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Silicon tetraazide
Chemical compound From Wikipedia, the free encyclopedia
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Silicon tetraazide is a thermally unstable binary compound of silicon and nitrogen with a nitrogen content of 85.7% (by molar mass). This high-energy compound combusts spontaneously and can only be studied in a solution.[1][2][3] A further coordination to a six-fold coordinated structure such as a hexaazidosilicate ion [Si(N3)6]2−[4] or as an adduct with bidentate ligands Si(N3)4·L2[2] will result in relatively stable, crystalline solids that can be handled at room temperature.
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Preparation
Silicon tetraazide is synthesized by conversion of silicon tetrachloride with sodium azide in benzene.[1][3]
The reaction of silicon tetrachloride with an excess of sodium azide at room temperature in acetonitrile will result in the formation of sodium hexaazidosilicate (Na2[Si(N3)6]) which by adding ligands such as 2,2′-bipyridine and 1,10-phenanthroline will result in stable silicon tetraazide adducts.[2] Other bases such as pyridine and tetramethylethylenediamine will not react with the hexaazidosilicate ion.[2]
Another preparation of a bis(triphenylphosphine)iminium hexaazidosilicate salt [(Ph3P)2N]2[Si(N3)6] is possible by conversion of bis(triphenylphosphine)iminium azide [(Ph3P)2N]N3 with silicon tetrachloride in acetonitrile, where Ph is phenyl.[4]
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Properties
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Perspective
Silicon tetraazide is a white crystalline compound that will detonate at even 0 °C.[1] The pure compound, and also silicon chloride triazide SiCl(N3)3 and silicon dichloride diazide SiCl2(N3)2 contaminated samples, can detonate spontaneously without clear cause.[5] The compound is susceptible to hydrolysis.[3] It is soluble in diethylether and benzene.[1]
The addition compound with 2,2′-bipyridine is much more stable. A melting point of 212 °C with a melting enthalpy of 110 J/g is recorded. The DSC measurement shows at 265 °C a sharp exothermic reaction with an enthalpy of −2400 J/g. Similar results are found for the addition compound with 1,10-phenanthroline. As the hemiacetonitrile solvatated isolated compound expels solvent at 100 °C, and shows then in the DSC measurement from 240 °C onwards a strong exothermic reaction with a generated heat of 2300 J/g.[2] The enthalpies are higher than that of sodium azide with −800 J/g,[6] but still lower than the values encountered with classic explosives such as RDX with −4500 J/g.[2] The addition compounds are stable in solution. It can be concluded from IR-spectroscopy and proton NMR data that no dissociation occurs in silicon tetraazide and 2,2'-bipyridine or for example 1,10-phenanthroline.[2] The bis(triphenylphosphino)iminium hexaazidosilicate salt [(Ph3P)2N]2[Si(N3)6] on the other hand is relatively stable. The compound melts at 214 °C and shows in the DSC measurement at 250 °C a reaction.[4] One mass spectrometry coupled thermogravimetric analysis investigation indicated as reaction products nitrogen, silicon tetraazide and hydrazoic acid.[4]
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Applications
A practical application of free silicon tetraazide is unlikely due to the high instability. In solution the compound has potential uses as raw material for nitrogen-rich materials.[2] One application as reagent in the manufacture of polyolefins has been patented.[7] The stabilized adducts can serve as energetic compounds as a replacement for lead azide.[2]
References
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