 Monomer (does not exist) | |
amide_Structural_formula_V1.svg) Cyclic trimer | |
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| Names | |
|---|---|
| Preferred IUPAC name Lithium 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide | |
| Other names Lithium hexamethyldisilazide Hexamethyldisilazane lithium salt | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider |
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| ECHA InfoCard | 100.021.569 |
| UNII | |
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| 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 theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Lithium bis(trimethylsilyl)amide is a lithiatedorganosilicon compound with the formulaLiN(Si(CH3)3)2. It is commonly abbreviated asLiHMDS orLi(HMDS) (lithiumhexamethyldisilazide - a reference to itsconjugate acidHMDS) and is primarily used as a strongnon-nucleophilic base and as aligand. Like many lithium reagents, it has a tendency to aggregate and will form acyclictrimer in the absence of coordinating species.
LiHMDS is commercially available, but it can also be prepared by the deprotonation ofbis(trimethylsilyl)amine withn-butyllithium.[1] This reaction can be performedin situ.[2]
Once formed, the compound can be purified bysublimation ordistillation.
LiHMDS is often used in organic chemistry as a strongnon-nucleophilic base.[3] Its conjugate acid has apKa of ~26,[4] making it is less basic than other lithium bases, such asLDA (pKa of conjugate acid ~36). It is relatively moresterically hindered and hence lessnucleophilic than other lithium bases. It can be used to form variousorganolithium compounds, includingacetylides[3] or lithiumenolates.[2]
where Me =CH3. As such, it finds use in a range of coupling reactions, particularly carbon-carbon bond forming reactions such as theFráter–Seebach alkylation and mixedClaisen condensations.
An alternative synthesis oftetrasulfur tetranitride entails the use ofS(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 andsulfur dichloride (SCl2).
TheS(N(Si(CH3)3)2)2 reacts with the combination ofSCl2 andsulfuryl chloride (SO2Cl2) to formS4N4,trimethylsilyl chloride, andsulfur dioxide:[5]
Li(HMDS) can react with a wide range ofmetal halides, by asalt metathesis reaction, to givemetal bis(trimethylsilyl)amides.
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 ofnonpolar organic solvents, this often makes them more reactive than the corresponding metal halides, which can be difficult to solubilise. Thesteric 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 complexcoordination compounds.[6]
LiHMDS is volatile and has been discussed for use foratomic layer deposition of lithium compounds.[7]
Like manyorganolithium reagents, lithium bis(trimethylsilyl)amide can form aggregates in solution. The extent of aggregation depends on the solvent. In coordinating solvents, such asethers[8] andamines,[9] themonomer anddimer are prevalent. In the monomeric and dimeric state, one or two solvent molecules bind to lithium centers. With ammonia as donor base lithium bis(trimethylsilyl)amide forms a trisolvated monomer that is stabilized by intermolecular hydrogen bonds.[10][11] In noncoordinating solvents, such asaromatics orpentane, the complexoligomers predominate, including the trimer.[9] In the solid state structure is trimeric.[12]
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 LiHMDS adduct withTMEDA | 2(THF)2.png) THF solvated dimer:[(LiHMDS)2(THF)2] | Trimer, solvent free:[(LiHMDS)3] | ||
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