Combining Performance with Thermal Stability: Synthesis and Characterization of 5-(3,5-Dinitro-1 H -pyrazol-4-yl)-1 H -tetrazole and its Energetic Derivatives

. In this study, we present the synthesis of 5-(3,5-dinitro-1 H -pyrazol-4-yl)-1 H -tetrazole and its energetic derivatives starting from 4-amino-3,5-dinitropyrazole, which was diazotized and cyanide substituted. A subsequent cycloaddition reaction with sodium azide led to 5-(3,5-dinitro-1 H -pyrazol-4-yl)-1 H -tetrazole ( 3 ). Several alkaline metal and nitrogen-rich salts were prepared and characterized by low-temperature X-ray diffraction. Additionally, all compounds were analyzed by vibrational spectroscopy (IR), 1 H, 13 C and 14 N NMR spec-Introduction funding enabled and organized by Projekt DEAL.


Introduction
The demand for new energetic materials has risen sharply in recent years, as the field of application has been extended not only to the military sector, but also to an increasing number of civilian sectors, such as aerospace technology and the automotive industry. [1][2][3] Increasingly specialized fields of application also constantly present new challenges in the development of suitable substances. [4,5] Some key characteristics every new HEDM to be developed should meet are a high decomposition temperature, which is especially important for temperature resistant materials. In addition, paired with low sensitivity to external stimuli, this is indispensable for the safety of the persons handling the materials. [6][7][8] Green chemistry is also becoming an increasingly important point to consider in the development of energetic materials. [3,9,10] Newly developed materials should therefore be completely free of toxic or environmentally harmful reactants in their synthesis. Of course, the toxicity of the final product is also important and should therefore be as harmless as possible. [11][12][13] Especially for the military sector, more performance-efficient substances are of interest. [5] In addition, production costs must be regarded as a criterion, as troscopy, elemental analysis and differential thermal analysis (DTA). Additionally, the heats of formation for selected compounds were calculated using the atomization method based on CBS-4M enthalpies as well as important detonation parameters by using the EXPLO5 code (V6.05). Furthermore, the sensitivities of 3 and all synthesized salts toward friction, impact and electrostatic discharge according to BAM (Bundesamt für Materialforschung) were determined and compared to RDX.
Energetic materials based on the 5-(pyrazol-4-yl)-tetrazole skeleton have not been mentioned in literature yet. Herein, we report on the synthesis of the first compound combining a dinitropyrazole derivative with a tetrazole via a C-C bond in a five-step reaction. In addition, various mono salts of H 2 DNPT (3) were synthesized and intensively characterized and compared to each other.
All compounds were fully characterized by IR and multinuclear NMR spectroscopy, mass spectrometry and differential thermal analysis. Further, selected compounds were analyzed using low-temperature single-crystal X-ray measurements.

Crystal Structures
Suitable crystals of compounds 2-5, 7, 8, 11 and 12 were obtained by recrystallization of the crude products from methanol or acetonitrile, respectively. Compounds 2, 11 and 12 crystallize with the inclusion of water molecules. The DAU and hydrazinium derivatives 9 and 10 maintained crystalline morphology, but the solution of the diffraction measurement data could not be completed due to structural disorder. Here, only the low temperature X-ray crystal structures of the neutral compound 3 and anhydrous derivatives 4, 5, 7 and 8 are discussed. The other solid-state structures can be found in the Supporting Information.
Compound 3 crystallizes in the orthorhombic space group Pbca with a cell volume of 1754.19(12) Å 3 and eight formula units per cell. The cell constants are a = 9.5893(3) Å, b = 10.5373(5) Å and c = 17.3604(7) Å, while the density is 1.712 g cm -3 at 123 K. Thus, the density is clearly below the value calculated by Ghule et al. [35] The nitro groups are almost in one plane with the pyrazole moiety (O2-N3-C3-C2 -3.9°, O3-N4-C1-N2 -2.4°). The pyrazole and tetrazole ring of H 2 DNPT each have a planar structure (C3-N1-N2-C1 0.6°, N6-N5-C4-N8 0.2°). However, both rings in the molecule are not coplanar to each other (C1-C2-C4-N8 129°). The twisting  prevents a regular stacking of the molecular units. This can be assumed to be the main reason for the low density of compound 3 (1.669 g cm -3 at 298 K). The bond lengths within the azole rings are between the expected values for C-C, C-N and N-N single and double bonds (C-C:1.47 Å, 1.34 Å, C-N: 1.47 Å, 1.22 Å; N-N: 1.48 Å, 1.20 Å). [36][37] The C-C bond between the two aromatic rings has the classic length of a single bond (C2-C4 1.465 Å) [36,38] (  ties in the ac plane form a strong intramolecular interaction with N8 at the tetrazole of the adjacent unit through the hydrogen at the pyrazole (N1-H1···N8 iii , 1.90 Å). The steric demand of the nitro groups also explains the tilting of the aromatic rings towards each other in the layering.
Compound 4 crystallizes is the monoclinic space group P2 1 with a cell volume of 910.47(8) Å 3 and two formula units per cell. The cell constants are a = 9.0333(4) Å, b = 11.3970(6) Å and c = 9.2529(4) Å, while the density is 1.928 g cm -3 at 131 K. Deprotonation occurs at the pyrazole ring, which indicates a more acidic character than the tetrazole proton. The pyrazole ring forms an almost flat plane with the two nitro groups (O3-N4-C3-N2 9.2°, O1-N3-C1-C2 176.6°). The ring moieties are not coplanar but around 61°tilted straight to each other. Compared to neutral compound 3, the twist is more distinctive with about 10 degrees more. All bond lengths in the azole rings are in the range of C-C, C-N or N-N single and double bonds. [36] Compared to the neutral compound 3 the bond lengths do not vary significantly ( Figure 5).
The anions HDNPTin 5 form dimers, which build two strong hydrogen bridges via the nitro group and the tetrazolium proton (N5 i -H5···O2, N5-H5···O1 i ). Four further nitrogen atoms (i.e., N1, N2, N6 and N8) of the anion are involved as acceptor atoms in further hydrogen bonds (Table 1), thereby resulting strong interactions with surrounding cations. The donors of the hydrogen bridges originate from the amine functionalities N13 and N14 as well as the secondary tetrazoleamine N12.   ture is shown in Figure 7. The crystal structure shows the formation of layers of cations and anions, respectively, along c axis. The anions HDNPTcreate an alternating chain-like structure, whereby one clear interaction N5-H5···N1 i is built. Two aminoguanidinium cations surround the tetrazole ring and interact with the accepting nitrogen atoms N6 and N8. Another cation forms a strong interaction with the deprotonated pyrazole nitrogen (N1 and N2). Surprisingly, only one nitro group with O3 and O4 forms hydrogen bridges with the cation.  Both, the cations and the anions form linear chains along c. In the anion structure strong interactions between the tetrazoles are visible (N5-H5···N8 i ). In the TABTrH chains, many interactions between the amines and the nitrogen atoms of the triazoles are detectable. In addition, certain hydrogen bridges between the accepting anion nitrogens N2, N3, N6 and N7 and the amino groups of the cation (N15-H15A, N15-H15B, N17-N17A, N18-H18A, N18-H18B) are formed.
The highly acidic protons of the pyrazole and tetrazole can only be detected as a broad signal at δ = 14.24 ppm for the neutral compound 3. The signal only became detectable with an extended measuring time and high substance concentration, since a constant exchange takes place due to the high acidity in DMSO. For all deprotonated compounds (4)(5)(6)(7)(8)(9)(10)(11)(12), the remaining tetrazole proton signal could not be observed in the 1 H spectra. While the 1,5-DATH cation proton resonances in 5 are located at δ = 7.58 ppm resulting in a broad signal, the guanidinium compound 6, representing 6 protons appears as sharp singlet at δ = 6.92 ppm. The aminoguanidinium cation shows four signals, in accordance to the four different types of protons, located at δ = 8.55 (NH-NH 2 ), 7.24 (NH 2 ), 6.72 (NH 2 ) and 4.68 ppm (NH-NH 2 ). The two different amino groups of the TABTrH cation can be assigned to sharp singlet signals at δ = 7.40 (N-NH 2 ) and 5.99 ppm (C-NH 2 ). The protons in the DAU compound 9 result in a broad signal at δ = 8.68 ppm. For the water containing compounds, the water signal can only be observed for the hydrazinium 10 and the hydroxylammonium 11 compound at δ = 3.36 and 3.39 ppm, respectively. Additionally, the cations of the water containing compounds show signals at δ = 7.13 ppm for the hydrazinium 10, δ = 11.33 and 10.06 ppm for the hydroxylammonium 11 and δ = 7.24 ppm for the ammonium 12 compound.
In all 13 C NMR spectra one signal for the tetrazole and according to symmetry two signals for the pyrazole can be observed. The signals are all in the range for C-substituted tetrazoles and 3,5-dinitropyrazoles. [26,28,39,40] For the neutral compound 3 C4 can be observed at δ = 153.6 ppm, C1/C3 at δ =  [41] The guanidinium and aminoguanidinium derivatives show a resonance at δ = 157.9 and 158.8 ppm, respectively. The TABTr derivative has two more signals in the 13 Figure 9 shows the 15 N NMR spectrum of compound 3. The assignments were based on comparison with theoretical calculations using Gaussian 09 [42] and literature values with similar 3,5-dinitropyrazoles and electron poor 5-substituted tetrazoles. [18,40,43] The spectrum shows two sharp signals at δ = -9.5 and -25.4 ppm. Additionally, two broad signals at δ = -93.8 and -98.0 ppm are observed. The sharp signals can be clearly assigned to the nitrogen N4/N4' (-9.5 ppm) of the tetrazole

ARTICLE
moiety and the nitrogen atoms of the nitro groups N2/N2Ј (-25.4 ppm). A defined assignment of the two wide signals to N1/N1Ј and N3/N3Ј is not possible. Due to the high acidity of the protons, no N-H couplings can be found in the spectrum. The associated rapid proton exchange in DMSO also explains the width of the signals.
The assignment of the respective oscillations in the IR spectra to the corresponding functional groups was checked with appropriate data. [44] The characteristic bands for the nitro groups (asymmetric and symmetric vibrations) can be found for all compounds investigated. They appear in the range of 1557-1514 cm -1 for the asymmetric stretching vibration and 1323-1312 cm -1 for the symmetric vibration, respectively. All compounds with an amino group containing cation (5-9) show significant absorption bands in the range of 3000 cm -1 for the NH 2 stretching vibration and in the region of 1600 cm -1 for the deformation vibration of the amino group.

Physicochemical Properties
As all compounds produced can be classified as energetic substances, the energetic properties must be investigated. The theoretically calculated and experimentally determined physicochemical values are shown in Table 3 and compared with the data of RDX. Computed values (detonation velocity, detonation pressure, ect.) are only given for compounds with a preserved crystal structure.

Thermal Behavior
The thermal behavior of all synthesized compounds was determined by differential thermal analysis experiments. The compound with the highest decomposition temperature is the potassium salt 4, which decomposes at a temperature of  burner. An initiation test, in which 50 mg of compound 4 was compressed onto 200 mg of PETN in a copper sleeve and an ignitor, which produced a jet of flame, did not initiate PETN but caused a deflagration.

Sensitivities
Additionally, the sensitivity values for external stimuli toward impact, friction and electrostatic discharge were determined following the BAM standards. [45][46] The potassium salt

Detonation Parameters
The detonation velocity V D and pressure p CJ were calculated using the EXPLO5 code. The densities used in the calculations were determined based on the respective crystal structures. However these values cannot compete with RDX (V D = 8801 m·s -1 ; p CJ = 336 kbar). All compounds listed in Table 3 show a high positive heat of formation from 816.4 kJ·kg -1 for 8 to 2618.2 kJ·kg -1 for 5, which clearly exceeds that of RDX (433.7 kJ·kg -1 ). The water containing hydroxylammonium derivative 11 with V D = 8648 m·s -1 and p CJ = 310 kbar is the only compound that shows values close to those of RDX. The other compounds show calculated values in the range of 7364 m·s -1 (8) to 8441 m·s -1 (5) for the detonation velocity and 188 kbar (8)

Conclusions
In this study, we report an innovative synthesis leading to a previously unknown C-C connection of a dinitropyrazole moiety with a tetrazole ring. The starting material for the synthesis was 4-amino-3,5-dinitropyrazole (ADNP, 1), which can be produced from pyrazole in 3 steps via an established synthesis pathway. Subsequently, a diazotization of 1 to the intermediate diazonium compound takes place, which is immediately converted to 4-cyano-3,5-dinitropyrazole (2) (3) with nitrogen-rich organic bases and potassium carbonate, nine ionic derivatives were synthesized. All compounds were fully characterized via vibrational (IR) and NMR spectroscopy as well as sensitivity towards impact, friction and electrostatic discharge. The thermal behavior was investigated using differential thermal analysis (DTA), with the potassium derivative 4 with a T dec = 281°C being particularly striking. For certain compounds the detonation parameters were calculated with the EXPLO5 code using the corresponding crystal structure data. For the anhydrous compounds, the values range from 7364 m·s -1 for TABTrH compound 8 to 8441 m·s -1 for 1,5-DATH compound 5. The neutral compound H 2 DNPT (3) shows passable values for the detonation velocity and pressure (V D = 8062 m·s -1 , p CJ = 260 kbar). Unfortunately, the apparently most promising cations yield material with included water molecules. Dehydration of these compounds was not possible because water was immediately trapped again when the compounds were dried and then exposed to air again. For the hydroxylammonium salt 11 with two crystal water moieties, the value for V D is still 8648 m·s -1 .

Experimental Section
General Procedures: 1 H, 13 C, 14 N and 15 N NMR spectra were recorded on JEOL 270 and BRUKER AMX 400 instruments. The samples were measured at room temperature in standard NMR tubes (Ø 5 mm). Chemical shifts are reported as d values in ppm relative to the residual solvent peaks of [D 6 ]DMSO (δ H : 2.50, δ C : 39.5). Solvent residual signals and chemical shifts for NMR solvents were referenced against tetramethylsilane (TMS, δ = 0 ppm) and nitromethane. Unless stated otherwise, coupling constants were reported in Hertz (Hz) and for the characterization of the observed signal multiplicities the following abbreviations were used: s (singlet), m (multiplet) and br (broad). Mass spectra were recorded on a JEOL MStation JMS700 using the EI or ESI technique. Infrared spectra (IR) were recorded from 4000 cm -1 to 400 cm -1 on a PERKIN ELMER Spectrum BX-59343 instrument with a SMITHS DETECTION DuraSamplIR II Diamond ATR sensor. The absorption bands are reported in wave-numbers (cm -1 ). Decomposition temperatures were measured via differential thermal analysis (DTA) with an OZM Research DTA 552-Ex instrument at a heating rate of 5 K·min -1 and in a range of room temperature to 400°C. All sensitivities toward impact (IS) and friction (FS) were determined according to BAM (German: Bundesanstalt für Materialforschung und Prüfung) standards using a BAM drop hammer and a BAM friction apparatus by applying the 1 of 6 method. All energetic compounds were tested for sensitivity towards electrical discharge using an Electric Spark Tester ESD 2010 EN from OZM. CAUTION! All investigated compounds are potentially explosive materials, although no hazards were observed during preparation and handling these compounds. Nevertheless, safety precautions (such as wearing leather coat, face shield, Kevlar sleeves, Kevlar gloves, earthed equipment and ear plugs) should be drawn.