The syntheses of 5,5′-azotetrazole and 5,5′-azotetrazolates were first reported by Thiele in 1898 . Numerous crystal structures of 5,5′-azotetrazolate salts possessing organic cations [2–15] have been reported. A series of metal salts with alkali and alkaline earth metals [16–18], Mn , Fe , Co and Ni , Cu and Cd , Ag , Zn , Sn , lanthanoids [25–27], Tl , and Pb  have been described. In continuation of our studies of nitrogen-rich heterocycles [3, 4, 30, 31], we investigated azotetrazolates, which are not typical explosives but still undergo thermal decomposition at reasonable temperatures. These compounds may be used as an alternative to the azide method for the preparation of nitridosilicates [32–34]. New approaches are desirable for the synthesis of nitridosilicates since no general methods are available. Next to the reaction of metals with silicon diimide, also liquid alkali metals (Na or Li) as fluxing agents or the decomposition of NaN3 or LiN3 as nitrogen sources (‘azide method’) have been employed for the syntheses of nitridosilicates [35–37]. Unfortunately, the introduction of nitrogen by means of azides is of course not compatible with synthetic starting mixtures containing metal additives due to explosion hazards. The use of nitrogen-rich salts as alternative nitrogen sources is envisioned for the preparation of these highly relevant compounds. In the present article, we report the preparation, crystal structures, and thermoanalyses of six new 5,5′-azotetrazolates with organic cations.
2 Results and discussion
Two synthetic pathways were followed for the preparation of the new 5,5′-azotetrazolates: either (a) oxidation of 5,5′-hydrazinebistetrazole by air  in the presence of a quaternary ammonium hydroxide or an organic free base, or (b) ion metathesis of an organic sulfate with sodium or barium 5,5′-azotetrazolate. An almost unlimited supply of cations from organic bases, such as imidazole, diazabicyclo[2.2.2]octane (DABCO), cystamine, and guanidines, is readily available when the oxidative method is used.
The crystallographic data and details of the refinements are summarized in Table 1. As expected, the crystal structure of 1 (Fig. 1) does not exhibit strong interactions between the ions due to the lack of hydrogen bond donor functionalities, but weak C–H···N contacts are present. In contrast, a strong interionic N–H···N hydrogen bond is observed in the crystal structure of the DABCO salt 2 (Fig. 2). Again, additional C–H···N contacts are found. The tetramethylguanidinium salt 3 crystallized as a dihydrate. A cyclic hydrogen bond motif involving two water molecules and two anions is observed, which can be described by a graph set [39–41] (Fig. 3). The cation donates another hydrogen bond to the water molecule. In the structure of 4, there are three phosphonium cations: P1 occupies a general position, whereas P2 and P3 are located at special positions (two-fold rotation axes) with occupancies of 0.5. The hydroxyl groups of the P1 cation donate hydrogen bridges to three azotetrazolate anions, and those of the P2 and the P3 cations to two azotetrazolate anions (Fig. 4). All N–H protons of the dication in the cystamine salt 5 are engaged in hydrogen bonds with azotetrazolate dianions, forming a three-dimensional network (Fig. 5). The screw-shaped dications exhibit a dihedral C–S–S–C angle of 84.6(1)°.
The imidazolium salt 6 crystallized as a monohydrate. The water molecule donates two hydrogen bonds to two azotetrazolate anions and, in turn, receives two hydrogen bonds from two imidazolium cations (Fig. 6). The details of the N–H···N, C–H···N, O–H···N, and N–H···O hydrogen bonds are summarized in Table 2. The phase purities of the bulk samples were confirmed by X-ray powder diffraction analyses. Pawley fits are depicted in Figs. 7–12, and the corresponding lattice parameters are collected in Table 3.
2.2 Thermal analysis
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were employed to characterize the new materials with respect to their potential use as gas generating agents. Exothermic decomposition of the solids was observed in all cases. In contrast to other 5,5′-azotetrazolates, which are sensitive to impact and friction [8, 42], the new compounds decompose only at elevated temperatures, which is evidence for the intended phlegmatizing effect of the low nitrogen content of the organic cations used. The dihydrate 3 shows the expected loss of water in the reasonable temperature range of 70–90°C, whereas the monohydrate 6 retains its water up to 160°C (Fig. 13). Thermograms are shown in Figs. 14–19. Therefore, these new materials may be promising candidates as moderate blowing agents and alternative precursors for the preparation of nitridosilicates.
3 Experimental section
3.1 Syntheses and characterization
5,5′-Hydrazinebistetrazole was prepared according to a published procedure [10, 38]. The sodium and barium 5,5′-azotetrazolates were synthesized by the original method of Thiele . All other chemicals were purchased from Sigma-Aldrich. NMR spectra were recorded with a Bruker Avance DPX 300 spectrometer. IR spectra were obtained with a Alpha FT instrument (Bruker). For hot-stage thermomicroscopic investigations (HTM) a Reichert Thermovar polarization microscope equipped with a Kofler hot-stage (Reichert) was used. Photographs were taken with an Olympus DP71 digital camera. DSC was performed with a DSC 7 (Perkin-Elmer) using the pyris 2.0 software. Samples (0.5–3 mg) were weighed (UM3 ultramicrobalance, Mettler) into Al pans (30 μL). Dry N2 was used as the purge gas (purge: 20 mL min–1). A heating rate 10°C min–1 was applied. The instrument was calibrated for temperature with pure benzophenone (m.p. 48.0°C) and caffeine (m.p. 236.2°C), and the energy calibration was performed with pure In (m.p. 156.6°C, heat of fusion 28.45 J g–1). TGA was carried out with a TGA 7 system (Perkin-Elmer) using the pyris 2.0 software. Samples (2–3 mg) were accurately weighed into a Pt pan. Two-point calibration of the temperature was performed with ferromagnetic materials (alumel and Ni, Curie-point standards, Perkin-Elmer). A heating rate of 10°C min–1 was applied and dry N2 was used as a purge gas (sample purge: 20 mL min–1, balance purge: 40 mL min–1).
Bis(tetramethylammonium) 5,5′-azotetrazolate (1) Tetramethylammonium hydroxide pentahydrate (530 mg, 2.92 mmol) and 5,5’-hydrazinebistetrazole (246 mg, 1.46 mmol) were dissolved in boiling H2O (10 mL). The mixture was stirred in air at ambient temperature for 20 h. The resulting clear solution was taken to dryness under reduced pressure. The yellow residue was recrystallized from hot EtOH to yield 367 mg (80%) of the product. Decomposition onset 235–237°C. – 1H NMR (300 MHz, [D6]DMSO): δ=3.11 (s, 24H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=54.4 (m, 8C), 173.5 (2C) ppm. – IR (neat): v=1487 m, 947 s, 724 m cm–1.
Bis(1-aza-4-azoniabicyclo[2.2.2]octane) 5,5′-azotetrazolate (2) DABCO (278 mg, 2.48 mmol) and 5,5′-hydrazinebistetrazole (208 mg, 1.24 mmol) were dissolved in boiling H2O (10 mL). The mixture was stirred in air at ambient temperature for 20 h. The resulting clear solution was taken to dryness under reduced pressure. The yellow residue was recrystallized from hot EtOH-H2O (5:1) to yield 395 mg (82%) of the product. Decomposition onset 206 °C. – 1H NMR (300 MHz, [D6]DMSO): δ=3.04 (s, 24H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=44.2 (12C), 173.2 (2C) ppm. – IR (neat): ν=1392 m, 1053 m, 727 m, 593 s cm–1.
Bis(tetramethylguanidinium) 5,5′-azotetrazolate dihydrate (3) 1,1,3,3-Tetramethylguanidine (288 mg, 2.50 mmol) and 5,5′-hydrazinebistetrazole (210 mg, 1.25 mmol) were dissolved in boiling H2O (10 mL). The mixture was stirred in air at ambient temperature for 20 h. The resulting clear solution was taken to dryness under reduced pressure. The orange residue was crystallized by addition of CH2Cl2 to yield 441 mg (82%) of the product. Decomposition onset 175–179°C. – 1H NMR (300 MHz, [D6]DMSO): δ=2.89 (s, 24H), 7.90 (s, 4H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=39.4 (8C), 161.0 (2C), 173.4 (2C) ppm. – IR (neat): ν=3048 m, 1571 s, 1392 s, 1035 s, 705 s cm–1.
Bis(tetrakis(hydroxymethyl)phosphonium) 5,5′-azotetrazolate (4) A solution of bis(tetrakis(hydroxymethyl)-phosphonium) sulfate (72% in H2O; 970 mg, 1.72 mmol) in H2O (5 mL) and a solution of disodium 5,5′-azotetrazolate pentahydrate (501 mg, 1.67 mmol) in H2O (20 mL) were mixed, and the solvent was evaporated under reduced pressure. The residue was recrystallized from warm H2O to yield 585 mg (74%) of the product as orange needles. Decomposition onset 126–133°C. – 1H NMR (300 MHz, [D6]-DMSO): δ=4.50 (d, J=2.3 Hz, 16H), 6.5 (br, 8H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=48.8 (d, J=50 Hz, 8C), 171.8 (2C) ppm. – 31P NMR (121 MHz, [D6]DMSO): δ=28.2 ppm. – IR (neat): ν=3151 m, 1039 s, 880 m, 669 m cm–1.
Cystamine 5,5′-azotetrazolate (5) A mixture of cystamine sulfate hydrate (342 mg, 1.27 mmol) and barium 5,5′-azotetrazolate pentahydrate (499 mg, 1.27 mmol) in H2O (15 mL) was stirred at room temperature for 1 h. The precipitate was removed by filtration, and the filtrate was taken to dryness under reduced pressure. The residue was recrystallized from hot H2O to yield 275 mg (68%) of the product as orange needles. Decomposition onset 193–194°C. – 1H NMR (300 MHz, [D6]DMSO): δ=2.95 (t, J=6.8 Hz, 4H), 3.16 (t, J=6.8 Hz, 4H), 6.6 (br, 6H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=34.9 (2C), 38.3 (2C), 173.1 (2C) ppm. – IR (neat): ν=2729 m, 1383 m, 938 m, 723 s cm–1.
Bis(imidazolium) 5,5′-azotetrazolate hydrate (6) Imidazole (406 mg, 5.96 mmol) and 5,5′-hydrazinebistetrazole (500 mg, 2.97 mmol) were dissolved in boiling H2O (10 mL). The mixture was stirred in air at ambient temperature for 48 h. The resulting suspension was taken to dryness under reduced pressure. The orange residue was crystallized from hot H2O to yield 665 mg (70%) of the product as yellow needles. Decomposition onset 172–173°C. – 1H NMR (300 MHz, [D6]DMSO): δ=7.61 (s, 4H), 8.91 (s, 2H), 9.8 (br, 4H) ppm. – 13C NMR (75 MHz, [D6]DMSO): δ=120.0 (4C), 134.7 (2C), 171.9 (2C) ppm. – IR (neat): ν=3114 w, 2643 w, 1393 m, 860 m, 783 m, 733 s, 634 s cm–1.
3.2 Crystal structure determinations
Single-crystal diffraction intensity data were recorded by ω scans with an Oxford Diffraction Gemini-R Ultra diffractometer (for 1, 2, 3, 5, and 6) or by ϕ- and ω-scans with a Nonius KappaCCD diffractometer (for 4) using MoKα or CuKα radiation (for 3). The crystal structures were solved by Direct Methods and refined by full-matrix least-squares techniques [43, 44]. All nonhydrogen atoms were assigned anisotropic displacement parameters in the refinement. The Flack parameter x of the noncentrosymmetric crystal structure of 4 was –0.02(8).
CCDC 1021720–1021725 (see Table 1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3.3 Powder X-ray diffraction
PXRD patterns were obtained using a X’Pert PRO diffractometer (PANalytical, Almelo, NL) equipped with a θ/θ-coupled goniometer in transmission geometry, programmable XYZ stage with well plate holder, CuKα1,2 radiation source with a focussing mirror, a 0.5° divergence slit and a 0.02° Soller slit collimator on the incident beam side, a 2-mm antiscattering slit and a 0.02° Soller slit collimator on the diffracted beam side and a 255 channel solid state PIXcel detector. The patterns were recorded at a tube voltage of 40 kV and a tube current of 40 mA, applying a step size of θ=0.007° with 80 s per step in the 2θ range between 2° and 40°. Pawley fits were performed with Topas Academic V5 . The background was modelled with Chebyshev polynomials, and the modified Thompson-Cox-Hastings pseudo-Voigt (TCHZ) function was used for peak shape fitting.
We gratefully acknowledge funding by the Österreichische Forschungsförderungsgesellschaft (FFG, project 841104) and Durst Phototechnik AG for additional funding. D. E. Braun gratefully acknowledges funding by the Hertha Firnberg Program of the Austrian Science Fund (FWF, project T593-N19).
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Published Online: 2015-01-10
Published in Print: 2015-02-01