Lithium bis(trimethylsilyl)amide
| Lithium bis(trimethylsilyl)amide | |
|---|---|
 | |
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| IUPAC name lithium bis(trimethylsilyl)azanide | |
| Systematic name Lithium 1,1,1,3,3,3-hexamethyldisilazan-2-ide | |
| Other names Lithium bis(trimethylsilyl)amide | |
| Identifiers | |
| CAS number | 4039-32-1  |
| PubChem | 2733832 |
| ChemSpider | 21170111  |
| Jmol-3D images | {{#if:C[Si](C)(C)[N-][Si](C)(C)C.[Li+]|Image 1 |
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| Properties | |
| Molecular formula | C6H18LiNSi2 |
| Molar mass | 167.326 g/mol |
| Appearance | White solid |
| Density | 0.86 g/cm3 at 25 °C |
| Melting point | 71-72 °C |
| Boiling point | 80 - 84 °C (0.001 mm Hg) |
| Solubility in water | decomposes |
| Solubility | Most aprotic solvents THF, hexane, toluene |
| Acidity (pKa) | 36 |
| Hazards | |
| Main hazards | flammable, corrosive |
| Related compounds | |
| Related compounds | Sodium bis(trimethylsilyl)amide Potassium bis(trimethylsilyl)amide |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa) | |
| 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.[1] This reaction can be performed in situ.[2]
- 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
In organic chemistry, LiHMDS is often used as a strong non-nucleophilic base. It has a similar pKa to that of LDA (~36) but it is more sterically hindered and hence less nucleophilic. It can be used to form compounds such as lithium acetylide,[3] or lithium enolates.[2]
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.
- MXx + 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.[4]
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[5] and amines[6] 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.[6] In the solid state lithium bis(trimethylsilyl)amide is trimeric.[7]
 | ||||
 LiHMDS adduct with TMEDA. |
2(THF)2.png) THF solvated dimer: (LiHMDS)2•THF2 |
Trimer, solvent free: (LiHMDS)3 | ||
See also
References
- ↑ Amonoo-Neizer, E. H.; Shaw, R. A.; Skovlin, D. O.; Smith, B. C. (1966). "Lithium Bis(Trimethylsilyl)Amide and Tris(Trimethylsilyl)Amine". Inorg. Synth. Inorganic Syntheses 8: 19–22. doi:10.1002/9780470132395.ch6. ISBN 978-0-470-13239-5.
- ↑ 2.0 2.1 Danheiser, R. L.; Miller, R. F.; Brisbois, R. G. (1990), "Detrifluoroacetylative Diazo Group Transfer: (E)-1-Diazo-4-phenyl-3-buten-2-one", Org. Synth. 73: 134; Coll. Vol. 9: 197
- ↑ Reich, Melanie (Aug 24, 2001). "Addition of a lithium acetylide to an aldehyde; 1-(2-pentyn-4-ol)-cyclopent-2-en-1-ol". ChemSpider Synthetic Pages. p. 137. Retrieved 5 September 2010.
- ↑ Michael Lappert, Andrey Protchenko, Philip Power, Alexandra Seeber (2009). Metal Amide Chemistry. Weinheim: Wiley-VCH. doi:10.1002/9780470740385. ISBN 0-470-72184-7.
- ↑ Lucht, Brett L.; Collum, David B. (1995). "Ethereal Solvation of Lithium Hexamethyldisilazide: Unexpected Relationships of Solvation Number, Solvation Energy, and Aggregation State". Journal of the American Chemical Society 117 (39): 9863–9874. doi:10.1021/ja00144a012.
- ↑ 6.0 6.1 Lucht, Brett L.; Collum, David B. (1996). "Lithium Ion Solvation: Amine and Unsaturated Hydrocarbon Solvates of Lithium Hexamethyldisilazide (LiHMDS)". Journal of the American Chemical Society 118 (9): 2217–2225. doi:10.1021/ja953029p.
- ↑ Rogers, Robin D.; Atwood, Jerry L.; Grüning, Rainer (1978). "The crystal structure of N-lithiohexamethyldisilazane, [LiN(SiMe3)2]3". J. Organomet. Chem. 157 (2): 229–237. doi:10.1016/S0022-328X(00)92291-5.