Coordinative Combination of Nitroamine and Gem-Dinitromethyl with Fused Ring for Enhanced Oxygen Balances and Detonation Properties

Oxygen balance and heat of formation are closely related to the nitrogen and oxygen content in a molecule and have a significant effect on the detonation performance of energetic materials. Here a new family of 1,2,4-triazolo [4,3-b][1,2,4,5]-tetrazine containing gem-dinitromethyl and nitroamine with high nitrogen-oxygen content was synthesized and characterized. Moreover, the structure of the guanidine salt (3) and TATOT salt (4) were confirmed by single-crystal X-ray diffraction. The nitrogen and oxygen content of ammonium salt 2 reached 82.5%, with a high density (1.805 g cm−3) and high detonation properties (D = 8900 m s−1; P = 32.4 GPa), which were similar to those of RDX.


Introduction
The design and synthesis of energetic compounds with both enhanced energy and safety are highly desired in the field of energetic materials. Due to the existence of the energy and safety contradiction, higher energy usually leads to lower safety. The energetic compounds involving a single explosophoric group always result in only one of the energy or safety properties being improved. For instance, although increased detonation performances were obtained for the nitroamine benchmark explosives such as 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX) or hexanitrohexaazaisowurtzitane , the large number of nitroamine groups in the structure resulted in a decreased safety with sensitivities located in the sensitive range ( Figure 1a). The coordinative combination of different explosophoric groups is beneficial for achieving enhanced comprehensive properties for energetic compounds; the representative examples, such as 2,4,6-triamino-1,3,5-trinitrobenzene (TATB) and 1,1-diamino-2,2-dinitroethene (FOX-7), are insensitive explosives due to the existence of C-NH 2 and C-NO 2 groups [1][2][3]. Demonstrating the coordinative combination of an explosophoric group is an effective way to tune the structures of energetic compounds to gain desired properties [4,5].
Due to the limited number of explosophoric groups, the construction of novel backbones is a commonly used strategy for designing new energetic compounds. The nitrogenrich conjugated coplanar system has the advantages of high heat of formation (HOF) and ring-strain energy to store more chemical energy, and has therefore become a considerable choice as the skeleton for high energy density materials (HEDMs) [6]. Lots of fused heterocyclic ring-based energetic compounds preserving good thermal stability and low sensitivities have been designed and synthesized [7]. A higher chemical energy storage for the skeleton which is reflected in forms of higher heat of formation usually consists of more nitrogen atoms in the conjugated system. However, the increase of the nitrogen atoms in Int. J. Mol. Sci. 2022, 23, 14337 2 of 8 the skeleton always results in a decreased reactivity and makes it difficult to introduce nitro groups directly bonded to the fused ring. To maintain the maximum nitrogen atoms as well as two substituted positions in the structure, the 1,2,4-triazolo [4,3-b] [1,2,4,5]tetrazine fused backbone, which can be readily constructed by reacting hydazinyl-1,2,4,5-tetrazines with cyanogen bromide, is an ideal option for designing novel energetic compounds.
compounds (Figure 1b,c) [10][11][12][13][14]. Most of those compounds exhibit improved detonation properties and oxygen balances; however, a high sense of unsatisfactory sensitivity was also observed, which limits their practical applications [15,16]. Incorporating a suitable backbone with gem-dinitromethyl and nitroamine groups to gain enhanced stabilities would be a good promotion for further applications. Triazole-tetrazine fused backbone has found utility in designing new HEDMs with enhanced detonation properties and stability for its large conjugated system [17,18]. In this work, a series of triazole-tetrazine energetic salts 1-4 containing gem-dinitromethyl and nitroamine energetic groups have been successfully synthesized (Figure 1d). Their properties were tuned by incorporation with different cations. The structures of the new compounds were characterized by multinuclear NMR, elemental analyses, IR and X-ray single crystal diffraction. It is interesting to note that all the prepared salts show good sensitivity to mechanical stimuli, which is a promising perspective for practical applications.

Synthesis
The synthetic route is shown in Scheme 1. The starting material, compound S1, was obtained using a special aminohydrazone cyclization strategy based on our previous research [18]. Then, the further nitration of S1 in 100% nitric acid creates the fused triazole- The combination of gem-dinitromethyl [8] and nitroamine [9] groups with bridged heterocycles or single five member ring such as furazan-1,2,4-triazole, furazan-1,2,4-oxadiazole, 1,2,4 triazole or furazan were previously reported to access novel energetic compounds (Figure 1b,c) [10][11][12][13][14]. Most of those compounds exhibit improved detonation properties and oxygen balances; however, a high sense of unsatisfactory sensitivity was also observed, which limits their practical applications [15,16]. Incorporating a suitable backbone with gem-dinitromethyl and nitroamine groups to gain enhanced stabilities would be a good promotion for further applications. Triazole-tetrazine fused backbone has found utility in designing new HEDMs with enhanced detonation properties and stability for its large conjugated system [17,18]. In this work, a series of triazole-tetrazine energetic salts 1-4 containing gem-dinitromethyl and nitroamine energetic groups have been successfully synthesized ( Figure 1d). Their properties were tuned by incorporation with different cations. The structures of the new compounds were characterized by multinuclear NMR, elemental analyses, IR and X-ray single crystal diffraction. It is interesting to note that all the prepared salts show good sensitivity to mechanical stimuli, which is a promising perspective for practical applications.

Synthesis
The synthetic route is shown in Scheme 1. The starting material, compound S1, was obtained using a special aminohydrazone cyclization strategy based on our previous research [18]. Then, the further nitration of S1 in 100% nitric acid creates the fused triazole-tetrazine nitroform compound S2. Pure S2 can be isolated by column chromatography; S2 is stable in solvent but will decompose fast in a solid state. The dipotassium salt 1 was readily prepared in good yield by reacting S2 with potassium iodide in methanol. However, if the potassium iodide was replaced by hydroxylamine hydrochloride or potassium hydroxide, the reduction of nitroform proceeded incompletely due to the weak reducibility of hydroxylamine. The attempt to prepare the hydrazinium salt by reacting diluted hydrazine hydrate, which has a strong reducing ability, resulted in a significant decomposition of S2. Subsequently, the diammonium salt 2 (yield: 83.2%), diguanidine salt 3 (yield: 82.1%) and di-TATOT salt 4 (yield: 79.5%) were prepared through a metathesis reaction using silver salts with corresponding chloride salt.
tetrazine nitroform compound S2. Pure S2 can be isolated by column chromatography S2 is stable in solvent but will decompose fast in a solid state. The dipotassium salt 1 was readily prepared in good yield by reacting S2 with potassium iodide in methanol. How ever, if the potassium iodide was replaced by hydroxylamine hydrochloride or potassium hydroxide, the reduction of nitroform proceeded incompletely due to the weak reducibil ity of hydroxylamine. The attempt to prepare the hydrazinium salt by reacting diluted hydrazine hydrate, which has a strong reducing ability, resulted in a significant decom position of S2. Subsequently, the diammonium salt 2 (yield: 83.2%), diguanidine salt 3 (yield: 82.1%) and di-TATOT salt 4 (yield: 79.5%) were prepared through a metathesis reaction using silver salts with corresponding chloride salt.  (3) (CCDC 2204894) were obtained from water. Compound 3 belongs to the monoclinic crystal system with space group Cc (Z = 4) symmetry with a crystal density of 1.629 g cm −3 (296 K). The X-ray structure and crystal parameters of com pound 3 are shown in Figure 2a and Table S1. From Figure 2b (3) (CCDC 2204894) were obtained from water. Compound 3 belongs to the monoclinic crystal system with space group Cc (Z = 4) symmetry with a crystal density of 1.629 g cm −3 (296 K). The X-ray structure and crystal parameters of compound 3 are shown in Figure 2a and Table S1. From Figure 2b tetrazine nitroform compound S2. Pure S2 can be isolated by column chromatography; S2 is stable in solvent but will decompose fast in a solid state. The dipotassium salt 1 was readily prepared in good yield by reacting S2 with potassium iodide in methanol. However, if the potassium iodide was replaced by hydroxylamine hydrochloride or potassium hydroxide, the reduction of nitroform proceeded incompletely due to the weak reducibility of hydroxylamine. The attempt to prepare the hydrazinium salt by reacting diluted hydrazine hydrate, which has a strong reducing ability, resulted in a significant decomposition of S2. Subsequently, the diammonium salt 2 (yield: 83.2%), diguanidine salt 3 (yield: 82.1%) and di-TATOT salt 4 (yield: 79.5%) were prepared through a metathesis reaction using silver salts with corresponding chloride salt.  (3) (CCDC 2204894) were obtained from water. Compound 3 belongs to the monoclinic crystal system with space group Cc (Z = 4) symmetry with a crystal density of 1.629 g cm −3 (296 K). The X-ray structure and crystal parameters of compound 3 are shown in Figure 2a and Table S1. From Figure 2b  Crystal of the TATOT salt (4·H 2 O) (CCDC 2204893) was grown from methanol with a crystal density of 1.760 g cm −3 (298K) (Figure 3a). Compound 4 crystallizes in the monoclinic crystal system and belongs to the P2 1 -c (Z = 4) symmetry. Similar to guanidine salt, the geminal dinitro group is twisted totally out of the plane with N18-C6-C5-N16 = 68.1 • and N17-C6-C5-N13 = 65.0 • (Figure 3b). As shown in Figure 3d, the dihedral angles of the fused ring plane B and another TATOT cation plane C with plane A are 75.51 • and 81.98 • , respectively, implying that the fused ring is substantially parallel to plane C. Meanwhile, the asymmetric unit of 4 contains one water molecule as a significant link by forming hydrogen bonds (HBs) interaction. First, the water forms three intramolecular HBs in the same layer. Then, the O7-H7C. . . N1 and N1-H1B. . . O7 HBs connect the water with another two molecules in a different direction (Figure 3c). Thus, abundant hydrogen bonds between compound 4 and H 2 O molecule aid in wave-like packing (Figure 3e). Crystal of the TATOT salt (4·H2O) (CCDC 2204893) was grown from methanol with a crystal density of 1.760 g cm −3 (298K) (Figure 3a). Compound 4 crystallizes in the monoclinic crystal system and belongs to the P21-c (Z = 4) symmetry. Similar to guanidine salt, the geminal dinitro group is twisted totally out of the plane with N18-C6-C5-N16 = 68.1° and N17-C6-C5-N13 = 65.0° (Figure 3b). As shown in Figure 3d, the dihedral angles of the fused ring plane B and another TATOT cation plane C with plane A are 75.51° and 81.98°, respectively, implying that the fused ring is substantially parallel to plane C. Meanwhile, the asymmetric unit of 4 contains one water molecule as a significant link by forming hydrogen bonds (HBs) interaction. First, the water forms three intramolecular HBs in the same layer. Then, the O7-H7C…N1 and N1-H1B…O7 HBs connect the water with another two molecules in a different direction (Figure 3c). Thus, abundant hydrogen bonds between compound 4 and H2O molecule aid in wave-like packing (Figure 3e).

Sensitivity
Sensitivity is an important physical index for the secure use of energetic materials. The sensitivity was evaluated by using standard BAM testers. As shown in Table 1, all the new synthesized compounds, including metal salt, have lower sensitivity. The guanidine salt (3) possesses an impact sensitivity of 40 J, and friction sensitivity of 360 N, which can be attributed to the strong hydrogen bond network. Due to the slip action of the wavelike stacking, the TATOT salt (4) shows a lower impact sensitivity (40 J) and friction sensitivity (160 N), indicating the contributions to reduced sensitivity.
To better understand the reasons for low sensitivity, Hirshfeld surfaces and the associated two-dimensional fingerprint plots were introduced using the software CrystalExplorer 17.5 as shown in Figure 4a,b [19,20]. The red spots on the Hirshfeld surface represent high close contact areas. The red dots are mainly concentrated on the sides of the surface, representing the close HBs interactions between O…H and N…H. The percentage contribution of each type of interaction is aggregated according to the 2D fingerprint. O…H and N…H interactions in salts 3 and 4 play a major role, accounting for 59.1% and 53.9% of the total weak interactions, respectively, indicating the advantage of the existence

Sensitivity
Sensitivity is an important physical index for the secure use of energetic materials. The sensitivity was evaluated by using standard BAM testers. As shown in Table 1, all the new synthesized compounds, including metal salt, have lower sensitivity. The guanidine salt (3) possesses an impact sensitivity of 40 J, and friction sensitivity of 360 N, which can be attributed to the strong hydrogen bond network. Due to the slip action of the wave-like stacking, the TATOT salt (4) shows a lower impact sensitivity (40 J) and friction sensitivity (160 N), indicating the contributions to reduced sensitivity.
To better understand the reasons for low sensitivity, Hirshfeld surfaces and the associated two-dimensional fingerprint plots were introduced using the software CrystalExplorer 17.5 as shown in Figure 4a,b [19,20]. The red spots on the Hirshfeld surface represent high close contact areas.  Figure S15 and Table 1.

Physicochemical and Energetic Properties
The heat of formation (HOF) for these new energetic salts was calculated by the Gaussian 09 (Revision E.01) suite of programs using isodesmic reactions (Scheme S1, Supplementary Materials). The potassium salt 1 (172.6 kJ mol −1 ) exhibits positive HOFs based on the extensive N-N bonds of this new triazole-tetrazine anion, higher than the traditional explosive RDX (70.3 kJ mol −1 ). Meanwhile, TATOT salt 4 has the highest heat of formation of 1627.3 kJ mol −1 , followed by guanidine salt 3 (485.1 kJ mol −1 ) and ammonium salt 2 (439.7 kJ mol −1 ). The detonation velocity (D) and detonation pressure (P) of these new compounds compared with traditional explosives RDX were summarized based on the density, which were measured by using a gas pycnometer (25 • C) in Table 1

Safety Precaution
In this work, all compounds are potential energetic materials that tend to explode under certain external stimuli. Therefore, the whole experimental process should be carried out by using proper safety equipment, such as safety shields, eye protection and leather gloves.

General Methods
1 H and 13 C NMR spectra were tested using Bruker Avance NEO 400 MHz spectrometer (400 and 100 MHz, respectively) in d-DMSO. Chemical shifts are reported as δ values relative to internal standard d-DMSO (δ 2.50 for 1 H NMR and 39.52 for 13 C NMR) using Bruker TopSpin 4.0.9. Infrared spectra (IR) were obtained on a PerkinElmer Spectrum BX FT-IR instrument equipped with an ATR unit at 25 • C using an Omnic software. Elemental analyses of C/H/N were investigated on a Vario EL III Analyzer. The onset decomposition temperature was measured using a TA Instruments discovery DSC25 differential scanning calorimeter at a heating rate of 5 • C min −1 under dry nitrogen atmosphere. Densities were determined at room temperature by a Micromeritics AccuPyc 1345 gas pycnometer. Impact and friction sensitivities were tested by a BAM fallhammer and friction tester. X-ray diffractions of all single crystals were carried out on a Bruker D8 VENTURE diffractometer using Mo-Kα radiation (λ = 0.71073 Å). The crystal structures were produced employing Mercury 2021.1.0 software and XP. All reagents used in the experiment were purchased from Aladdin manufacturers.

General Procedures for Synthesis of Compounds 2, 3 and 4
The silver salt was synthesized by reacting with AgNO 3 . To a suspension of the silver salt (2.00 mmol) in water, 3.80 mmol of corresponding hydrochloride salt was added. After stirring for 8 h at room temperature, the solid was removed by filtration. The filter cake was washed with water (100 mL) and the filtrate was removed, then dried under vacuum to give the corresponding compound.  (Figures S3, S4, S10 and S14).