& Energetic Materials

N-Oxide 1,2,4,5-Tetrazine-Based High-Performance EnergeticMaterials

Hao Wei, Haixiang Gao, and Jean’ne M. Shreeve*[a]

Abstract: One route to high density and high performanceenergetic materials based on 1,2,4,5-tetrazine is the intro-duction of 2,4-di-N-oxide functionalities. Based on several ex-amples and through theoretical analysis, the strategy of re-gioselective introduction of these moieties into 1,2,4,5-tetra-zines has been developed. Using this methodology, variousnew tetrazine structures containing the N-oxide functionalitywere synthesized and fully characterized using IR, NMR, andmass spectroscopy, elemental analysis, and single-crystal X-ray analysis. Hydrogen peroxide (50 %) was used very effec-tively in lieu of the usual 90 % peroxide in this system togenerate N-oxide tetrazine compounds successfully. Compar-ison of the experimental densities of N-oxide 1,2,4,5-tetra-zine compounds with their 1,2,4,5-tetrazine precursors

shows that introducing the N-oxide functionality is a highlyeffective and feasible method to enhance the density ofthese materials. The heats of formation for all compoundswere calculated with Gaussian 03 (revision D.01) and thesevalues were combined with measured densities to calculatedetonation pressures (P) and velocities (nD) of these energet-ic materials (Explo 5.0 v. 6.01). The new oxygen-containingtetrazines exhibit high density, good thermal stability, ac-ceptable oxygen balance, positive heat of formation, and ex-cellent detonation properties, which, in some cases, are su-perior to those of 1,3,5-tritnitrotoluene (TNT), 1,3,5-trinitro-triazacyclohexane (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX).

Introduction

Modern high-energy density materials (HEDMs) have beenwidely studied for civilian and military applications, and contin-ue to attract considerable attention.[1] The performance ofHEDMs is evaluated by detonation pressure (P) and velocity(nD), which are related to density, oxygen balance, and heat offormation.[2] Performance is highly dependent on the density.The velocity of detonation increases linearly with density andthe detonation pressure increases with the density squared.[3]

For energetic material synthesis, to obtain higher density andbetter detonation properties, introducing more dense andmore energy-rich functional groups as substituents into candi-date 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 ofinsensitivity and stability along with introducing more energy-rich functional groups are quite often contradictory to eachother.[5]

The synthesis of N-oxides is a rather recent methodology.[6]

The N�O bond of N-oxide is a relatively strong bond possess-ing significant double bond character owing to p-back-bond-ing by the lone oxygen pair. On the other hand, the formation

of a heterocyclic N-oxide also changes the charge distributionof the entire molecule which enhances the aromaticity of thering system, thus stabilizing the entire molecule.[7] For exam-ple, 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 com-pound, 4,4’-dinitro-3,3’-diazenofuroxan (DDF), the latter showssuperior 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-di-amino-3,5-dinitropyrazine (ANPZ) (nD (calcd) ca. 7892 m s�1, 1=

1.84 g cm�3) with the more dense N-oxide 2,6-diamino-3,5-dini-tropyrazine-1-oxide (LLM-105), shows a higher energetic per-formance for the oxygen-containing species (nD (calcd) ca.8516 m s�1, 1= 1.92 g cm�3).[9] Thus, the N-oxide functionalitynot only increases oxygen balance, but also allows bettercrystal packing, and efficiently enhances detonationperformance.[10]

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

[a] Dr. H. Wei, Prof. Dr. H. Gao, Prof. Dr. J. M. ShreeveDepartment of Chemistry, University of Idaho875 Perimeter Dr. , MS 2343, Moscow, ID 83844–2343 (USA)E-mail : jshreeve@uidaho.edu

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201405122.

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Full PaperDOI: 10.1002/chem.201405122

1,2,4,5-Tetrazines, also known as s-tetrazines, were first syn-thesized in 1893,[11] and have attracted the attention ofmany.[12] Researchers at the Los Alamos National Laboratoryare the main contributors to energetic s-tetrazine chemistry.[13]

Energetic materials based on 1,2,4,5-tetrazines show desirableproperties associated with high N-atom content, positive heatof formation, and thermal stability. However, due to limitedstability, the introduction of energy-rich functional groups isvery difficult. For example, 3,6-dinitramine 1,2,4,5-tetrazine isreadily hydrolyzed and reverts to 3,6-diamino 1,2,4,5-tetrazinewith even traces of moisture,[6e] and was found to be too un-stable to isolate.[13e] 1,2,4,5-Tetrazines have many advantages interms of performance and stability, but due to the lack ofenergy-rich functional groups, energetic materials based solelyon 1,2,4,5-tetrazine usually exhibit low density and negativeoxygen balance. The introduction of at least one N-oxidemoiety into a 1,2,4,5-tetrazine system provides an effective andpractical way of overcoming this problem. Surprisingly thissystem is relatively unexploited. The few tetrazines extensivelyevaluated as explosives include: 3,6-diamino-1,2,4,5-tetrazine-1,4-dioxide (LAX-112),[13b] N-oxides of 3,3’-azobis(6-amino-1,2,4,5-tetrazine) (DAATO3.5)[13d] and 3,6-diguanidino-1,2,4,5-tet-razine-1,4-di-N-oxide (DGT-DO)[13a] (Figure 2). One explanationis that the oxidative step is most often accompanied by a lowyield, and a complex mixture that makes purificationdifficult.[13d, 14]

In order to understand the relationship of the structure of1,2,4,5-tetrazine and the oxidative result, we selected a few tet-razines as theoretical subjects. The strategy of regioselectiveintroduction of 2,4-di-N-oxide into 1,2,4,5-tetrazine was put for-ward. Natural bond orbital (NBO) analysis (charge) also sup-ports this result. By employing this strategy, a number of un-known and insensitive compounds with good thermal stabili-ties as well as high detonation performances were designedand synthesized. These compounds were characterized by X-ray diffraction (in some cases), IR and multinuclear NMR spec-troscopy, elemental analysis, and DSC. Their properties as ener-getic materials were studied and evaluated using the experi-mental values obtained for the thermal decomposition and thesensitivity data as well as calculated performance characteris-tics. Calculations and experimental values confirm that intro-ducing N-oxide is an effective method to enhance densities

and to improve energetic performance. In this paper, weexpand the field of tetrazine chemistry by suggesting for thefirst time a relationship between the substituents on the tetra-zine ring and the resulting oxidation product.

Results and Discussion

Synthesis

Based on the literature, the reagents available for introducingthe nitrogen–oxygen bond into tetrazines are hypofluorousacid [HOF],[6] Caro’s acid (peroxomonosulfuric acid, H2SO5),[13d]

peroxytrifluoroacetic acid (PTFA), and Oxone�

(2 KHSO5·KHSO4·K2SO4).[13e] HOF is very effective but elementalfluorine is needed for its synthesis. Oxone� is often not a suffi-ciently strong oxidizer. The strong acid properties of Caro’sacid tend to limit its application. Therefore, PTFA was our re-agent of choice. PTFA is most often prepared by using 90 %hydrogen peroxide and trifluoroacetic anhydride. For the firsttime, we now have replaced 90 % with 50 % hydrogen perox-ide and can report that the oxidizing power of this more dilutemixture continues to be very effective in yielding N-oxide(s)products in moderate to good yields. This change greatly im-proves the safety and economy, and enhances the practicalityof the methodology.

Several well-known 1,2,4,5-tetrazine derivatives were select-ed, such as tetrazole (1), 3,5-dimethylpyrazol-l-yl (2) or 5-amino-3-nitro-1H-1,2,4-trizol-l-yl (3), with heterocyclic substitu-ents bonded to both of the carbon atoms of the tetrazine ring(Scheme 1).

However, N-oxide products were not obtained with 1, 2, or3 even when a large excess of oxidant (10 � ) with a prolongedreaction time at an elevated temperature was used. In order toinitiate the reaction, a nitrogen atom on the tetrazine ringmust attack an oxygen atom of PTFA.[15] Based on that assump-tion, it follows that the distribution of electron density hasa strong influence on the oxidation reaction. Electron-deficientgroups decrease the nucleophilicity of the ring discouragingeffective attack on PTFA. Therefore, electron-rich groups wereintroduced into the tetrazine system. It was hoped that elec-tron-rich groups would enhance the electron density initiatingthe reaction and forming hydrogen bonds to stabilize the

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

Scheme 1. Oxidized tetrazine compounds that contain two heterocyclicsubstituents.

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entire molecule. 3,6-Diamino-1,2,4,5-tetrazine (4 a), 3,6-dihydra-zino-1,2,4,5-tetrazine (5 a), 3-amino-6-(1H-,1,2,3,4-tetrazol-5-yl-amino)1,2,4,5-tetrazine (6 a), and 3-amino-6-guanidino-1,2,4,5-tetrazine (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-dihydrazino-1,2,4,5-tetrazine (5 a) decomposed in the oxidation process,possibly because of the ease of reducing the hydrazino group(Table 1, entry 2).

In order to introduce additional N-oxides into the tetrazinering, further enhancement of charge density on the ring orother methods were required. One example that attracted ourattention was that the treatment of 3-amino-6-chlorotetrazinewith peroxytrifluoroacetic acid gave the 2,4-di-N-oxide productsuccessfully.[6e] While this result was not rationalized in thisstudy and no additional examples were reported, we assumedthat the electron-deficient group (chloro) played an importantrole in the oxidation process. Therefore, an electron-deficientgroup (heterocyclic ring) was introduced on one carbon atomand an electron-rich group (amino) on the other carbon atomto 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 systemfor two reasons: 1) the heterocyclic rings could behave as elec-tron-deficient groups mimicking the chloro group so that 2,4-di-N-oxide moieties could be introduced into the 1,2,4,5-tetra-zine successfully giving products 8–10 ; 2) recently, the designand synthesis of novel energetic materials are focused on het-

erocyclic compounds. Energetic compounds composed of het-erocycles are dramatic not only owing to their higher heats offormation, density, thermal stability, and oxygen balance, butalso due to their greater environmental acceptability, sincethey produce a high percentage of nitrogen gas in a blast orburn. The amino group was retained as an electron-rich group,which can provide sufficient electron density to initiate the re-action, form intramolecular hydrogen bonds with N-oxides tostabilize the entire molecule, and decrease steric hindranceduring the oxidation process. Results have shown the accuracyand effectiveness of this strategy. All substrates gave 2,4-di-N-oxide products in high yields using 50 % hydrogen peroxideand trifluoroacetic acid anhydride in dichloromethane at roomtemperature (Table 2).

In an attempt to rationalize these results, the charge distri-bution data were calculated by natural bond orbitals (NBO)charges analysis for compounds 4 a, 9 a, and 10 a and the re-sults are given in Tables S19, S20, and S21, respectively (Sup-porting Information). In the case of 4 a, for example, NBO anal-ysis shows that N1, and N4 are more negatively charged thanN2 and N3 (Figure 3). However, only a mono-oxide productwas obtained under standard conditions. This observation canbe rationalized as follows: the product, 4, formed by the intro-duction of a single mono-oxide is subsequently protonated bythe more strongly acidic trifluoroacetic acid. As a result, furtheroxidation of the N-oxide product 4 to the 2,4-di-N-oxide prod-uct 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, dis-play a more negative charge density than N1 and N3

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

Substrate Product Yield[%][b]

1 64

2 — —

3 54

4 55

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

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

Substrate Product Yield[%][b]

1 74

2 82

3 78

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

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(Figure 2). The electron-withdrawing groups (pyrazole and tri-azole) considerably reduce the electron charge on the nearbynitrogen atoms due to conjugative and resonance effects ofthe ring. Evidently, the 2,4-di-N-oxide products should beformed during the oxidation process. The NBO analysis sup-ports the experimental results showing that N2 and N4 on 9 aand 10 a are more negatively charged than N atoms on com-pound 4 a. Therefore, the electron-withdrawing groups actuallyenhance the electronic densities of N2 and N4 in the tetrazinesystem. Based on the above, it can be concluded that whena tetrazine ring is bonded to an electron-deficient group onone carbon atom and an electron-rich group on the othercarbon atom, the 2,4-di-N-oxide products will be the main oxi-dation products. Several heterocycles with different substitu-ents were introduced into the tetrazine system, such as 3,5-di-amino-1,2,4-triazole (in 11 a), 3,5-dimethyl-4-nitropyrazole (in12 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 deriva-tives (11–14) were obtained. It should be noted that oneamino group on the triazole moiety in compound 11 a wasconverted to nitro. The structures of 11·DMSO and 14·H2Owere confirmed by X-ray crystallographic analysis (SupportingInformation). Finally, the N-oxide product can be obtained byintroducing an electron-deficient functional group on onecarbon atom and an amino moiety on the other carbon atom.Compounds 15 a and 16 a, which contain cyano and nitrogroups, respectively, were designed and synthesized (Table 3,entries 5, 6). The desired 2,4-dioxide products 15 and 16 wereformed in high yields

Surprisingly, a completely different transformation occurredwith a guanidine group substituent on one carbon atom andan amino group on the other. Stirring 3-guanidine-6-amino-1,2,4,5-tetrazine (17 a) under standard conditions for 12 h, gave3-guanidine-6-nitro-1,2,4,5-tetrazine-2,4-di-N-oxide (17)(Scheme 2). However, when the reaction time was shortenedto 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 isoxidized to nitro, and under the impact of guanidine (electron-rich group) and nitro group (electron-deficient group), 2,4-di-N-oxide moieties are successfully introduced onto the ring.

Spectroscopy

The structures of 6–17 are supported by IR, 1H and 13C NMRspectroscopy as well as elemental analysis. Additionally, 15NNMR spectra were recorded for compounds 9, 14, 15, 16, and17 in [D6]DMSO; chemical shifts are given with respect toCH3NO2 as external standard (Figure 4). All the compoundswith a 2,4-di-N-oxide structure are symmetric ; therefore thereare only two nitrogen signals attributable to the tetrazine ring.The N2 and N4 signals are upfield relative to N1 and N3. Com-pounds 16 and 17 each have a nitro group bonded to the tet-razine ring assigned at �27.95 and �3.55 ppm, respectively.

Figure 3. NBO analysis of 4 a, 9 a and 10 a (charge densities: Tables S14, S15and S16 in the Supporting Information).

Table 3. Design and synthesis of some energetic materials based on thestrategy.[a]

Substrate Product Yield[%][b]

1 25

2 55

3 62

4 70

5 82

6 52

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

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

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Single-crystal X-ray structure analysis

Crystals of 11·DMSO and 14·H2O, suitable for single-crystal X-ray diffraction, were obtained by slow evaporation of solutionsof the compounds in DMSO or water, respectively, at roomtemperature (Figures 5 and 6). The crystallographic data andrefinement details can be found in the Supporting Information.The crystal structure of 14·H2O appears as if it has two protonson the tetrazole ring; however, each tetrazole ring has onlyone proton per moiety (Figure 5). This occurs because the hy-

drogen on N4 or N7 occupieseach position only one half ofthe time. The N-oxide bondlengths in 11·DMSO are1.2564(13) and 1.2604(12) �, in14·H2O are 1.2703(16) and1.2683(15) �. In the structures of11·DMSO and 14·H2O, the N�Nbond lengths in the tetrazinering are 1.33(13), 1.34(13),1.33(17), and 1.33 (17), which isnormal in tetrazines without N-oxide. The angles of N-N-O are119.3, 119.05, 119.96, and 119.958for 11·DMSO and 14·H2O, respec-tively. For 11·DMSO and 14·H2O,it is interesting to note that thetetrazine ring and triazole (tor-sion angle N5-N4-C4-N8:178.148) or tetrazole ring (torsionangle N7-C2-C3-N8: �0.88) arecoplanar.

Thermal behavior

The phase-transition tempera-tures and thermal stabilities ofcompounds 6–17 were deter-mined by differential scanningcalorimetric (DSC) measurementsscanning at 5 8C min�1, usingabout 1.0 mg of material(Table 4). Most of these com-pounds show sharp exothermicpeaks, which indicate rapid de-composition. All compounds arethermally stable with decompo-sition temperatures (onset tem-peratures) ranging from 110 to252 8C. Compound 16 has thelowest onset decompositiontemperature at 110 8C, and 8 thehighest at 252 8C.

Properties

The experimentally determineddensities of 6–17 range between

1.76 and 1.92 g cm�3 equal or exceed that of common explo-sives. 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 nitrogroups being involved in multiple intermolecular hydrogenbonding 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 com-pounds 6–17 in a bar graph comparison with those of the tet-

Figure 4. 15N spectra of compounds 9, 14, 15, 16 and 17 in [D6]DMSO.

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razine precursors 6 a–17 a. As indicated in this figure, a markeddensity increase is seen and this supports the advantage of in-troduction of the N-oxide group into the 1,2,4,5-tetrazinesystem. Compounds 17 a/17 appear to exhibit the largest in-crease. In this case, in addition to the N-oxide contribution, theamino group on the tetrazine ring was also converted toa nitro group during the oxidation process, which also contrib-utes to hydrogen bonding. Based on the above, introducingthe N-oxide moiety into the 1,2,4,5-tetrazine system is a highlyeffective and feasible method to obtain high density tetrazinecompounds.

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

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

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

Td[b]

[8C]1[c]

[g cm�3]OB[d]

[%]DfH

[e]

[kJ mol�1/kJ g�1]

nD[f]

[m s�1]P[g]

[GPa]IS[h]

[J]FS[i]

[N]

6 192 1.76 �32.6 351.8/1.79 8429 27.3 35 3607 177 1.77 �18.6 627.8/2.92 8580 29.1 26 3608 252 1.76 �68.1 347.6/1.56 8180 23.6 35 3609 237 1.79 �45.1 427.0/2.19 8304 26.4 27 36010 221 1.80 �32.4 448.6/2.29 8438 27.7 24 36011 168 1.84 �12.5 430.6/1.68 8707 32.0 17 24012 211 1.82 �41.7 328.3/1.23 8413 27.5 19 36013 161 1.82 �20.2 781.7/3.29 8731 30.9 8 6014 181 1.85 �20.3 539.4/2.73 8884 32.4 14 12015 191 1.84 �21.3 378.6/2.46 8635 31.3 15 24016 110 1.92 9.2 225.7/1.29 9316 39.4 3 1017 134 1.91 �7.4 325.5/1.50 9157 37.5 20 240TNT 295 1.65 �24.7 �67.0/�0.30 6881 19.5 15 –RDX 210 1.82 0 80.0/0.36 8748 34.9 7.4 120HMX 280 1.91 0 104.8/0.36 9320 39.5 7.4 120

[a] All new compounds are anhydrous except 14 and 16 which are mono-hydrates. [b] Thermal decomposition temperature (onset) under nitrogengas (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 = mo-lecular weight of compound. [e] Calculated heat of formation. [f] Detona-tion velocity (Explo5 v.6.01). [g] Detonation pressure (Explo5 v.6.01).[h] Impact sensitivity. [i] Friction sensitivity.

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

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In this study, the oxygen balance (OB) values fall in therange between �68.1 to 9.2. Heats of formation were calculat-ed by using the Gaussian 03 (Revision D. 01) suite of pro-grams.[17] The detonation pressures (P) and velocities (nD) werecalculated by using EXPLO5 v6.01. As can be seen fromTable 4, the calculated detonation velocities lie between nD =

8180 and nD = 9316 m s�1. The highest values in terms of deto-nation velocity were observed for compounds 16 (9316 m s�1)and 17 (9157 m s�1), all of which exceed 1,3,5-trinitrotriazacy-clohexane (RDX). In comparison with other tetrazine deriva-tives, an improved performance is seen resulting from the in-troduction of N-oxide groups. The detonation pressures of N-oxide and di-N-oxide tetrazine derivatives lie in the range 23.6and 39.4 GPa (compared to RDX 34.9 GPa, and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) 39.5 GPa).

For initial safety testing, the sensitivities of 6–17 towardimpact (IS) and friction (FS) were measured. Impact sensitivitymeasurements were made using standard BAM Fall hammertechniques.[18] Compound 16 is the most impact- and friction-sensitive compound (IS: 3 J, FS: 10 N), Nevertheless, 17 whichhas a structure that is similar to 16, is less sensitive (IS: 20 J,FS: 240 N). The remainder of the compounds are much lessimpact sensitive (8–35 J) than RDX and HMX, which suggeststhat 6, 7, 8, 9, 10, 12, 14, 15, and 17 could serve as promisingcandidates for safe energetic materials. With the exception of13 and 16, the friction sensitivities of all compounds are allmore positive than 120 N which makes them less sensitivethan 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 systemwas found to be an effective method. Based on several experi-mental examples and theoretical analysis, a strategy for regio-selective introduction of 2,4-di-N-oxide moieties into 1,2,4,5-tetrazines was established. Computational results in terms ofNBO charge analysis also support this methodology, which isuseful in the design and synthesis of a number of new com-pounds. Using this methodology, various new tetrazine com-pounds containing the N-oxide moiety were synthesized andfully characterized using IR and NMR spectroscopy, elementalanalysis, and X-ray single-crystal structure analysis. Hydrogenperoxide (50 %) was used for the first time in this system togenerate N-oxide tetrazine compounds successfully. Thesecompounds exhibit good physical and detonation properties,such as moderate thermal stabilities, high densities, high heatsof formation, and high detonation pressures and velocities. Amajority of these compounds shows an equivalent or higherdensity (in the range of 1.76–1.92 g cm�3) than RDX and com-pounds 16 and 17 are comparable with HMX. It is seen thatthe introduction of the N-oxide functionality into the 1,2,4,5-tetrazine system is a highly effective and feasible method toobtain highly dense tetrazine compounds. Calculated detona-tion values for these compounds are comparable to those ofexplosives such as TNT, RDX, and HMX. All compounds werealso characterized with respect to impact and friction sensitivi-

ty, and thermal stability. With the exception of 16, all are lesssensitive than RDX and HMX, which suggests that these com-pounds could be of interest for future applications as environ-mentally friendly and high-performing nitrogen or oxygen-richmaterials and may serve as a series of promising alternativesto RDX and HMX.

Experimental Section

Safety precautions

While we have experienced no difficulties in syntheses and charac-terization of these materials, proper protective measures should beused. Manipulations must be carried out in a hood behind a safetyshield. Face shield and leather gloves must be worn. Cautionshould be exercised at all times during the synthesis, characteriza-tion, and handling of any of these materials. Mechanical actions in-volving scratching or scraping must be avoided. All of the energet-ic compounds must be synthesized only in small amounts.

X-ray crystallography

A yellow prism of dimensions 0.148 � 0.335 � 0.423 mm3 for11·DMSO and a yellow prism of dimensions 0.148 � 0.335 �0.423 mm3 for 14·H2O were used for the X-ray crystallographicanalysis. The X-ray intensity data were collected by a Bruker Apex2 CCD system equipped with a graphite monochromator anda MoKa fine focus tube (l= 0.71073 �). An Oxford Cobra low-tem-perature device was used to maintain the crystals of 14·H2O ata constant 173 K during data collection. The data for 11·DMSOwere collected at 298 K. The frames were attached to the BrukerSAINT software package[19] using a narrow-frame algorithm anddata were corrected for absorption effects using the multiscanmethod (SADABS[20]). The structures were solved and refined withthe aid of the programs in the Bruker SHELXTL Software Package.CCDC 1011379 (11·DMSO) and 1011381 (14·H2O) contain the sup-plementary crystallographic data for this paper. These data can beobtained free of charge from The Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif.

General methods

Analytical grade reagents were purchased from Aldrich and AcrosOrganics and were used as received. 1H and 13C NMR spectra wererecorded using a 300 MHz (Bruker AVANCE 300) NMR spectrometeroperating 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 sol-vent and locking solvent unless otherwise stated. Chemical shiftsin the 1H, and 13C spectra are reported relative to Me4Si and 15NNMR to MeNO2. The melting and decomposition (onset) pointswere obtained with a differential scanning calorimeter (TA Instru-ments Co., model Q10) at a scan rate of 5 8C min�1. IR spectra wererecorded using KBr pellets for solids on a BIORAD model 3000 FTSspectrometer. Density was measured at room temperature usinga Micromeritics AccuPyc 1330 gas pycnometer. Elemental analysiswas carried out on an Exeter CE-440 elemental analyzer. Com-pounds 1,[2e] 2,[13b] 3,[21a] and 5[6e] were synthesized according to lit-erature procedures.

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Synthesis

General procedure for the synthesis of compounds 9 a, 10 a, and12 a : Ammonia was bubbled through a solution of the 1,2,4,5-tet-razine 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 collectedby filtration, washed with toluene, and dried to give compounds9 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): C36.81, 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);13C NMR: d= 164.2, 154.9, 153.3, 143.5 ppm; elemental analysiscalcd (%) for C4H4N8 (164.06): C 29.27, H 2.46, N 68.27; found: C29.00, 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) wereadded 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 ina sealed tube in 120 8C for 4 h. The solvent was removed byvacuum 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): C19.68, 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 for20 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 resultingmixture was stirred at room temperature for 12 h. The dark slurrywas 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.

General procedure for the synthesis of compounds 4, 6–18 : Tri-fluoroacetic anhydride (4 mL, 28 mmol) was added to a slurry of50 % 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 atroom temperature. The solvent was removed and the residue waswashed with ether and then air-dried.

3,6-Diamino-1,2,4,5-tetrazine-1-N-oxide (4):[13e] Compound 4 was pre-pared using the general procedure and the starting material was3,6-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;1H NMR: d= 6.56 (s, 2 H), 7.10 ppm (s, 2 H); 13C NMR: d= 149.3,158.8 ppm; 15N NMR: d-61.6, �81.9, �86.9, �91.3, �316.2,�322.2 ppm.

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 andthe starting material was 3-(1H-1,2,3,4-tetrazol-5-ylamino)-6-amino-1,2,4,5-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 7was prepared using the general procedure and the starting materi-al 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);13C 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, H2.29, 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 andthe 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): C37.14, 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): Com-pound 9 was prepared using the general procedure and the start-ing 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): C30.77, 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 thestarting 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;1H 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 (%) forC4H4N8O2·H2O (214.14): C 22.43, H 2.82, N 52.35; found: C 22.20, H2.72, N 51.24.

3-Amino-6-(3-amino-5-nitro-1H-1,2,4-triazol-1-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide monohydrate (11): Compound 11 was prepared usingthe general procedure and purified on silica gel (1:1 hexane-EtOAc). The starting material was 3-amino-6-(3,5-diamino-1H-1,2,4-triazol-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 (%) forC4H4N10O4·H2O (274.05): C 17.52, H 2.21, N 51.09; found: C 17.87, H2.53, N 51.87.

3-Amino-6-(4-nitro-3,5-dimethylpyrazol-l-yl)-1,2,4,5-tetrazine-2,4-di-N-oxide (12): Compound 12 was prepared using the general proce-dure and the starting material was 3-amino-6-(4-nitro-3,5-dimethyl-pyrazol-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);13C NMR: d= 12.3, 13.9, 132.2, 143.6, 144.5, 147.0, 147.1 ppm; ele-

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mental analysis calcd (%) for C7H8N9O4 (281.07): C 31.35, H 3.01, N41.78; 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 andthe 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): C20.26, 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 thestarting 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 analysiscalcd (%) 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 15was prepared using the general procedure and the starting materi-al 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 (%) forC3H2N6O2 (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 16was prepared using the general procedure and the starting materi-al 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:d= 149.3, 150.4 ppm; 15N NMR: d=�27.9, �90.2, �94.6,�309.4 ppm; elemental analysis calcd (%) for C2H2N6O4 (174.01): C13.82, 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 thestarting 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 analysiscalcd (%) for C3H6N8O4·H2O (236.06): C 15.39, H 2.58, N 47.86;found: C 15.47, H 2.93, N 46.07.

Acknowledgements

The authors gratefully acknowledge the support of ONR(NOOO14-12-1-0536) and DTRA (HDTRA 1-11-1-0034). We areindebted to Scott Economu for considerable assistance withcrystal structuring and Dr. Xinhao Zhang and Juan Du forcalculations.

Keywords: energetic materials · nitrogen heterocycles · N-oxides · structure elucidation · tetrazines

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

Chem. Eur. J. 2014, 20, 16943 – 16952 www.chemeurj.org � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim16952

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