Thermal Stabilization of Energetic Materials by the Aromatic Nitrogen-Rich 4,4’,5,5’-Tetraamino-3,3’-Bi-1,2,4-Triazolium Cation

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Introduction
The highest attention in the research of modern energetic materials in the 20th century was paid on cyclic or caged nitramines, for example 1,3,5-trinitro-1,3,5-triazinane (RDX) and 2,4,6,8,10,12hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) shown in Figure 1. [1]Since the development of RDX, newly synthesized energetic compounds have to be able to compete with RDX, especially in terms of detonation pressure and detonation velocity, being very important parameters when investigating secondary explosives. [2]Furthermore, stability towards temperature, high density, safe handling, and cheap synthesis are additional properties a new secondary explosive should possess.A high density is vital for a good performance of the energetic material, since the detonation pressure is proportional to its squared density. [2]Next to equal or even enhanced physical properties, the demand for "green", nitrogen-rich energetic materials steadily rises.To reduce the pollution of the environment through commonly used toxic and carcinogenic explosives, like RDX or lead azide, explosives releasing mostly dinitrogen after decomposition gain more and more importance. [3]ne approach for synthesizing new energetic compounds is the preparation of energetic salts, which mostly exhibit high densities and high stabilities, due to their high lattice energy. [2]Other than employing alkali metals (Na + , K + ) [4] as cations, the use of nitrogenrich cations became more popular, due to (i) the potential hydrogen bond network resulting in less sensitive materials [5] and (ii) a high positive heat of formation.Latter one mostly results in a high energetic performance as well as a decent oxygen balance. [6]gure 1.Chemical structure of the commonly used secondary 50 explosive hexogen (RDX, left) and octogen (HMX, middle) as well as CL-20 (right), a potential high-performing alternative.
One approach for synthesizing new energetic compounds is the preparation of energetic salts, which mostly exhibit high densities 55 and high stabilities, due to their high lattice energy. [2]Other than employing alkali metals (Na + , K + ) [4] as cations, the use of nitrogenrich cations became more popular, due to (i) the potential hydrogen bond network resulting in less sensitive materials [5] and (ii) a high positive heat of formation.Latter one mostly results in a high 60 energetic performance as well as a decent oxygen balance. [6]xamples for commonly used nitrogen-rich cations are guanidinium (G + ), aminoguanidinium (AG + ), diaminoguanidinium (DAG + ), triaminoguanidinium (TAG + ), ammonium (NH 4 + ), hydroxylammonium (NH 3 OH + ) and hydrazinium (N 2 H 5 row G + , AG + , DAG + and TAG + whereas the performance increases with increasing number of energetic N-N bonds. [7]To improve the energetic performance of explosives the employment of NH 4 + , NH 3 OH + and N 2 H 5 + cations is a valuable strategy.Unfortunately, this usually leads to an increase in mechanical sensitivity and a decrease in temperature stability from NH 4 + over N 2 H 5 + to NH 3 OH + . [5,8]Since a high thermal stability seems to be accompanied by a decrease in the energetic performance of energetic salts, currently used nitrogen-rich salts are limited to either their thermal stability or sufficient energetic performance in comparison to RDX.Thus, it seems that in many energetic materials, a good energetic performance and low sensitivity exclude each other. [8]Exemplarily this can be observed in the series of five-membered azoles from pyrazole to pentazole.3a,3f] R. Centore et al. reported the facile synthesis of 4,4',5,5'tetraamino-3,3'-bi-1,2,4-triazole (1). [10]nterestingly this compound has never been described as building block in the development of new energetic materials so far.It only has been characterized by its decomposition point, elemental analysis and mass spectroscopy.A detailed characterization using X-Ray diffraction, elemental analysis and 13 C NMR spectrometry as well as the investigation of the energetic properties of compound 1 are missing in literature.Herein we report the synthesis of the temperature stable nitrogen-rich salts of 4,4',5,5'-tetraamino-3,3'bi-1,2,4-triazole and a detailed investigation on the properties of the resulted energetic materials.

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The density of compound 2 is in the same range as of previously reported dinitramides.For example ADN with a density of 1.856 g cm -3 at 173K [12] or hydrazinium dinitramide 1.83 g cm -3 (298K), [13] show similar densities as the newly reported compound 2. The torsion angle of N3-C2-C2 i -N1 i equals 0.5(2)° showing 85 that a nearly planar ring-system is formed by the two triazoles.Through the aromaticity of the ring system, the triazoles form almost regular pentagons with angles near 108° and with almost equal bond lengths between the ring atoms of 1.3-1.4Å.The connecting C-C bond of the triazole-rings with a length of 90 1.445(3) Å, is significantly shorter than a C-C single bond (1.54 Å).Similar C-C bond lengths can be observed in salts 3-10 and 13.8a, 14] The distance C1-N5 amounts to 1.317(2) Å and thus is remarkably shorter than the distance of the carbon to the nitrogen of the nitro group in 3,3'-dinitro-1,1'-dihydroxy-5,5'-bi-1,2,4-triazole and its derivatives with bond lengths between 5 1.454(5) Å and 1.427(5) Å. [8a] Moreover, C1-N5 is shorter than other C (triazole) -N (amino) distances (1.351(3) Å) of comparable compounds reported in the literature. [15]Compounds 3-10 and 13 also show bond lengths around 1.31 Å for the equivalent C-N distance.8c] The torsion angle of N9-C3-C3 i -N6 i is 0°, making the bicyclic ring system completely planar similar to that of the cation in compound 2. The angles of the ring-system and the bond lengths behave in the same way as described for compound 2.
Likewise the bond lengths and bond angles of the anion correspond 35 to the data reported in literature. [5]The molecular unit is shown in Figure 3, with selected bond lengths and bond angles in the caption.The energetic compound 4,4',5,5'-tetraamino-3,3'-bi-1,2,4triazolium dinitrate (5) crystallizes anhydrously from water in the triclinic space group P-1 with one molecule per unit cell and a density of 1.779 g cm -3 at 173 K.In comparison to compound 2 and 3, the density of compound 5 is smaller.Compared to 55 hydroxylammonium nitrate with a density of 1.841 g cm -3 , [16] compound 5 exhibits a smaller density, whereas compound 5 reveals a higher density than guanidinium nitrate (1.410 g cm -3 ). [17]ith a torsion angle of 1.4(2)° of the plane N1-C1-C1 i -N3 i the two triazoles are tilted only slightly more towards each other than 60 in the compounds 2-4.Bond angles and bond lengths of the bicyclic ring system match the data reported above.Figure 4 represents the molecular unit of compound 5.Only the hydrazinium salt of tetranitrobisimidazolate with a density of 1.826 g cm -3 (298K) [18] showed a density in the range of compound 6, whereas other nitrogen-rich salts like the guanidinium (ρ = 1.701 g cm -3 at 298K) 80 or aminoguanidinium salt (ρ = 1.698 g cm -3 at 298K) exhibit clearly lower densities. [18]The torsion angles of the two bicyclic ring systems show that two nearly planar moieties are formed.The C-C bond of the anion, linking the two rings is in the same dimension as the bond of the cation and comparable to the bond hydrogen bond lengths in its caption.4,4',5,5'-Tetraamino-3,3'-bi-1,2,4-triazolium 5,5'-bitetrazole-1,1'-dioxide (7) forms anhydrous crystals as well as crystals containing two water moieties when crystallized from water (7•2 H 2 O).The water free compound 7 crystallizes in the triclinic space 15 group P-1 with a density of 1.686 g cm -3 at 173 K and one molecule per unit cell.The structure of the dianion has been discussed previously in the literature. [20,21]Compared to other energetic salts of 5,5'-bitetrazole-1,1'-dioxide, e.g. the hydroxylammonium salt TKX-50 (ρ 173 K = 1.915 g cm -3 ), the 20 density of compound 7 is fairly low. [20]However, in comparison to the thermally stable diguanidinium salt 5,5'-bitetrazole-1,1'dioxide with a density of 1.639 g cm -3 , [21] compound 7 exhibits a slightly higher density.Figure 6 shows the molecular unit of 7, with selected bond lengths and torsion angles in the caption.The   The anhydrous energetic compound 4,4',5,5'-tetraamino-3,3'-bi-1,2,4-triazolium 1,1'-dinitramino-5,5'-bitetrazolate (8) crystallizes from water in the triclinic space group P-1 with a density of 1.778 g cm -3 and one formula unit per unit cell.4a] The anion of 8 exhibits a larger torsion angle than the corresponding cation, forming a planar cation and a slightly tilted anion.The molecular unit of compound 8 is illustrated in Figure 7.

Thermal Analysis and compatibility
To identify the decomposition temperatures of compound 1-13 differential thermal analysis (DTA) with a heating rate of 5°C min - 1 was employed.The results are displayed in Figures 8 and 10.
The decomposition temperature of the neutral compound 1 60 amounts to a very high temperature of 342°C and thus exceeds the decomposition temperature of the explosives RDX (T dec.= 204 °C) [22] and even hexanitrostilbene (T dec.= 316°C). [23]The dinitramide 2 decomposes at 200°C.Dinitramide based ionic energetic materials oftentimes lack in thermal stability.One 65 exception is FOX-12.Its decomposition temperature of 215°C is reported at a heating rate of 10°C min -1 . [24]For better comparability of 2 with FOX-12, ammonium dinitramide (ADN) and guanidinium dinitramide (GDN), all of them were remeasured on our instrument at a heating rate of 5°C min -1 (see Figure 8).FOX-70 12 reveals an onset decomposition temperature of 201°C, which is virtually at the same temperature as compound 2. GDN and ADN exhibit lower decomposition temperatures of 149°C and 147°C.Table 2 compares the outstanding thermal stability (although being a 1:2 dinitramide salt) with other nitrogen-rich dinitramides.75 Compounds 4, 9 and 10 containing two crystal water moieties each dehydrate at 89°C, 93°C and 104°C respectively.The highest decomposition temperature of the energetic salts of compound 1 could be measured for compound 6 (T dec .= 290°C).Next to compound 6, compound 5, 7, 11 and 12 also show very high 80 thermal stabilities, with onset decomposition temperatures of 275°C, 279°C, 286°C and 284°C, respectively.In comparison to the 1-methylnitriminotetrazolate 10, which exhibits a fairly low onset at 232°C.This value is in accordance with the decomposition temperatures of reported nitrogen-rich 1-methylnitriminotetrazolates with onset temperatures of around 210°C. [25]able 2. Decomposition temperatures of various nitrogen-rich 10 dinitramides [28] in comparison to compound 2.

Dinitramides T dec. (onset) [°C]
2 200 FOX-12 201 (215 at 10 °C min -1 ) [24] A DN 147 G DN 149 TAG DN 180 [28a] 1,5-DAT DN 135 [28b] 1-Me-AT DN 145 [28b] 2-Me-AT DN 148 [28c] 5-AT DN 117 [28c] Tz DN 110 [28c] 3,5-DATr DN 164 The nitrotetrazolate 4 decomposes at a temperature of 225°C and thus shows a higher thermal stability than all other reported non-20 metal nitrotetrazolate salts, with the guanidinium nitrotetrazolate being the most temperature stable (T dec.= 212°C). [5]Through cation metathesis a thermal stabilization of the anion in compound 8 could be achieved.While the potassium salt K 2 DNABT decomposes at 200°C [4a] , the new energetic compound 8 is thermally stable up to a 25 temperature of 223°C.In comparison to the nitrogen-rich salts, only the ammonium salt (NH 4 ) 2 DNABT lies in the range of the thermal stability of compound 8, whereas the hydroxylammonium salt (NH 3 OH) 2 DNABT reveals a much lower decomposition temperature of 170°C. [26]The onset decomposition temperature of 30 the nitrotetrazolate-2N-oxide 3 equals 220°C showing a much higher thermal stability than other comparable nitrotetrazolate-2Noxides, as the guanidinium salt (T dec.= 211°C), the aminoguanidinium salt (T dec.= 185°C), the diamino-guanidinium salt (T dec.= 174°C), the triaminoguanidinium salt (T dec.= 153°C) or 35 the ammonium salt (T dec.8c] In comparison to RDX with a decomposition temperature of 204°C [22] , salts 2-12 have at least an equivalent thermal stability.Only the nitroformate 13 decomposes at 94°C, which is slightly lower than the ammonium (116°C), guanidinium (113°C) and triaminoguanidinium (105°C) 40 salt but higher than the corresponding aminoguanidinium (71°C) and diaminoguanidinium (82°C) salt. [27]l explosives have to be coated for practical applications.Compounds 1-13 were tested for their sensitivity towards friction and impact by employment of BAM methods as described in the supporting information.Compounds 1-13 show a wide range of sensitivities towards impact.While the neutral compound 1 as well as compound 6, 7 and 9 can be classified as insensitive towards 60 impact (<40 J), compounds 2, 3, 8 and 11 show very high sensitivities from 6 J (3) to up to 3 J (8).8c] Compound 8 exhibits the highest sensitivity towards impact stimuli with 3 J, which arises from the highly sensitive anion.The corresponding hydroxylammonium and ammonium salts reveal an impact sensitivity of 2 J, and thus lie in the same range as compound 8. [26] Figure 10.DTA plots of compounds 1 and 3-13 measured with a heating rate of 5 °C min -1 .
Compound 13 exhibits a sensitivity of 4 J. Compounds 4, 10 and 15 12 are fairly insensitive with sensitivities >30 J.In comparison to previously reported nitrogen-rich salts of 1-methyl-5-nitriminotetrazole, an impact sensitivity of up to 35 J is very high, especially because the aminoguanidinium or triaminoguanidinium salts exhibit values in the range of 10 J. [25] The nitrate salt 5 (15 J), 20 shows an moderate sensitivity towards friction.All compounds thus at least lie in the range (2, 3, 8, 11 and 13) of RDX (7.5 J) [30] , or reveal higher stabilities towards impact.Except for compound 8 and 11, the energetic salts as well as the neutral compound 1 are insensitive towards friction (360 N).

Energetic Performance
The values for the enthalpies of formation are calculated with the atomization method, using electronic energies (CBS-4M method) 45 at room temperature (see supplementary information).The heats and energies of formation for compounds 1-3, 5-8, 11 and 13 are given in Table 3. Calculation of the detonation parameters of 1-3, 5-8, 11 and 13 was performed with the program package EXPLO5 (version 50 6.02). [31]The program is based on the chemical equilibrium, steady-state model of detonation.It uses the Becker-Kistiakowsky-Wilson equation of state (BKW EOS) for gaseous detonation products and Cowan-Fickett's equation of state for solid carbon.For these calculations low temperature X-ray 55 densities were converted to room temperature values with the equation ρ 298K = ρ T / (1+α V (298-T 0 ); α V = 1.5 10 -4 K -1 . [32]In order to verify this approximation, crystals of compound 2 have been measured at 100K, 173K and 298K (see SI).The measured X-ray density of 1.819 g cm -3 (298K) is virtually the same as the 60 recalculated density of 1.826 g cm -3 (298K).The marginal influence on the calculated detonation performances is illustrated in Table 3.
The calculated detonation parameters are summarized in Table 3 and compared to the values calculated for FOX-12 and RDX.The , or hydroxylammonium dinitramide with a heat of formation of -34 kJ mol -1 , the calculated heat of formation of compound 2 is significantly higher. [13]Additionally in comparison to RDX and FOX-12 with heats of formation of 70 kJ mol -1 and -355 kJ mol -1 , a higher value is obtained as well.

Journal of Materials Chemistry A Accepted Manuscript
8c] The nitrate 5 has a detonation velocity of 5 8334 m s -1 and a detonation pressure of 260 kbar.The lowest detonation pressure (221 kbar) and velocity (8081 m s -1 ) of the compounds discussed is exhibited by compound 7.
A small scale reactivity test (SSRT, for a detail setup description see supporting information) was conducted to assess the explosive 10 performance of 2 in comparison to RDX and FOX-12.From measuring the volumes of the dents (Table 4), it can be concluded that the small scale explosive performance of 2 is slightly lower than that of commonly used RDX, but exceeds that of FOX-12 by far. 15 The toxicity to aquatic life was investigated using the luminescent marine bacterium Vibrio fischeri (for a detail setup description see SI). [33] For the dinitramide 2 and FOX-12 we observed EC 50 values of 3.78 g L -1 and 3.58 g L -1 after an incubation time of 30 min.With a 20 value higher than 1.00 g L -1 a compound can be considered as non toxic.The toxicity test demonstrates the low toxicity of 2 and FOX-12 compared to RDX (Table 4).the chemical shifts already reported in literature (5.90 and 5.80 ppm). [10]Carbon resonances of poorly soluble 1 could only be observed in a 13 C NMR long-time measurement (pulse delay >2 s, > 8000 scans).The resulting spectrum shows two sharp peaks at 155.4 (C-NH 2 ) and 139.4 ppm (C-C). 35 The IR and Raman spectra for compounds 1-13 were measured and the frequencies were assigned according to commonly observed values in the literature. [5,34] In the IR-spectrum of compound 1 the stretching vibration of the N-H bond is observed between 3500 and 3300cm -1 , whereas the deformation vibration shows a strong band at 1537 cm -1 in the IR spectrum and a very weak band at 1551 cm -1 in the Raman spectrum.The strongest band observed in the IR spectrum is the 60 C=N stretch at 1626 cm -1 .Another characteristic band of the triazole-ring is observed at 1317 cm -1 representing the C-N stretch.These values are very similar to the ones reported for substituted 1,2,4-triazoles. [35]The vibration of the carbon-amine bond appears at 1085 cm -1 and the C-C vibration of the carbons 65 linking the two rings together occurs at 1022 cm -1 .These bands can all be observed for the cations of compounds 2-13 at very similar values.
The 13 C NMR peaks as well as the IR and Raman bands of the anions all match the values of the literature and are discussed in 70 detail in the supplementary information.

Experimental Part
General methods and procedures as well as synthesis of 3-13 are described in the SI.

4,4',5,5'-Tetraamino-3,3'-bi-1,2,4-triazole (1)
4,4',5,5'-Tetraamino-3,3'-bi-1,2,4-triazole (1) was synthesized slightly modified according to the literature: 10 Phosphorus pentoxide (10 g, 70.4 mmol) was slowly dissolved in phosphoric acid (30 g, 306 mmol), which was preheated to 50 °C.A finely ground mixture of oxalic acid dihydrate (3.15 g, 25.0 mmol, 1.0 eq) 10 and diaminoguanidine monohydrochloride (8.29 g, 66 mmol, 2.6 eq.) was slowly added to the preheated solution After complete addition, the viscous mixture was slowly heated to 120 °C and gas evolution of HCl was observed.The mixture was kept at 120 °C for 4 h and was then cooled to room temperature under stirring.150 mL ice water was poured into the mixture and a white precipitate was formed.About 75 mL of 10 M NaOH was used to neutralize the reaction mixture, changing the color of the suspension from white to brown.The precipitate was filtered, washed repeatedly with water and air dried to obtain crude compound 1 as a brownish 20 solid.Yield: 1.39 g, 7.09 mmol, 28%.For purification the crude product was recrystallized with hydrochloric acid or glacial acetic acid.Compound 1 (1000 mg, 5.10 mmol, 1.00 eq.) was added slowly to glacial acid.The mixture was heated until compound 1 completely dissolved.The mixture was removed from the heating 25 bath and was left to cool to room temperature.After filtration and repeated washing with water the residue was dried in a nitrogen flow before drying the substance in oven at 100 °C over night.

Conclusions
The aromatic, nitrogen-rich 4,4',5,5'-tetraamino-3,3'-bi-1,2,4triazole (1) was synthesized starting from commercially available 70 diaminoguanidine hydrochloride and oxalic acid in poly phosphoric acid. 1 shows an amazing thermal stability in its neutral (342 °C) as well as protonated form.Through simple anion metathesis a number of new energetic salts (2-13) were obtained and characterized in detail.These energetic ionic derivatives were 75 extensively characterized for their physico-chemical properties (e.g.stability, sensitivity, compatibility) and detonation parameters based on computed enthalpies of formation were calculated with the EXPLO5 computer code.The dinitramide salt 2 has a heat of formation of 301.5 kJ mol -1 , a detonation pressure of 338 kbar and 80 a detonation velocity of 9053 m s -1 , which are remarkably high compared to other nitrogen-rich dinitramide salts as FOX-12.Moreover, dinitramide 2 was measured to be less toxic tan RDX in aqueous media.Fundamental compatibility tests demonstrate the compatibility of 2 with 2,4,6-trinitrotoluene (TNT).

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The high decomposition temperature (200 °C, determined by DTA at a heating rate of 5 °C min -1 ) of dinitramide 2 is superior over nearly all dinitramides described in literature so far.The great thermal stability of the described energetic salts demonstrate the great value of 1 over other nitrogen rich cations as guanidine, 90 aminoguanidine or triaminoguanidine in future synthesis of new ionic energetic materials.methods for energetic materials and with Dr. Muhamed Sucesca (Brodarski Institute, Croatia) in the development of new computational codes to predict the detonation and propulsion parameters of novel explosives.We are indebted to and thank Drs. Betsy M. Rice and Brad Forch (ARL, Aberdeen, Proving Ground, 5 MD) for many inspired discussions.The authors want to thank Stefan Huber for measuring the sensitivities, Regina Scharf for measuring the toxicities and Daniel W Terwilliger for his contributions.

Notes and references
10

Figure 8 .
Figure 8. DTA plots of compound 2 in comparison with other5
65 energetic performances of compounds 6-8, 11 and 13 are discussed in detail in the Supporting Information.Compared to the energetic salts, compound 1 reveals a fairly high heat of formation of 472 kJ mol -1 .The dinitramide 2 exhibits a heat of formation of 302 kJ mol -1 .Compared to ADN with a heat of 70 formation of -150 kJ mol -1

75Compound 3 for dinitramide 2 .
comprises a highly positive heat of formation of 761 kJ mol -1 .For neutral 1 a fairly high detonation velocity of 8944 m s -1 and a detonation pressure of 285 kbar was calculated.The highest detonation velocity of 9053 m s -1 within this work was calculated 80 It is also higher than that of RDX (8861 m s -1 ) and FOX-12 (8323 m s -1 ).When comparing the detonation pressures, compound 2 shows a value in the range of RDX (345 kbar) and a much higher value than FOX-12 (265 kbar).Compared to RDX, compound 3 has a very similar detonation 85 velocity of 8857 m s -1 and a slightly lower detonation pressure of 306 kbar.The corresponding hydroxylammonium salt has higher Page 7 of 11 Journal of Materials Chemistry A

Table 4 .
Values for the SSRT and toxicity test of compound 2 compared to FOX-12 and RDX 1H NMR spectra of compounds 2-5, 8 and 11-13, which all contain only N-connected protons, each reveals two similar broad signals with chemical shifts of 8.64-8.56ppm and 6.16-6.08ppm.With two broad signals at 8.10 and 6.17 ppm the energetic salt 6 shows chemical shifts, which are slightly shifted upfield compared

Table 3 .
Energetic Properties and detonation parameters of compounds 1