5-Nitrotetrazol and 1,2,4-Oxadiazole Methylene-Bridged Energetic Compounds: Synthesis, Crystal Structures and Performances

A new structural type for melt cast materials was designed by linking nitrotetrazole ring with 1,2,4-oxadiazole through a N-CH2-C bridge for the first time. Three N-CH2-C linkage bridged energetic compounds, including 3-((5-nitro-2H-tetrazol-2-yl) methyl)-1,2,4-oxadiazole (NTOM), 3-((5-nitro-2H-tetrazol-2-yl)methyl)-5-(trifluoromethyl)-1,2,4 -oxadiazole (NTOF) and 3-((5-nitro-2H-tetrazol-2-yl)methyl)-5-amine-1,2,4-oxadiazole (NTOA), were designed and synthesized through a two-step reaction by using 2-(5-nitro-2H-tetrazole -2-yl)acetonitrile as the starting material. The synthesized compounds were fully characterized by NMR (1H, 13C), IR spectroscopy and elemental analysis. The single crystals of NTOM, NTOF and NTOA were successfully obtained and investigated by single-crystal X-ray diffraction. The thermal stabilities of these compounds were evaluated by DSC-TG measurements, and their apparent activation energies were calculated by Kissinger and Ozawa methods. The crystal densities of the three compounds were between 1.66 g/cm3 (NTOA) and 1.87 g/cm3 (NTOF). The impact and friction sensitivities were measured by standard BAM fall-hammer techniques, and their detonation performances were computed using the EXPLO 5 (v. 6.04) program. The detonation velocities of the three compounds are between 7271 m/s (NTOF) and 7909 m/s (NTOM). The impact sensitivities are >40 J, and the friction sensitivities are >360 N. NTOM, NTOF and NTOA are thermally stable, with decomposition points > 240 °C. The melting points of NTOM and NTOF are 82.6 °C and 71.7 °C, respectively. Hence, they possess potential to be used as melt cast materials with good thermal stabilities and better detonation performances than TNT.


Introduction
Melt cast explosives are widely used in military and civilian fields such as grenades, mortars, warheads and antipersonnel mines [1]. Although the melting points of melt cast explosives are generally between 70 and 120 • C, a compound possessing melting point below 100 • C is ideal in order to use steam for low-cost melting operations [2]. 2,4,6-Trinitrotoluene (TNT) and dinitroanisole (DNAN) are two kinds of widely investigated and applied melt cast materials. However, their further applications are limited by their relative shortcomings, such as the toxicity and environmental problems of TNT [3,4] and low detonation performances of DNAN [5,6]. Therefore, there is continued interest in developing new melt cast materials with better explosive performances.

Results and Discussions
2.1. Synthesis 5-nitrotetrazol-3-acetonitrile (1) was prepared according to the method we developed previously [17]. The intermediate compound N'-hydroxy-2-(5-nitro-2H-tetrazol-2-yl)acetimidamide (NTAA) was synthesized from compound one in the presence of hydroxylamine hydrochloride with a yield of 96%. Based on NTAA, three methyl-bridged nitrotetrazole and 1,2,4-isofurazan compounds including NTOM, NTOF and NTOA were synthesized with yields of 82%, 79% and 62%, respectively. In order to synthesize energetic compounds with better energy performances, we tried to oxidize and nitrate the amino group of NTOA to the nitro or nitramine group, but failed to obtain the targeted compounds (Scheme 1).
ing new strategy for constructing energetic compounds that possess good detonation parameters, better thermal stabilities and lower sensitivity toward impact and friction. In 2019, we firstly synthesized NTOA [17], but detailed structural characteristics and performance investigations were not investigated. Herein, based on our previous studies, two new energetic compounds (NTOF and NTOM) were firstly designed and synthesized ( Figure 1c). All of the synthesized compounds in this study were characterized by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. X-ray crystallographic measurements of the three compounds were performed, providing insight into structural characteristics as well as intramolecular and intermolecular interactions of these compounds. The thermal stabilities were evaluated by DSC-TG measurements, and the detonation and sensitivity performances were investigated comprehensively.

X-ray Crystallography
The crystal structures of NTOM, NTOF and NTOA were obtained and analysed by X-ray single crystal diffraction ( Figure 2). The crystal data and structure refinement parameters were given in Tables S1-S4 in the SI.

X-ray Crystallography
The crystal structures of NTOM, NTOF and NTOA were obtained and analysed by X-ray single crystal diffraction ( Figure 2). The crystal data and structure refinement parameters were given in Tables S1-S4 in the SI. performance investigations were not investigated. Herein, based on our previous studies, two new energetic compounds (NTOF and NTOM) were firstly designed and synthesized ( Figure 1c). All of the synthesized compounds in this study were characterized by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. X-ray crystallographic measurements of the three compounds were performed, providing insight into structural characteristics as well as intramolecular and intermolecular interactions of these compounds. The thermal stabilities were evaluated by DSC-TG measurements, and the detonation and sensitivity performances were investigated comprehensively.

X-ray Crystallography
The crystal structures of NTOM, NTOF and NTOA were obtained and analysed by X-ray single crystal diffraction ( Figure 2). The crystal data and structure refinement parameters were given in Tables S1-S4 in the SI. NTOM, NTOF and NTOA crystalized in the monoclinic system and contained four molecules in a unit cell. The space group of NTOM, NTOF and NTOA are P(1)2(1)/n(1), P2(1)/c and P2(1)/n, respectively. The crystal density of NTOM is 1.76 g·cm −3 at 296(2) K. After introduction of -CF 3 , crystal density was obviously improved to 1.87 g·cm −3 . However, the introduction of -NH 2 into NTOM causes crystal density of NTOA to decrease to 1.66 g·cm −3 . As illustrated in Figure 2c, the amino group and 1,2,4-oxadiazole are in the same plane, and the atoms in 5-nitrotetrazole are essentially planar with very small torsion angle (−1.76 • ) of N3-C1-N1-O2. Meanwhile, the C-N bond length and the N-N bond length in oxadiazole and tetrazole rings are found to be shorter than the normal C-N single bond (1.47 Å) and longer than the C = N double bond (1.22 Å). These results indicate that electronic conjugation is formed in amino-1,2,4-oxadiazole and 5-nitrotetrazole structure.
From Figure         In order to obtain further information about the thermal decomposition processes of NTOM, NTOA and NTOF, the reaction kinetic parameters during the heating process were studied by Kissinger's [18] and Ozawa-Doyle's method [19]. The DSC curves at different hating rates (2.5, 5, 10 and 15 • C min −1 ) were shown in Figure 5. The apparent activation energy E for the thermal decomposition of NTOM calculated by Kissinger and Ozawa method is 121.54 kJ mol −1 and 121.56 kJ mol −1 , respectively (Table 1). After the introduction of -CF 3 and -NH 2 , the apparent activation energies for the thermal decomposition of NTOF and NTOA were both increased. Notably, the linear correlation coefficients (r) calculated by Kissinger's method and Ozawa's method of the three compounds are very close and greater than 0.98. Thus, the calculated results are credible and provide a good reference for the thermal safety of NTOM, NTOA and NTOF. Notes: E is the apparent activation energy; A k is pre-exponential factor; r is the liner correlation coefficient.  In order to obtain further information about the thermal decomposition processes of NTOM, NTOA and NTOF, the reaction kinetic parameters during the heating process were studied by Kissinger's [18] and Ozawa-Doyle's method [19]. The DSC curves at different hating rates (2.5, 5, 10 and 15 °C min −1 ) were shown in Figure 5. The apparent activation energy E for the thermal decomposition of NTOM calculated by Kissinger and Ozawa method is 121.54 kJ mol −1 and 121.56 kJ mol −1 , respectively (Table 1). After the introduction of -CF3 and -NH2, the apparent activation energies for the thermal decomposition of NTOF and NTOA were both increased. Notably, the linear correlation coefficients

Sensitivities
The sensitivities of NTOM, NTOF and NTOA toward impact and friction were tested using BAM methods, and the results are listed in Table 2. The three compounds possess satisfactory sensitivities with the IS of > 40 J and FS > 360 N. The impact sensitivity of energetic materials are very closely related to their electrostatic potential (ESP) [20][21][22][23][24][25]. We calculated the ESP of NTOM, NTOF and NTOA using the Gaussian 09 method at the theoretical level of B3LYP/6-311g (d, p) before the sensitivity test.
It can be observed from Figure 6 that the negative charges of the three compounds are mainly distributed around nitrotetrazole, while the positive charges are concentrated on the N-CH 2 -C linkage. Positive charge is distributed on the amino group connected with isofurazan in the NTOA compound and the hydrogen atom connected with isofurazan in NTOM. The trifluoromethyl group in NTOF is distributed with partial positive charge. In most N-O compound systems, especially those containing nitro groups, the concentration of more positive charges around N atoms will result in an imbalance, which will theoretically result in higher impact sensitivity. The N atoms of the three compounds have no obvious positive charge distribution. Therefore, we believe that the sensitivities of the three compounds are low. NTOM. The trifluoromethyl group in NTOF is distributed with partial positive charge. In most N-O compound systems, especially those containing nitro groups, the concentration of more positive charges around N atoms will result in an imbalance, which will theoretically result in higher impact sensitivity. The N atoms of the three compounds have no obvious positive charge distribution. Therefore, we believe that the sensitivities of the three compounds are low.

Hirshfeld Analysis
The Hirshfeld surfaces (dnorm) of NTOM, NTOF and NTOA were calculated by Crys-talExplorer [26] (Figure 7a−c). The 3D dnorm surface was used to identify close intermolecular interactions of compounds. The blue and red regions represent closer and longer contacts, respectively, and the white regions mean the distance of contacts equal to the vdW separation with a dnorm value of zero. The green dotted lines represent the hydrogen bonds formed in the molecule, and the red dotted lines are close contacts.
The red dots appeared on the surfaces of NTOM and NTOA are mainly resulted from intermolecular hydrogen bonds formed in the compounds, while the red areas on the surfaces of NTOF are caused by close contacts of the compound. The red dots on NTOM and NTOA attributed to HBs are dark, which indicates the presence of strong hydrogen bonds. As observed from Figure 7a−c, the three compounds have relatively few "hot spots," indicating that their sensitivities to stimuli are relatively low.
The 2D fingerprint plots clearly illustrate the different contributions of intermolecu-

Hirshfeld Analysis
The Hirshfeld surfaces (d norm ) of NTOM, NTOF and NTOA were calculated by Crystal-Explorer [26] (Figure 7a−c). The 3D d norm surface was used to identify close intermolecular interactions of compounds. The blue and red regions represent closer and longer contacts, respectively, and the white regions mean the distance of contacts equal to the v dW separation with a d norm value of zero. The green dotted lines represent the hydrogen bonds formed in the molecule, and the red dotted lines are close contacts.

Physiochemical Properties and Detonation Performances of NTOM, NTOF and NTOA
The physiochemical and energetic properties of NTOM, NTOF and NTOA compared with TNT and BODN are summarized in Table 2. Density is one of the most important factors for evaluating the detonation performances of energetic compounds [28,29]. The crystal densities of NTOM, NTOF and NTOA were 1.66, 1.76 and 1.87 g/cm 3 , respectively. The introduction of the -CF3 group makes the density of NTOF apparently higher than that of NTOM, while the introduction of -NH2 makes the density of NTOA lower than that of NTOM. Moreover, the densities of these three compounds are higher than that of TNT (1.65 g/cm 3 ). The solid-state enthalpies of formation of NTOM and NTOA are posi- The red dots appeared on the surfaces of NTOM and NTOA are mainly resulted from intermolecular hydrogen bonds formed in the compounds, while the red areas on the surfaces of NTOF are caused by close contacts of the compound. The red dots on NTOM and NTOA attributed to HBs are dark, which indicates the presence of strong hydrogen bonds. As observed from Figure 7a−c, the three compounds have relatively few "hot spots," indicating that their sensitivities to stimuli are relatively low.
The 2D fingerprint plots clearly illustrate the different contributions of intermolecular interactions (Figure 7d [27]. The percentage of contribution of O···H, N···H, N···O and N···N type interactions in the total Hirshfeld surface of NTOM are 27.6%, 17.5%, 19.2% and 16.4%, respectively. These values are similar to the results of NTOA (the percentage of contribution of O···H, N···H, N···O and N···N type interactions in the total Hirshfeld surface of NTOA is 30%, 20.7%, 20% and 12.4%, respectively). As there are only two H atoms exist in NTOF, the percentage of contribution of O···H and N···H interactions are only 9.2% and 10.6%. The percentage of N···O and N···N type interactions of NTOF are 19.6% and 11.4%, respectively. The N···C and O···C type of interactions contribute asmall amount in the total surface of the three compounds. The O···O contact interactions of NTOM, NTOA and NTOF are only 4.1%, 0.8% and 2.9%, which indicate that the three compounds are insensitive to external mechanical stimuli. The Hirshfeld analysis is in good consistency with the ESP analysis and experiment results.

Physiochemical Properties and Detonation Performances of NTOM, NTOF and NTOA
The physiochemical and energetic properties of NTOM, NTOF and NTOA compared with TNT and BODN are summarized in Table 2. Density is one of the most important factors for evaluating the detonation performances of energetic compounds [28,29]. The crystal densities of NTOM, NTOF and NTOA were 1.66, 1.76 and 1.87 g/cm 3 , respectively. The introduction of the -CF 3 group makes the density of NTOF apparently higher than that of NTOM, while the introduction of -NH 2 makes the density of NTOA lower than that of NTOM. Moreover, the densities of these three compounds are higher than that of TNT (1.65 g/cm 3 ). The solid-state enthalpies of formation of NTOM and NTOA are positive and obtained as 275.8 kJ/mol and 344.9 kJ/mol, respectively. NTOM possesses a detonation velocity of 7909 m/s and detonation pressure of 24.80 Gpa. The detonation velocity of NTOA is 7451 m/s, and the detonation pressure is 29.3 GPa. However, the solid-state enthalpy of formation of NTOF is negative (−328.8 kJ/mol), which renders the detonation pressure and detonation velocity of NTOF lower than those of NTOM and NTOA.
The sensitivities of the three compounds to impact and friction were tested by BAM drop hammer and BAM friction tester. The impact sensitivities of NTOM, NTOA and NTOF are >40 J and the friction sensitivities are >360 N. In comparison with TNT, NTOM and NTOF have better detonation performances, thermal stabilities and sensitivities. In terms of detonation performance, BODN has more outstanding detonation velocity (D: 8180 m/s) and detonation pressure (P: 29.4 GPa) than those of NTOM and NTOF. The thermal decomposition temperatures of NTOM and NTOF are greater than 240 • C, which are much higher than that of BODN (T d : 183.4 • C). Meanwhile, the impact sensitivities of NTOM and NTOF are apparently lower than that of BODN (IS: 8.7 J, FS: 282 N).

Experiments
General caution! Although we have experienced no explosion accidents in synthesis and characterization of these materials, proper protective measures should be adopted.

Materials and Measurements
2-(5-Nitro-2H-tetrazol-2-yl)acetonitrile (1) was prepared according to the literature [17]. Hydroxylamine hydrochloride, sodium bicarbonate, methanol, potassium bicarbonate, cyanogen bromide, trichloroacetic acid (TFA), tetrahydrofuran (THF), triethyl orthoformate((EtO) 3 CH) and BF 3 ·Et 2 O were commercially available and used without further purification. 1 H NMR and 13 C NMR of NTAA, NTOA, NTOM and NTOF were recorded on 500 MHz (Bruker AVANCE 500) nuclear magnetic resonance spectrometers. The melting and decomposition points were determined using a differential scanning calorimeter (TA Instruments Company, Model DSC-Q200) at a flow rate of N 2 at 50 mL min −1 . About 0.3 mg of the sample was sealed in aluminium pans for DSC analysis. Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000~400 cm −1 . Elemental analyses (C, H and N) were performed on a VARI-El-3 elementary analysis instrument. The impact and friction sensitivities were determined by using the BAM method.

X-ray Crystallography
The diffraction data of NTOA, NTOM and NTOF were collected on a BRUKER SMART Apex II CCD X-ray diffractometer equipped with a Mo Kα radiation (λ = 0.71073 A) using the ω-θ scan mode. The structures were solved by the direct method using SHELXS-97 and refined with full-matrix least-squares procedures on F2 with SHELXL-97. The crystal data and structure refinement parameters were listed in

Synthesis of NTOA
NTAA (0.3 g, 1.6 mmol) was added to a solution of KHCO 3 (0.53 g, 5.3 mmol) in water (10 mL). The reaction solution was heated to clear (45°C), and cooled to room temperature. BrCN (0.26 g, 2.4 mmol) was added portionwise. The reaction solution was stirred overnight; then, the light brown solid was formed and filtered. The filter cake was washed with a small amount of cold water and diethyl ether and dried naturally to provide the product NTOA (0.2 g, 62%). 1

Synthesis of NTOM
Triethyl orthoformate (3.96 g, 26.7 mmol) and NTAA (0.5 g, 2.67 mmol) were added into three-necked flask, then BF 3 ·Et 2 O was added dropwise at room temperature. After 4 h, the reaction solution was poured into ice, white solid was precipated and filtered. Washed with small amount of cold water, 0.43 g NTOM was obtained with a yield of 82%. 13

Synthesis of NTOF
NTAA (0.72 g, 3.85 mmol) was added to dry THF (4 mL), then TFAA (2.4 g, 11.55 mmol) was added dropwise. The solution was heated to reflux for 24 h. Twenty-five percent aqueous ammonia solution was added to neutralize the reaction solution to pH = 7-8. Saturated NaCl aqueous solution was added, and ethyl acetate was used for extraction. The solvent evaporated, and 0.8 g NTOF was obtained with a yield of 79%. 13

Conclusions
In this study, a new structural type for melt cast materials was designed by linking, for the first time, a nitrotetrazole ring with 1,2,4-oxadiazole through a N-CH 2 -C bridge. Three N-CH 2 -C-bridged energetic compounds, namely NTOM, NTOF and NTOA, were synthesized starting from 2-(5-nitro-2H-tetrazole-2-yl)acetonitrile. The three compounds were fully characterized by NMR, IR spectroscopy and elemental analysis. In addition, the single crystals of NTOM, NTOF and NTOA were successfully obtained and investigated by using single-crystal X-ray diffraction. Crystal densities of the three compounds are in the range of 1.66 g/cm 3 (NTOA)~1.87 g/cm 3 (NTOF). Calculated detonation velocities are between 7271 m/s (NTOF) and 7909 m/s (NTOM). The impact sensitivities are >40 J, and the friction sensitivities are >360 N. NTOM, NTOF and NTOA are thermally stable with decomposition points above 240 • C. These compounds possess good thermal stabilities. The melting points of NTOM and NTOF are 82.6 • C and 71.7 • C, respectively. Hence, these compounds hold the potential to be used as melt cast materials with better performances than TNT. This study demonstrates that the construction of N-CH 2 -C linkage in heterocycles is an effective strategy to reduce sensitivity and improve thermal stabilities of energetic materials.