German Edition: DOI: 10.1002/ange.201608723Energetic MaterialsInternational Edition: DOI: 10.1002/anie.201608723

An Energetic Triazolo-1,2,4-Triazine and its N-OxideDavin G. Piercey, David E. Chavez,* Brian L. Scott, Greg H. Imler, and Damon A. Parrish

Abstract: The reaction of 3-amino-5-nitro-1,2,4-triazole withnitrous acid produces the corresponding diazonium salt. Whenthe diazonium salt is treated with nitroacetonitrile, a subsequentcondensation and cyclization reaction occurres to produced 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine (DPX-26). X-ray crystallographic analysis shows that the DPX-26 hasa density of 1.86 gcm@3, while it is calculated to have a heat offormation of 398.3 kJmol@1. DPX-26 is predicted to approachthe explosive performance of RDX but displays significantlybetter safety properties. Oxidation of DPX-26 using hypofluo-rous acid produces 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4]triazine 4-oxide (DPX-27), which is also predicted to be a high-performance material with enhanced safety properties.

Energetic materials, that is propellants, explosives, andpyrotechnics, have long attracted intense research efforts inthe chemical sciences, including interest from early scientistssuch as Liebig, Berzelius, and Gay-Lussac.[1,2] Much as thesefoundational researchers focused on understanding the prop-erties of new materials, todayQs energetic materials research-ers strive to understand how to break the correlation betweenhigh-performance explosives and high sensitivity.[3] Whilethere are some outlier energetic materials that do not followthis trend,[4] these materials have not yet reached theperformance of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazo-cine (HMX) while displaying the insensitivity of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Additionally, theability to intentionally design such an outlier from firstprinciples is still in its infancy.[5] Within our own laboratory,the rational design of new energetic materials with tailoredsensitivity and performance has been a long-term objective.Within this context, a major goal in the field is the replace-ment of cyclotrimethylenetrinitramine (RDX) with a materialthat shows lower sensitivity and comparable or higherperformance, and yet avoids toxicity and carcinogenicity.

It is known that vicinal C-amino/C-nitro functionalitiestend to lead to increased insensitivity through both inter- andintramolecular hydrogen bonding, as seen in the insensitivehigh explosive 1,3,5-triamino-2,4,6-trinitrobenzene (impactsensitivity> 40 J, friction sensitivity> 360 N, electrostatic dis-

charge (ESD) sensitivity> 0.125 J).[6] 1,1-Diamino-2,2-dini-troethylene (FOX-7) is another example where vicinal aminoand nitro groups combine to provide an energetic materialwith good performance (detonation velocity and pressure)and safety properties (impact sensitivity 15–20 J, frictionsensitivity 340 N).[7] Recently, we began an investigation ofother heterocyclic architectures incorporating vicinal aminoand nitro groups.[8] In prior work, we developed an efficientsynthesis of 4-amino-3,7,8-trinitropyrazolo-[5,1-c][1,2,4] tria-zine, which was originally synthesized by Dalinger and co-workers.[9] We have also investigated the density, enthalpy,and safety properties of this compound. From the standpointof chemical reactivity, nitro groups attached to a ring carbonadjacent to a ring nitrogen in triazines and tetrazines areknown to be hydrolytically unstable.[10] Interestingly, thepresence of the neighboring C-amino group leads to thestabilization of 4-amino-3,7,8-trinitropyrazolo-[5,1-c][1,2,4]triazine and significantly reduces the susceptibility of thenitro group to hydrolysis as a result of electron donation fromthe amine group nitrogen lone pair into the ring system.

We now wish to report the synthesis of 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine (DPX-26) and its oxida-tion product 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine4-oxide (DPX-27). We began with the diazotization of 3-amino-5-nitro-triazole,[11] which produces an intermediatezwitterionic species that precipitates from the reactionmixture. Subsequent addition of nitroacetonitrile[12] leads tocondensation with the diazonium moiety. The reaction thenproceeds through a cyclization process to give the triazolo-triazine bicyclic system (Scheme 1).

DPX-26 was found to be thermally stable up to 232 88C,and was stable under vacuum at 120 88C for 48 hours. Singlecrystals of DPX-26 suitable for X-ray crystallographic anal-ysis were obtained through recrystallization from nitrome-thane. The molecule crystallizes in the monoclinic crystalsystem and belongs to the Cc space group. It is planar, withtwelve formula units residing in the unit cell, and displaysa density of 1.86 gcm@3 at 293 K (1.88 gcm@3 at 100 K;Figure 1).

The introduction of an N-oxide moiety is another strategyto increase density and promote additional hydrogen-bondinginteractions.[13] Peracids have been shown to oxidize 1,2,4

Scheme 1. Synthesis of DXP-26.

[*] Dr. D. G. PierceyNalas Engineering, Inc., Centerbrook, CT 06409 (USA)

Dr. D. E. ChavezM Division, Los Alamos National LaboratoryLos Alamos, NM 87545 (USA)E-mail: dechavez@lanl.gov

Dr. B. L. ScottC Division, Los Alamos National LaboratoryLos Alamos, NM 87545 (USA)

Dr. G. H. Imler, Dr. D. A. ParrishNaval Research Laboratory, Washington, DC 20375 (USA)

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triazines at the N1 or N2 positions.[14] Oxidation of DPX-26was investigated using peroxytrifluoroacetic acid and Oxoneas the oxidants. Unfortunately, these oxidizing reagents werenot active enough to oxidize the substrate. Hypofluorous acid(HOF) one of the most powerful oxidizers and oxygen-transfer reagents that has been described.[15] HOF can beprepared in situ by bubbling a stream of 10% F2 in an inertcarrier gas (such as helium or nitrogen) into a 10% solution ofwater in acetonitrile below @5 88C.[16] DPX-26 was added toa solution of HOF in acetonitrile at @10 88C and after severalhours, 4-amino-3,7-dinitrotriazolo-[5,1-c][1,2,4] triazine 4-oxide began to precipitate as a bright yellow solid (Scheme 2).

The product was determined to be a complex of DPX-27and two molecules of water. The identity of the complex wasconfirmed by X-ray crystallography, as was the position of theN-oxidation on the triazine ring.[17] X-ray crystallographicanalysis of the DPX-27·2H2O complex shows that themolecule crystallizes in the Pbca space group and theorthorhombic crystal system (Figure 2). The bond lengthsbetween several atoms in DPX-26 and DPX-27 were com-

pared. The bonds between the triazolo ring and the triazinering in DPX-26 had lengthened from 1.363 (C4–N4) and1.345 c (C2–N5) in DPX-26 to 1.382 and 1.377 c in DPX-27·2H2O. These two effects lead to an increased separation ofthe 1,2,4-triazine ring from the triazole ring. The density ofthe H2O solvate was calculated to be 1.882 gcm@3 at 100 K.The unit cell of DPX-27 is displayed in Figure 3 and shows theherring bone crystal lattice structure of DPX-27, whichcontains eight molecules in the unit cell.

Recrystallization from nitromethane provided a 1:1 nitro-methane energetic co-crystal as orange crystals. The crystalforms in the P212121 space group and the orthorhombic crystalsystem (Figure 4).[18] The nitromethane oxygen atoms (O6and O7) form hydrogen bonds with a single proton on theamino group (N8) of DPX-27, with distances between nitro-methane oxygen atom and the amino group of DPX-27 of 2.41and 2.68 c. The density of the solvate was determined to be1.765 gcm@3 at 150 K and 1.727 gcm@3 at 293 K.[17] Thenitromethane solvate displays a similar lengthening of thebonds connecting the triazole to the 1,2,4 triazine (see above).This lengthening of the bonds connecting the triazole to the1,2,4-triazine ring may be the reason that DPX-27 exhibits

Figure 1. a) Molecular structure of DPX-26 as it appears in the crystalstructure top view. b) Edge view of DPX-26. Non-H atomic-displace-ment ellipsoids are shown at 50 % probability. c) Unit cell of DPX-26.

Scheme 2. N-oxidation of DPX-26 using HOF·CH3CN.

Figure 2. Molecular structure of DPX-27·2H2O solvate as it appears inthe crystal structure. Non-H atomic-displacement ellipsoids are shownat 50% probability.

Figure 3. A view of the packing down the c axis of the unit cell forDPX-27·2H2O.

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a lower decomposition temperature (138 88C vs. 232 88C)

compared to DPX-26, since the bond order has decreased.Attempts to crystallize DPX-27 to provide a solvate-freestructure were unsuccessful.

The sensitivity data for compounds DPX-26 and DPX-27are displayed in Table 1. DPX-26 was found to be relativelyinsensitive to impact, and insensitive to spark and friction,while DPX-27 was found to be less sensitive to impact andfriction than RDX.

The heats of formation of DPX-26 and DPX-27 werecalculated using the method developed by Rice.[19] The heat offormation of DPX-26 was calculated to be 387 kJmol@1 andthe heat of formation of DPX-27 was calculated to be378 kJmol@1. The crystal density of DPX-27 was also deter-mined by using the Rice method,[20] and was predicted to be1.904 gcm@3. The Cheetah thermochemical code was thenused to predict the explosive performance of DPX-26 andDPX-27.[21] DPX-26 was predicted to have a detonationpressure (PCJ) of 32 GPa and a detonation velocity (VD) of8.7 km s@1. The detonation pressure for DPX-27 was predictedto be 35.4 GPa and the detonation velocity was predicted tobe 8.97 km s@1. These data compare favorably with theperformance properties of the insensitive TATB (VD =

7.6 km s@1, PCJ = 27 GPa) and less-sensitive FOX-7 (VD =

8.8 km s@1, PCJ = 35. GPa).[21]

In conclusion, we carried out the condensation of thediazonium salt of 3-amino-5-nitro-1,2,4-triazole with nitro-acetonitrile, which then leads to a cyclization reaction to formthe product fused triazolo-triazine bicycle DPX-26. Hypo-fluorous acid was then used to install an N-oxide on the

triazine N1 position. The materials display performancesimilar to or better than RDX but show excellent insensitivitytoward destructive mechanical stimuli.

Experimental SectionCaution! The materials DPX-26 and DPX-27 are explosive

materials. Additionally, the diazonium intermediate in Scheme 1 isvery sensitive when dry. Personnel protective equipment such asleather or Kevlar gloves, blast shields and safety glasses should beused when working with these materials.

DPX-26: 3.87 g (0.030 mol) of 3-amino-5-nitro-1,2,4-triazole wasslurried in 30 mL water and 9 mL of concentrated hydrochloric acidwas added. This solution was cooled to 5 88C, and a solution of 2.4 g ofsodium nitrite in 15 mL of distilled water was added dropwise, whilemaintaining this temperature. After the addition, the reactionmixture was stirred at this temperature for 30 minutes. To thissolution was added (dropwise) a solution of 2.58 g (0.030 mol) ofnitroacetonitrile and 1.2 g of sodium hydroxide in 20 mL water. Themixture was allowed to warm to room temperature and stirred forthree days, after which time, a tan solid had precipitated. Afterfiltration, 3.52 g (48%) of DPX-26 was obtained. An analytically puresample was produced by recrystallization from nitromethane. DSC(5 88Cmin@1): 232 88C (dec). 1H NMR ([D6]DMSO) d = 10.15 (s, H,NH2), 10.88 ppm (s, H, NH2); 13C NMR ([D6]DMSO) d = 163.5, 155.4,140.0, 139.0 ppm; IR (KBr) n = 3439, 3412, 3312, 3290, 3325, 3143,1656, 1560, 1495, 1476, 1456, 1439, 1408, 1332, 1290, 1252, 1210, 1174,1091, 1006, 842, 775, 770, 752, 704, 667, 623, 585, 532 cm@1. m/z : (ESI)242.0 (M); Elemental Analysis calc’d for C4H2N8O4 : C 19.84, N 46.28,H 0.83%, found: C 19.83, N 46.30, H 0.82%.

DPX-27: A mixture of 100 mL of acetonitrile and 10 mL of waterwas cooled to @5 88C with a dry-ice bath. A 10% fluorine solution (innitrogen) was bubbled through the acetontrile solution for 1.5 hoursat @5 88C. Concentrations of 0.25m to 0.35m solutions were typicallyobtained. DPX-26 (1.21 g, 5.0 mmol) was added portionwise. Thereaction was stirred at 0 88C for 1 hour and then allowed to warm toroom temperature. A yellow orange precipitate began to form. Themixture was stirred for additional 2 hours at ambient temperature,concentrated, and filtered. The filter cake was washed with tert-butylmethyl ether and air dried. The solid was then taken up in acetonitrile,treated with trimethylsilyl chloride, and filtered to give DPX-27(0.235 g (80%). M.P. 138 88C (dec.).1H NMR ([D6]DMSO) d = 9.95, (s,1H, NH2), 10.56 ppm (s, 1H, NH2); 13C NMR ([D6]acetone) d = 127.0,137.5, 148.2, 161.1 ppm. IR (KBr) n = 3614, 3573, 3476, 3386, 1670,1560, 1507, 1499, 1432, 1376, 1311, 1282, 1250, 1213, 1124 cm@1.Elemental Analysis calcd for C4H2N8O5 : C 19.84, H 0.83, N 46.28;found: C 19.87, H 0.85, N 46.25.

Acknowledgements

We would like to thank Thomas W. Myers for providingsupport in finalizing the crystal structure for DPX-27-2H2O,Stephanie Hagelberg (elemental analysis) for characteriza-tion, Hongzhao Tian and Jose G. Archuleta, (sensitivitytesting), and Mary Sandstrom (thermal analysis). We wouldalso like to thank the Joint Munitions Technology Develop-ment Program and The Laboratory Directed Research andDevelopment Program for funding this work. Los AlamosNational Laboratory is operated by Los Alamos NationalSecurity (LANS, LLC) under contract No. DE-AC52-06NA25396 for the U.S. Department of Energy. The authorsalso thank the Office of Naval Research (Award No. N00014-15-WX-0-0149)

Figure 4. Molecular structure of DPX-27·nitromethane as it appears inthe crystal structure. Non-H atomic-displacement ellipsoids are shownat 50% probability.

Table 1: Sensitivity data for DPX-26 and DPX-27.

Impact [J][a] Spark [J][b] Friction [N][c]

DPX-26 29 0.125 >360DPX-27 10.3 0.062 258PETN 2.5 0.062 92RDX 4.6 0.062 157

[a] LANL type 12, 50 % drop height, 2.5 kg. [b] ABL spark 3.4% thresholdinitiation level (TIL). [c] 50% load Bruceton up/down method.

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Keywords: 1,2,4-triazines · 1,2,4-triazoles · bicyclic compounds ·energetic materials · nitroacetonitriles

How to cite: Angew. Chem. Int. Ed. 2016, 55, 15315–15318Angew. Chem. 2016, 128, 15541–15544

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[17] CCDC 1481755 (DPX-26) and 1481754 (DPX-27-2H2O) containsupplementary crystallographic data for this paper. These datacan be obtained free of charge from The CambridgeCrystallographic Data Centre.

[18] CCDC1501740 (DPX-27-nitromethane) contains supplemen-tary crystallographic data for this paper. These data can beobtained free of charge from The Cambridge CrystallographicData Centre.

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Received: September 6, 2016Published online: November 9, 2016

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