Lithium bis(trimethylsilyl)amide

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Lithium bis(trimethylsilyl)amide

Monomer (does not exist)

Cyclic trimer
Names
Preferred IUPAC name
Lithium 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide
Other names
Lithium hexamethyldisilazide
Hexamethyldisilazane lithium salt
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.021.569 Edit this at Wikidata
UNII
  • InChI=1S/C6H18NSi2.Li/c1-8(2,3)7-9(4,5)6;/h1-6H3;/q-1;+1
  • ionic monomer: C[Si](C)(C)[N-][Si](C)(C)C.[Li+]
  • cyclic trimer: C[Si](C)(C)[N+]0([Si](C)(C)C)[Li-][N+]([Si](C)(C)C)([Si](C)(C)C)[Li-][N+]([Si](C)(C)C)([Si](C)(C)C)[Li-]0
Properties
LiN(Si(CH3)3)2
Molar mass 167.33 g·mol−1
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)
decomposes
Solubility Most aprotic solvents
THF, hexane, toluene
Acidity (pKa) 26
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 flammable, corrosive
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).
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Lithium bis(trimethylsilyl)amide is a lithiated

conjugate acid 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

n-butyllithium.[1] This reaction can be performed in situ.[2]

HN(Si(CH3)3)2 + C4H9Li → LiN(Si(CH3)3)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

organolithium compounds, including acetylides[3] or lithium enolates.[2]

where Me = CH3. 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.

An alternative synthesis of tetrasulfur tetranitride entails the use of S(N(Si(CH3)3)2)2 as a precursor with pre-formed S–N bonds. S(N(Si(CH3)3)2)2 is prepared by the reaction of lithium bis(trimethylsilyl)amide and sulfur dichloride (SCl2).

2 LiN(Si(CH3)3)2 + SCl2 → S(N(Si(CH3)3)2)2 + 2 LiCl

The S(N(Si(CH3)3)2)2 reacts with the combination of SCl2 and sulfuryl chloride (SO2Cl2) to form S4N4, trimethylsilyl chloride, and sulfur dioxide:[5]

2 S(N(Si(CH3)3)2)2 + 2 SCl2 + 2 SO2Cl2 → S4N4 + 8 (CH3)3SiCl + 2 SO2

As a ligand

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

MXn + n Li(HMDS) → M(HMDS)n + n LiX

where 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

coordination compounds.[6]

Niche uses

LiHMDS is volatile and has been discussed for use for atomic layer deposition of lithium compounds.[7]

Structure

Like many

oligomers predominate, including the trimer.[9] In the solid state structure is trimeric.[12]

TMEDA
THF
solvated dimer: [(LiHMDS)2(THF)2]
Trimer, solvent free: [(LiHMDS)3]

See also

References

  1. ISBN 978-0-470-13239-5. {{cite book}}: |journal= ignored (help
    )
  2. ^ a b Danheiser, R. L.; Miller, R. F.; Brisbois, R. G. (1990). "Detrifluoroacetylative Diazo Group Transfer: (E)-1-Diazo-4-phenyl-3-buten-2-one". Organic Syntheses. 73: 134; Collected Volumes, vol. 9, p. 197.
  3. ^
    PMID 16836294
    .
  4. doi:10.1021/jo00217a050
    .
  5. ISBN 9780471208259
    .
  6. ISBN 978-0-470-72184-1.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  7. doi:10.1149/2.052203jes
    .
  8. doi:10.1021/ja00144a012
    .
  9. ^
    doi:10.1021/ja953029p
    .
  10. PMID 27457218
    .
  11. ^ Neufeld, R.: DOSY External Calibration Curve Molecular Weight Determination as a Valuable Methodology in Characterizing Reactive Intermediates in Solution. In: eDiss, Georg-August-Universität Göttingen. 2016.
  12. doi:10.1016/S0022-328X(00)92291-5
    .
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