Silicon tetraazide

Silicon tetraazide

Names
Other names Tetraazidosilane
Identifiers
CAS Number	27890-58-0
3D model (JSmol)	Interactive image
ChemSpider	35764491
PubChem CID	57461327
CompTox Dashboard (EPA)	DTXSID701029721
InChI InChI=1S/N12Si/c1-5-9-13(10-6-2,11-7-3)12-8-4 Key: SZJFGTWFLXTOHF-UHFFFAOYSA-N
SMILES [N-]=[N+]=N[Si](N=[N+]=[N-])(N=[N+]=[N-])N=[N+]=[N-]
Properties
Chemical formula	Si(N3)4
Molar mass	196.1659 g/mol
Appearance	White crystals
Melting point	212 °C (414 °F; 485 K)
Solubility in water	Reacts
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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 bicationic ligands Si(N₃)₄·L₂^[2] will result in relatively stable, crystalline solids that can be handled at room temperature.

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]

Properties

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]

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