Hexamethyldisilazide

Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula CH₃)₃Sisub>2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An electron diffraction study shows that silicon-nitrogen bond length (173.5 pm) and Si-N-Si bond angle (125.5°) to be similar to disilazane (in which methyl groups are replaced by hydrogen atoms) suggesting that steric factors are not a factor in regulating angles in this case. This colorless liquid is a reagent and a precursor to bases that are popular in organic synthesis and organometallic chemistry. Additionally, HMDS is also increasingly used as molecular precursor in chemical vapor deposition techniques to deposit silicon carbonitride thin films or coatings.

Synthesis and derivatives

Bis(trimethylsilyl)amine is synthesized by treatment of trimethylsilyl chloride with ammonia:
:2 (CH₃)₃SiCl + 3 NH₃ → CH₃)₃Sisub>2NH + 2 NH₄Cl
Ammonium nitrate together with triethylamine can be used instead. This method is also useful for ¹⁵N isotopic enrichment of HMDS.

Alkali metal bis(trimethylsilyl)amides result from the deprotonation of bis(trimethylsilyl)amine. For example, lithium bis(trimethylsilyl)amide (LiHMDS) is prepared using n-butyllithium:
: CH₃)₃Sisub>2NH + BuLi → CH₃)₃Sisub>2NLi + BuH
LiHMDS and other similar derivatives: sodium bis(trimethylsilyl)amide (NaHMDS) and potassium bis(trimethylsilyl)amide (KHMDS) are used as a non-nucleophilic bases in synthetic organic chemistry.

Use as reagent

Hexamethyldisilazane is employed as a reagent in many organic reactions:

1) HMDS is used as a reagent in condensation reactions of heterocyclic compounds such as in the microwave synthesis of a derivative of xanthine:

:

2) The HMDS mediated trimethylsilylation of alcohols, thiols, amines and amino acids as protective groups or for intermediary organosilicon compounds is found to be very efficient and replaced TMSCl reagent.

Silylation of glutamic acid with excess hexamethyldisilazane and catalytic TMSCl in either refluxing xylene or acetonitrile followed by dilution with alcohol (methanol or ethanol) yields the derived lactam pyroglutamic acid in good yield.

HMDS in the presence of catalytic iodine facilitates the silylation of alcohols in excellent yields.

3) HMDS can be used to silylate laboratory glassware and make it hydrophobic, or automobile glass, just as Rain-X does.

4) In gas chromatography, HMDS can be used to silylate OH groups of organic compounds to increase volatility, this way enabling GC-analysis of chemicals that are otherwise non-volatile.

Other uses

In photolithography, HMDS is often used as an adhesion promoter for photoresists. Best results are obtained by applying HMDS from the gas phase on heated substrates.

In electron microscopy, HMDS can be used as an alternative to critical point drying during sample preparation.

In pyrolysis-gas chromatography- mass spectrometry, HMDS is added to the analyte to create silylated diagnostic products during pyrolysis, in order to enhance detectability of compounds with polar functional groups.

In plasma-enhanced chemical vapor deposition (PECVD), HMDS is used as a molecular precursor as a replacement to highly flammable and corrosive gasses like SiH₄, CH₄, NH₃ as it can be easily handled. HMDS is used in conjunction with a plasma of various gases such as argon, helium and nitrogen to deposit SiCN thin films/coatings with excellent mechanical, optical and electronic properties.

See also

* Hexamethyldisiloxane
* Metal bis(trimethylsilyl)amides

References

{{Reflist

Amines
Trimethylsilyl compounds
Reagents for organic chemistry