Synthesis and Spectroscopic Characterization of Beryllium Azide and Two Derivatives

SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF BERYLLIUM AZIDE AND TWO DERIVATIVES

Thomas M. Klapötke* and Thomas Schütt

Institut für Anorganische Chemie der Universität München, Butenandtstr. 5-13 (Haus D), D-81377 München, Germany

Abstract Beryllium azide (1) has been synthesized by the reaction of beryllium chloride with neat trimethylsilyl azide. Be(THF)2(N3)2 (2) and Be(py)2(N3)2 (3) (py = pyridine) have been prepared by reacting beryllium chloride with trimethylsilyl azide in the respective solvent. These compounds have been characterized by Raman, IR and multinuclear NMR spectroscopy. Their decomposition has also been investigated. 1 probably forms endless chains, where the beryllium atom is tetrahedrally coordinated through end-on, bridging azide ligands. The empirical formula is Be(N3)2. 2 and 3 have a monomeric structure. All prepared compounds form structures with the highest possible coordination number of 4 for the beryllium-atom.

Introduction In contrast to the chemistry of halogen azides and heavy-metal azides, which has been extensively explored in the last years,12 studies on beryllium azide compounds are still very limited. Beryllium azide was first synthesized by Curtius et al.3 by reacting barium azide with beryllium sulfate in water. The water could not be removed without destroying the product. E. Wiberg et al* prepared the compound by reacting dimethylberyllium and HN3 in absolute etheric solution. The compound was identified by microanalysis, but no spectroscopic investigations were undertaken. Beryllium azide has been synthesized in a different way and characterized by IR spectroscopy by 5 N. Wiberg et a/ . Furthermore the compound BeCI(N3)(Et20) has been investigated by IR and Raman spectroscopy. In our studies we have synthesized beryllium azide, according to equation (1), by a modified literature synthesis5. We have carried out IR, Raman, and 14N NMR spectroscopic investigations on this product. We also report on the synthesis and IR, Raman and multinuclear NMR spetra of Be(THF)2(N3)2 (2) and Be(py)2(N3)2 (3).

„ neat, reflux _ , ... BeCI2 + 2 (CH3)3SiN3 ^ Be(N3)2 (1) - 2 (CH3)3SiCI

Materials and Methods CAUTION: Azides are potentially explosives. Safety equipment such as leather gloves and face shield is recommended. Materials: Beryllium chloride (Aldrich) and trimethylsilyl azide (Aldrich) were used as supplied. THF (Merck) was dried over sodium. Pyridine (Fluka) was dried over potassium hydroxide. Both solvents were distilled prior to use. All manipulations were routinely performed under an inert gas atmosphere (N2, dry box). Spectroscopy: Infrared spectra were recorded at 20°C, as Nujol mulls between KBr plates, or as KBr pellets on a Nicolet 520 FT-IR. Raman spectra were recorded on a Perkin Elmer 2000R NIR-FT. NMR-spectra were measured on a Jeol EX400 Delta (1H, 13C, chemical shifts refer to 14 = 0.00 according to the chemical shifts of residual solvent signals; N, external standard: 5CH NC,2 = 0.00). CHN-analyses were carried out in a Analysator Elementar Vario EL. Preparation of BefNJs· BeCI2 (0.66, 8.26 mmol) was suspended in trimethylsilyl azide (10 mL, 75.30 mmol). The suspension was heated under reflux for 6 days. The by-product trimethylsilyl chloride and the excess of trimethylsilyl azide were removed under dynamic vacuum. Colorless 1 remained. Yield: 0.767 g, 100%, m.p. 356°C (decomp.), Anal. Calc. for BeN6: N, 90.32. Found: N, 89.55. Preparation of Be(THF)2(N3)2\ BeCI2 (0.33, 4.13 mmol) and trimethylsilyl azide (2.0 mL, 15.1 mmol) were suspended in THF (10 mL, 125 mmol). The suspension was heated under reflux for 6 days. After evaporation, a colorless powder (2) remained, which was dried for 20 hours in dynamic vacuum. Yield: 0.980 g, 100%, m.p. 112°C, Anal. Calc. for C10H10BeN8: C, 47.80; H, 4.01; N, 44.60. Found: C, 47.16; H, 3.74; N, 45.36.

357 Vol. 22, No. 6, 1999 Synthesis and Spectroscopic Characterization of Beryllium Azide and Two Derivatives

Preparation of Be(py)2(N3)2: BeCI2 (0.33, 4.13 mmol) and trimethylsilyl azide (2.0 mL, 15.1 mmol) were suspended in Pyridine (8 mL, 99 mmol). The suspension was heated under reflux for 6 days. After evaporation, a pale yellow powder (3) remained, which was dried for 17 hours in dynamic vacuum. Yield: 1.038 g, 100%, m.p. 139°C, Anal. Calc. for C8H16BeN602: C, 40.50; H, 6.80; N, 35.42. Found: C, 41.22; H, 6.92; N, 34.82.

Results and Discussion Be(N3)2 In our investigations, beryllium azide was synthesized according to equation (1) by a modified literature synthesis5. Beryllium azide is a colorless powder that decomposes at a temperature of 356°C. The IR spectrum of beryllium azide is well known5. The identity of 1 was therefore confirmed by IR spectroscopy. 1 has been further characterized by Raman and 14N NMR spectroscopy. In the IR spectrum the antisymmetric stretching vibration of the azide group appears at 2147 cm"1 (2128 cm"1)5. The symmetric stretching vibration of the azide group shows a signal at 1263 cm"1 (1255 cm-1)5. The strong intensity of the symmetric stretching vibration that is IR forbidden for the azide ion, points to a covalent beryllium-azide bond6. The Raman spectrum show the antisymmetric stretching vibration of the azide group at 2127 cm"1 and the symmetric stretching vibration of the azide group can be seen at 1273 cm"1. Further signals can not be assigned. The fact that there is only one signal for the antisymmetric stretching vibration of the azide group suggests that the beryllium atom is surrounded by chemically equivalent azide groups, see figurel. Beryllium azide is insoluble in methylene chloride and other non-basic solvents. This property indicates a polymeric structure. Compound 1 probably crystallizes in chains, in which the beryllium atom is surrounded in tetrahedral fashion by bridging azide groups (figure 1).

Νγ Ν

Νβ Ν

Ν Ν

Fig. 1: Structure model of 1.

Beryllium azide is soluble in basic solvents like pyridine, or THF. The 14N NMR spectrum of 1, measured in THF, shows the expected signals for covalently bonded azides. The nitrogen resonances are assigned as described in the literature7,8. The ß-N atom shows a resonance at δ = -135 ppm, the γ-Ν atom at δ = -181 ppm, and the a-N atom a very broad resonance at δ = -327 ppm. The solution of 1 in THF shows the same 14N NMR resonances as measured for 2. Probably THF coordinates at the beryllium center to form 2.

Be(THF)2(N3)2 To prove the supposition that beryllium azide in THF forms 2 with a beryllium-oxygen bond, we tried to synthesize beryllium azide bis(tetrahydrofuran). Should comparable chemical shifts be found for beryllium azide in THF and beryllium azide bis(tetrahydrofuran) in the 14N NMR spectrum, the supposition mentioned above should be proven.

358 Thomas Μ. Klapotke and Thomas Schutt Main Group Metal Chemistry

Beryllium azide bis(tetrahydrofuran), 2, was synthesized by the reaction of beryllium chloride with trimethylsilyl azide in THF according to equation 2. THF, reflux BeCI, 2 (CH3)3SiN3 Be(THF)2(N3)2 (2) 2 (CH3)3SiCI This colorless hygroscopic product 2 (m.p.: 112°C ) was dried for 20 hours under dynamic vacuum. The solvent remained coordinated. Its IR spectrum shows two bands at 3181 cm"1 and 2990 cm-1 that can be assigned to the C- H stretching vibrations of the coordinated THF. The antisymmetric stretching vibration of the azide group can be seen at 2150 cm"1. The IR spectrum does not provide any further knowledge about the symmetric stretching vibration of the azide group, the azide deformation vibration and the Be-N stretching vibration. The signals at 1017 cm"1, 872 cm"1 and 658 cm"1 can be assigned to the coordinated THF. The symmetric stretching vibration could only be seen in the Raman spectrum at 1259 cm"1. 1 The H NMR spectrum of compound 2 measured in acetone-d6 shows the expected signals at δ = 3.64 ppm (Ha) and δ = 1.79 ppm (Hb) (see figure 2 for the labeling of the hydrogen and 13 carbon atoms). In the C NMR spectrum, two signals can be seen at δ = 68.1 ppm (Ca) and δ = 14 26.1 ppm (Cb). The N NMR spectrum of compound 2 shows the expected three signals for a covalently bonded azide at a chemical shift of δ = -135 ppm for the β-Ν atom, δ = -180 ppm for the γ-Ν atom and δ = -326 ppm for the α-bonded N-atom. These resonances are narrow and strong for the β- and γ-Ν atoms and very broad for the α-N. The chemical shifts of compound 2 are identical to chemical shifts of 1 in THF showing that 1, when dissolved in THF, becomes 2. The fact that there is only one signal for the antisymmetric stretching vibration of the azide group for compound 2 too, points to chemical equivalent azide groups. From the elemental analysis and the results of the spectroscopic investigations, a monomeric structure (figure 2), is proposed for compound 2, in which the beryllium atom is tetrahedrally coordinated to two azide groups and two THF molecules.

Fig. 2: Structure model of 2.

Be(py)2(N3)2 Beryllium azide bis(pyridine), 3, has been synthesized by reacting beryllium chloride and trimethylsilyl azide in pyridine under reflux for six days according to equation 3.

pyridine, reflux BeCI, 2 (CH3)3SiN3 Be(py)2(N3)2 (3) - 2 (CH3)3SiCI This pale very hygroscopic yellow compound 3 (m.p.: 139°C ) could not be separated from the pyridine ligands, even after drying for 17 hours under dynamic vacuum. The IR spectrum of 3 shows a very weak signal at 3078 cm"1 for the aromatic stretching vibration of the C-H bond. At 2137 cm"1 a very strong signal appears which can be assigned to the antisymmetric stretching vibration of the azide group. Compound 3 probably contains equivalent azide groups, because only one signal is evident for this stretching mode. Signals for the symmetric stretching vibration, the deformation vibration of the azide group and for the Be-N

359 Vol. 22, No. 6, 1999 Synthesis and Spectroscopic Characterization of Beryllium Azide and Two Derivatives stretching vibration cannot be assigned. The symmetric stretching vibration of the azide group can only be seen in the Raman spectrum at 1373 cm"1. 1 The H NMR spectrum of compound 3 measured in acetone-d6 shows the expected signals for the aromatic protons at a chemical shift of δ = 8.70 ppm (Ha), δ = 8.24 ppm (Hc) and δ = 7.80 13 ppm (Hb). See figure 3 for the labeling of the hydrogen and carbon atoms. The C NMR spectrum shows the resonances for the coordinated pyridine rings at chemical shifts of δ = 148.2 ppm (Ca), δ = 142.1 ppm (Cc) and δ = 126.8 ppm (Cb). The covalent character of the azide groups can be proved in the 14N NMR spectrum. The strong signal at δ = -135 ppm is due to the β-Ν atom of the azide group. The γ-Ν atom shows a resonance at δ = -245 ppm. The a-N atom shows a broad resonance at δ = -316 ppm, because of the coupling with the beryllium atom. The resonance of the coordinated pyridine rings can be seen at δ = -142 ppm. The elemental analysis and the high solubility indicate that compound 3 is monomeric. This proposal is further confirmed by the results of the vibrational and NMR spectroscopic investigations. Compound 3 must have equivalent azide groups, which are covalently bonded. 3 is probably surrounded in a tetrahedral fashion by two azide groups and two pyridine rings.

He C

Fig. 3: Structure model of 3.

Acknowledgments We gratefully acknowledge the support of the Fonds der Chemischen Industrie and the University of Munich. We thank Mrs. A. MacKenzie for her help with the manuscript.

References

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Received: April 1,1999 - Accepted: April 29,1999 - Accepted in revised camera-ready format: April 30,1999

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