Preparation and Crystal Structure of 5-azido-3-nitro-1h-1,2,4- Triazole, Its Methyl Derivative and Potassium Salt

5-Azido-3-nitro-1H-1,2,4-triazole, its methyl derivative and potassium salt were synthesized and characterized by various spectroscopic methods. The crystal structures were determined by low temperature single crystal X-ray diffraction. The interactions between the molecules or ions were analyzed and discussed. Furthermore, all compounds were tested according to BAM (Bundesanstalt für Materialforschung und-prüfung) methods.


Introduction
New non-toxic and environmentally-friendly primary explosives, such as lead azide replacement, are of recent interest in our research group.In this study we report on the preparation and characterization of 5-azido-3-nitro-1H-1,2,4-triazole and its potassium salt, which shows the desired characteristics for primary explosives.Besides high sensitivities to various stimuli, for example impact or friction, a main characteristic is a fast deflagration-to-detonation transition.Although the title compound is already literature known at least since 1974 [1], we were not able to find reliable data on its preparation or characterization (spectroscopy, X-ray, sensitivities, etc.).
While suitable single crystals of the parent compound 1 were obtained upon removal of the ethyl acetate solvent in a rotary evaporator as colorless blocks, the methylated derivative 2 yielded colorless, fine needles directly from the reaction mixture upon cooling down to room temperature.Recrystallization of the potassium salt 3 from acetone yielded pale yellow needles.The crystallographic data for all compounds 1-3 are compiled in Table 1.Thermal ellipsoids in structure depictions are drawn with 50% probability.Selected bond lengths, bond angles and torsion angles of all three compounds are available as supplementary information, together with the CIF files.5-Azido-3-nitro-1H-1,2,4-triazole (1) crystallizes in the orthorhombic space group Pnma with a cell volume of 580.85(9)Å 3 and four molecules in the unit cell.The calculated density at −100 °C is 1.774 g cm −3 , which is lower than that of the reactant (ANTA, 1.841 g cm −3 ).This may be due to fewer hydrogen bonds.The asymmetric unit consists of one complete molecule (Figure 1).The structure of 1 in the crystalline state is characterized by infinite chains of alternating molecules along the a-axis in the a-c plane, connected by bifurcated hydrogen bonds (Figure 2 and Table 2).The N1-H71•••O1 i contact is less directed with an angle of 135( 2
The chains build up a layer structure where one layer consists of identical parallel chains with no interaction between them.The second layer consists of anti-parallel chains, in relation to the first layer, with the molecules being above the gaps of the first layer.This leads to the layers being connected by an O-O short contact O1•••O1 iii where the atoms are almost directly above each other along the b-axis.The distance of 2.9633(4) Å is only slightly shorter than the sum of the van der Waals radii (r w (O) + r w (O) = 3.0 Å), indicating a weak interaction.The packing of the molecules is depicted in Figure 3, together with the O-O short contacts.
A discussion of hydrogen bonds is omitted due to the hydrogen atoms being calculated as an idealized methyl group (HFIX 33), although the crystal packing shows the possibility of a very weak electrostatic interaction between C3 and N3 ii (Figure 5).
The structure is again comprised of parallel chains along the a-axis forming a layer structure, whereby the alternating layers have antiparallel chains (Figure 5), quite similar to 1.But contrary to 1, the molecules are not above the gaps of the surrounding layers, but rather directly above each other.This is the result of four O-C and N-N short contacts (O1-C1 i : 3.0351(9) Å; N3-N4 i : 3.0448(8) Å), all of which are well below the sum of the respective van der Waals radii (r w (O) + r w (C) = 3.3 Å).

Experimental Section
All chemicals were used as supplied (ABCR, Acros Organics, AppliChem, Sigma-Aldrich, VWR).NMR spectra were recorded using the spectrometers JEOL Eclipse 400 and JEOL ECX 400.The measurements were conducted in regular glass NMR tubes (Ø 5 mm) and, if not stated otherwise, at 25 °C.Tetramethylsilane ( 1 H, 13 C) and nitromethane ( 14/15 N) were used as external standards.As an additional internal standard, the reference values of the partially deuterated solvent impurity ( 1 H) and the fully deuterated solvent ( 13 C) were used [7].Data analysis was performed using MestReNova [8].
IR spectra were recorded using a PerkinElmer BX FT IR spectrometer on a Smiths DuraSamplIR II diamond ATR unit with pure samples.Raman spectra were recorded using a Bruker RAM II, with 300 mW laser power (Nd:YAG laser, λ = 1064 nm) and 30 scans in open glass tubes (Ø 1 mm).
The determinations of the carbon, hydrogen and nitrogen contents were carried out by combustion analysis using an Elementar Vario EL.The theoretical values are given in parentheses.
Differential scanning calorimetry was conducted with a Linseis DSC-PT10 in closed aluminum pans, equipped with a hole (Ø 0.1 mm) for gas release, and a heating rate of 5 °C min −1 .
CAUTION!Most compounds prepared herein are energetic compounds sensitive to impact, friction and electric discharge.Therefore proper protective measures (ear protection, Kevlar ® gloves, face shield, body armor and earthed equipment) should be used, especially when handling the primary explosive 2.