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Crystals 2012, 2(1), 127-136; doi:10.3390/cryst2010127

Synthesis and Crystal Structures of New 5,5'-Azotetrazolates
Gerhard Laus 1, Volker Kahlenberg 2, Klaus Wurst 1, Herwig Schottenberger 1, Niko Fischer 3, Jörg Stierstorfer 3 and Thomas M. Klapötke 3,*
Faculty of Chemistry and Pharmacy, University of Innsbruck, 6020 Innsbruck, Austria; Email:
Institute of Mineralogy and Petrography, University of Innsbruck, 6020 Innsbruck, Austria; Email:
Department of Chemistry, University of Munich (LMU), 81377 Munich, Germany; Email:
Author to whom correspondence should be addressed; Email: Tel.: +49-89-2180-77491; Fax: +49-89-2180-77492 (T.M.K.).
Received: 27 December 2011; in revised form: 21 February 2012 / Accepted: 29 February 2012 / Published: 15 March 2012


: Five new 5,5'-azotetrazolate salts (amminsilver, trimethylsulfonium, tetramethyl-phosphonium, trimethylsulfoxonium, 2-(hydroxyethyl)trimethylammonium) were prepared and characterized. The crystal structures were determined by X-ray diffraction. Interactions between the ions are identified and discussed. The sensitivities of the highly energetic silver salt were measured by BAM (Bundesanstalt für Materialforschung und-prüfung) methods.
azotetrazolate; cholin; phosphonium; silver; sulfonium; sulfoxonium

1. Introduction

Nitrogen-rich “energetic salts” have received considerable attention as propellants or gas generators [1,2]. These salts typically contain cations such as hydrazinium [3,4,5], ammonium and guanidinium [6], triazolium [7,8,9,10], tetrazolium [11,12], or tetraaminopiperazinium [13], sometimes involving additional azido groups [6,10]. Preferred anions are—beside azide, nitrate, perchlorate and picrate—dinitramide [7], nitroazolates [7,10,14,15], dianions such as 5,5'-bis(tetrazolate) [16] and, in particular, 5,5'-azotetrazolate [3,4,5,6,11,12,16]. The synthesis of 5,5'-azotetrazolates was first reported by Thiele [17]. Beyond the much acclaimed use as explosives, nitrogen-rich heterocycles are also of interest as ligands in coordination chemistry. With respect to their performance as potential explosives or propellants powerful materials are obtained when nitrogen-rich cations (e.g., hydrazinium, guanidinium) are used. When ions with lower nitrogen and higher carbon content are employed, the resulting less energetic materials are still relevant. Due to their electrochemical or optical properties, technical applications such as molecular electronics are envisioned [18]. They also have potential as precursors of functional materials, for example in the synthesis of low-density, nanoporous metal foams [19].

Numerous crystal structures of 5,5'-azotetrazolate salts [3,4,5,6,7,8,11,12,13,16,20,21] have been reported, including a series of metal salts; specifically, alkali and alkaline earth metals [22,23], lanthanoids [24,25,26], Pb [27], Tl [28], Mn [29], Fe [30], Cu and Cd [31] form crystalline salts.

In the present work 5,5'-azotetrazolate salts comprising cations based on sulfur or phosphorus are reported. A not yet described salt of choline and a new energetic silver complex are also disclosed, the latter one showing high sensitivities belonging to the class of primary explosives.

2. Results and Discussion

The silver complex was prepared by slow diffusion of the components. The incorporated ammonia molecule has a phlegmatizing effect causing the product to be less sensitive than the pure silver salt. The other salts were synthesized by two metathetical steps (the Ag2SO4/Ba azotetrazolate method) starting from the respective halogenides. Satisfactory crystals could be obtained with little effort by slow evaporation of solutions in water or methanol. The structures reported herein are centrosymmetric. In all cases the asymmetric unit contained one half of the planar azotetrazolate ion which is completed by inversion. The crystallographic data and structure refinement parameters of all structures 15 are gathered in Table 1.

Table 1. Crystal data and structure refinement details for compounds 15.
Table 1. Crystal data and structure refinement details for compounds 15.
CCDC no.846911846912846913846914846915
Chemical formulaAg2(NH3)2 (C2N10)(C3H9S)2 (C2N10)(C4H12P)2·(C2N10)(C3H9OS)2·(C2N10)(C5H14NO)2·(C2N10)
Crystal shape, colorplate, orangeprism, yellowplate, orangefragment, yellowfragment, yellow
Crystal size/mm30.1 × 0.1 × 0.060.44 × 0.36 × 0.240.36 × 0.32 × 0.120.2 × 0.2 × 0.20.40 × 0.24 × 0.24
Crystal systemmonoclinictriclinicmonoclinicmonoclinictriclinic
Space groupC2/cP Crystals 02 00127 i001P21/cP21/nP Crystals 02 00127 i001
V3967.4(6)358.88(7)892.4 (2)775.34 (5)461.53(6)
Dx/g cm–32.841.471.291.501.34
DiffractometerNonius KappaCCDGemini UltraGemini-R UltraGemini-R UltraGemini-R Ultra
Data collection methodφand ω scansω scansω scansΩ scansω scans
h, k, l range–19 ≤ h ≤ 20–6 ≤ h ≤ 7–5 ≤ h ≤ 7–6 ≤ h ≤ 6–6 ≤ h ≤ 5
–4 ≤ k ≤ 3–8 ≤ k ≤ 5–14 ≤ k ≤ 16–18 ≤ k ≤ 17–10 ≤ k ≤ 10
–16 ≤ l ≤ 17–10 ≤ l ≤ 11–15 ≤ l ≤ 13–13 ≤ l ≤ 13–12 ≤ l ≤ 10
Absorption correctionnonemulti-scanmulti-scanmulti-scannone
Measured reflections19272168362663323070
Independent reflections (Rint)739 (0.028)1288 (0.024)1760 (0.021)1723 (0.021)1686 (0.030)
Observed reflections [I≥ 2σ(I)]6201162146115731449
Restraints / parameters0/750/940/1040/1030/123
R1/wR2[I 2σ(I)]0.037/0.0970.030/0.0700.030/0.0790.028/0.0700.035/0.095
R1/wR2 (all data)0.046/0.1040.035/ 0.0730.039/0.0820.031/0.0720.042/0.098
Goodness of fit1.
Δρmax/min/e Å–31.38/–0.810.24/–0.250.27/–0.240.34/0.310.19/–0.17

2.1. Bis(amminsilver(I)) 5,5'-Azotetrazolate (1)

The silver ion in compound 1 coordinates to two nitrogen atoms of two azotetrazolate anions and to the ammonia molecule. Short contacts observed are Ag...N1 (2.255(5) Å), Ag...N2i (2.342(5) Å), and Ag...N6 (2.205(6) Å), respectively (Figure 1). Symmetry operation i: 1/2 – x, 1/2 + y, 1/2 – z. These interionic contacts assemble a layer structure parallel to the bc-plane.

Figure 1. Packing diagram of Ag salt 1. Dashed lines represent the Ag...N interactions. Layers are arranged parallel to the bc-plane.
Figure 1. Packing diagram of Ag salt 1. Dashed lines represent the Ag...N interactions. Layers are arranged parallel to the bc-plane.
Crystals 02 00127 g001 1024

2.2. Bis(trimethylsulfonium) 5,5'-Azotetrazolate (2)

In contrast, no directional interactions are found in salt 2. This aggregate consists of discrete ions and is predominantly stabilized by electrostatic forces. The (CH3)3S+ cation is pyramidal and exhibits approximately 3m symmetry as found in other trimethylsulfonium salts in the literature [32]. The C–S bond lengths are equal within the experimental error (1.782(2) Å), and the C–S–C angles differ only slightly (100.3, 101.6 and 102.4°). As discussed previously [33], the electron lone pair is not a structure-determining factor. The shortest distance between neighbouring S atoms is 3.742 Å. The unit cell is shown in Figure 2.

Figure 2. Packing diagram of trimethylsulfonium salt 2.
Figure 2. Packing diagram of trimethylsulfonium salt 2.
Crystals 02 00127 g002 1024

2.3. Bis(tetramethylphosphonium) 5,5'-Azotetrazolate (3)

The (CH3)4P+ cation in 3 shows only small deviations from ideal tetrahedral geometry. The C–P bond lengths range from 1.772 to 1.777 Å, and the C–P–C angles from 107.9 to 110.4°. The shortest distance between neighbouring P atoms is 5.736 Å. The crystal packing is shown in Figure 3a. There is a number of weak C–H...N contacts in this structure with three of them being considerably shorter than the sum of van der Waals radii. Thus, C4–H...N5 (d(H...A) = 2.494 and d(D...A) = 3.404 Å, <(D–H...A) = 154.4°), C5–H...N1 (2.566 and 3.540 Å, 172.4°), and C5–H...N5ii (2.517 and 3.403 Å, 150.2°) are the major interactions (Figure 3b). Symmetry operation ii: –1 + x, y, z.

Figure 3. (a) Packing diagram of tetramethylphosphonium salt 3. (b) Dashed lines represent the C–H...N contacts. Only N atoms engaged in major interactions are numbered.
Figure 3. (a) Packing diagram of tetramethylphosphonium salt 3. (b) Dashed lines represent the C–H...N contacts. Only N atoms engaged in major interactions are numbered.
Crystals 02 00127 g003 1024

2.4. Bis(trimethylsulfoxonium) 5,5'-Azotetrazolate (4)

The (CH3)3SO cation in 4 again adopts a pyramidal geometry. It has neither a mirror plane nor a rotation axis. Nevertheless, the 3m symmetry is approximately fulfilled in good agreement with known trimethylsulfoxonium salts [34]. The shortest S–S distance is 5.245 Å. The packing is presented in Figure 4a. Probably due to the enhanced dipolar character of this cation, it participates in a series of significant C–H...N interactions with the anion. All nitrogen atoms of the anion serve as acceptors building a three-dimensional network (Figure 4b): C3–H...N4 (2.477 and 3.289 Å, 140.0°), C2–H...N5iii (2.411 and 3.352 Å, 160.8°), C4–H...N1iv (2.634 and 3.407 Å, 136.1°), C2–H...N3v (2.497 and 3.428 Å, 158.6°), C4–H...N2v (2.563 and 3.499 Å, 159.8°), C3–H...N1vi (2.435 and 3.334 Å, 152.3°), C4–H...N2vi (2.446 and 3.320 Å, 148.4°). Symmetry codes iii: 1 – x, 1 – y, 1 – z; iv: 1/2 – x, -1/2 + y, 1/2 – z; v: –1/2 + x, 1/2 – y,1/2 + z; vi: 3/2 – x, –1/2 + y, 1/2 – z.

Figure 4. (a) Packing diagram of trimethylsulfoxonium salt 4. (b) Dashed lines represent the short C–H...N contacts (for symmetry operators see text).
Figure 4. (a) Packing diagram of trimethylsulfoxonium salt 4. (b) Dashed lines represent the short C–H...N contacts (for symmetry operators see text).
Crystals 02 00127 g004 1024

2.5. Bis(2-(hydroxyethyl)trimethylammonium) 5,5'-azotetrazolate (5)

In the structure of 5, a strong hydrogen bond with the parameters O1–H...N1 (1.984 and 2.791 Å, 160.8°) between the cation and the dianion is observed. Figure 5 shows the unit cell of 5.

Figure 5. Packing diagram of cholinium salt 5. Dashed lines represent the strong inter­molecular O–H···N hydrogen bonds.
Figure 5. Packing diagram of cholinium salt 5. Dashed lines represent the strong inter­molecular O–H···N hydrogen bonds.
Crystals 02 00127 g005 1024

3. Experimental Section

The starting materials were obtained from Sigma-Aldrich and used as received. The NMR spectra were recorded with a Bruker AC 300 spectrometer. IR spectra were obtained with a Nicolet 5700 FT instrument in ATR mode. The impact and friction sensitivity of silver salt 1 was determined by the BAM drophammer (method 1 of 6) [35,36,37] and BAM friction tester [35,36,37] respectively. The sensitivity towards electrostatic discharge was measured using an OZM small scale electrostatic discharge device [38]. X-Ray diffraction data were collected on Oxford Diffraction Gemini-R Ultra and Nonius Kappa CCD diffractometers using Mo-Kα radiation. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 [39,40]. CCDC 846911-846915 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

3.1. Bis(amminsilver(I)) 5,5'-Azotetrazolate (1)

Caution: this compound is a primary explosive with extremely high sensitivities towards friction and also electrostatic discharge when dry. Proper protective measures (safety glasses, face shield, leather coat, earthened equipment and shoes, Kevlar® gloves and ear plugs) should be used during the handling of this compound. Concentrated NH3 (2 mL) was layered over a solution of AgNO3 (51 mg, 0.30 mmol) in H2O (2 mL) and concentrated NH3 (1 mL). A solution of sodium azotetrazolate pentahydrate (45 mg, 0.15 mmol) in H2O (5 mL) and concentrated NH3 (1 mL) was added cautiously on top of the mixture. The mixture was set aside, and orange-red crystals grew overnight at 20 °C. Yield: 10 mg (16%). No melting below 230 °C (dec). No NMR spectra could be recorded due to insolubility in common solvents. IR (neat, cm1): 1396 (s), 1214 (w), 1184 (m), 1168 (m), 1049 (w), 1032 (w), 765 (m), 737 (s), 719 (m). BAM drophammer: 2 J. BAM friction tester (<5 N). ESD: 5 mJ.

3.2. Preparation of 5,5'-Azotetrazolates (2–5) (General Procedure)

Ag2SO4 (78 mg, 0.25 mmol) was added to a solution of the respective organic halide (0.50 mmol) in H2O (10 mL). The mixture was stirred at 50 °C for 10 min and ultrasonicated for 5 min. Subsequently, the precipitate was removed by centrifugation. Barium azotetrazolate pentahydrate (94 mg, 0.24 mmol) was added to the supernatant, and the mixture was again stirred at 50 °C for 10 min and ultrasonicated for 5 min. After centrifugation, the supernatant solution was brought to dryness in a rotary evaporator, the temperature not exceeding 50 °C. The yellow residue was recrystallized from MeOH, collected by filtration and vacuum-dried.

3.3. Bis(trimethylsulfonium) 5,5'-Azotetrazolate (2)

Yield: 61 mg (80%), m.p. 155 °C (dec.). 1H NMR (DMSO-d6, 300 MHz): δ 2.90 (s). 13C NMR (DMSO-d6, 75 MHz): δ 26.2 (3C), 173.3. IR (neat): v 2992 (m), 2976 (m), 1417 (w), 1401 (s), 1196 (w), 1163 (w), 1039 (s), 773 (m), 739 (s), 733 (s) cm–1.

3.4. Bis(tetramethylphosphonium) 5,5'-Azotetrazolate (3)

Yield: 70 mg (84%), m.p. 215 °C (dec.). 1H NMR (DMSO-d6, 300 MHz, ppm): δ 1.85 (d, JH–P = 15.4 Hz). 13C NMR (DMSO-d6, 75 MHz, ppm): δ 8.9 (d, JC–P = 55 Hz, 4C), 173.4. IR (neat, cm1): v 2985 (m), 2915 (w), 1434 (m), 1374 (m), 1289 (m), 1174 (w), 1149 (w), 1015 (m), 973 (s), 772 (m), 725 (m).

3.5. Bis(trimethylsulfoxonium) 5,5'-Azotetrazolate (4)

Yield: 75 mg (89%), m.p. 192–193 °C (dec.). 1H NMR (DMSO-d6, 300 MHz, ppm): δ 3.92 (s). 13C NMR (DMSO-d6, 75 MHz, ppm): δ 39.2 (3C), 173.2. IR (neat, cm–1): v 2959 (s), 2878 (m), 1403 (s), 1225 (s), 1036 (s), 950 (s), 756 (m), 741 (m).

3.6. Bis(2-(hydroxyethyl)trimethylammonium) 5,5'-Azotetrazolate (5)

Yield: 65 mg (73%), m.p. 126–129 °C. 1H NMR (DMSO-d6, 300 MHz, ppm): δ 3.15 (s, 9H), 3.45 (m, 2H), 3.82 (m, 2H), 5.76 (s, 1H). 13C NMR (DMSO-d6, 75 MHz, ppm): δ 53.2 (3C), 55.1, 66.9, 173.3. IR (neat, cm–1): v 3177 (w), 3028 (w), 2902 (w), 1471 (m), 1384 (s), 1338 (w), 1179 (w), 1146 (w), 1095 (s), 1028 (w), 950 (s), 875 (m), 722 (m).

4. Conclusions

A large number of crystalline azotetrazolates is known today and can also be found in a recently published review article [41]. This versatile dianion grants access to an almost unlimited diversity of intriguing structures. Certainly, the five new salts presented in this work will be succeeded by others in due course, stimulating both materials science and crystallography. Silver salt 1 was characterized to be a thermally stable primary explosive which detonates in flame. The sensitivities towards outer stimuli are in the range of those observed for lead azide.

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