& Energetic Materials

3,6,7-Triamino-[1,2,4]triazolo[4,3-b][1,2,4]triazole: A Non-toxic,High-Performance Energetic Building Block with ExcellentStability

Thomas M. Klapçtke,* Philipp C. Schmid, Simon Schnell, and Jçrg Stierstorfer[a]

Abstract: A novel strategy for the design of energetic mate-rials that uses fused amino-substituted triazoles as energeticbuilding blocks is presented. The 3,6,7-triamino-7H-[1,2,4]tri-azolo[4,3-b][1,2,4]triazolium (TATOT) motif can be incorporat-ed into many ionic, nitrogen-rich materials to form salts withadvantages such as remarkably high stability towards physi-cal or mechanical stimuli, excellent calculated detonation ve-locity, and toxicity low enough to qualify them as “green ex-plosives”. Neutral TATOT can be synthesized in a convenientand inexpensive two-step protocol in high yield. To demon-

strate the superior properties of TATOT, 13 ionic derivativeswere synthesized and their chemical- and physicochemicalproperties (e.g. , sensitivities towards impact, friction andelectrostatic discharge) were investigated extensively. Lowtoxicity was demonstrated for neutral TATOT and its nitratesalt. Both are insensitive towards impact and friction and thenitrate salt combines outstanding thermal stability (decom-position temperature = 280 8C) with promising calculated en-ergetic values.

Introduction

Since the discovery of trinitrotoluene (TNT, Figure 1), the devel-opment of explosives and other new energetic materials hasbeen a continuously growing research topic in the large fieldof materials science.[1] A new secondary explosive should pos-sess a high heat of formation and density, which results ina high detonation pressure and detonation velocity. Further-more, it must be less sensitive towards impact and frictionthan RDX (Figure 1) and its thermal stability should surpassthe 200 8C benchmark. Recently, environmental concerns havebecome increasingly important, which means that new explo-sive materials should only release environmentally benign de-composition products on detonation.[2]

Several concepts have been used in the past two decades toimprove the properties of secondary explosives[3] (Figure 1):i) ionic compounds with nitrogen-rich cations, such as ammoni-um, hydrazinium, and hydroxylammonium, can lead to higherdensities and stabilities due to their high lattice energy (e.g. ,AN, ADN,[3a, b] FOX-12,[3c] DA-4-MeTDN),[3d] ii) nitrogen-rich het-erocycles (AzT)[3e] with an N-oxide functionality (NT2O,[3f, g]

TKX-50),[3h] iii) large hydrogen-bonding networks in the solidstate (e.g. , TATB,[3i] FOX-12), and iv) use of aromaticity to fur-ther increase the stability of energetic compounds (e.g. , TKX-50, TNT, TATB, TNTTB,[3j] TADTr (DN)2).[3k]

However, the most recent studies focus on new energeticanions that are often functionalized with nitro- (e.g. , NT2O) ornitramino (e.g. , TNTTB) moieties combined with known, non-aromatic, nitrogen-rich cations, such as ammonium, hydroxyl-ammonium (e.g. , TKX-50), or hydrazinium.[1a, b, 3f, h, 4] New ionicexplosives that comprise nitro- and nitramino moieties coupledwith hydrazinium or hydroxylammonium cations might notmeet the criteria for “green explosives”.

Examples of aromatic nitrogen-rich cations in energetic com-pounds are di- or triaminotriazolium ions, as well as amino- ordiaminotetrazolium ions.[5] Recently, Shreeve et al. reported en-ergetic materials based on tris(triazolo)benzene[3j] and 3,6-dini-tropyrazolo[4,3-c]pyrazole,[3l] which showed good thermal sta-bilities. Recently, we reported TADTr, which is exceptionallythermally stable, up to 342 8C in its neutral form.[3k]

In our continuing effort to synthesize energetic materialswith high thermal stability and good energetic performance,we decided to investigate 3,6,7-triamino-7H-[1,2,4]triazolo[4,3-b][1,2,4]triazole (TATOT), a system consisting of two fused tri-azole rings and three amino moieties. Potts and Hirsch first re-ported the synthesis of TATOT in the 1960s;[6] recent investiga-tions focused on TATOT and similar compounds have beenconducted by Centore et al.[7] Herein, we report a detailed in-vestigation of the triaminotriazolotriazole system that utilizesTATOT as an energetic building block. Scheme 1 shows the syn-thesis and characterization of numerous ionic derivatives,which all meet the required criteria for mechanical (impactsensitivity (IS)�7.5 J, friction sensitivity (FS)�120 N) or thermal(decomposition temperature (Tdec.)�200 8C) stimuli, and mostshow excellent explosive properties.

[a] Prof. Dr. T. M. Klapçtke, P. C. Schmid, S. Schnell, Dr. J. StierstorferDepartment of Chemistry, Energetic Materials ResearchLudwig Maximilian University, Butenandtstr. 5-13 (D)81377 Mìnchen (Germany)E-mail : tmk@cup.uni-muenchen.de

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201500982.

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Full PaperDOI: 10.1002/chem.201500982

Results and Discussion

TATOT (2) is synthesized by a quick and convenient reactionusing water as the solvent.[6] Triaminoguanidine hydrochloride(1 a) is dissolved in 1 m hydrochloric acid, then cyanogen bro-mide is added. The reaction mixture is first heated to 60 8C,then heated at reflux for 1 h. The mixture is allowed to cool toambient temperature and, after alkaline workup with sodiumcarbonate, neutral 2 is obtained in good yield (Scheme 1,route 2). Nitrate 6 can be synthesized by a less-expensive,more-feasible, and even-more-efficient route from triaminogua-nidine nitrate (1 b) solvated in 1 M nitric acid (Scheme 1,route 1). All the products can be used as obtained and do notrequire further purification (e.g. , column chromatography).Compound 2 and its salts show low water solubility, which fa-cilitates isolation and purification.

The energetic salts 3–5, 7, 9–11, and 13–14 were obtainedby simple anion metathesis of 1 c with different energeticanions, whereas compounds 6, 8, and 12 were obtained by ad-dition of the appropriate acid to the free base 2 (Scheme 1).

TATOT dinitramide (3) is obtained by a 1:1 stoichiometric re-action with the monohydrochloride salt 1 c in ethanol. TATOT5-nitrotetrazole-2-oxide (4), TATOT 5-nitrotetrazolate monohy-drate (5), TATOT 5-nitriminotetrazolate (10), and TATOT 1-methyl-5-nitriminotetrazolate (11) are obtained by a 1:1 stoi-chiometric reaction of 1 c with the required anion in water.TATOT nitrate (6) and TATOT perchlorate (12) salts are formed

by addition of nitric acid (1 equiv) or perchloric acid (1 equiv),respectively, to 2, whereas di-TATOT bitetrazole-1-oxide (8) isobtained by a 2:1 stoichiometric reaction of the free base 2with 1,1’-dihydroxy-5,5’-bitetrazole. Di-TATOT tetranitro-biimd-azolate (7), di-TATOT 1,1’-dinitramino-5,5’-bitetrazolate (9), di-TATOT 5,5’-azotetrazolate (13), and di-TATOT 5,5’-azotetrazole-1,1’-dioxide (14) are formed by dissolving 1 (2 equiv) and theappropriate anion (1 equiv) in water.

Crystal structures

During this work the crystal structures of 2, 4–6, 8, 9, and 11–13 were obtained. Selected data and parameters from the low-temperature X-ray data collection and refinement are given inTables S1–S3 (Supporting Information).[8]

Compound 2 crystallizes from water in the triclinic spacegroup P1 with a density of 1.757 g cm¢3 at 173 K and two mol-ecules per unit cell. The molecular unit of 2 with selectedbond lengths and angles is shown in Figure 2. With bondlengths of 1.306(2) to 1.434(9) æ, the distances between thering atoms of the 1,2,4-triazoles lie between the length offormal C¢N and N¢N single and double bonds (C¢N: 1.47 æ,C=N: 1.22 æ; N¢N: 1.48 æ, N=N: 1.20 æ), which indicates the ar-omaticity of the ring system.[9] The bond angles in the tworings all lie near 1088 ; the N1-N2-C1 angle shows the greatestdeviation (100.97(13)8). A planar ring system is formed with anN5-N4-C2-N3 torsion angle of 180.00(19)8. Thus, like bitri-

Figure 1. Selected chemical structures of secondary explosives, which give an overview of different concepts and research strategies used during the develop-ment of new secondary explosives. A combination of strategies, i) ionic compounds (AN, ADN, FOX-12, DA-4-MeTDN), ii) nitrogen-rich heterocycles (AzT)with N-oxides (NT2O, TKX-50), iii) introduction of amino- (TADTr (DN)2, TATB), nitro- (TNT, TATB), and nitramino functional groups (RDX, CL-20, TNTTB), iv) hy-drogen-bonding network (FOX-12, TATB), v) caged (CL-20) and fused (TNTTB) molecules, results in the ionic, aromatic, nitrogen-rich, energetic, environmen-tally friendly, fused heterocyclic compound TATOT NO3.

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azoles,[9] fused triazoles form planar ring systems. The torsionangle of the amino groups is slightly larger than 08 (1–58), thusthe amino groups are slightly tilted out of the plane of thering system. All the amino groups participate in hydrogenbonds, whereas in the ring system only N2, N4, and N5 act as

hydrogen-bond acceptors. The lengths of the hydrogen bondsare in the range of the sum of the van der Waals radii(rw(N)++rw(N) = 3.20 æ), thus a strong hydrogen-bond network isformed.[9]

Compound 4 crystallizes from water in the orthorhombicspace group P212121 with a density of 1.765 g cm¢3 at 173 Kand four molecules per unit cell. The molecular unit of 4 withand selected hydrogen bond lengths, is illustrated in Figure 3.Compared to the high-energy hydroxylammonium salt (1=

1.850 g cm¢3), this density is fairly low. On the other hand, am-monium nitrotetrazole-2N-oxide shows a lower density (1=

1.730 g cm¢3), similar to the more-stable nitrogen-rich guanidi-nium- (1= 1.698 g cm¢3), aminoguanidinium- (1= 1.697 g cm¢3),diaminoguanidinium- (1= 1.687 g cm¢3), and triaminoguanidi-nium salts (1= 1.6391 g cm¢3).[3f] The hydrogen-bond networkin 4 is built up a little differently due to the new donor intro-duced into the ring-system through protonation. Only N10 ofthe cation acts as a hydrogen-bond acceptor, whereas theother hydrogen bonds are all intermolecular bonds to the ni-trotetrazolate-2-oxide anion. Additionally, the geometry of thecation is slightly different than in the free base 2. The proton-ated triazole component is slightly distorted relative to neutral2. The bond length N8¢C4 (1.347(3) æ) is slightly shorter,whereas the C4¢N9 bond (1.344(3) æ) is significantly longer

Scheme 1. Synthesis of nitrate 6 (route 1; total yield >60 %) and compounds 2–14 (route 2).

Figure 2. Molecular unit of 2. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ] and torsion angle of 2 [8]: N1¢C2 1.343(2),N1¢N2 1.408(5), N2¢C1 1.327(2), C1¢N3 1.383(2), N3¢C2 1.370(2), C2¢N41.306(2), N4¢N5 1.434(9), N5¢C3 1.318(2), C3¢N1 1.366(1) ; N5-N4-C2-N3:180.

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than the corresponding lengths in 2 (C3¢N1: 1.366(1) æ, N5¢C3: 1.318(2) æ). Also, the N9¢N10 bond length in 4 (1.412(2) æ)shows a deviation from that in neutral 2 (1.434(9) æ). Similarly,the bond angles around the protonated nitrogen atom showsmall deviations of around 48 relative to in the free base 2. Thenitrotetrazolate-2N-oxide anion shows the same bond lengthsand bond angles as reported in literature.[3f]

Compound 5 crystallizes as monohydrate from water in themonoclinic space group C2/c and eight molecular units perunit cell. The molecular unit of 5 with selected bond lengthsand angles is illustrated in Figure 4. The density at 173 K is

1.742 g cm¢3, which is much higher than the density of the cor-responding triaminoguanidinium monohydrate (1=

1.599 g cm¢3).[10] The bond lengths and angles of the counter-ion correspond to the data reported in literature.[10] Likewise,the geometry of the cation lies in the range of the data report-ed for compound 4, showing small deviations from the freebase 2 near the protonated nitrogen atom N8. Additionally,the length of the C2¢N9 bond, at which the two triazoles arefused together is slightly longer than in the free base 2 or in 4.The C3-N8-N7 bond angle (113.69(13)8) is even larger than thecorresponding angle in 4, whereas the angle N9-C3-N8 showsa similar value to that in 4.

Anhydrous compound 6 crystallizes from water in the ortho-rhombic space group Pnma with four formula units per unit

cell. The molecular unit of 6 with selected bond lengths andangles is illustrated in Figure 5. The density of 6 (1 =

1.812 g cm¢3 at 173 K) is higher than the density of guanidini-um nitrate (1 = 1.410 g cm¢3),[11] and is similar to that of thehighly energetic compound hydroxylammonium nitrate (1=

1.841 g cm¢3).[12] Strikingly the torsion angles of this moleculereveal a perfectly planar molecule, with every torsion angle ofthe cation equal to 0.008. The geometry of the cation is com-parable to the geometry of the cation in compound 4, and hassimilar bond angles and lengths.

The energetic salt 8 crystallizes from water in the triclinicspace group P1 with two water moieties and one molecule perunit cell. The molecular unit and selected bond lengths andangles is illustrated in Figure 6. The density of the crystal is

1.692 g cm¢3 at 173 K. As reported for the cations above, theprotonated ring is slightly distorted relative to the free base 2.The length of the C¢C bond connecting the two tetrazole moi-eties in the anion is between a formal C¢C single bond anda C=C double bond (C1ii¢C1iii = 1.439(0) æ), which lies in therange of bond lengths reported for neutral 1,1’-dihydroxy-5,5’-bitetrazole (C¢C = 1.454(2) æ).[13] The cation ring system andthe anion form two nearly planar structures with N6-N5-C3-N8and N1ii-C1ii-C1iii-N4iii torsion angles of 179.11(10)8 and 1.3(2)8,respectively.

Compound 9 crystallizes from water in the monoclinic spacegroup P21/n with two formula units per unit cell. The molecular

Figure 3. Molecular unit of 4. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ], hydrogen-bond lengths [æ], and angles [8]:C2¢N8 1.350(3) ; N12¢H12A···N10 2.989(3), N13¢H13A···N1 2.941(2), N9¢H9A···O1 2.808(2) ; N8-C4-N9 104.14(16), C4-N9-N10 112.82(17).

Figure 4. Molecular unit of 5. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ] and angles [8]: C2¢N9 1.353(1), N9¢C31.349(2), C3¢N8 1.336(2), N8¢N7 1.415(5); N9-C3-N8 104.26(12), N8-N7-C299.87(12).

Figure 5. Molecular unit of 6. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ] and angles [8]: C2¢N5 1.350(2), C1¢N21.341(8), N2¢N1 1.413(4) ; C1-N2-N1 113.47(12), N5-C1-N2 104.13(13).

Figure 6. Molecular unit of 8. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ] and angles [8]: C3¢N5 1.352(3), N5¢C41.349(9), C4¢N9 1.343(2), N9¢N8 1.410(7); N5-C4-N9 104.24(10), C4-N9-N8113.15(10), N9-N8-C3 100.67(10).

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unit of 9 with selected bond lengths and angles is shown inFigure 7. The density at 173 K is 1.792 g cm¢3, which is lowerthan that of the potassium salt K2DNABT (1 = 2.172 g cm¢3 at100 K).[14] The hydroxylammonium salt (NH3OH)2 DNABT andthe ammonium salt (NH4)2 DNABT also show higher densities(1= 1.877 and 1.814 g cm¢3, respectively).[15] The tetrazole unitsof the anion form a completely planar structure with an N11-C4-C4ii-N14ii torsion angle of 0.0(2)8. The nitro groups are twist-ed out of this plane by 78.66(15)8, as reported in the litera-ture.[14] The triazole rings of the cation are slightly tilted to-wards each other with a torsion angle of 177.94(11)8. The N4i¢N5i bond length of 1.405(8) æ is slightly smaller than in com-pounds 2–8. The C4¢C4ii bond length is equal to that inK2DNABT (1.451(5) æ).[14]

Anhydrous compound 11 crystallizes from water in themonoclinic space group P21/c with four molecules per unitcell. The molecular unit of 11 with selected bond lengths,bond angles, and torsion angles is illustrated in Figure 8. Thiscompound displays the lowest density of the compounds inthis report (1= 1.685 g cm¢3 at 173 K). However, the other ni-trogen-rich guanidinium, diaminoguanidinium, and triamino-guanidinium 1-methylnitriminotetrazolate salts exhibit signifi-cantly lower densities (1= 1.550, 1.605, and 1.569 g cm¢3, re-spectively).[16] The C3-N9-C4-N7 torsion angle of the cation(177.88(10)8) reveals an almost-planar ring system. The tetra-zole ring of the anion also forms a planar moiety, whereas thenitramine unit is twisted out of the plane by 10.9(2)8. In con-trast, the amine units of the cation lie in the same plane as thering system. The bond lengths in the tetrazolate ring system

are between 1.29 and 1.37 æ and match the data reported inliterature.[16] The bond lengths and angles between the ringatoms in the cation have similar values to those of compound9.

Perchlorate salt 12 crystallizes from water in the triclinicspace group P1 with two formula units per unit cell. The mo-lecular unit of 12 with selected bond lengths, bond angles,and torsion angles is illustrated in Figure 9. The density at

173 K (1= 1.808 g cm¢3) is relatively small compared with thedensity reported for the commonly used salt ammonium per-chlorate (1 = 1.951 g cm¢3),[17] whereas the calculated density ofhydroxylammonium perchlorate (1= 2.13 g cm¢3)[18] is signifi-cantly higher. However, compound 12 has a similar density ashydrazinium perchlorate (1= 1.84 g cm¢3) and a much higherdensity than triaminoguanidinium perchlorate (1=

1.67 g cm¢3).[19] The two triazoles are tilted slightly towardseach other with an N2-N3-C1-N4 torsion angle of 176.96(18)8.The amino groups are significantly twisted out of the plane upto 10.38. The bond angles and lengths of the cation are similarto the values reported for 9. The perchlorate anion has a slight-ly distorted tetrahedral geometry with O-Cl-O bond angles be-tween 111.998 and 107.568 and Cl¢O bond lengths between1.406 and 1.459 æ.

Salt 13 crystallizes from water in the monoclinic spacegroup P21/c with two molecular moieties per unit cell. The mo-lecular unit of 13 with selected bond lengths, angles, torsionangles, and hydrogen-bond lengths is illustrated in Figure 10.The density of 13 (1= 1.756 g cm¢3) is significantly higher thanthe density of the corresponding ammonium, guanidinium, ortriaminoguanidinium salts (1= 1.562, 1.569, and 1.634 g cm¢3,respectively).[20] A direct comparison with the hydroxylammoni-um salt was not possible because the hydroxylammonium saltcocrystallizes with two water moieties.[21] The two rings of thecation are tilted slightly towards each other with a torsionangle of 176.67(17)8, whereas the anion displays a completely

Figure 7. Molecular unit of 9. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ] and angles [8]: N3i¢C1i 1.349(7), N3i¢C3i

1.345(3), C3i¢N4i 1.336(7), N4i¢N5i 1.405(8), C4¢C4ii 1.451(5) ; N3i-C3i-N4i

104.28(10), C3i-N4i-N5i 113.63(10).

Figure 8. Molecular unit of 11. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ], bond angles [8] , and torsion angles [8]: C4¢N9 1.353(9), N9¢C3 1.353(9), C3¢N10 1.340(2), N10¢N11 1.404(7) ; N10-N11-C4 100.41(10), N11-N10-C3 113.53(10), N10-C3-N9 104.39(11) ; N12-N7-C4-N110.6(3), N12-N7-C5-N13 3.1(2), N8-N9-C3-N14 1.6(3), N6-N5-C1-N4 10.9(2), N2-N1-C1-N4 0.72(14).

Figure 9. Molecular unit of 12. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ], bond angles [8] , and torsion angles [8]: C1¢N3 1.355(3), N3¢C3 1.341(3), C3¢N5 1.337(3), N5¢N4 1.409(2), Cl1¢O11.438(3), Cl1¢O2 1.435(2), Cl1¢O3 1.459(3), Cl1¢O4 1.406(3) ; N3-C3-N5104.20(18), N4-N5-C3 113.58(18), N5-N4-C1 100.38(16), O1-Cl1-O4 111.99(16),O2-Cl1-O4 111.17, O3-Cl1-O4 107.89(17); N6-N1-C1-N4 10.3(5), N3-N2-C2-N7178.4(2), N2-N3-C3-N8 7.6(4).

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planar system. The amino groups of the cation are slightly lesstwisted outward than in 12. The geometry of the cation is verysimilar to those of the other cations in this report. The bondlength of the diazene bond (N5=N5i : 1.255(2) æ) lies in therange found for the corresponding diguanidinium and di(tri-aminoguanidinium) salts.[20] The crystal structure analysis alsorevealed numerous N¢H···N hydrogen bonds. Several hydrogenbonds are formed in the crystal structure, including one intra-molecular bond, a cation–cation bond, and various anion–cation bonds.

Compound 14 crystallizes from water in the monoclinicspace group C2/c with four cation/anion pairs per unit cell.The molecular unit of 14 with selected bond lengths, bondangles, torsion angles, and hydrogen bond lengths is repre-sented in Figure 11. The density of 14 (1= 1.763 g cm¢3) isslightly higher than the density of azotetrazolate 13, which re-veals that introduction of a 1,1’-dioxide functionality slightlyimproves the density. A slightly lower density is observed com-pared with the corresponding hydroxylammonium (1=

1.778 g cm¢3) or ammonium salts (1 = 1.800 g cm¢3).[22] The tor-sion angle of the cation reveals an almost-planar ring systemwith the amino groups slightly twisted out of the plane. Theanion reveals a planar system in which the oxygen atoms areslightly twisted outwards at an angle of 1.68. The bond lengthsand angles of the cation are consistent with the data reportedfor the energetic salts. The only exception is that the C1¢N4bond is slightly longer than the reported distances for the cor-responding bond lengths in the other cations studied. TheN13i¢N13ii diazene bond length (1.269(2) æ) is even longerthan the diazene bond length in 13. The hydrogen-bond net-work formed in 14 is different to that found for 13 due to thepresence of the oxygen atoms. In addition to hydrogen bondsto oxygen, two very strong intermolecular hydrogen bonds be-tween the triazoles of the two cations are formed (N5ii¢H5ii···N2: 2.890(2) æ and N6¢H6···N4iii : 2.867(2) æ).

Spectroscopy

NMR spectroscopy

All NMR spectra were recorded in solution in [D6]DMSO. In ad-dition to 1H and 13C NMR spectroscopy, HMBC experimentswere carried out to ensure the correct assignment of all theprotons and carbon atoms. For example (Figure 12), the1H NMR spectrum of 6 reveals four signals at d= 13.31 (HA),8.19 (HB), 7.23 (HC), and 5.78 ppm (HD). In the 13C NMR spec-trum nitrate 6 shows resonances at d= 160.3 (C1), 147.6 (C2),and 141.3 ppm (C3). The assignments were confirmed bygauge independent atomic orbital (GIAO) quantum chemicalcalculations. The chemical shifts of 3–5 and 7–14 all lie in thesame range and are discussed in the Supporting Information.

IR and Raman spectroscopy

IR and Raman spectra for compounds 2–14 were measuredand the frequencies were assigned according to commonly ob-served values reported in the literature.[23] Compound 2 showsthe N¢H bond stretching vibration between n= 3500 and3100 cm¢1 in the IR spectrum, whereas the deformation vibra-tion of the N¢H bond is found as the strongest band at n=

1569 cm¢1 in the IR spectrum. In the Raman spectrum, this de-formation vibration appears as a weak band at n= 1584 cm¢1.The C=N stretching vibration of the triazole ring is visible asa very strong band at n= 1616 cm¢1 in the IR spectrum, where-as the same vibration is present as a weak band at n =

1611 cm¢1 in the Raman spectrum. The second important vi-bration of the ring system is the C¢N stretch, which appears atn= 1293 cm¢1 in both vibrational spectra. Similar values for thevibrational data have been found for previously reported sub-stituted 1,2,4-triazoles.[24] The band at n= 1502 cm¢1 arisesfrom the N¢NH2 stretch in the IR spectrum, whereas the char-acteristic C¢NH2 vibration is found at n= 1078 cm¢1 in both vi-brational spectra. In compounds 3–14, similar values to those

Figure 11. Molecular unit of 14. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ], hydrogen-bond lengths [æ], bond angles[8] , and torsion angles [8]: C1¢N1 1.348(2), N1¢C3 1.363(2), C3¢N5 1.338(3),N5¢N4 1.413(2), N9i¢O1i 1.306(2), N13¢N13ii 1.269(2) ; N5iiI¢H5iIi···N2 2.890(2),N6¢H6···N4iii 2.867(2), N7¢H7···N12i 2.853(3); N1-C3-N5 104.04(16), C3-N5-N4113.27(16), N5-N4-C1 100.75(15); N2-N1-C3-N6 4.2(4), N1-N2-C2-N7 179.2(2),N8-N3-C1-N4 10.1(4).

Figure 10. Molecular unit of 13. Ellipsoids are drawn at the 50 % probabilitylevel. Selected bond lengths [æ], hydrogen bonds [æ], bond angles [8] , andtorsion angles [8]: C2¢N9 1.345(2), N9¢C4 1.357(2), C4¢N7 1.335(2), N8¢N71.406(2) ; N7ii¢H7ii···N2 2.824(2), N11¢H11A···N4’ 3.211(3), N11¢H11A···N5i

3.026(2), N11¢H11B···N1 3.179(2), N12¢H12A···N4 3.163(2), N12¢H12B···N32.967(2), N13ii¢H13ii

A···N8 2.883(2), N13ii¢N13iiB···N1 2.989(2) ; N9-C4-N7

104.65(15), C4-N7-N8 113.02(15), N7-N8-C2 100.74(15); N11-N10-C2-N8 5.4(4),N6-N9-C4-N13 7.4(4), C2-N10-C3-N12 175.19(16), C1-N5-N5i-C1i 0.0(9).

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described for compound 2 were observed for the cations andare discussed in the Supporting Information.

Physicochemical properties

All the materials investigated are highly energetic compounds,therefore their energetic behaviors were investigated.

Thermal behavior and sensitivities

The thermal behavior and sensitivities of all the compoundsstudied were investigated and reported. A detailed descriptionof the setup in presented in the Supporting Information. It canbe seen from Table 1 that all the compounds have a decompo-

sition temperature (Tdec.)�200 8Cand, therefore, meet the re-quired criteria for replacing RDX.Nitrate salt 6 shows an excep-tionally high thermal stability(Tdec. = 280 8C). The long-termstability of 6 was determined(75 8C for 48 h), according to in-terim hazard classification (IHC).No mass loss was observed andno significant change in the ele-mental composition was ob-served before or after the mea-surement.

In terms of the sensitivities, allthe compounds, except 9, meetthe required criteria towards me-chanical stimuli (IS�7.5 J andFS�120 N). Notably, neutralcompound 2, as well as the en-

ergetic salts 4–8, 10, 11, 13, and 14, are insensitive to friction(FS�360 N).

Heats of formation and detonation parameters

The heats of formation were calculated theoretically by usingthe atomization equation [Eq. (1)] and CBS-4M electronic en-thalpies.

DHf�ðg,M,298Þ ¼ HðM,298Þ¢SH�ðatoms,298ÞþSDHf

�ðatoms,298Þ ð1Þ

Calculation of the detonation parameters was performedwith the EXPLO5 (version 6.02) program package.[25] The pa-rameters of the Becker–Kistiakowsky–Wilson equation of state

Figure 12. Peak assignment in the HMBC spectrum of 6.

Table 1. Energetic properties and detonation parameters of compounds 2, 4, 6, 9, 11–14, and RDX (for comparison).

2 4 6 9 11 12 13 14 RDX

formula C3H6N8 C4H7N13O3 C3H7N9O3 C8H14N28O4 C5H10N14O2 C3H7N8O4Cl C8H14N26 C8H14N26O2 C3H6N6O6

FW [g mol¢1] 154.13 285.18 217.15 566.38 298.23 254.59 474.37 506.37 222.12IS [J][a] 40 25 40 7.5 8 9 20 30 7.5FS [N][b] 360 360 360 108 360 216 360 360 120ESD [J][c] 1.5 1.0 0.75 0.08 1.0 0.6 1.25 1.5 0.20N [%][d] 72.70 63.85 58.05 69.24 65.75 44.01 76.77 71.92 37.84W [%][e] ¢93.43 ¢47.69 ¢47.89 ¢53.67 ¢69.75 ¢34.57 ¢77.58 ¢66.35 ¢21.61Tdec. [8C][f] 245 222 280 224 237 264 200 210 2101 [g cm¢3] (298 K)[g] 1.725 1.732 1.779 1.759 1.654 1.775 1.724 1.731 1.806DHf8 [kJ mol¢1][h] 446.7 691.7 261.5 1845.5 783.9 314.6 1811.8 1834.1 70.3DUf8 [kJ kg¢1][i] 2872.0 2525.3 1312.7 3359.1 2736.7 1331.1 3924.0 3724.9 417.0EXPLO5V6.02 values :¢DEU8 [kJ kg¢1][j] 3559 5040 4663 5170 4602 5072 4327 4926 5845TE [K][k] 2284 3412 3062 3388 2970 3654 2776 3154 3810pC-J [kbar][l] 297 290 302 320 269 281 307 315 345D [m s¢1][m] 9385 8814 9005 9242 8760 8312 9360 9289 8861V0 [L kg¢1][n] 813 835 870 834 845 817 811 824 785

[a] Impact sensitivity (Bundesanstalt fìr Materialforschung und -prìfung (BAM) drophammer, 1 of 6) ; [b] friction sensitivity (BAM friction tester, 1 of 6) ;[c] electrostatic discharge (ESD) device (OZM Research) ; [d] nitrogen content; [e] oxygen balance; [f] decomposition temperature from DSC (b= 5 8C); [g] re-calculated from low temperature X-ray densities (1298K =1T/(1++aV(298¢T0) ; aV = 1.5 Õ 10¢4 K¢1) and controlled with pycnometric measurements at rt ; [h] cal-culated (CBS-4 m) heat of formation; [i] calculated energy of formation; [j] energy of explosion; [k] explosion temperature; [l] detonation pressure; [m] deto-nation velocity; [n] volume of detonation gases, assuming only gaseous products.

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(BKW EOS) in EXPLO5 V6.02 are calibrated particularly for theformation of nitrogen, which is the main product from com-pounds with a high nitrogen content.[26] The EXPLO5 detona-tion parameters of compounds 2, 4, 6, 9, and 11–14 were cal-culated by using the room-temperature density values ob-tained from the X-ray structures as described in Table 1 and inreference [27] and are summarized in Table 1, alongside a com-parison with the values calculated for RDX. For a complete dis-cussion on the methods used see the Supporting Information.

With regards to the calculated detonation velocities (D), sur-prisingly, the free base 2 exhibits the highest value (D =

9385 m s¢1. The detonation pressure (pC-J) of 297 kbar lies inthe range of the detonation pressures of the energetic salts,which have values between 269 and 315 kbar. The neutralcompound 2 possess a lower detonation pressure than RDX(pC-J RDX = 345 kbar) but a significantly higher detonation veloci-ty (DRDX = 8861 m s¢1). The detonation velocity of 4 (D =

8814 m s¢1) is similar to that of RDX, whereas the detonationpressure (pC-J = 290 kbar) is slightly lower. The correspondingstable, nitrogen-rich guanidinum (G++), aminoguanidinium, di-aminoguanidinium, and triaminoguanidinium (TAG++) salts allshow lower detonation velocities (D = 8201 [G++NT2O] to8768 m s¢1 [TAG++NT2O]) and detonation pressures (pC-J = 266[G++NT2O] to 294 kbar [TAG++NT2O]) than 4.[3f] However, 4 haspoorer energetic properties than highly energetic hydroxylam-monium nitrotetrazolate-2N-oxide salt (D = 9499 m s¢1; pC-J =

390 kbar).[3f] The nitrate salt 6 exhibits a very high detonationvelocity (D = 9005 m s¢1), which is significantly higher than thedetonation velocity of the highly energetic hydrazinium hy-drate salt (D = 8690 m s¢1) or the commonly used salt ammoni-um nitrate (D = 5270 m s¢1).[28] Unfortunately, the detonationpressure of 6 (pC-J = 302 kbar) is somewhat lower than that ofRDX (pC-J = 345 kbar). The highly energetic compound 9 revealspromising energetic properties with a detonation velocity of9242 m s¢1 and the highest detonation pressure of the com-pounds studied (pC-J = 320 kbar). The detonation velocity is sig-nificantly higher than the detonation velocity of the dipotassi-um salt (D = 8330 m s¢1) or RDX (D = 8861 m s¢1), whereas thedetonation pressure is in the same range as the detonationpressure of K2DNABT (pC-J = 317 kbar) and only slightly lowerthan the detonation pressure of RDX. Compound 11 revealsa relatively low detonation velocity (D = 8760 m s¢1) and thelowest detonation pressure (pC-J = 269 kbar). These values aresimilar to the detonation pressure (pC-J = 273 kbar) and the det-onation velocity (D = 8770 m s¢1) of the corresponding triami-noguanidinium salt.[16] The more-stable guanidinium, amino-guanidinium, and diaminoguanidinium salts all reveal lowervalues for the detonation velocity and detonation pressure.[16]

Perchlorate salt 12 had the lowest detonation velocity (D =

8312 m s¢1) and a fairly low detonation pressure (pC-J =

281 kbar), therefore does not fulfill the requirements to replaceRDX. In contrast, azotetrazolate 13 possesses the highest deto-nation velocity (D = 9360 m s¢1), which greatly exceeds the det-onation velocity of RDX and even of the highly energetic dihy-drazinium salt (D = 6330 m s¢1).[29] Compound 14 exhibitsa slightly lower detonation velocity (D = 9289 m s¢1) than com-pound 13, whereas the detonation pressure of 14 (pC-J =

315 kbar) is slightly higher than the detonation pressure of 13.Compound 14 possesses a competitive detonation pressurecompared to the corresponding highly energetic ammonium,hydroxylammonium, and hydrazinium salts,[22] which exhibithigh detonation pressures of 338, 375, and 329 kbar, respec-tively. The detonation velocities of the ammonium and hydrazi-nium salts (D = 9032 and 9066 m s¢1, respectively) are slightlylower, whereas the dihydroxylammonium salt exhibits thehighest value (D = 9348 m s¢1).[22] RDX cannot compete withthe detonation velocity of 14, but has a slightly higher detona-tion pressure.

Toxicity assessment

Neutral 2 and nitrate salt 6 exhibit high thermal stability, insen-sitivity towards impact and friction, as well as good energeticperformances. The toxicity of 2 and 6 was determined byusing the known method with Vibrio fischeri NRRL-B-11177.[9]

The half-maximal effective concentration (EC50) of both com-pounds after 30 min of incubation time (2 : 4.83 g L¢1; 6 :3.36 g L¢1) is much higher than toxic RDX (0.22 g L¢1) and alsoabove the benchmark of 1.0 g L¢1 at which a compound is con-sidered non-toxic.

Conclusion

Crucially, a higher detonation velocity was calculated for 2, 6,9, 13, and 14 than for RDX. It can be concluded from the re-sults shown in Table 1 that all the compounds studied, except9, meet the required criteria for mechanical (IS�7.5 J, FS�120 N) or thermal (Tdec.�200 8C) stimuli, which demonstratesthe great advantage of this aromatic system that forms manyhydrogen bonds. The most promising compounds for potentialapplications are 6 and the azotetrazolates 13 and 14. Salts 6,13, and 14 combine good stability with high calculated deto-nation velocities (D�9000 m s¢1) and detonation pressures (pC-J

�300 kbar).It has been demonstrated that TATOT cation can be used as

an inexpensive alternative to commonly used nitrogen-rich cat-ions, such as guanidines and amines, due to several key fea-tures: stability towards high temperatures and mechanicalstimuli, low (or no) toxicity, and high energetic performance.These advantages make TATOT a next-generation energeticbuilding block.

Experimental Section

Complete experimental details can be found in the Supporting In-formation.

Caution! All the compounds investigated are potentially explosiveenergetic materials, although no hazards were observed duringthe preparation and handling of these compounds. Nevertheless,this necessitates additional meticulous safety precautions (earthedequipment, Kevlar gloves, Kevlar sleeves, face shield, leather coat,and ear plugs).

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Compound 1 c[6]

Compound 1 a (70 g, 0.5 mol, 1.00 equiv) was dissolved in 2 M hy-drochloric acid (750 mL). Cyanogen bromide (105.9 g, 1.0 mol,2.00 equiv) was added and the solution was heated to 60 8C withstirring, then kept at 60 8C for 2 h. After heating the reaction mix-ture at reflux for 1 h the solution was cooled to 0 8C in an ice bath.At 0 8C, 1 c (50.3 g, 0.26 mol, 53 %) was deposited as white plates.1H NMR ([D6]DMSO): d= 13.41 (br s, 1 H; H-2), 8.22 (s, 2 H; H-9), 7.30(s, 2 H; H-11), 5.84 ppm (s, 2 H; H-10); 13C NMR ([D6]DMSO): d=160.2 (s ; C-6), 147.4 (s; C-8), 141.1 ppm (s; C-3); m/z (FAB¢): 35.0(Cl¢) ; m/z (FAB++): 155.2 (C3H6N8

++).

Compound 2[6]

Neutral compound 2 was obtained by neutralizing a solution of 1 c(50.3 g, 0.264 mol, 2.00 equiv) in water (500 mL) with sodium car-bonate (14.0 g, 0.132 mol, 1.00 equiv). The solution was heated atreflux, then left to crystallize to give compound 2 (32.3 g,0.210 mol, 80 %). DTA (5 8C min¢1) onset Tdec. = 245 8C; 1H NMR([D6]DMSO): d= 6.38 (s, 2 H; H-11), 5.59 (s, 2 H; H-10), 5.46 ppm (s,2 H; H-9); 13C NMR ([D6]DMSO): d= 158.5 (s; C-6), 149.1 (s; C-8),143.5 ppm (s; C-3); m/z (DEI++): 154.1 (C3H6N8) ; IR (ATR): n= 3411(w),3298 (m), 3181 (m), 3141 (m), 2735 (w), 2361 (w), 2339 (w), 1616(vs), 1569 (vs), 1502 (s), 1403 (m), 1346 (m), 1293 (m), 1241 (w),1158 (w), 1106 (m), 1078 (w), 968 (m), 946 (m), 847 (w), 729 (w),704 (m), 667 (w), 667 (w), 659 cm¢1 (w); Raman (1064 nm, 300 mW,25 8C): n= 3300(7), 3191(14), 3148(6), 1660(11), 1646(14), 1611(27),1584(21), 1498(5), 1413(10), 1402(14), 1296(18), 1247(12), 1121(12),1081(4), 984(7), 949(7), 852(44), 711(5), 677(21), 624(22), 604(26),401(19), 306(22), 306(22), 177(10), 109(100) cm¢1; elemental analysiscalcd (%) for C3H6N8 (154.13): C 23.38, H 3.92, N 72.20; found: C23.67, H 3.85, N 72.20; Bundesanstalt fìr Materialforschung und-prìfung (BAM) impact: 40 J; BAM friction: 360 N, electrostatic dis-charge (ESD): 1.5 J (at grain sizes <100 mm).

Compound 4

Ammonium nitrotetrazolate-2-oxide (296 mg, 2.00 mmol,1.00 equiv)[9] and 1 c (381 mg, 2.00 mmol, 1.00 equiv) were dis-solved in water and heated to 90 8C. The resulting clear solutionwas left to crystallize overnight to yield 4 (404 mg, 1.42 mmol,71 %) as yellow needles. DTA (5 8C min¢1) onset Tdec. = 222 8C;1H NMR ([D6]DMSO): d= 13.37 (br s, 1 H; H-2), 8.21 (s, 2 H; H-9), 7.24(s, 2 H; H-11), 5.77 ppm (s, 2 H; H-10); 13C NMR ([D6]DMSO): d=160.2 (s; C-6), 157.2 (s; Canion), 147.4 (s ; C-8), 141.1 ppm (s; C-3); m/z(FAB¢): 130.0 (CN5O3

¢) ; m/z (FAB++): 155.2 (C3H7N8++) ; IR (ATR): n=

3430 (m), 3362 (m), 3295 (m), 3244 (m), 3161 (m), 3066 (m), 2968(m), 2734 (m), 2363 (w), 2342 (w), 1702 (s), 1693 (s), 1664 (s), 1643(vs), 1604 (m), 1552 (s), 1529 (s), 1471 (s), 1457 (m), 1422 (vs), 1395(m), 1368 (m), 1319 (s), 1319 (s), 1232 (s), 1148 (m), 1080 (m), 1048(m), 1025 (m), 1008 (m), 976 (m), 845 (s), 799 (w), 773 (s), 723 (m),697 (s), 658 cm¢1 (w); Raman (1064 nm, 300 mW, 25 8C): n=3246(2), 1706(4), 1668(3), 1645(4), 1561(4), 1534(4), 1482(5),1475(5), 1459(5), 1423(100), 1403(21), 1386(5), 1321(10), 1264(9),1151(2), 1101(41), 1052(12), 1009(68), 978(3), 849(14), 764(4), 726(1),682(2), 624(10), 604(7), 492(5), 431(2), 396(7), 350(1), 309(5), 261(4),242(5), 200(6), 171(14), 96(34), 74(14), 65(12) cm¢1; elemental analy-sis calcd (%) for C4H7N13O3 (285.18): C 16.85, H 2.47, N 63.85;found: C 17.29, H 2.50, N 63.76; BAM impact: 25 J, BAM friction:360 N, ESD: 1.0 J (at grain sizes 100–500 mm).

Compound 6

HNO3 (2 M, 1 mL) was added to a suspension of 2 (2.0 mmol,308 mg) in water (20 mL). The mixture was heated at reflux andthe solution was left to stand. Compound 6 (400 mg, 1.84 mmol,92 %) crystallized as colorless plates. DTA (5 8C min¢1) onset Tdec. =280 8C; 1H NMR ([D6]DMSO): d= 13.31 (br s, 1 H; H-2), 8.20 (s, 2 H; H-9), 7.24 (s, 2 H; H-11), 5.78 ppm (s, 2 H; H-10); 13C NMR ([D6]DMSO):d= 160.2 (s; C-6), 147.5 (s ; C-8), 141.2 ppm (s; C-3); m/z (FAB¢):62.0 (NO3

¢) ; m/z (FAB++): 155.0 (C3H7N8++) ; IR (ATR): n= 3348 (m),

3290 (m), 3223 (m), 3166 (m), 3134 (m), 3053 (m), 2998 (m), 2873(m), 2745 (m), 2362 (m), 2342 (m), 1773 (vw), 1767 (vw), 1698 (m),1670 (s), 1634 (vs), 1583 (m), 1552 (m), 1512 (m), 1427 (w), 1398 (s),1344 (vs), 1300 (s), 1163 (m), 1073 (m), 1050 (m), 978 (m), 912 (s),843 (m), 819 (m), 767 (m), 724 (w), 715 (m), 700 (m), 676 (w),668 cm¢1 (w); Raman (1064 nm, 300 mW, 25 8C): n= 3352(3),3294(3), 3246(4), 3233(5), 3205(3), 1707(6), 1671(13), 1639(10),1586(7), 1559(9), 1513(3), 1458(8), 1427(5), 1396(4), 1383(5),1377(5), 1305(9), 1255(25), 1166(4), 1082(7), 1072(8), 1055(68),914(3), 846(44), 718(11), 700(3), 680(9), 616(40), 603(19), 395(14),336(5), 309(16), 255(9), 200(15), 164(47), 98(100) cm¢1; elementalanalysis calcd (%) for C3H7N9O3 (217.15): C 16.59, H 3.25, N 58.05;found: C 16.87, H 3.22, N 57.81; BAM impact: 40 J, BAM friction:360 N, ESD: 0.75 J (at grain sizes 100–500 mm).

Acknowledgements

Financial support of this work by the Ludwig-Maximilian Uni-versity of Munich (LMU), the U.S. Army Research Laboratory(ARL) [grant number W911NF-09-2-0018], the Armament Re-search, Development and Engineering Center (ARDEC) [grantnumber W911NF-12-1-0467] , and the Office of Naval Research(ONR) [grant numbers ONR.N00014-10-1-0535 andONR.N00014-12-1-0538] is gratefully acknowledged. We ac-knowledge collaborations with Dr. Mila Krupka (OZM Research,Czech Republic) for the development of new testing and eval-uation methods for energetic materials and Dr. Muhamed Su-ceska (Brodarski Institute, Croatia) for the development of newcomputational codes to predict the detonation and propulsionparameters of novel explosives. We are indebted to and thankDr. Betsy M. Rice and Dr. Brad Forch (ARL, Aberdeen, ProvingGround, MD) for many inspired discussions. Last but not leastwe thank Mr. St. Huber and Mrs. Regina Scharf for carrying outthe sensitivity and toxicity measurements, respectively.

Keywords: cations · crystal structures · energetic materials ·nitrogen · triazoles

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Received: March 18, 2015

Published online on May 26, 2015

Chem. Eur. J. 2015, 21, 9219 – 9228 www.chemeurj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim9228

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