Silicon tetraazide

Silicon tetraazide

Identifiers
InChI InChI=1S/N12Si/c1-5-9-13(10-6-2,11-7-3)12-8-4 Key: SZJFGTWFLXTOHF-UHFFFAOYSA-N
Jmol-3D images	Image
SMILES [N-]=[N+]=N[Si](N=[N+]=[N-])(N=[N+]=[N-])N=[N+]=[N-]
Properties
Chemical formula	SiN 12
Molar mass	196.1659 g mol−1
Appearance	White crystals
Melting point	212 °C (414 °F; 485 K)
Solubility in water	Reacts
Except where noted otherwise, data is 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%. 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 like a hexaazido silicide [Si(N₃)₆]^{2− [4]} or as an adduct with bicationic ligands Si(N₃)₄L₂ ^[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 hexaazidosilicide 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 hexaazido silicide ion.^[2]

Another preparation of a bis(triphenylphosphino)iminiumhexaazido silicide salt ((PPN)₂Si(N₃)₆, [Ph₃P=NPPh₃][Si(N₃)₆]) is possible by conversion of bis(triphenylphosphino)iminium azide (PPNN₃, [Ph₃P=NPPh₃]⁺N₃⁻) with silicon tetrachloride in acetonitrile.^[4]

Properties

Silicon tetraazide is a white crystalline compound that will detonate at even 0 °C.^[1] The pure compound but also silicon chloride triazide and silicon dichloride diazide contaminated samples can without clear cause detonate spontaneously ^[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⁻¹ is recorded. The DSC measurement shows at 265 °C a sharp exothermic reaction with an enthalpy of −2400 J·g⁻¹. Similar results are found for the addition compound with 1,10-phenanthroline. The as 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⁻¹.^[2] The enthalpies are higher than that of sodium azide with −800 J·g⁻¹,^[6] but still lower than the values encountered with classic explosives such as RDX with −4500 J·g⁻¹.^[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)iminiumhexaazidosilicate salt ((PPN)₂Si(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