DOI: 10.1002/chem.201405122

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& Energetic Materials

N-Oxide 1,2,4,5-Tetrazine-Based High-Performance Energetic Materials Hao Wei, Haixiang Gao, and Jean’ne M. Shreeve*[a]

Abstract: One route to high density and high performance energetic materials based on 1,2,4,5-tetrazine is the introduction of 2,4-di-N-oxide functionalities. Based on several examples and through theoretical analysis, the strategy of regioselective introduction of these moieties into 1,2,4,5-tetrazines has been developed. Using this methodology, various new tetrazine structures containing the N-oxide functionality were synthesized and fully characterized using IR, NMR, and mass spectroscopy, elemental analysis, and single-crystal Xray analysis. Hydrogen peroxide (50 %) was used very effectively in lieu of the usual 90 % peroxide in this system to generate N-oxide tetrazine compounds successfully. Comparison of the experimental densities of N-oxide 1,2,4,5-tetrazine compounds with their 1,2,4,5-tetrazine precursors

Introduction Modern high-energy density materials (HEDMs) have been widely studied for civilian and military applications, and continue to attract considerable attention.[1] The performance of HEDMs is evaluated by detonation pressure (P) and velocity (nD), which are related to density, oxygen balance, and heat of formation.[2] Performance is highly dependent on the density. The velocity of detonation increases linearly with density and the detonation pressure increases with the density squared.[3] For energetic material synthesis, to obtain higher density and better detonation properties, introducing more dense and more energy-rich functional groups as substituents into candidate compounds is an effective and widely used method. These functional groups include -NO2 (-CNO2, -NNO2, and -ONO2), -N3, -N=N-, and -NF2.[4] However, the requirements of insensitivity and stability along with introducing more energyrich functional groups are quite often contradictory to each other.[5] The synthesis of N-oxides is a rather recent methodology.[6] The N O bond of N-oxide is a relatively strong bond possessing significant double bond character owing to p-back-bonding by the lone oxygen pair. On the other hand, the formation [a] Dr. H. Wei, Prof. Dr. H. Gao, Prof. Dr. J. M. Shreeve Department of Chemistry, University of Idaho 875 Perimeter Dr., MS 2343, Moscow, ID 83844–2343 (USA) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405122. Chem. Eur. J. 2014, 20, 16943 – 16952

shows that introducing the N-oxide functionality is a highly effective and feasible method to enhance the density of these materials. The heats of formation for all compounds were calculated with Gaussian 03 (revision D.01) and these values were combined with measured densities to calculate detonation pressures (P) and velocities (nD) of these energetic materials (Explo 5.0 v. 6.01). The new oxygen-containing tetrazines exhibit high density, good thermal stability, acceptable oxygen balance, positive heat of formation, and excellent detonation properties, which, in some cases, are superior to those of 1,3,5-tritnitrotoluene (TNT), 1,3,5-trinitrotriazacyclohexane (RDX), and octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine (HMX).

of a heterocyclic N-oxide also changes the charge distribution of the entire molecule which enhances the aromaticity of the ring system, thus stabilizing the entire molecule.[7] For example, when comparing 4,4’-dinitro-3,3’-azobisfurazan (DNAzBF) (nD (calcd) ca. 8733 m s 1, 1 = 1.85 g cm 3) with the N-oxide compound, 4,4’-dinitro-3,3’-diazenofuroxan (DDF), the latter shows superior performance with high density (nD (calcd) ca. 10 000 m s 1, 1 = 2.02 g cm 3)[8] (Figure 1). Comparison of 2,6-diamino-3,5-dinitropyrazine (ANPZ) (nD (calcd) ca. 7892 m s 1, 1 =

Figure 1. Energetic compounds DNAzBF, DDF, ANPZ, and LLM-105.

1.84 g cm 3) with the more dense N-oxide 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), shows a higher energetic performance for the oxygen-containing species (nD (calcd) ca. 8516 m s 1, 1 = 1.92 g cm 3).[9] Thus, the N-oxide functionality not only increases oxygen balance, but also allows better crystal packing, and efficiently enhances detonation performance.[10]

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Full Paper 1,2,4,5-Tetrazines, also known as s-tetrazines, were first synthesized in 1893,[11] and have attracted the attention of many.[12] Researchers at the Los Alamos National Laboratory are the main contributors to energetic s-tetrazine chemistry.[13] Energetic materials based on 1,2,4,5-tetrazines show desirable properties associated with high N-atom content, positive heat of formation, and thermal stability. However, due to limited stability, the introduction of energy-rich functional groups is very difficult. For example, 3,6-dinitramine 1,2,4,5-tetrazine is readily hydrolyzed and reverts to 3,6-diamino 1,2,4,5-tetrazine with even traces of moisture,[6e] and was found to be too unstable to isolate.[13e] 1,2,4,5-Tetrazines have many advantages in terms of performance and stability, but due to the lack of energy-rich functional groups, energetic materials based solely on 1,2,4,5-tetrazine usually exhibit low density and negative oxygen balance. The introduction of at least one N-oxide moiety into a 1,2,4,5-tetrazine system provides an effective and practical way of overcoming this problem. Surprisingly this system is relatively unexploited. The few tetrazines extensively evaluated as explosives include: 3,6-diamino-1,2,4,5-tetrazine1,4-dioxide (LAX-112),[13b] N-oxides of 3,3’-azobis(6-amino1,2,4,5-tetrazine) (DAATO3.5)[13d] and 3,6-diguanidino-1,2,4,5-tetrazine-1,4-di-N-oxide (DGT-DO)[13a] (Figure 2). One explanation is that the oxidative step is most often accompanied by a low yield, and a complex mixture that makes purification difficult.[13d, 14]

and to improve energetic performance. In this paper, we expand the field of tetrazine chemistry by suggesting for the first time a relationship between the substituents on the tetrazine ring and the resulting oxidation product.

Results and Discussion Synthesis Based on the literature, the reagents available for introducing the nitrogen–oxygen bond into tetrazines are hypofluorous acid [HOF],[6] Caro’s acid (peroxomonosulfuric acid, H2SO5),[13d] peroxytrifluoroacetic acid (PTFA), and Oxone [13e] (2 KHSO5·KHSO4·K2SO4). HOF is very effective but elemental fluorine is needed for its synthesis. Oxone is often not a sufficiently strong oxidizer. The strong acid properties of Caro’s acid tend to limit its application. Therefore, PTFA was our reagent of choice. PTFA is most often prepared by using 90 % hydrogen peroxide and trifluoroacetic anhydride. For the first time, we now have replaced 90 % with 50 % hydrogen peroxide and can report that the oxidizing power of this more dilute mixture continues to be very effective in yielding N-oxide(s) products in moderate to good yields. This change greatly improves the safety and economy, and enhances the practicality of the methodology. Several well-known 1,2,4,5-tetrazine derivatives were selected, such as tetrazole (1), 3,5-dimethylpyrazol-l-yl (2) or 5amino-3-nitro-1H-1,2,4-trizol-l-yl (3), with heterocyclic substituents bonded to both of the carbon atoms of the tetrazine ring (Scheme 1).

Figure 2. Energetic materials that contain N-oxide tetrazines.

In order to understand the relationship of the structure of 1,2,4,5-tetrazine and the oxidative result, we selected a few tetrazines as theoretical subjects. The strategy of regioselective introduction of 2,4-di-N-oxide into 1,2,4,5-tetrazine was put forward. Natural bond orbital (NBO) analysis (charge) also supports this result. By employing this strategy, a number of unknown and insensitive compounds with good thermal stabilities as well as high detonation performances were designed and synthesized. These compounds were characterized by Xray diffraction (in some cases), IR and multinuclear NMR spectroscopy, elemental analysis, and DSC. Their properties as energetic materials were studied and evaluated using the experimental values obtained for the thermal decomposition and the sensitivity data as well as calculated performance characteristics. Calculations and experimental values confirm that introducing N-oxide is an effective method to enhance densities Chem. Eur. J. 2014, 20, 16943 – 16952

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Scheme 1. Oxidized tetrazine compounds that contain two heterocyclic substituents.

However, N-oxide products were not obtained with 1, 2, or 3 even when a large excess of oxidant (10  ) with a prolonged reaction time at an elevated temperature was used. In order to initiate the reaction, a nitrogen atom on the tetrazine ring must attack an oxygen atom of PTFA.[15] Based on that assumption, it follows that the distribution of electron density has a strong influence on the oxidation reaction. Electron-deficient groups decrease the nucleophilicity of the ring discouraging effective attack on PTFA. Therefore, electron-rich groups were introduced into the tetrazine system. It was hoped that electron-rich groups would enhance the electron density initiating the reaction and forming hydrogen bonds to stabilize the

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Full Paper entire molecule. 3,6-Diamino-1,2,4,5-tetrazine (4 a), 3,6-dihydrazino-1,2,4,5-tetrazine (5 a), 3-amino-6-(1H-,1,2,3,4-tetrazol-5-ylamino)1,2,4,5-tetrazine (6 a), and 3-amino-6-guanidino-1,2,4,5tetrazine (7 a) were designed and synthesized. Compounds 4 a, 6 a, and 7 a were oxidized to the mono-N-oxide products (4, 6, and 7; Table 1, entries 1, 3, and 4), while 3,6-dihydrazino1,2,4,5-tetrazine (5 a) decomposed in the oxidation process, possibly because of the ease of reducing the hydrazino group (Table 1, entry 2).

Table 2. Oxidized tetrazine compounds that contain an electron-deficient group on one carbon atom and an electron rich group on the other.[a]

Substrate

Product

Yield [%][b]

1

74

2

82

3

78

Table 1. Oxidized tetrazine compounds that contain two electron-rich substituents.[a]

Substrate

Product

1

2

Yield [%][b]

64

—

3

[a] Reaction conditions: Trifluoroacetic anhydride (4 mL, 28 mmol) was added to 50 % hydrogen peroxide (1.3 mL, 25 mmol) in methylene chloride (20 mL) with stirring at < 10 8C. The tetrazine compound (7 mmol) was added at 0 8C. [b] Yield of isolated product.

—

54

4

55

[a] Reaction conditions: Trifluoroacetic anhydride (4 mL, 28 mmol) was added to 50 % hydrogen peroxide (1.3 mL, 25 mmol) in methylene chloride (20 mL) with stirring at < 10 8C. The tetrazine compound (7 mmol) was added at 0 8C. [b] Yield of isolated product.

In order to introduce additional N-oxides into the tetrazine ring, further enhancement of charge density on the ring or other methods were required. One example that attracted our attention was that the treatment of 3-amino-6-chlorotetrazine with peroxytrifluoroacetic acid gave the 2,4-di-N-oxide product successfully.[6e] While this result was not rationalized in this study and no additional examples were reported, we assumed that the electron-deficient group (chloro) played an important role in the oxidation process. Therefore, an electron-deficient group (heterocyclic ring) was introduced on one carbon atom and an electron-rich group (amino) on the other carbon atom to form, for example, 3-amino-6-(3,5-dimethylpyrazol-l-yl)1,2,4,5-tetrazine (8 a), 3-amino-6-pyrazol-l-yl-1,2,4,5-tetrazine (9 a), and 3-amino-6-1,2,4-triazol-l-yl-1,2,4,5-tetrazine (10 a) (Table 2). Heterocyclic rings were introduced into this system for two reasons: 1) the heterocyclic rings could behave as electron-deficient groups mimicking the chloro group so that 2,4di-N-oxide moieties could be introduced into the 1,2,4,5-tetrazine successfully giving products 8–10; 2) recently, the design and synthesis of novel energetic materials are focused on hetChem. Eur. J. 2014, 20, 16943 – 16952

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erocyclic compounds. Energetic compounds composed of heterocycles are dramatic not only owing to their higher heats of formation, density, thermal stability, and oxygen balance, but also due to their greater environmental acceptability, since they produce a high percentage of nitrogen gas in a blast or burn. The amino group was retained as an electron-rich group, which can provide sufficient electron density to initiate the reaction, form intramolecular hydrogen bonds with N-oxides to stabilize the entire molecule, and decrease steric hindrance during the oxidation process. Results have shown the accuracy and effectiveness of this strategy. All substrates gave 2,4-di-Noxide products in high yields using 50 % hydrogen peroxide and trifluoroacetic acid anhydride in dichloromethane at room temperature (Table 2). In an attempt to rationalize these results, the charge distribution data were calculated by natural bond orbitals (NBO) charges analysis for compounds 4 a, 9 a, and 10 a and the results are given in Tables S19, S20, and S21, respectively (Supporting Information). In the case of 4 a, for example, NBO analysis shows that N1, and N4 are more negatively charged than N2 and N3 (Figure 3). However, only a mono-oxide product was obtained under standard conditions. This observation can be rationalized as follows: the product, 4, formed by the introduction of a single mono-oxide is subsequently protonated by the more strongly acidic trifluoroacetic acid. As a result, further oxidation of the N-oxide product 4 to the 2,4-di-N-oxide product is precluded.[13e] In compounds 9 a and 10 a, the N2 and N4 nitrogen atoms, which are vicinal to the amino-substituted carbon atom, display a more negative charge density than N1 and N3

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Full Paper Table 3. Design and synthesis of some energetic materials based on the strategy.[a] Substrate

Figure 3. NBO analysis of 4 a, 9 a and 10 a (charge densities: Tables S14, S15 and S16 in the Supporting Information).

(Figure 2). The electron-withdrawing groups (pyrazole and triazole) considerably reduce the electron charge on the nearby nitrogen atoms due to conjugative and resonance effects of the ring. Evidently, the 2,4-di-N-oxide products should be formed during the oxidation process. The NBO analysis supports the experimental results showing that N2 and N4 on 9 a and 10 a are more negatively charged than N atoms on compound 4 a. Therefore, the electron-withdrawing groups actually enhance the electronic densities of N2 and N4 in the tetrazine system. Based on the above, it can be concluded that when a tetrazine ring is bonded to an electron-deficient group on one carbon atom and an electron-rich group on the other carbon atom, the 2,4-di-N-oxide products will be the main oxidation products. Several heterocycles with different substituents were introduced into the tetrazine system, such as 3,5-diamino-1,2,4-triazole (in 11 a), 3,5-dimethyl-4-nitropyrazole (in 12 a), azido-1,2,4-triazole (in 13 a), and tetrazole (in 14 a) (Table 3, entries 1–4). Under the experimental conditions used, the reactions occurred as predicted and 2,4-di-N-oxide derivatives (11–14) were obtained. It should be noted that one amino group on the triazole moiety in compound 11 a was converted to nitro. The structures of 11·DMSO and 14·H2O were confirmed by X-ray crystallographic analysis (Supporting Information). Finally, the N-oxide product can be obtained by introducing an electron-deficient functional group on one carbon atom and an amino moiety on the other carbon atom. Compounds 15 a and 16 a, which contain cyano and nitro groups, respectively, were designed and synthesized (Table 3, entries 5, 6). The desired 2,4-dioxide products 15 and 16 were formed in high yields Surprisingly, a completely different transformation occurred with a guanidine group substituent on one carbon atom and an amino group on the other. Stirring 3-guanidine-6-amino1,2,4,5-tetrazine (17 a) under standard conditions for 12 h, gave 3-guanidine-6-nitro-1,2,4,5-tetrazine-2,4-di-N-oxide (17) (Scheme 2). However, when the reaction time was shortened to 2 h, only 3-guanidine-6-nitro-1,2,4,5-tetrazine-2-N-oxide (17 b) was obtained. Here the amino group of compound 17 is oxidized to nitro, and under the impact of guanidine (electronrich group) and nitro group (electron-deficient group), 2,4-diN-oxide moieties are successfully introduced onto the ring.

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Product

Yield [%][b]

1

25

2

55

3

62

4

70

5

82

6

52

[a] Reaction conditions: Trifluoroacetic anhydride (4 mL, 28 mmol) was added to 50 % hydrogen peroxide (1.3 mL, 25 mmol) in methylene chloride (20 mL) with stirring at < 10 8C. Tetrazine compound (7 mmol) was added at 0 8C. [b] Yield of isolated product.

Scheme 2. Synthetic route proposed for the oxidation reaction of compound 17 a.

Spectroscopy The structures of 6–17 are supported by IR, 1H and 13C NMR spectroscopy as well as elemental analysis. Additionally, 15N NMR spectra were recorded for compounds 9, 14, 15, 16, and 17 in [D6]DMSO; chemical shifts are given with respect to CH3NO2 as external standard (Figure 4). All the compounds with a 2,4-di-N-oxide structure are symmetric; therefore there are only two nitrogen signals attributable to the tetrazine ring. The N2 and N4 signals are upfield relative to N1 and N3. Compounds 16 and 17 each have a nitro group bonded to the tetrazine ring assigned at 27.95 and 3.55 ppm, respectively.

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Full Paper drogen on N4 or N7 occupies each position only one half of the time. The N-oxide bond lengths in 11·DMSO are 1.2564(13) and 1.2604(12) , in 14·H2O are 1.2703(16) and 1.2683(15) . In the structures of 11·DMSO and 14·H2O, the N N bond lengths in the tetrazine ring are 1.33(13), 1.34(13), 1.33(17), and 1.33 (17), which is normal in tetrazines without Noxide. The angles of N-N-O are 119.3, 119.05, 119.96, and 119.958 for 11·DMSO and 14·H2O, respectively. For 11·DMSO and 14·H2O, it is interesting to note that the tetrazine ring and triazole (torsion angle N5-N4-C4-N8: 178.148) or tetrazole ring (torsion angle N7-C2-C3-N8: 0.88) are coplanar. Thermal behavior The phase-transition temperatures and thermal stabilities of compounds 6–17 were determined by differential scanning calorimetric (DSC) measurements scanning at 5 8C min 1, using about 1.0 mg of material (Table 4). Most of these compounds show sharp exothermic peaks, which indicate rapid decomposition. All compounds are thermally stable with decomposition temperatures (onset temperatures) ranging from 110 to 252 8C. Compound 16 has the lowest onset decomposition temperature at 110 8C, and 8 the highest at 252 8C. Properties Figure 4. 15N spectra of compounds 9, 14, 15, 16 and 17 in [D6]DMSO.

Single-crystal X-ray structure analysis Crystals of 11·DMSO and 14·H2O, suitable for single-crystal Xray diffraction, were obtained by slow evaporation of solutions of the compounds in DMSO or water, respectively, at room temperature (Figures 5 and 6). The crystallographic data and refinement details can be found in the Supporting Information. The crystal structure of 14·H2O appears as if it has two protons on the tetrazole ring; however, each tetrazole ring has only one proton per moiety (Figure 5). This occurs because the hyChem. Eur. J. 2014, 20, 16943 – 16952

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The experimentally determined densities of 6–17 range between 1.76 and 1.92 g cm 3 equal or exceed that of common explosives. Moreover, the outstanding high densities of 16 (1.92 g cm 3, literature data[13e] 1.919 g cm 3) and 17 (1.91 g cm 3) are a consequence of the N-oxide and nitro groups being involved in multiple intermolecular hydrogen bonding interactions and are comparable with HMX (1.91 g cm 1). The densities of 6 a–17 a were also determined. Plotted in Figure 7 are density values of the N-oxide compounds 6–17 in a bar graph comparison with those of the tet-

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Full Paper

Figure 6. a) A view of the molecular unit of 14·H2O. b) Unit cell view along the b axis; hydrogen bonds are indicated as dotted lines. Figure 5. a) A view of the molecular unit of 11·DMSO. b) Unit cell view along the a axis; hydrogen bonds are indicated as dotted lines.

Table 4. Physical properties of 6–17[a] compared with RDX and HMX.

6 7 8 9 10 11 12 13 14 15 16 17 TNT RDX HMX

Td[b] [8C]

1[c] OB[d] [g cm 3] [%]

DfH[e] [kJ mol 1/ kJ g 1]

nD[f] [m s 1]

P[g] [GPa]

IS[h] [J]

FS[i] [N]

192 177 252 237 221 168 211 161 181 191 110 134 295 210 280

1.76 1.77 1.76 1.79 1.80 1.84 1.82 1.82 1.85 1.84 1.92 1.91 1.65 1.82 1.91

351.8/1.79 627.8/2.92 347.6/1.56 427.0/2.19 448.6/2.29 430.6/1.68 328.3/1.23 781.7/3.29 539.4/2.73 378.6/2.46 225.7/1.29 325.5/1.50 67.0/ 0.30 80.0/0.36 104.8/0.36

8429 8580 8180 8304 8438 8707 8413 8731 8884 8635 9316 9157 6881 8748 9320

27.3 29.1 23.6 26.4 27.7 32.0 27.5 30.9 32.4 31.3 39.4 37.5 19.5 34.9 39.5

35 26 35 27 24 17 19 8 14 15 3 20 15 7.4 7.4

360 360 360 360 360 240 360 60 120 240 10 240 – 120 120

32.6 18.6 68.1 45.1 32.4 12.5 41.7 20.2 20.3 21.3 9.2 7.4 24.7 0 0

[a] All new compounds are anhydrous except 14 and 16 which are monohydrates. [b] Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 8C min 1). [c] Density measured by gas pycnometer (25 8C). [d] OB = oxygen balance (%); for CaHbOcNd : 1600(c a b/2)/Mw, Mw = molecular weight of compound. [e] Calculated heat of formation. [f] Detonation velocity (Explo5 v.6.01). [g] Detonation pressure (Explo5 v.6.01). [h] Impact sensitivity. [i] Friction sensitivity.

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razine precursors 6 a–17 a. As indicated in this figure, a marked density increase is seen and this supports the advantage of introduction of the N-oxide group into the 1,2,4,5-tetrazine system. Compounds 17 a/17 appear to exhibit the largest increase. In this case, in addition to the N-oxide contribution, the amino group on the tetrazine ring was also converted to a nitro group during the oxidation process, which also contributes to hydrogen bonding. Based on the above, introducing the N-oxide moiety into the 1,2,4,5-tetrazine system is a highly effective and feasible method to obtain high density tetrazine compounds.

Figure 7. Bar diagram comparing the densities of 6–17 with 6 a–17 a.

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Full Paper In this study, the oxygen balance (OB) values fall in the range between 68.1 to 9.2. Heats of formation were calculated by using the Gaussian 03 (Revision D. 01) suite of programs.[17] The detonation pressures (P) and velocities (nD) were calculated by using EXPLO5 v6.01. As can be seen from Table 4, the calculated detonation velocities lie between nD = 8180 and nD = 9316 m s 1. The highest values in terms of detonation velocity were observed for compounds 16 (9316 m s 1) and 17 (9157 m s 1), all of which exceed 1,3,5-trinitrotriazacyclohexane (RDX). In comparison with other tetrazine derivatives, an improved performance is seen resulting from the introduction of N-oxide groups. The detonation pressures of Noxide and di-N-oxide tetrazine derivatives lie in the range 23.6 and 39.4 GPa (compared to RDX 34.9 GPa, and octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) 39.5 GPa). For initial safety testing, the sensitivities of 6–17 toward impact (IS) and friction (FS) were measured. Impact sensitivity measurements were made using standard BAM Fall hammer techniques.[18] Compound 16 is the most impact- and frictionsensitive compound (IS: 3 J, FS: 10 N), Nevertheless, 17 which has a structure that is similar to 16, is less sensitive (IS: 20 J, FS: 240 N). The remainder of the compounds are much less impact sensitive (8–35 J) than RDX and HMX, which suggests that 6, 7, 8, 9, 10, 12, 14, 15, and 17 could serve as promising candidates for safe energetic materials. With the exception of 13 and 16, the friction sensitivities of all compounds are all more positive than 120 N which makes them less sensitive than RDX and HMX as well.

Conclusion In order to obtain highly dense 1,2,4,5-tetrazine compounds, the introduction of the N-oxide functionality into this system was found to be an effective method. Based on several experimental examples and theoretical analysis, a strategy for regioselective introduction of 2,4-di-N-oxide moieties into 1,2,4,5tetrazines was established. Computational results in terms of NBO charge analysis also support this methodology, which is useful in the design and synthesis of a number of new compounds. Using this methodology, various new tetrazine compounds containing the N-oxide moiety were synthesized and fully characterized using IR and NMR spectroscopy, elemental analysis, and X-ray single-crystal structure analysis. Hydrogen peroxide (50 %) was used for the first time in this system to generate N-oxide tetrazine compounds successfully. These compounds exhibit good physical and detonation properties, such as moderate thermal stabilities, high densities, high heats of formation, and high detonation pressures and velocities. A majority of these compounds shows an equivalent or higher density (in the range of 1.76–1.92 g cm 3) than RDX and compounds 16 and 17 are comparable with HMX. It is seen that the introduction of the N-oxide functionality into the 1,2,4,5tetrazine system is a highly effective and feasible method to obtain highly dense tetrazine compounds. Calculated detonation values for these compounds are comparable to those of explosives such as TNT, RDX, and HMX. All compounds were also characterized with respect to impact and friction sensitiviChem. Eur. J. 2014, 20, 16943 – 16952

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ty, and thermal stability. With the exception of 16, all are less sensitive than RDX and HMX, which suggests that these compounds could be of interest for future applications as environmentally friendly and high-performing nitrogen or oxygen-rich materials and may serve as a series of promising alternatives to RDX and HMX.

Experimental Section Safety precautions While we have experienced no difficulties in syntheses and characterization of these materials, proper protective measures should be used. Manipulations must be carried out in a hood behind a safety shield. Face shield and leather gloves must be worn. Caution should be exercised at all times during the synthesis, characterization, and handling of any of these materials. Mechanical actions involving scratching or scraping must be avoided. All of the energetic compounds must be synthesized only in small amounts.

X-ray crystallography A yellow prism of dimensions 0.148  0.335  0.423 mm3 for 11·DMSO and a yellow prism of dimensions 0.148  0.335  0.423 mm3 for 14·H2O were used for the X-ray crystallographic analysis. The X-ray intensity data were collected by a Bruker Apex 2 CCD system equipped with a graphite monochromator and a MoKa fine focus tube (l = 0.71073 ). An Oxford Cobra low-temperature device was used to maintain the crystals of 14·H2O at a constant 173 K during data collection. The data for 11·DMSO were collected at 298 K. The frames were attached to the Bruker SAINT software package[19] using a narrow-frame algorithm and data were corrected for absorption effects using the multiscan method (SADABS[20]). The structures were solved and refined with the aid of the programs in the Bruker SHELXTL Software Package. CCDC 1011379 (11·DMSO) and 1011381 (14·H2O) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

General methods Analytical grade reagents were purchased from Aldrich and Acros Organics and were used as received. 1H and 13C NMR spectra were recorded using a 300 MHz (Bruker AVANCE 300) NMR spectrometer operating at 300.13, and 75.48 MHz, respectively. A 500 MHz (Bruker AVANCE 500) NMR spectrometer operating at 50.69 MHz was used to obtain 15N spectra. [D6]DMSO was employed as solvent and locking solvent unless otherwise stated. Chemical shifts in the 1H, and 13C spectra are reported relative to Me4Si and 15N NMR to MeNO2. The melting and decomposition (onset) points were obtained with a differential scanning calorimeter (TA Instruments Co., model Q10) at a scan rate of 5 8C min 1. IR spectra were recorded using KBr pellets for solids on a BIORAD model 3000 FTS spectrometer. Density was measured at room temperature using a Micromeritics AccuPyc 1330 gas pycnometer. Elemental analysis was carried out on an Exeter CE-440 elemental analyzer. Compounds 1,[2e] 2,[13b] 3,[21a] and 5[6e] were synthesized according to literature procedures.

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Full Paper Synthesis General procedure for the synthesis of compounds 9 a, 10 a, and 12 a: Ammonia was bubbled through a solution of the 1,2,4,5-tetrazine compound (9 b,[21b] 10 b,[21a] or 12 b[21a]) (5 mmol) in toluene (50 mL) with stirring for 30 min. The resulting solid was collected by filtration, washed with toluene, and dried to give compounds 9 a, 10 a, or 12 a. 3-Amino-6-(1,2-pyrazol-l-yl)-1,2,4,5-tetrazine (9 a): Red solid; yield: 95 %; IR (KBr): n˜ = 3308, 3207, 1641, 1525, 1489, 1397, 1196, 1035, 951 cm 1; 1H NMR: d = 6.55(s, 1 H), 7.82 (s, 1 H), 8.05(s, 2 H), 8.55 ppm (s, 1 H); 13C NMR: d = 164.0, 156.0, 143.0, 128.9, 108.7 ppm; elemental analysis calcd (%) for C5H5N7 (163.06): C 36.81, H 3.09, N 60.10; found: C 36.76, H 3.12, N 59.43. 3-Amino-6-(1,2,4-triazol-l-yl)-1,2,4,5-tetrazine (10 a): Red solid; yield: 93 %; IR (KBr): n˜ = 3342, 3165, 2755, 2252, 1976, 1665, 1638, 1437, 1234, 970 cm 1; 1H NMR: d = 8.29 (s, 2 H), 9.26 ppm (s, 1 H); 13 C NMR: d = 164.2, 154.9, 153.3, 143.5 ppm; elemental analysis calcd (%) for C4H4N8 (164.06): C 29.27, H 2.46, N 68.27; found: C 29.00, H 2.58, N 66.86. 3-Amino-6-(4-nitro-3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine (12 a): Red solid; yield: 96 %; IR (KBr): n˜ = 3353, 3134, 1628, 1625, 1518, 1423, 1046, 926 cm 1; 1H NMR: d = 8.35 (s, 2 H), 2.31 (s, 3 H), 2.23 ppm (s, 3 H); 13C NMR: d = 163.3, 156.2, 147.0, 143.0, 132.2, 13.8, 12.4 ppm; elemental analysis calcd (%) for C7H8N8O2 (236.08): C 35.60, H 3.41, N 47.44; found: C 35.70, H 3.45, N 46.80. 3-Amino-6-(1H-tetrazol-5-yl)-1,2,4,5-tetrazine monohydrate (14 a): NaN3 (0.13 g, 1.98 mmol) and NH4Cl (1.04 g, 1.98 mmol) were added to a solution of 3-amino-6-cyano-1,2,4,5-tetrazine (15 a)[21c] (0.24 g, 1.98 mmol) in DMF (4 mL), and the reaction was stirred in a sealed tube in 120 8C for 4 h. The solvent was removed by vacuum distillation and the residue was purified on silica gel (EtOAc), to give the product 14 a as a red solid (0.20 g, 1.21 mmol, 61 %). IR (KBr): n˜ = 3322, 3209, 1614, 1570, 1508, 1413, 1055, 956 cm 1; 1H NMR: d = 8.62 ppm (s, 3 H); 13C NMR: d = 162.2, 151.8, 150.3 ppm; elemental analysis calcd (%) for C3H3N9·H2O (183.06): C 19.68, H 2.75, N 68.84; found: C 19.81, H 2.44, N 67.68. 3-Guanidino-6-amino-1,2,4,5-tetrazine (17 a): Methanol (100 mL), guanidine hydrochloride (4.75 g, 0.05 mol) and sodium methoxide (200 mg, 60 % dispersion in mineral oil, 0.05 mmol) were stirred for 20 min. 3-Amino-6-(3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine (8 a)[13b] (8 g, 42 mmol) was added in one portion and the resulting mixture was stirred at room temperature for 12 h. The dark slurry was filtered, washed with water, and air-dried (5.82 g, 90 % yield). Red solid; IR (KBr): n˜ = 3393, 3184, 1658, 1615, 1548, 1433, 1066, 936 cm 1; 1H NMR: d = 6.83 (s, 2 H), 6.67 ppm (s, 4 H); 13C NMR: d = 164.3, 160.1, 157.6 ppm; elemental analysis, calcd (%) for C3H6N8 (154.07): C 23.38, H 3.92, N 72.70; found: C 23.65, H 4.00, N 69.46.

3-(1H-1,2,3,4-Tetrazol-5-ylamino)-6-amino-1,2,4,5-tetrazine-1-N-oxide (6): Compound 6 was prepared using the general procedure and the starting material was 3-(1H-1,2,3,4-tetrazol-5-ylamino)-6-amino1,2,4,5-tetrazine (6 a).[6e] Red solid; yield: 54 %; IR (KBr): n˜ = 3396, 3304, 3199, 2970, 2810, 1616, 1491, 1348, 1311, 1244, 1099, 1047, 833 cm 1; 1H NMR: d = 7.14 (s, 3 H), 7.65 ppm (s, 1 H); 13C NMR: d = 148.6, 154.3, 161.4 ppm; elemental analysis calcd (%) for C3H4N10O (196.13): C 18.37, H 2.06, N 71.42; found: C 18.56, H 2.41, N 69.38. 3-Amino-6-nitroguanyl-1,2,4,5-tetrazine-1-N-oxide (7): Compound 7 was prepared using the general procedure and the starting material was 3-amino-6-nitroguanyl-1,2,4,5-tetrazine (7 a).[13a] Red solid; yield: 55 %; IR (KBr): n˜ = 3365, 3306, 3193, 2852, 1629, 1589, 1348, 1240, 1034, 947 cm 1; 1H NMR: d = 7.14 (s, 3 H), 7.65 ppm (s, 1 H); 13 C NMR: d = 148.6, 154.3, 161.4 ppm; elemental analysis calcd (%) for C3H5N8O3 ( 201.05): C 16.75, H 2.34, N 58.60; found: C 17.05, H 2.29, N 58.84. 3-Amino-6-(3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (8): Compound 8 was prepared using the general procedure and the starting material was 3-amino-6-(3,5-dimethylpyrazol-l-yl)1,2,4,5-tetrazine (8 a).[13b] Yellow solid; yield: 74 %; IR (KBr): n˜ = 3283, 3234, 3148, 1674, 1499, 1416, 1326, 1304, 1268, 1224, 1083, 1003, 949 cm 1; 1H NMR: d = 2.21 (s, 3 H), 2.44 (s, 3 H), 6.17 (s, 1 H), 8.65 ppm (s, 2 H); 13C NMR: d = 11.6, 13.3, 109.2, 141.9, 145.7, 146.5, 150.5 ppm; elemental analysis calcd (%) for C7H9N7O2 (223.19): C 37.14, H 4.06, N 43.93; found: C 37.14, H 3.98, N 43.37. 3-Amino-6-(1,2-pyrazol-l-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (9): Compound 9 was prepared using the general procedure and the starting material was 3-amino-6-(1,2-pyrazol-l-yl)-1,2,4,5-tetrazine (9 a). Yellow solid; yield: 82 %; IR (KBr): n˜ = 3130, 1629, 1539, 1483, 1390, 1416, 1336, 1306, 1107, 1079, 1035, 1003, 974 cm 1; 1H NMR: d = 6.56 (dd, 1 H, J = 2.6, 1.5 Hz), 7.82 (d, 1 H, J = 1.5 Hz), 8.33 (d, 1 H, J = 2.6 Hz), 8.52 ppm (s, 2 H); 13C NMR: d = 109.4, 129.4, 143.7, 145.9, 146.0 ppm; 15N NMR: d = 77.7, 90.8, 100.9, 170.8, 311.9 ppm; elemental analysis calcd (%) for C5H5N7O2 (195.14): C 30.77, H 2.58, N 50.24; found: C 30.48, H 2.56, N 49.71. 3-Amino-6-(1,2,4-triazol-l-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (10): Compound 10 was prepared using the general procedure and the starting material was 3-amino-6-(1,2,4-triazol-l-yl)-1,2,4,5-tetrazine (10 a).Yellow solid; yield: 78 %; IR (KBr): n˜ = 3365, 3133, 3013, 1647, 1505, 1418, 1346, 1333, 1278, 1205, 1107, 1006, 946, 888 cm 1; 1 H NMR: d = 8.29 (s, 1 H), 8.79 (s, 2 H), 9.24 ppm (s, 2 H); 13C NMR: d = 146.4, 148.9, 149.8, 159.3 ppm; elemental analysis calcd (%) for C4H4N8O2·H2O (214.14): C 22.43, H 2.82, N 52.35; found: C 22.20, H 2.72, N 51.24.

General procedure for the synthesis of compounds 4, 6–18: Trifluoroacetic anhydride (4 mL, 28 mmol) was added to a slurry of 50 % hydrogen peroxide (1.3 mL, 25 mmol) in methylene chloride (20 mL) with stirring at < 10 8C. The tetrazine compound (7 mmol) was added at 0 8C and stirred for 30 min; then for several hours at room temperature. The solvent was removed and the residue was washed with ether and then air-dried.

3-Amino-6-(3-amino-5-nitro-1H-1,2,4-triazol-1-yl)-1,2,4,5-tetrazine-2,4di-N-oxide monohydrate (11): Compound 11 was prepared using the general procedure and purified on silica gel (1:1 hexaneEtOAc). The starting material was 3-amino-6-(3,5-diamino-1H-1,2,4triazol-1-yl)-1,2,4,5-tetrazine (11 a).[21b] Yellow solid; yield: 25 %; IR (KBr): n˜ = 3442, 3314, 1638, 1572, 1527, 1474, 1341, 1311, 1105, 844 cm 1; 1H NMR: d = 8.62 (s, 2 H), 7.82 ppm (s, 2 H); 13C NMR: d = 144.0, 148.6, 151.0, 162.7 ppm; elemental analysis calcd (%) for C4H4N10O4·H2O (274.05): C 17.52, H 2.21, N 51.09; found: C 17.87, H 2.53, N 51.87.

3,6-Diamino-1,2,4,5-tetrazine-1-N-oxide (4):[13e] Compound 4 was prepared using the general procedure and the starting material was 3,6-diamino-1,2,4,5-tetrazine (4 a).[13e] Yellow solid; yield: 70 %; IR (KBr): n˜ = 3415, 3294, 3175, 1629, 1462, 1395, 1051, 835, 742 cm 1; 1 H NMR: d = 6.56 (s, 2 H), 7.10 ppm (s, 2 H); 13C NMR: d = 149.3, 81.9, 86.9, 91.3, 316.2, 158.8 ppm; 15N NMR: d-61.6, 322.2 ppm.

3-Amino-6-(4-nitro-3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine-2,4-di-Noxide (12): Compound 12 was prepared using the general procedure and the starting material was 3-amino-6-(4-nitro-3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine (12 a). Yellow solid; yield: 55 %; IR (KBr): n˜ = 3356, 1627, 1583, 1509, 1462, 1327, 1151, 1109, 1012, 806 cm 1; 1H NMR: d = 8.72 (s, 2 H), 2.52 (s, 3 H), 2.43 ppm (s, 3 H); 13 C NMR: d = 12.3, 13.9, 132.2, 143.6, 144.5, 147.0, 147.1 ppm; ele-

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Full Paper mental analysis calcd (%) for C7H8N9O4 (281.07): C 31.35, H 3.01, N 41.78; found: C 31.01, H 2.99, N 39.69. 3-Amino-6-(5-azido-1,2,4-triazol-l-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (13): Compound 13 was prepared using the general procedure and the starting material was 3-amino-6-(5-azido-1,2,4-triazol-l-yl)1,2,4,5-tetrazine (13 a).[21d] Yellow solid; yield: 62 %; IR (KBr): n˜ = 3343, 2144, 1631, 1531, 1327, 1186, 1109, 1014, 734 cm 1; 1H NMR: d = 9.21(s, 1 H), 8.78 ppm (s, 2 H); 13C NMR: d = 153.4, 145.6, 145.1, 143.6 ppm; elemental analysis calcd (%) for C4H3N11O2 (237.05): C 20.26, H 1.28, N 64.97; found: C 19.85, H 1.55, N 64.76. 3-Amino-6-(1H-tetrazol-5-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (14): Compound 14 was prepared using the general procedure and the starting material was 3-amino-6-(1H-tetrazol-5-yl)-1,2,4,5-tetrazine (14 a). Yellow solid; yield: 70 %; IR (KBr): n˜ = 3203, 3127, 2957, 1637, 1516, 1426, 1326, 1317, 1176, 1053, 950, 732 cm 1; 1H NMR: d = 9.12 ppm (s, 3 H); 13C NMR: d = 152.3, 147.6, 140.6 ppm; 15N NMR: d = 9.6, 81.6, 89.6, 102.8, 309.2 ppm; elemental analysis calcd (%) for C4H3N11O2.H2O (255.06): C 16.75, H 2.34, N 58.60; found: C 17.05, H 2.29, N 58.84. 3-Amino-6-cyano-1,2,4,5-tetrazine-2,4-di-N-oxide (15): Compound 15 was prepared using the general procedure and the starting material was 3-amino-6-cyano-1,2,4,5-tetrazine (15 a).[21c] Yellow solid; yield: 82 %; IR (KBr): n˜ = 3434, 3234, 2853, 2254, 1650, 1640, 1496, 1431, 1365, 1328, 1122, 942, 802 cm 1; 1H NMR: d = 9.60 ppm (s, 2 H); 13C NMR: d = 149.7, 129.5, 112.0 ppm; 15N NMR: d = 75.4, 90.6, 121.0, 301.3 ppm; elemental analysis calcd (%) for C3H2N6O2 (154.02): C 23.38, H 1.31, N 54.54; found: C 23.45, H 1.29, N 54.67. 3-Amino-6-nitro-1,2,4,5-tetrazine-2,4-di-N-oxide (16): Compound 16 was prepared using the general procedure and the starting material was 3-amino-6-nitro 1,2,4,5-tetrazine (16 a).[21e] Yellow solid; yield: 52 %; IR (KBr): n˜ = 3421, 3314, 1649, 1585, 1532, 1432, 1348, 1337, 1125, 840, 819 cm 1; 1H NMR: d = 7.88 ppm (s, 2 H); 13C NMR: 90.2, 94.6, d = 149.3, 150.4 ppm; 15N NMR: d = 27.9, 309.4 ppm; elemental analysis calcd (%) for C2H2N6O4 (174.01): C 13.82, H 1.16, N 48.28; found: C 13.79, H 1.14, N 47.59. 3-Guanidino-6-nitro-1,2,4,5-tetrazine-2,4-di-N-oxide·monohydrate (17): Compound 17 was prepared using the general procedure and the starting material was 3-guanidino-6-amino-1,2,4,5-tetrazine (17 a).Yellow solid; yield: 70 %; IR (KBr): n˜ = 3396, 3219, 1696, 1609, 1404, 1315, 1201, 779 cm 1; 1H NMR: d = 7.94 (s, 4 H), 8.52 ppm (s, 2 H); 13C NMR: d = 154.5, 146.3, 145.3 ppm; 15N NMR: d = 3.6, 90.9, 100.5, 281.8, 295.8, 312.9 ppm; elemental analysis calcd (%) for C3H6N8O4·H2O (236.06): C 15.39, H 2.58, N 47.86; found: C 15.47, H 2.93, N 46.07.

[3] [4]

[5]

[6]

[7]

[8] [9] [10]

[11] [12]

[13]

Acknowledgements The authors gratefully acknowledge the support of ONR (NOOO14-12-1-0536) and DTRA (HDTRA 1-11-1-0034). We are indebted to Scott Economu for considerable assistance with crystal structuring and Dr. Xinhao Zhang and Juan Du for calculations. Keywords: energetic materials · nitrogen heterocycles · Noxides · structure elucidation · tetrazines

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Received: September 10, 2014 Published online on October 21, 2014

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