Lithium bis(trimethylsilyl)amide

Lithium bis(trimethylsilyl)amide

200px Monomer (does not exist)
200px Cyclic trimer
200px
Names
Other names lithium hexamethyldisilazide Hexamethyldisilazane lithium salt
Identifiers
CAS Number	4039-32-1 Y
ChemSpider	21170111 N
Jmol 3D model	Interactive image
PubChem	2733832
InChI InChI=1S/C6H18NSi2.Li/c1-8(2,3)7-9(4,5)6;/h1-6H3;/q-1;+1
SMILES C[Si](C)(C)[N-][Si](C)(C)C.[Li+]
Properties
Chemical formula	C6H18LiNSi2
Molar mass	167.326 g/mol
Appearance	White solid
Density	0.86 g/cm3 at 25 °C
Melting point	71 to 72 °C (160 to 162 °F; 344 to 345 K)
Boiling point	80 to 84 °C (176 to 183 °F; 353 to 357 K) (0.001 mm Hg)
Solubility in water	decomposes
Solubility	Most aprotic solvents THF, hexane, toluene
Acidity (pKa)	26
Vapor pressure	{{{value}}}
Related compounds
Related compounds	Sodium bis(trimethylsilyl)amide Potassium bis(trimethylsilyl)amide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula LiN(SiMe₃)₂. It is commonly abbreviated as LiHMDS (Lithium HexaMethylDiSilazide - a reference to its starting material HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species.

Preparation

LiHMDS is commercially available, but it can also be prepared by the deprotonation of bis(trimethylsilyl)amine with n-butyllithium. This reaction can be performed in situ.

HN(SiMe₃)₂ + C₄H₉Li → LiN(SiMe₃)₂ + C₄H₁₀

Once formed, the compound can be purified by sublimation or distillation.

Reactions and applications

As a base

LiHMDS is often used in organic chemistry as a strong non-nucleophilic base. It has a pKa of ~26 making it is less basic that other lithium bases, such as LDA (pKa ~36), but it is more sterically hindered and hence less nucleophilic. It can be used to form various organolithium compounds including acetylides, or lithium enolates.

As such it finds use in a range of coupling reactions; particularly carbon-carbon bond forming reactions such as the Fráter–Seebach alkylation and mixed Claisen condensations.

As a ligand

LiHMDS can react with a wide range of metal halides, via a salt metathesis reaction, to give metal bis(trimethylsilyl)amides.

MX_x + x Li(hmds) → M(hmds)_x + x LiX

(X = Cl, Br, I and sometimes F)

Metal bis(trimethylsilyl)amide complexes are lipophilic due to the ligand and hence are soluble in a range of nonpolar organic solvents, this often makes them more reactive than the corresponding metal halides, which can be difficult to solubilise. The steric bulk of the ligands causes their complexes to be discrete and monomeric; further increasing their reactivity. Having a built-in base, these compounds conveniently react with protic ligand precursors to give other metal complexes and hence are important precursors to more complex coordination compounds.

Niche uses

LiHMDS is volatile and has been discussed for use for atomic layer deposition of lithium compounds.

Structure

Like many organolithium reagents, lithium bis(trimethylsilyl)amide can form aggregates in solution. The extent of aggregation depends on the solvent. In coordinating solvents such as ethers and amines the so-called monomer and dimer are prevalent. In the monomeric and dimeric state, one or two solvent molecules bind to lithium centers. In noncoordinating solvents, such as aromatics or pentane, the complex oligomers predominate, including the trimer. In the solid state lithium bis(trimethylsilyl)amide is trimeric.

750px
LiHMDS adduct with TMEDA.		200px THF solvated dimer: (LiHMDS)2•THF2		200px Trimer, solvent free: (LiHMDS)3

See also

• Lithium amide
• Lithium diisopropylamide
• Lithium tetramethylpiperidide