar to those of TNT. As a result of the introduction of the second trinitromethyl group, 2 (IS = 9 J, FS = 192 N) is more sensitive than 1 but still less sensitive than RDX (IS = 7.4 J, FS = 120 N). As noted, the formation of salts is often an effective means of reducing sensitivity . However, salt 4 possesses similar sensitivity to its parent compound 1, and is even slightly more sensitive regarding impact sensitivity. In contrast, salt 5 is significantly less sensitive than 2, especially with regard to FS.
OB plays an important role in determining the detonation properties of energetic compounds. Comparing the OB values of 1‒5, 2 has the highest value of −14.85%, exceeding that of 2,4,6-trinitropyridine-1-oxide (TNPyO), which has the best OB (−27.8%) among the previously-reported energetic pyridine derivatives. The OB values of the hydrazinium salts are lower due to the addition of cations and the loss of nitro groups, although salt 5 still has a good OB comparable to that of 1.
Density is another important physical property of energetic materials. The densities of all the compounds were measured using a gas pycnometer at 25 °C, and were found to be in the range of 1.56 (4)–1.77 g·cm−3 (2). Although the bulky trinitromethyl group usually has a negative effect on crystal packing, the densities of these compounds increase in proportion to the accumulation of trinitromethyl groups, such that 2 possesses a significantly improved density compared to 1. The enthalpies of formation were computed based on the Gaussian 03 program package  and each compound exhibited a positive enthalpy of formation. Interestingly, the byproduct 3 had the highest value (314.22 kJ·mol−1), most likely due to the high bond energy of the cyanogroup. Based on the calculated enthalpies of formation and the measured densities at ambient temperature, the detonation properties of all the compounds were determined using the EXPLO5 (v6.01) program . The calculated detonation velocities (D) and detonation pressures (P) were in the range of 7438–8700 m·s−1 and 19.6–33.2 GPa, respectively. Compound 1 exceeds TNT in terms of detonation properties, while 2 exhibits superior performance to 1
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