Energetic alliance of tetrazole-1-oxides and 1 , 2 , 5-oxadiazoles

The connection of highly endothermic heterocycles with high nitrogen as well as oxygen content is a recent trend in the development of new energetic materials in order to increase densities and stabilities. Bis(1-hydroxytetrazolyl)furazane (9) and bis(1-hydroxytetrazolyl)furoxane (10) were synthesized for the first time from dicyanofurazane and dicyanofuroxane, respectively. Several nitrogen-rich compounds (e.g. ammonium and hydroxylammonium) and metal salts thereof were prepared. Most compounds were characterized by single crystal X-ray diffraction. In addition all compounds were analyzed by vibrational spectroscopy (IR and Raman), multinuclear NMR spectroscopy, elemental analysis and DSC measurements. The heats of formation of 4, 5, 15–16, 20 and 24 were calculated using the atomization method based on CBS-4M enthalpies. With these values and the experimental (X-ray) densities several detonation parameters such as the detonation pressure, velocity, energy and temperature were computed using the EXPLO5 code (V.5.05). In addition, the sensitivities towards impact, friction and electrical discharge were tested using the BAM drop hammer and friction tester as well as a small scale electrical discharge device.


Introduction
Research towards insensitive replacements for hexogen (RDX), octogen (HMX) and nitropenta (PETN) is still of particular interest in our and many other research groups worldwide.RDX has been identified as toxic and possibly carcinogenic. 1Several attempts to synthesize appropriate replacements for RDX have been made in the recent past using tetrazole oxides. 2-4The purpose of this study was to combine furazanes (1,2,5-oxadiazoles) and furoxanes (1,2,5-oxadiazole-2-oxides) with tetrazole oxides.The connection of highly endothermic heterocycles with high nitrogen as well as oxygen content is a recent trend in the development of new energetic materials in order to obtain powerful materials with great density and appropriate oxygen balance on the one hand, and perfect stability on the other hand.Tetrazoles (without N-oxide) have already been attached to furazanes 5 and furoxanes. 6 The resulting literature known compounds 24 and 25 as well as the new ones 9 and 10 are displayed in Fig. 1.
Various nitrogen rich salts of 9 and 10 as well as metal salts were synthesized to investigate their properties as potential energetic ingredients.The resulting 3,4-(1-oxidotetrazolyl)furoxanes and furazanes are capable and fairly stable compounds in their deprotonated form.

Synthesis
Compounds 9 and 10 were synthesised using a similar protocol from dicyanofurazane 7 and dicyanofuroxane 8 as depicted in Scheme 1.
Hydroximoylamines 3 and 4 are made from the nitriles by exothermic addition of aqueous hydroxylamine in ethanol in about 85% yield.Hydroximoyl chlorides 5 and 6 were synthesized from 3 and 4 by diazotization in 15% HCl and subsequently Fig. 1 Compounds 9 and 10 and the literature known compounds 24 5  and 25. 6  extracted into diethyl ether.The reaction of 5 and 6 with sodium azide in aqueous ethanol affords hydroximoylazides 7 and 8, respectively, which are also extracted into diethyl ether.The dried ether phase was saturated with gaseous HCl at 0 1C and stirred for 24 h in order to close the desired aromatic tetrazole-oxide rings.Compounds 9 and 10 are obtained as slightly yellow sticky oils after removal of the ethereal HCl solution.
Salts of compounds 9 and 10 could be easily prepared by the addition of a base or the corresponding carbonates/bicarbonates to aqueous solutions of 9 and 10.The silver salt of 10 precipitated upon the addition of aqueous silver nitrate.An overview of the salts prepared in this work is given in Scheme 2.

Crystal structures
Single crystals for XRD of compounds 3-6, 11, 12, 14-17, 20 and 23 could be obtained during this work.Crystallographic data and parameters as well as CCDC numbers are given in Tables S1 and S2 in the ESI.‡ In general, all bond lengths and angles were observed as expected and are comparable to similar crystal structures of furazanes, 9 furoxanes, 10 and tetrazole-oxides 11 in the literature.Compound 3 crystallizes in the monoclinic space group C2/c with four molecules in the unit cell.The molecular unit is generated by C 2 symmetry through atom O1 and bond C1-C1 i .The density (1.667 g cm À3 at 100 K) of 3 is significantly smaller than that of the corresponding furazane 7 (1.780g cm À3 at 173 K).Compound 4 crystallizes in the monoclinic space group P2 1 /n.The molecular moieties are depicted in Fig. 2.
Hydroximoyl chlorides 5 and 6 crystallize in the monoclinic (P2 1 /n) and orthorhombic (P2 1 2 1 2 1 ) crystal systems with four molecules in the unit cell.The molecular units are shown in Fig. 3.The density of furazane (1.909 g cm À3 at 100 K) again is slightly lower than that of furoxane (1.949 g cm À3 at 100 K).
Crystal structures of both potassium salts were obtained.The structure of furazane 11, which crystallizes in the monoclinic space group P2 1 /c, contains two crystal water molecules resulting in a lower density of 1.926 g cm À3 (at 100 K).For furoxane 12 (triclinic, P% 1) a density of 2.156 g cm À3 at 100 K has been calculated.In both structures two rings (containing atoms C1 and C2/C3) are almost in plane while the third one is significantly deviated.Molecular moieties of 11 and 12 are depicted in Fig. 4.
Compound 14ÁH 2 O could only be obtained in the crystalline form with inclusion of one crystal water molecule (Fig. 5).It crystallizes in the monoclinic space group P2 1 with a density of 1.794 g cm À3 at 173 K.
Bisammonium salt 15 crystallizes in the orthorhombic space group Pbca with a calculated density of 1.686 g cm À3 at 298 K.The corresponding bisammonium furoxane salt 16 crystallizes with a higher density of 1.748 g cm À3 (at 293 K) in the monoclinic space group P2 1 /c.Both structures shown in Fig. 6 are dominated by strong hydrogen bonds involving all NH 4 + protons.
The products of deprotonation of 9 with hydrazine and different guanidinium bases have not been obtained as single crystals.The hydrazinium salt of 9 crystallizes in the triclinic (P% 1) crystal system and with a density of 1.727 g cm À3 at 236 K without inclusion of crystal water.The molecular moiety is shown in Fig. 7.
The bisguanidinium salt 20 (Fig. 8) crystallizes in the monoclinic space group C2/c and with a density of 1.739 g cm À3 at 100 K (Fig. 8).
In contrast to 20, the aminoguanidinium salt 23 shows a slightly lower crystal density of 1.692 g cm À3 .The asymmetric unit of the monoclinic (P2 1 /c) cell is shown in Fig. 9.

NMR spectroscopy
All 1 H NMR and 13 C NMR shifts of compounds 1-23 are gathered in Table 1.The 1 H NMR spectra of 9 and 10 exhibit both a broad singlet at around 7 ppm although 10 has no Sensitivities were measured using a BAM drop hammer, a BAM friction tester 12 and a OZM electrostatic discharge device 13 (see also the Experimental part, General methods).
Detonation parameters were calculated using the computer code EXPLO5.05 14using X-ray densities which were converted to room temperature values according to eqn (2).A coefficient of volume expansion 15 a v of 1.5 Â 10 À4 K À1 was used.The structures of 15 and 16 were already measured at room temperature.Further explanations are gathered in the ESI.‡ Only the physicochemical properties of compounds 4, 5, 15-16, 20 and 23 are discussed since (i) they consist only of CHNO atoms and (ii) anhydrous crystal structures were obtained.The energetic parameters in comparison with RDX (cyclotrimethylene-trinitramine) are summarized in Table 2.All compounds investigated show improved sensitivities to RDX (IS 7.4 J, FS 120 N).Especially 20 is classified as insensitive towards impact and friction.The highest heat of formation was calculated for hydrazinium salt 17 (D f H1 (s) = 947.5 kJ mol À1 ).For energetic materials it is more convenient to look for mass based enthalpies or energies.Also the highest mass based energy of formation value (D f U1 3245.4 kJ kg À1 ) was calculated for 17.The most important detonation parameters (heat of detonation, detonation temperature, pressure, velocity of detonation, and volume of detonation gases) were calculated with the EXPLO5.05code and are summarized in Table 2. Based on these computations, compound 17 (8843 m s À1 ) has higher velocity of detonation than RDX (8763 m s À1 ).However, with respect to the synthetic expenditures and the assessment of all important energetic properties (sensitivities, stabilities and performance) probably none of the compounds will be used as an explosive filler by itself.

Conclusions
From this combined experimental and theoretical study the following conclusions can be drawn.
-The combination of furazanes or furoxanes with tetrazole-1-oxides is a suitable strategy in order to generate new triheterocyclic high-performing energetic materials due to their large positive heats of formation and appropriate densities.
-Generally the investigated furoxanes show mostly higher densities but lower thermal stabilities than the corresponding furazanes.Therefore, furazanes mostly are the better choice as energetic backbone heterocycles.

Fig. 3
Fig. 3 Molecular moiety of 5 and 6.Thermal ellipsoids are drawn at the 50% probability level.

C
2V symmetry anymore.The oximes in compounds 3-8 are observed in low field regions from 10.33 to 13.00 ppm.In the case of the oxime and amide protons splitting of the signals of the furoxane compounds can be observed because of the lower symmetry.In the13 C NMR spectrum 9 exhibits two resonances at 142.1 ppm caused by furazane and 136.6 ppm caused by 1-hydroxytetrazole.10 shows four resonances in the 13 C NMR spectra because of the lower symmetry at 136.8 and 134.4 ppm for the tetrazole oxides and at 143.4 and 103.4 ppm for the furoxane ring.Upon deprotonation the furazane signal of 9 is shifted towards lower fields up to 148.7 ppm and the tetrazole-oxide signal is shifted towards higher fields down to 132.8 ppm.The same trend was observed for 10.Deprotonation led to a shift towards lower fields up to 149.4 and 107.7 ppm for furoxane but to a shift towards higher fields down to 133.2 and 130.3 ppm for the tetrazole-oxide resonances.The 15 N NMR spectrum of 9 is depicted in Fig. 10.The chemical shifts are assigned to the particular nitrogen atoms.The carbon signals of the salts 11-23 are shifted to lower fields in comparison with their acids.No irregularities in the 1 H NMR and 13 C NMR shifts of the nitrogen-rich cations were observed.Energetic properties Thermal behavior (DSC).The thermal behaviour of the most important salts (with respect to their energetic behavior) of 9 and 10 is depicted in Fig. 11.The highest thermal stabilities of bis(1-oxidotetrazolyl)furaz(ox)anes are reached by the potassium salts at 277 1C (11) and 265 1C (12), respectively.Heats of formation, sensitivities and detonation parameters.Gas phase heats of formation (D f H1 (g,M,298) ) were calculated theoretically using the atomization eqn (1) and CBS-4M electronic enthalpies.Details of the computations and the conversion of gas phase values into solid state values are given in the ESI.‡ D f H1 (g,M,298) = H (M,298) À SH1 (atoms,298) + SD f H1 (atoms,298)