Crystal Structures of Furazanes

Several nitrogen-rich salts of 3-nitramino-4-nitrofurazane and dinitraminoazoxyfurazane were synthesized and characterized by various spectroscopic methods. The crystal structures were determined by low temperature single crystal X-ray diffraction. Moreover the sensitivities toward thermal and mechanical stimuli were determined by differential thermal analysis (DTA) and BAM (Bundesanstalt für Materialforschung und-prüfung) methods. The standard enthalpies of formation were calculated for all compounds at the CBS-4M level of theory, and the energetic performance was predicted with the EXPLO5 V6.02 computer code.


Introduction
Modern energetic materials need to fulfill several criteria to become potential replacements for commonly used Research Department explosive (RDX).In addition to having excellent detonation velocities and pressures, new secondary explosives must comply with certain safety criteria, such as a decomposition temperature over 200 °C and low sensitivity to mechanical stimuli as impact (>7.5 J) or friction (>120 N).In recent years, the growth in environmental concern has resulted in the desire for energetic materials that release environmentally benign dinitrogen after decomposition [1].

Crystal Structure
The crystal structures of compounds 3-6, 8 and 12 were determined.Tables S1-S2 in the Supporting Information gather selected data and parameters of the X-ray measurements.Once crystals were obtained for each compound, we did not attempt to search for additional polymorphs of each.
The compound 3-Amino-4-nitraminofurazan 3 crystallizes in the orthorhombic space group Pna21 with a cell volume of 938.26 Å 3 and eight molecular units per unit cell (Figure 2).The calculated density at 173 K is 1.842 g•cm −3 , which is similar to the previously reported density of 1.84 g•cm −3 (X-ray analysis at 153 K) [11].Compound 3 is planar, only the nitro moiety is slightly tilted against the furazan plane by a torsion angle of O2-N3-C1-C2 = 4.4°.Short hydrogen bonding can be observed between the amino and nitro groups, similar bond distances have been reported for other furazanes in the literature [6].A more detailed description of the structure can be found in the literature [11].The compound 3-Nitramino-4-nitrofurazan 4 crystallizes in the orthorhombic space group Pbca with a cell volume of 1177.45Å 3 and eight molecular units per unit cell.The calculated density at 173 K is 1.975 g•cm −3 , which is comparable to the density reported in the literature: 1.93 g•cm −3 (gas pycnometer) [5] and 1.95 g•cm −3 [7].The small discrepancies are likely due to the different temperatures at which the densities were measured.The molecular unit of 4 is displayed in Figure 3.
The nitro-and nitramino groups in compound 4 are only slightly tilted against the furazan ring as shown by the torsion angles O4-B5-C2-C1 = 4.2°, C1-N3-N4-O2 = 179.4°and C1-N3-N4-O2 = 3.9°.Together with the planar furazan ring (N2-O1-N1-C1 = 0.5°) the complete compound forms an almost planar unit, which might be one of the reasons that can explain the high density of this compound.Another reason might be the hydrogen-bonds of N3−H3…O4 and N3−H3…O3.The bond distances of the furazan ring (C1-C2 = 1.420Å) and the nitro group (N5-C2 1.444 Å, O5-N5 = 1.215Å) are similar to those in other nitrofurazanes as compound 3.The bond distances of the nitramino moiety (O2-N4 = 1.210Å, N3-N4 = 1.370Å) are also in the same range compared to other nitramino moieties in heterocyclic ring systems [6,12].Hydroxylammonium 3-nitramino-4-nitrofurazan 5 crystallizes in the monoclinic space group Pc with a cell volume of 361.86 Å 3 and two molecular units per unit cell.The calculated density at 173 K is 1.910 g•cm −3 , which is comparable to the reported values in the literature: 1.875 g•cm −3 (X-ray analysis) [5] and 1.89 g•cm −3 [7].The slightly higher density values of 5 might be because of the different temperatures at which the densities were measured.The molecular unit of 5 is displayed in Figure 4.
Similar to neutral compound 4, the furazan unit in 5 is planar (N2-O2-N1-C1 = 1.1°), whereas the nitro-and nitramino groups are slightly tilted against the furazan ring as indicated by the torsion angles of O5-N5-C2-N2 = 5.1° and N4-N3-C1-C2 = 171.7°,C1-N3-N4-O3 = 2.5°.Hydrogen-bonding can for example be observed between the hydrogen atoms of the hydroxylammonium cation and the O4 of the nitramino moiety.The bond distances of the furazan ring (C1-C2 = 1.432Å), the nitro group   5), C1-N3-N4-O3 2.5 (8).Ammonium 3-nitramino-4-nitrofurazan 6 crystallizes in the monoclinic space group P21/n with a cell volume of 704.59 Å 3 and four molecular units per unit cell.The calculated density at 173 K is 1.811 g•cm −3 , which is virtually the same as the reported density in the literature: 1.82 g•cm −3 [7], but significantly lower than of hydroxylammonium salt 5.The molecular unit of compound 6 is illustrated in Figure 5.As in salt 5 and neutral compound 4, the furazan ring in 6 has a planar structure (N2-O1-N1-C1 = 0.27°).While the nitramino group is only slightly tilted against the furazan plane (N5-N4-C1-N1 = 5.5°, C1-N4-N5-O5 = 2.9°), the nitro group is tilted strongly against the plane by a torsion angle of O2-N3-C2-N2 =29.5°.A reason for this might be the strong hydrogen bonds between the hydroxylammonium cation and the oxygen atoms O3 and O4 of the nitro group.More hydrogen bonds can be observed between the cation and the nitramino moiety.Even though the nitro group is strongly tilted against the plane, the bond distances of the furazan ring The calculated density at 173 K is 1.766 g•cm −3 , which is lower than the density of salts 5 and 6.The molecular unit of compound 8 is shown in Figure 6.
In compound 8, the furazan ring is also planar (N2-O1-N1-C1 = 0.27°), whereas the nitro (O5-N3-C2-C1 = 157.9°)and nitramino (N5-N4-C1-C2 = 50.6°,O2-N5-N4-C1 = 7.4°) moieties are strongly tilted out of the furazan plane.The cation has a planar structure except for the two protons located at N13 as reported previously in the literature [4].All the three amino groups in the cation form hydrogen bonds to the nitro and nitramino groups of the anion, resulting in a strong hydrogen-bonding network.This effect was expected because it was already observed for other energetic salts of the  ]triazolium dinitraminoazoxyfurazan 12 crystallizes in the triclinic space group P−1 with a cell volume of 561.13 Å 3 and one molecular units per unit cell.The calculated density at 173 K is 1.807 g•cm −3 , which is somewhat lower than the density of the corresponding hydroxylammonium salt: 1.883 g•cm −3 (X-ray analysis at 173 K). [6] The molecular unit of compound 12 is displayed in Figure 7.
In compound 12, the N=N connected furazan rings are tilted against each other by a torsion angle of 38.2° (N3i-N3-NC2-C1).O4 is in the same plane as the furazan ring which is located further away (O4-N3-C3i-C2i = 1.0°) and twisted against the furazan ring which is closer to O4 (O4-N3-C2-C1 = 37.0°).The two nitramino moieties are also tilted out of the furazan plane by 44.0° (N6-N5-C1-C2).The two cations have a planar structure except for the two protons located at N13 as reported previously in the literature [4].As in salt 8, an intensive hydrogen bond network is formed between the three amino groups of the cation and the nitro and nitramino groups of the anion, which makes the

Thermal Analysis and Sensitivities
Considering safety issues explosives should be heat resistant.To identify the decomposition temperatures of compounds 3-12, differential thermal analysis (DTA) with a heating rate of 5 °C•min −1 was used.The results are shown in Figure 8.The decomposition temperatures are given as absolute onset temperatures.Sensitivities of compounds 3-12 were determined and are displayed in Table 1.The impact sensitivities for the derivatives of 3-nitramino-4-nitrofurazan 4 range from 3 J (7) to 10 J (5).The two energetic salts of dinitraminoazoxyfurazan 10 show similar and slightly lower impact sensitivities with 15 J (11) and 10 J (12).The friction sensitivities lie between 120 N (4) and 360 N (12), except for the hydroxylammonium 5 and ammonium 6 salt, both of which comprise a friction sensitivity of 72 N.
Although all structures are dominated by a large variety of hydrogen bonds and other electrostatic interactions all investigated compounds were found to be sensitive toward various outer stimuli.Except for energetic polymorphs in our opinion it is still very challenging to correlate sensitivities and structural motives.For neutral compounds some strategies have been described e.g. by Politzer et al.They correlated impact sensitivity versus electrostatic surface potentials or molecular volumes [13].However, these methods have not successfully been applied for energetic salts, which deviate significantly from standard energetic motifs, such as those found in traditional explosives, such as TNT and RDX.

Physiochemical and Energetic Properties
The heats and energies of formation are given in Table 1.Calculation of the detonation parameters as detonation velocity D and detonation pressure pCJ was performed with the program package EXPLO5 (version 6.02) [14].The EXPLO5 detonation parameters of compounds 2-12 were calculated by using pycnometrical measured densities of the water free compounds or the room-temperature density values obtained from the X-ray structures as described in Table 1 and in reference [15].For a complete discussion on the methods used see the Supporting Information.The densities range from 1.69 g•cm −3 (7) to 1.939 g•cm −3 (4) and are summarized in Table 1 with the calculated explosion parameters for 3-8 and 11-12, alongside a comparison with the values calculated for RDX.The recalculated low temperature X-ray densities of compounds 4-6 have only small deviation compared to the values reported in the literature: 1.939 g•cm −3 (4) versus 1.93 g•cm −3 (gas pycnometer) [5] and 1.95 g•cm −3 [7]; 1.875 g•cm −3 (5) versus 1.875 g•cm −3 (X-ray analysis) [5] and 1.89 g•cm −3 [7]; 1.778 g•cm −3 6 versus 1.82 g•cm −3 [7].
All compounds reported in Table 1 show a highly positive heat of formation from 156.7 kJ•mol −1 (3) to 1706.7 kJ•mol −1 (12), which are all higher than the heat of formation of RDX (70.3 kJ•mol −1 ).The high heat of formation of salt 12 can be explained by the large number of N-N bonds.

Experimental Section
Compound 9 and 10 were prepared as described in the literature [6,10].

3-Nitramino-4-nitrofurazan (4)
Compound 4 was synthesized via a revised literature procedure [5].A dry 100ml three-neck flask was equipped with a stirrer, thermometer and two stoppers.60-70 mL of methylene chloride was added and cooled to −20 °C.To the cooled methylene chloride was added N2O5 (2.3 g, 21.3 mmol, 1.2 equiv.).When the N2O5 was fully dissolved, 3 (2.3 g, 17.7 mmol, 1.0 equiv.) was added slowly over a period of 2-5 minutes, while the temperature was kept at −20 °C.The yellowish solution was put in ice bath and warmed slowly to 0-5 °C, then it was stirred for 3 h at 0-5 °C.The solvents were removed under a constant nitrogen flow to yield 2.04 g (11.65 mmol, 66%) of 4.

Conclusions
Energetic derivatives of 3-nitramino-4-nitrofurazan and dinitraminoazoxyfurazan were investigated in the search for more thermally stable materials.The crystal structures were determined by low temperature single crystal X-ray diffraction and show the stabilizing effect of a strong hydrogen-bonding network formed by the 3,6,7-triamino- [1,2,4]triazolo [4,3-b][1,2,4]triazolium cation.Moreover, the X-ray structures of highly dense hydroxylammonium and ammonium 3-nitramino-4nitrofurazan were reported.Even though neutral 4 and salt 5 have excellent detonation parameters (D 9500 m•s −1 , pCJ 400 kbar), their application is unlikely because of their low thermal stability.To our dismay, thermally more stable salts 7 and 8 fell short of a decomposition temperature of 200 °C.However the dinitraminoazoxyfurazan 12 shows a high thermal stability due to an extensive hydrogen bonding network, with sufficient stability against impact and friction, and yet a detonation velocity D 9000 m•s −1 , which is similar to RDX.

Compound 3
decomposes at 143 °C, whereas 3-nitramino-4-nitrofurazan 4 as well as its hydroxylammonium 5 and hydrazinium salt 6 decompose between 65 °C and 140 °C.The thermal stability of 3-nitramino-4-nitrofurazan could be enhanced by introducing thermally stable cations as demonstrated by compounds 7 and 8 with a decomposition onset at 176 °C and 193 °C.Moreover compounds 11 and 12 surpass a decomposition temperature of 200 °C, which can be considered as benchmark for new potential secondary explosives.

Table 1 .
Physico-chemical properties properties of compounds 3-8 and 11-12 compared with those of Research Department explosive (RDX).