The nitration pattern of energetic 3,6-diamino-1,2,4,5-tetrazine derivatives containing azole functional groups

Abstract

One of the successful strategies for the design of promising new energetic materials is the incorporation of both fuel and oxidizer moieties into the same molecule. Therefore, during recent years, synthesis of various nitro-azole derivatives, as compounds with a more balanced oxygen content, has become very popular. In the framework of this effort, we studied nitration of N³,N⁶-bis(1H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (BTATz; 5) and its alkylated derivative N³,N⁶-bis(2-methyl-2H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 12, using a ¹⁵N-labeled nitration agent and monitoring and analyzing products of these reactions by ¹⁵N NMR. It was seen that the nitration of both compounds takes place only on the exocyclic (“bridging”) secondary amine groups. Possible tetranitro derivative isomers N,N′-(1,2,4,5-tetrazine-3,6-diyl)bis(N-(1-nitro-1H-tetrazol-5-yl)-nitramide) 6 and N,N′-(1,2,4,5-tetrazine-3,6-diyl)bis(N-(2-nitro-2H-tetrazol-5-yl)nitramide) 7, both of which have OB = 0% and calculated VODs of 9790 and 9903 m s⁻¹, respectively, could not be observed in the reaction mixtures, during the in situ¹⁵N NMR monitoring of nitration of 5, using ¹⁵N-labeled nitrating agents. Following a similar strategy, a new analog of BTATz – N³,N⁶-Bis(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 15 was obtained and its nitration was studied. The reaction of 15 with a HNO₃–Ac₂O nitration mixture resulted in the formation of a new N³,N⁶-bis(3-nitro-1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine derivative 20 in a moderate yield. Structures and properties of 15 (in the form of its perchlorate salt, 16) and 20 were measured by FTIR, multinuclear NMR, MS, DSC and X-ray crystallography. It is important to note that compound 20 exhibits exothermic decomposition at 302 °C (DSC) and >353 N (sensitivity to friction), making it a highly-promising thermally-insensitive energetic material for further development.

---

Introduction

The history of the discovery and development of new energetic materials goes back to gunpowder, which was invented in Imperial China around the 9^th century, in an attempt to create an immortality potion.¹ This ground-breaking discovery led to the invention of fireworks and a variety of weapons in China and, later, throughout the world.² Since the earliest findings, tremendous progress has been made in the rational design and preparation of novel energetic materials and compositions.³ Important properties of such materials, which are commonly taken into account by the designers are span heat of formation (HOF), density, melting and decomposition temperatures, sensitivity to friction and shock, velocity of detonation (VOD), synthetic complexity,⁴ as well as carbon content, nitrogen content and oxygen balance (OB).⁵

Energetic materials typically contain both oxidizing and reducing functional groups in their molecular structure (or in the structure of their components). Under high temperature and pressure conditions, these materials would transform into more thermodynamically-stable products, including small molecules with low heats of formation, such as H₂O, N₂, CO, CO₂, SO₂ and metal oxides.⁶ The OB is a mathematical formula used to calculate the degree to which a given explosive or propellant could be oxidized. A “zero” OB value will be calculated when the chemical composition of the calculated energetic material will have the exact amount of oxygen atoms needed to convert all the carbon atoms to CO₂, all hydrogen atoms to H₂O, all sulphur atoms to SO₂ and all metal atoms (if present in the material) to metal oxides. An energetic material would have a positive OB value if it contains more oxygen atoms than required for complete combustion and a negative OB value when the amount of oxygen atoms is insufficient for complete oxidation. The results of OB calculations were shown to have an excellent correlation with both sensitivity properties and the performance of energetic compounds and their formulations, which have a tendency to reach their best values as their OB values get closer to “zero”.⁷ When an energetic material has a negative OB value (an insufficient amount of oxygen for complete oxidation), it will typically exhibit an incomplete combustion, resulting in the formation of large amounts of toxic CO gas, smoke, soot and solid residues. Commonly, as the OB values for a certain explosive get lower, poorer performance for this explosive is observed, and the VOD and generated pressure for this explosive also become smaller.

In cases where an energetic material contains “too much oxygen” (has a positive OB value), the O₂ produced during explosion absorbs a significant amount of energy, substantially reducing its explosive performance.⁸

There are several fascinating examples of energetic compounds possessing an OB value of 0%, such as the most potent chemical explosive known – octanitrocubane (ONC; R.E. factor = 2.38).⁹ Other examples include the recently prepared compound nitryl cyanide¹⁰ and the still synthetically-elusive nitrogen-rich compounds – 3,6-dinitro-1,2,4,5-tetrazine,¹¹ 2,4,6-trinitro-1,3,5-triazine,¹² [1,2,3,4]-tetrazino-[5,6-e][1,2,3,4]-tetrazine-1,3,5,7-tetraoxide (TTTO),¹³ and (5-nitro-2H-tetrazol-2-yl)-methyl nitrate (Fig. 1).¹⁴

Fig. 1 Structures of ONC, nitryl cyanide, 3,6-dinitro-1,2,4,5-tetrazine, 2,4,6-trinitro-1,3,5-triazine, TTTO and (5-nitro-2H-tetrazol-2-yl)-methyl nitrate.

Due to their high density, highly positive enthalpy of formation, good detonation performances and excellent thermal stability in comparison with the conventional energetic materials, nitrogen-rich energetic compounds attract considerable scientific attention.¹⁵ The latter compounds were extensively explored during recent years for their great potential for both civilian and military applications such as in gas generators, low-signature propellants, as well as additives to pyrotechnics and explosives.¹⁶ Among popular building blocks used in the construction of nitrogen-rich energetic materials are tetrazole¹⁷ and tetrazine¹⁸ functional groups. However, the majority of nitrogen-rich organic compounds are oxygen-deficient. Since one of the most popular strategies for the design of promising new energetic materials is incorporating both fuel and oxidizer properties into a single molecule, one of the important challenges in this field is the preparation of new nitrogen-rich compounds with a low OB value. Therefore, in recent years, the synthesis and evaluation of various nitro-azole derivatives as compounds with a balanced oxygen content has become highly-popular (Fig. 2).¹⁹

Fig. 2 Structures of compounds 1 (OB = −7.0%), 2 (OB = +6.1%), 3 (OB = −12.3%) and 4 (OB = −3.1%).

Results and discussion

In our perspective, as part of the framework of these efforts, one of the interesting challenges was exploring the synthesis of unreported tetra-nitro derivatives of N³,N⁶-bis(1H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (BTATz 5; OB = −64.5%)²⁰ – isomers N,N′-(1,2,4,5-tetrazine-3,6-diyl)bis(N-(1-nitro-1H-tetrazol-5-yl)-nitramide) 6 and N,N′-(1,2,4,5-tetrazine-3,6-diyl)bis(N-(2-nitro-2H-tetrazol-5-yl)nitramide) 7, both of which have OB values of 0% and calculated VODs of 9790 and 9903 m s⁻¹, respectively (Fig. 3, Table 1). Although it would seem that BTATz analogues exhibiting better OB values should be of significant interest, only a single example of such derivatives – 1,4-di-N-oxide 8 (OB = −45.7%) – was prepared by Chaves and coworkers.^21a Unfortunately, compound 8 displayed decomposition at 134 °C (versus decomposition of BTATz at 318 °C), indicating that other derivatisation strategies should be investigated.^21b

Fig. 3 Structures of compounds 5, 6, 7 and 8.

Table 1 Hydrogen bonds present in the crystal structure of 16

D–H⋯A	D–H/Å	H⋯A/Å	D⋯A/Å	D–H⋯A/°
N1–H1⋯O4	0.83	2.20	2.9669	153.7
N1–H1⋯O1	0.83	2.799	2.973	93.8
N2–H2⋯O2	0.87	2.10	2.9401	162.1
N6–H3⋯O1	0.80	2.06	2.8303	163
N1–H1⋯N4	0.83	2.265	2.732	116

---

Approaches to prepare energetic molecules with improved OB values are frequently based on the conversion of NH groups in the starting materials into N–NO₂ groups in the corresponding, more energetic, derivatives.²² Since the BTATz molecule has two pairs of NH groups in its structure, we decided to probe whether the nitration of this molecule would lead to the development of more potent energetic compounds (ultimately, to compounds 6 and 7). There are many methods for the conversion of amines to nitramines: using nitrating agents such as HNO₃, mixtures of HNO₃ and H₂SO₄, acetic anhydride and HNO₃, nitrated silica gel and many others.²³ Thus, our initial efforts were focused on the direct nitration of compound 5 under various reaction conditions. More specifically, we evaluated a series of nitrating conditions and temperature regimes which included the use of red fuming HNO₃ and mixtures of either HNO₃ and H₂SO₄ (1 : 1 v/v), HNO₃ and acetic anhydride (1 : 1 v/v) or HNO₃ and trifluoroacetic anhydride (1 : 1 v/v). However, all examined reaction conditions (and all examined temperature regimes) led to one of the two results: recovery of only the starting compound 5 at the end of the reaction or decomposition of 5.

We further attempted to establish whether any nitration of compound 5 actually takes place, with the formed nitramines hydrolysing back to the starting material upon dilution of the purpose, we conducted in situ studies of the nitration of 5 by ¹⁵N NMR using Na¹⁵NO₃ in concentrated H₂SO₄. A reference mixture of Na¹⁵NO₃ in concentrated H₂SO₄ exhibited two ¹⁵N NMR signals at 383 ppm and 248 ppm, indicating the presence of ¹⁵NO₃⁻ and ¹⁵NO₂⁺, respectively, and was consistent with previous reports.²⁴ Subsequent addition of BTATz to this nitration mixture at 0 °C resulted in the appearance of a new ¹⁵N NMR signal at 336 ppm, which was assigned to the formation of a N–NO₂ adduct. Although it was obvious that, under the explored conditions, no further nitration of 5 could be obtained, it was not clear which amine group in this compound underwent nitration (Fig. 4). Since the in situ¹⁵N NMR measurements did not help us to determine whether edocyclic 9/10 (routes A and B, Fig. 4) or exocyclic 11 (route C, Fig. 4) nitramines were formed, and since the resulting product could not be isolated for further analysis due to its instability, other approaches were required. Therefore, we synthesized a model compound – N³,N⁶-bis(2-methyl-2H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 12 – in which both endocyclic NH groups in the tetrazole rings were methylated (Fig. 5). The successful conversion of 5 to 12 was achieved by using dimethyl-sulfate in aqueous NaHCO₃ at room temperature. A comparison of ¹H and ¹³C NMR data obtained for 12 with published reports for 2-methyl-2H-tetrazole derivatives (versus 1-methyl-1H-tetrazole derivatives) strongly suggested that the methylation of 5 took place on the second nitrogen atom of the tetrazole ring.

Fig. 4 Possible nitrated derivatives of compound 5. Reaction conditions: Na¹⁵NO₃, conc. H₂SO₄.

Fig. 5 Synthesis of bis-N-methylated compound 12, with its subsequent nitration. Reaction conditions: (i) dimethylsulfate, NaHCO₃, H₂O, r.t. (ii) conc. HNO₃, Ac₂O.

Further nitration of 12 with a mixture of concentrated HNO₃ and acetic anhydride (1 : 2 v/v) resulted in the formation of the new compound N,N′-(1,2,4,5-tetrazine-3,6-diyl)-bis(N-(2-methyl-2H-tetrazol-5-yl)-nitramide 13 (Fig. 5). Unfortunately, nitramide 13 could not be fully characterized due to its very high sensitivity to impact and friction (primary explosive!). Also, based on ¹H NMR analysis in a solution of DMSO-d₆ or CD₃CN, 13 underwent relatively quick hydrolysis back to the parent compound 12. ¹³C and ¹⁵N NMR studies in DMSO-d₆ of the precipitate obtained in the nitration of 12 with a mixture of Na¹⁵NO₃/HNO₃ (prepared separately) and acetic anhydride also showed only the presence of the starting material 12. Yet, in situ¹⁵N NMR studies of the nitration of 12 with Na¹⁵NO₃ in concentrated H₂SO₄ exhibited a new peak at 336 ppm (at the identical position of the nitramine's nitrate peak in compound 11), strongly supporting our hypothesis that both compounds 5 and 12 undergo nitration on their exocyclic NH groups.

After realizing that the nitration of 5 (and its bis-N-methyl-tetrazole analog 12) could not lead to the formation of stable nitramine products, we explored whether the nitration of the unreported 1,2,4-triazole analog of compound 5 – N³,N⁶-bis(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 15 would produce better results. The synthesis of compound 15 was achieved in an 81% yield, in a similar fashion to the synthesis of 5, by reacting 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetrazine (BPT, 14) with 1H-1,2,4-triazole-5-amine in sulfolane at 135 °C (Fig. 6). The corresponding, more soluble, energetic perchlorate salt 16 and nitrate salt 17 were prepared by treating 15 with HClO₄ and HNO₃, respectively (Fig. 6). The structure of 16 was confirmed by X-ray crystallography (Fig. 9). Subsequently, a direct nitration of compound 15 was explored under various reaction conditions, which included either HNO₃, a mixture of H₂SO₄ and HNO₃ (1 : 1; v/v), a mixture of CF₃CO₂H and HNO₃ (1 : 1; v/v) or a mixture of NaNO₃ and H₂SO₄. Under all tested temperature regimes, all aforementioned nitrating reagents led invariably to the formation of stable nitrate salt 17.

Fig. 6 Preparation of compound 15 and its perchlorate salt 16 and nitrate salt 17. Reaction conditions: (i) 1H-1,2,4-triazole-5-amine, sulfolane, 135 °C; (ii) HClO₄ or HNO₃.

In situ ¹⁵N NMR studies of the nitration of 15 with Na¹⁵NO₃ in concentrated H₂SO₄ (at 0 °C) displayed a peak at 336 ppm (at the identical position of the nitramine's nitrate peak in compound 13), clearly indicating that the first nitration reaction takes place on the exocyclic NH groups of 15 and not on the CH or NH groups of its triazole rings (Fig. 7). This interesting observation demonstrates a similar nitration pattern for structurally related compounds 5, 12 and 15. We believe that similarly to 11 and 13, attempts for isolation of compound 18 result in a quick hydrolysis of the exocyclic nitramine group back to the original amine. In contrast to previous attempts, the nitration of 15 with a mixture of HNO₃ and acetic anhydride (1 : 1; v/v) at 0 °C resulted in the formation of a new nitramine – N,N′-(1,2,4,5-tetrazine-3,6-diyl)bis(N-(3-nitro-1H-1,2,4-triazol-5-yl)-nitramide) 19, which was isolated by precipitation (Fig. 8). The latter compound could not be fully characterized due to its high sensitivity to impact and friction (primary explosive!). The solubility properties, sensitivity and thermal behaviour of 19 suggested its analogous structure to the exocyclic nitramine 13. Controlled gradual heating of approximately 0.2 mg of wet 19 in a glass capillary (suitable for melting point measurements) resulted via its explosion at 110–112 °C, shuttering the capillary.

Fig. 7 Formation of the proposed compound 18. Reaction conditions: (i) Na¹⁵NO₃, H₂SO₄.

Fig. 8 Synthesis of compounds 19, 20 and 19A. Reaction conditions: (i) conc. HNO₃, Ac₂O; (ii) CH₃CN, 65 °C; (iii) Na¹⁵NO₃/HNO₃, Ac₂O.

¹⁵N NMR studies of the precipitate 19 (dissolved in DMSO-d₆), obtained from nitration of 15 with a mixture of Na¹⁵NO₃/HNO₃ and acetic anhydride, showed the appearance of a new peak at 356.8 ppm (corresponding to C–¹⁵NO₂ nitrogen), indicating the formation of a new derivative with nitrated triazole rings N³,N⁶-bis(3-nitro-1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 20. No exocyclic nitramine derivatives (N–¹⁵NO₂) of compound 15 were observed upon dissolution of 19 in DMSO-d₆, most reasonably due to the hydrolysis of 19 into the hydrolytically stable 20. Also, the dissolution of 19 in hot CH₃CN (with further heating at 65 °C for 30 min) immediately after its separation from the nitration reaction mixture, resulted in the formation of a new compound – N³,N⁶-bis(3-nitro-1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 20 (Fig. 8). The structure of 20 was confirmed by X-ray crystallography (Fig. 10).

Fig. 9 (top): Molecular structure of compound 16, (bottom): interactions between 16 and surrounding perchlorate anions.

Fig. 10 (top): Molecular structure of compound 20, (bottom): interactions between 20 and surrounding DMF molecules.

In order to check whether the structure of 19 was correct, in situ¹⁵N NMR studies of the “back nitration” of 20 into exocyclic ¹⁵N nitramine 19A were conducted. In these experiments, a solid 20 was slowly added to a mixture of Na¹⁵NO₃ in concentrated H₂SO₄ at 0 °C and, after 30 min, the reaction mixture was analysed by ¹⁵N NMR. A new peak at 336.3 ppm (corresponding to the exocyclic N–¹⁵NO₂ nitrogen) appeared in the spectrum, indicating the formation of nitramine 19A (Fig. 8), which perfectly matches our previous observations and conclusions.

Crystal structures

X-ray measurements for single crystals of compounds 16 (perchlorate salt of compound 15; CCDC 1041768) and 20 (CCDC 1041769) were performed. The data for the crystals of 16 and 20 were collected using Mo Kα radiation (L = 0.71073 nm). An Oxford low-temperature device was used to keep the crystals at a constant temperature of 110 K during the entire data collecting period. Details of the X-ray data collection and structure refinements are summarized in the ESI.† A detailed examination of the crystal structures obtained for 16 and 20 showed no significant differences in bond lengths and torsion angles reported for structures of other compounds containing 1,2,4-triazole and 1,2,4,5-tetrazine functional groups.²⁵ Both 16 and 20 molecules were found to be completely planar, due to an extensive delocalization of electrons in these molecules.

Compound 16 was crystalized as solvent-free crystals with the monoclinic space group P2(1)/c and a cell volume of 757.49 ų. A crystal unit cell of 16 contains eight molecules of nitrogen-rich cations and four perchlorate anions. The measured density for 16 was found to be 1.960 g cm⁻³. The nitrogen-rich cation is protonated at nitrogen atom N6 in both triazole rings. Fig. 9 shows interactions between hydrogen atoms in compound 16 and oxygen atoms in perchlorate anions (each nitrogen-rich cation interacts with eight perchlorate anions).

The hydrogen bonding parameters of these interactions are detailed in Table 1. The triazole moiety of 16 points towards nitrogen atom N6 and participates in an intramolecular hydrogen bond N1–H1⋯N4 with a D⋯A length of 2.732 Å and a D–H⋯A angle of 116°.

Compound 20 was crystalized as a monoclinic space group P2(1)/c and a cell volume of 1546.76 ų was calculated. The crystal unit cell of this compound contains eight molecules of 20 and ten molecules of DMF. The unit cell contains two additional disordered DMF molecules, which could not be reliably modelled by discrete atoms. Correspondingly, its contribution was subtracted from the diffraction pattern by the Squeeze technique, using the PLATON software.²⁶ As a result, the density measured was 1.35 g cm⁻³, which is significantly lower than 1.87 g cm⁻³ – the calculated density for this compound.²⁷Fig. 10 shows interactions between nitrogen and hydrogen atoms of compound 20 and molecules of DMF (each molecule of 20 interacts with 10 molecules of DMF). The bond length of the bridge corresponds to a C–N single bond (C10–N9 1.359 Å, N9–C7 1.370 Å). The triazole moiety points towards atom N6 and participates in an intramolecular hydrogen bond – N6–H6⋯N12 – which has a D⋯A length of 2.7781 Å and a D–H⋯A angle of 117° and an intermolecular hydrogen bond with the solvent (Table 2).

Table 2 Hydrogen bonds present in the crystal structure of 20

D–H⋯A	D–H/Å	H⋯A/Å	D⋯A/Å	D–H⋯A/°
Symmetry codes: (i) 1 − x, −1/2 + y, 1/2 − z (ii) 1 + x, y, z (iii) −1 + x, y, z (iv) 1 − x, 1/2 + y, 1/2 − z (v) −1 + x, 1/2 − y, −1/2.
N6–H6⋯N12	0.88	2.27	2.7781	117
N6–H6⋯O19^(i)	0.88	1.94	2.6939	143
N9–H9⋯O13^(ii)	0.88	1.86	2.7333	175
C14–H14⋯N8^(iii)	0.95	2.43	3.2085	139
C16–H16A⋯O13	0.98	2.46	2.8005	100
C20–H20⋯N12^(iv)	0.95	2.58	3.2438	127
C20–H20⋯N11^(v)	0.95	2.43	3.3178	155 = z

---

Theoretical calculations, performance characteristics and thermal stabilities

Calculations were performed using the Gaussian 09 software.²⁸ The geometric optimization of the structures and frequency analyses were carried out by using the B3LYP functional with the 6-311+G(d,p) basis set.²⁹ The optimized structures of all materials correspond to, at least, a local energy minimum on the potential energy surface. In order to predict the energetic properties of new materials, the EXPLO5_v6.01³⁰ software was used. This software utilizes an algorithm based on the Becker–Kistiakowsky–Wilson equation of state (BKW EOS) for gaseous detonation products. Thermal stability is a very important property of energetic materials. The onset temperatures for thermal decomposition of compounds 5, 12, 15, 16, 17 and 20 were determined using a differential scanning calorimeter (DSC). The results of these calculations and the experimental data are summarized in Table 3.

Table 3 Physico-chemical properties of compounds 5, 6, 7, 9/10, 11, 12, 13, 15, 16, 17, 18, 19 and 20

	5	6	7	9/10	11	12	13	15	16	17	18	19	20
a Nitrogen content. b Measured by DSC start of decomposition temperature. c Calculated density; (c*) density of dried powdered compounds was measured by gas pycnometry at 25 °C; (c**) density value that was obtained by a single-crystal X-ray diffraction. d Oxygen balance (for CaHbNcOdΩ = (a − 2b − 0.5d) × 1600/Mw). e Calculated enthalpy of formation. f Calculated energy of formation. g Calculated heat of detonation. h Calculated temperature of detonation i Calculated pressure of detonation. j Calculated volume of gaseous products. k Calculated velocity of detonation.
Mw [g mol^−1]	248	428	428	338	338	276	366	246	447	372	336	426	336
%N^a [wt%]	79	60	60	61.5	61.5	71	61	68	33.3	41.1	50	50	58
T Dec [°C]	309	—	—	—	—	306	—	350	246	167	N/A	—	302
P [g cm^−3]	1.88	2.03	2.03	1.98	1.98	1.77	1.88	1.36*	1.96**	1.74*	1.87	1.95	1.87
Ω O2  [%]	−64.6	0	0	−23.6	−23.6	−92.6	−48	−97.5	−25	−43	−47.6	−18.7	−47.6
ΔHf ^e [kJ mol^−1]	918	1148	1334	113	964	1022	996	831	−504	−655	965	864	1032
ΔU^°f^f [kJ kg^−1]	3789	2756	3190	3436	2933	3801	2808	3468	−1056	−1667	2952	2103	3151
EXPLO5_v6.01 values:
−ΔEU° ^g [kJ kg^−1]	4002	6048	6400	5492	5032	4185	5097	3728	2947	1921	5212	5315	5398
T E ^h [K]	2905	4632	4856	4101	3872	2763	3597	2805	2630	1873	3782	4091	3891
P C–J [kbar]	301	443	456	415	394	265	333	124	251	161	329	390	339
V Det . [m s^−1]	8809	9790	9903	9604	9429	8492	8932	6240	6711	6833	8830	9310	8903
Gas vol.^j [L kg^−1]	758	759	762	794	789	743	778	734	751	792	753	777	753

---

In contrast to compounds 15, 16 and 17, dinitro derivative 20 exhibited high thermal stability (T_decomp. of 302 °C), high detonation velocity (VOD of 8903 m s⁻¹) and high detonation pressure, performing comparably to BTATz 5. In addition, BAM measurements of dropping-hammer (7.67 Nm) and friction (>353 N) for compound 20 indicate a relatively high stability of this energetic material under mechanical stress (for comparison, the equivalent values for RDX are 7.5 Nm and 120 N, respectively).

Conclusions

A methodical study of nitration patterns was conducted on a series of 3,6-diamino-1,2,4,5-tetrazine derivatives, revealing the specificity of the nitration in this type of nitrogen-rich compounds, as well as the stability of the formed nitramines towards hydrolysis. The general motivation of this study was to evaluate whether energetic nitrogen-rich compounds, structurally-related to the N³,N⁶-bis(1H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (BTATz; 5), with improved oxygen balance characteristics, could be prepared. By using ¹⁵N-labeling methodology, we found that although nitramine derivatives of 5, as well as its dimethyl analogue N³,N⁶-bis(2-methyl-2H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (12), are formed in the reaction mixture, these nitramines could not be successfully isolated and quickly undergo hydrolysis back to the starting materials. Therefore, subsequent efforts were focused on more stable nitro derivatives of the newly synthesized compound N³,N⁶-bis(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (15). The nitration of 15 was studied by ¹⁵N NMR, showing a similar initial reactivity pattern of 5 and 12. Yet, due to the presence of 1,2,4-triazole functional groups in the structure of 15, a new C–NO₂ derivative N³,N⁶-bis(3-nitro-1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (20) was obtained in a moderate yield. The structures of 16 (perchlorate salt of 15) and 20 (with 4 molecules of DMF) were determined by X-ray crystallography. Importantly, it was found that compound 20 exhibited high stability towards friction (>353 N) and very high thermal stability, decomposing at 302 °C (by DSC). The standard heats of formation for all compounds in this study were calculated on the B3LYP 6311G+d,p level of theory, showing highly-positive values of 831.5 kJ mol⁻¹ and 1032.9 kJ mol⁻¹ for 15 and 20, respectively. Detonation parameters for all compounds were calculated with the EXPLO5_v6.01 program. Theoretical density of 1.87 g cm⁻³ and velocity of detonation of 8903 m s⁻¹ were calculated for 20, making it a highly promising new energetic compound.

Experimental section

Caution! As certain triazole, tetrazole and tetrazine derivatives are unstable and can ignite or explode under the influence of impact, friction or heating, proper safety precautions should be taken when working with these materials. Lab personnel and equipment should be properly grounded and protective equipment including protective coat, Kevlar gloves, ear protection and face shield should be used.

General information

All used chemical reagents and solvents were of analytical grade and were used as supplied, without further purification. ¹H, ¹³C and ¹⁵N NMR spectra were recorded on a 400 MHz NMR spectrometer at 25 °C. The chemical shifts are given relative to the residual DMSO-d₆ peaks or formamide (¹⁵N) as external standards and coupling constants are given in hertz (Hz). Infrared (IR) spectra were recorded on a FTIR spectrometer equipped with a diamond ATR unit. Mass spectra were recorded on a high-resolution ESI-Q-ToF machine. Elemental analyses (CHN) were performed at the service facility of the Hebrew University. Decomposition points were determined by differential scanning calorimetry (DSC). Measurements were performed at a heating rate of 5 °C min⁻¹ in closed aluminium sample pans with a small hole in the lid under a nitrogen flow of 20 mL min⁻¹ using an empty aluminium sample pan as a reference. Melting points were measured on a melting point apparatus in open glass capillaries. The impact sensitivities were tested according to STANAG 4489 using a BAM drop-hammer. The friction sensitivities were tested according to STANAG 4487 using a BAM friction tester. Experimental densities were obtained by pycnometry measurements at ambient temperature.

General procedures

N ³,N⁶-Bis(2-methyl-2H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (12). A solid N³,N⁶-di(1H-tetrazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (816 mg, 3.29 mmol) was added to a solution of NaHCO₃ (690 mg, 8.21 mmol) in H₂O (30 mL) and stirred at RT for 30 min, until dissolution. To the resulting solution, dimethylsulfate (1.04 g, 8.22 mmol) was added and the reaction mixture was stirred at RT for 3 days. Then, the precipitate formed was collected by filtration, washed with a mixture of triethylamine in MeOH (1 : 20 v/v; 3 × 10 mL) and dried under vacuum to yield pure 12 (536 mg; 59%) as a red solid. DSC (5 °C min⁻¹) 306 °C (decomp.). ¹H NMR (400 MHz, DMSO-d₆): δ 4.32 (s, 6H). ¹³C NMR (100 MHz, DMSO-d₆): δ 40.2, 159.0, 160.6. ¹³C DEPT135 (100 MHz, DMSO-d₆): δ 40.2 (CH3). HRMS (ESI⁺): m/z = 277.1127 [M + H⁺]. Elemental analysis: calcd (%) for C₆H₈N₁₄ C 26.09, H 2.96; found. C 26.29, H 2.86. FTIR (ATR): ν 3235 (w), 3018 (w), 2865 (w), 1588 (s), 1538 (s), 1428 (s), 1262 (m), 1197 (m), 1049 (s), 950 (s), 898 (w), 815 (w), 744 (m), 672 (m), 547 (s) cm⁻¹.

N ³,N⁶-Di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (15). A solid 1H-1,2,4-triazol-5-amine (4.23 g, 50.31 mmol) was added to a solution of 3,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,2,4,5-tetrazine 14 (6.46 g, 23.90 mmol) in sulfolane (150 mL) and the reaction mixture was heated at 135 °C for 24 h. Then, the reaction mixture was cooled down to RT, DMF (150 mL) was added and the mixture was stirred for 1 h. The precipitate formed was collected by filtration, washed with MeOH (3 × 80 mL) and dried under vacuum to yield 15 (4.77 g; 81%) as an orange solid. DSC (5 °C min⁻¹) 350 °C (decomp.). HRMS (ESI⁻): m/z: 245.0697 [M − H⁻]. Elemental Analysis: calcd (%) for C₆H₆N₁₂: C 29.27, N 68.27, H 2.46; found: C 29.92, N 68.73, H 2.55. FTIR (ATR): ν 2807 (m), 2359 (w), 1597 (s), 1542 (s), 1486 (s), 1434 (s), 1312 (m), 1268 (s), 1207 (w), 1062 (s), 1047 (s), 1027 (s), 962 (s), 896 (s), 763 (m), 719 (s), 649 (m), 557 (s). Due to the very low solubility of 15 in all tested organic solvents, the free base was converted into the corresponding nitrate and perchlorate salts for further analysis.

N ³,N⁶-Di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine perchlorate salt (16). A solid N³,N⁶-di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 15 (500 mg, 2.03 mmol) was added to aqueous HClO₄ (5 mL, 70%) and the mixture was stirred at RT for 15 min. To the resulting mixture, CH₃CN (5 mL) was added and the reaction mixture was stirred at RT for an additional 10 min. Then, the precipitate formed was collected by filtration, washed with CH₃CN (3 × 3 mL) and dried under vacuum to yield pure 16 (653 mg; 72%) as an orange solid. DSC (5 °C min⁻¹) 246 °C (decomp.). ¹H NMR (400 MHz, DMSO-d₆): δ 8.56 (s), 9.49 (br s). ¹³C NMR (100 MHz, DMSO-d₆): δ 144.7, 151.0, 158.6. FTIR (ATR): ν 2761 (w), 1602 (s), 1552 (s), 1499 (s), 1435 (s), 1312 (s), 1268 (s), 1207 (m), 1062 (s), 964 (s), 854 (m), 740 (s), 620 (s), 557 (s).

N ³,N⁶-Di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine nitrate salt (17). A solid N³,N⁶-di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 15 (500 mg, 2.03 mmol) was added to HNO₃ (70%; 5 mL) and the mixture was stirred at RT for 15 min. To the resulting solution, CH₃CN (10 mL) was added and the reaction mixture was stirred at RT for 15 min. After that time, the precipitate formed was collected by filtration, washed with CH₃CN (3 × 3 mL) and dried under vacuum to yield pure 17 (557 mg; 74%) as an orange solid. DSC (5 °C min⁻¹) 167 °C (decomp.). ¹H NMR (400 MHz, DMSO-d₆): δ 8.65 (s), 11.35 (br s). ¹³C NMR (100 MHz, DMSO-d₆): δ 143.4, 150.1, 158.6. FTIR (ATR): ν 2943 (w), 1697 (s), 1621 (s), 1551 (s), 1407 (s), 1303 (s), 1060 (s), 1041 (s), 999 (s), 937 (s), 855 (s), 713 (s), 557 (s).

N ³,N⁶-Bis(3-nitro-1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine (20). HNO₃ (99%; 2.5 mL, 60.02 mmol) was added to acetic anhydride (7.5 mL, 79.49 mmol) at 0 °C and the mixture was stirred for 30 min. To this mixture, a solid N³,N⁶-di(1H-1,2,4-triazol-5-yl)-1,2,4,5-tetrazine-3,6-diamine 15 (494 mg, 2.0 mmol) was slowly added at 0 °C and the reaction mixture was stirred at this temperature for 2 h under anhydrous conditions. Then, the reaction mixture was allowed to warm up to RT and the orange precipitate formed (very sensitive explosive!) was collected by filtration, quickly washed with CF₃CO₂H (2 × 2 mL) and immediately re-dissolved in hot CH₃CN (30 mL). The resulting CH₃CN solution was heated at 65 °C for 30 min and then cooled to RT. The precipitate formed was collected by filtration, washed with CH₃CN (3 × 5 mL) and dried under vacuum to yield pure 20 (211 mg; 31%) as an orange solid. DSC (5 °C min⁻¹) 302 °C (decomp.). ¹³C NMR (100 MHz, DMSO-d₆): δ 149.7, 158.2, 160.5. HRMS (ESI⁻): m/z: 335.0492 [M − H⁻]. Elemental Analysis: calcd (%) for C₆H₄N₁₄O₄: C 21.44, N 58.33, H 1.20; found: C 21.75, N 57.56, H 1.30. FTIR (ATR): ν 3247 (w), 2996 (w), 1601 (s), 1541 (s), 1494 (s), 1435 (s), 1417 (s), 1312 (s), 1296 (s), 1151 (m), 1072 (s), 1050 (s), 1018 (m), 950 (s), 891 (m), 835 (s), 748 (m), 692 (s), 570 (s).

Acknowledgements

The authors would like to acknowledge the financial support provided by the Tel Aviv University and Dr S. Lipstman, Dr C. Evangelisti, Dr D. Krepel, Dr H. Titi and S. Fishman for their valuable contribution.

Notes and references

1. http://chemistry.about.com/od/historyofchemistry/a/gunpowder.htm .
2. http://kaleidoscope.cultural-china.com/en/136Kaleidoscope1.html .
3. (a) L. E. Fried, M. R. Manaa, P. F. Pagoria and R. L. Simpson, Ann. Rev. Mater. Res., 2001, 31, 291–321; (b) Q. Wu, W. Zhu and H. Xiao, J. Mater. Chem. A, 2014, 2, 13006–13015; (c) H. Martinez, Z. Zheng and W. R. Dolbier, J. Fluorine Chem., 2012, 143, 112; (d) A. Dippold and T. Klapötke, J. Am. Chem. Soc., 2013, 135, 9931; (e) V. Thottempudi, H. Gao and J. M. Shreeve, J. Am. Chem. Soc., 2011, 6464.
4. (a) P. F. Pagoria, G. S. Lee, A. R. Mitchell and R. D. Schmidt, Thermochim. Acta, 2002, 384, 187; (b) A. K. Sikder and N. Sikder, J. Hazard. Mater., 2004, 112, 1; (c) Z. Zeng, H. Gao, B. Twamley and J. M. Shreeve, J. Mater. Chem., 2007, 17, 3819; (d) G. Wang, H. Xiao, X. Ju and X. Gong, Propellants, Explos., Pyrotech., 2006, 31, 361; (e) G. X. Wang, H. M. Xiao, X. J. Xu and X. H. Ju, Propellants, Explos., Pyrotech., 2006, 31, 102; (f) J. Zhang and J. M. Shreeve, J. Am. Chem. Soc., 2014, 136, 4437.
5. (a) D. E. Chavez and M. A. Hiskey, J. Energy Mater., 1999, 17, 357–377; (b) Q. Zhang, J. Zhang, D. A. Parrish and J. M. Shreeve, Chem. – Eur. J., 2013, 19, 11000.
6. (a) D. M. Badgujar, M. B. Talawar, S. N. Asthana and P. P. Mahulikar, J. Hazard. Mater., 2008, 151, 289; (b) M. J. Rossi, J. C. Bottaro and D. F. McMillen, Int. J. Chem. Kinet., 1993, 25, 549; (c) C. Darwich, T. M. Klapötke and C. M. Sabaté, Chem. – Eur. J., 2008, 14, 5756.
7. (a) Q. L. Yan and S. Zeman, Int. J. Quantum Chem., 2013, 113, 1049; (b) M. Cartwright and J. Wilkinson, Propellants, Explos., Pyrotech., 2010, 35, 326; (c) Q. Wu, W. Zhu and H. Xiao, J. Chem. Eng. Data, 2013, 58, 2748.
8. (a) M. Genetier, A. Osmont and G. Baudin, J. Phys.: Conf. Ser., 2014, 500, 192001; (b) M. J. Kamlet and H. G. Adolph, Propellants Explos., 1979, 4, 30; (c) E. S. Shanley and G. A. Melhem, Process Saf. Prog., 1995, 14, 29.
9. (a) C. Zhang, Y. Shu, Y. Huang, X. Zhao and H. Dong, J. Phys. Chem. B, 2005, 109, 8978; (b) B. Tan, X. Long and J. Li, Comput. Theor. Chem., 2012, 993, 66; (c) M. X. Zhang, P. E. Eaton and R. Gilardi, Angew. Chem., Int. Ed., 2000, 39, 401.
10. M. Rahm, G. Bélanger-Chabot, R. Haiges and K. O. Christe, Angew. Chem., Int. Ed., 2014, 53, 6893.
11. T. Wei, W. Zhu, X. Zhang, Y. F. Li and H. Xiao, J. Phys. Chem. A, 2009, 113, 9404.
12. A. A. Korkin and R. J. Bartlett, J. Am. Chem. Soc., 1996, 118, 12244.
13. C. Steffen, K. Thomas, U. Huniar, A. Hellweg, O. Rubner and A. Schroer, J. Comput. Chem., 2010, 31, 2967.
14. Y. Li, W. Liu and S. Pang, Molecules, 2012, 17, 5040–5049.
15. (a) T. M. Klapötke, J. Stierstorfer and A. U. Wallek, Chem. Mater., 2008, 20, 4519; (b) E. G. Gillan, Chem. Mater., 2000, 12, 3906; (c) Y. H. Joo, B. Twamley, S. Garg and J. M. Shreeve, Angew. Chem., Int. Ed., 2008, 47, 6236; (d) Y. Guo, H. Gao, B. Twamley and J. M. Shreeve, Adv. Mater., 2007, 19, 2884; (e) G. Steinhauser and T. M. Klapötke, Angew. Chem., Int. Ed., 2008, 47, 3330; (f) D. Chand, D. Parrish and J. M. Shreeve, J. Mater. Chem. A, 2013, 1, 15383; (g) R. P. Singh, R. D. Verma, D. T. Meshri and J. M. Shreeve, Angew. Chem., Int. Ed., 2006, 45, 3584.
16. (a) D. Srinivas, V. D. Ghule and K. Muralidharan, RSC Adv., 2014, 4, 7041; (b) G. H. Tao, B. Twamley and J. M. Shreeve, J. Mater. Chem., 2009, 19, 5850.
17. (a) T. M. Klapötke and D. G. Piercey, Inorg. Chem., 2011, 50, 2732; (b) Y. H. Joo and J. M. Shreeve, Angew. Chem., Int. Ed., 2010, 49, 7320; (c) Y. H. Joo and J. M. Shreeve, Angew. Chem., Int. Ed., 2009, 48, 564.
18. (a) H. Torii, M. Nakadai, K. Ishihara, S. Saito and H. Yamamoto, Angew. Chem., Int. Ed., 2004, 43, 1983; (b) G. Fischer, G. Holl, T. M. Klapötke and J. Weigand, J. Thermochim. Acta, 2005, 437, 168; (c) T. M. Klapötke and J. Stierstorfer, Helv. Chim. Acta, 2007, 90, 2132; (d) T. M. Klapötke, A. Preimesser and J. Stierstorfer, Z. Naturforsch., B: Chem. Sci., 2013, 68, 1310–1320.
19. (a) P. M. Cormick, M. Rovera and E. N. Durantini, J. Photochem. Photobiol. Chem., 2008, 194, 220; (b) M. Esseffar, E. Quintanilla, J. Z. Dvalos, J. L. M. Abboud and M. Yez, New J. Chem., 2002, 26, 1567; (c) V. D. Ghule, R. Sarangapani, P. M. Jadhav and S. P. Tewari, J. Mol. Model., 2011, 17, 1507; (d) D. Srinivas, V. D. Ghule, S. P. Tewari and K. Muralidharan, Chem. – Eur. J., 2012, 18, 15031; (e) H. Hao Wei, H. Gao and J. M. Shreeve, Chem. – Eur. J., 2014, 20(51), 16943–16952; (f) R. Haiges, G. Bélanger-Chabot, S. M. Kaplan and K. O. Christe, Dalton Trans., 2015, 44, 7586; (g) R. Haiges and K. O. Christe, Inorg. Chem., 2013, 52, 7249.
20. A. Saikia, R. Sivabalan, B. G. Polke, G. M. Gore, A. Singh, A. Subhananda Rao and A. K. Sikder, J. Hazard. Mater., 2009, 170, 306.
21. (a) D. E. Chavez, M. A. Hiskey and D. L. Naud, Propellants, Explos., Pyrotech., 2004, 29, 209; (b) T. Wei, J. Wu, W. Zhu, C. Zhang and H. Xiao, J. Mol. Model., 2012, 18(8), 3467–3479.
22. M. Masuda, H. F. Mower, B. Pignatelli, I. Celan, M. D. Friesen, H. Nishino and H. Ohshima, Chem. Res. Toxicol., 2000, 13, 301.
23. (a) G. Olah, A. Orlinkov, A. B. Oxyzoglou and G. K. S. Prakash, J. Org. Chem., 1995, 60, 7348; (b) J. G. Hoggett, R. B. Moodie, J. R. Penton and K. Schofield, Nitration and aromatic reactivity, 1971.
24. G. K. S. Prakash, L. Heiliger and G. A. Olah, Inorg. Chem., 1990, 29, 4965–4968.
25. (a) A. A. Dippold and T. M. Klapötke, Chem. – Eur. J., 2012, 18, 16742–16753; (b) A. B. Sheremetev, N. V. Palysaeva, M. I. Struchkova and K. Y. Suponitsky, Mendeleev Commun., 2012, 22, 302–304.
26. A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148–155.
27. M. H. Keshavarz, J. Hazard. Mater., 2007, 145, 263–269.
28. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci and G. A. Petersson, et al., GAUSSIAN 09 (Revision D.01), Gaussian, Inc., Wallingford, CT, 2009.
29. A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
30. M. Sućeska, Mater. Sci. Forum, 2004, 325, 465–466.

---

Footnote

† Electronic supplementary information (ESI) available: Contains copies of ¹H and ¹³C NMR spectra for isolated compounds and ¹⁵N NMR spectra for ¹⁵N-labeled compounds formed in situ in the nitration experiments, and crystallographic data for compounds 16 and 20. CCDC 1041768 and 1041769. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01641j

---