Lithium bis(trimethylsilyl)amide

{{chembox

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 450703366

| ImageFile1 = Li-HMDS.svg

| ImageCaption1 = Monomer (does not exist)

| ImageSize1 = 150px

| ImageFile2 = Cyclic Trimer of Lithium bis(trimethylsilyl)amide Structural formula V1.svg

| ImageCaption2 = Cyclic trimer

| ImageFile3 = File:The real "Li N(Sitms2 )2".png

| PIN = Lithium 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide

| OtherNames = Lithium hexamethyldisilazide
Hexamethyldisilazane lithium salt

|Section1={{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 4039-32-1

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = RC4N1I108M

| PubChem = 2733832

| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}

| ChemSpiderID = 21170111

| SMILES = C[Si](C)(C)[N-][Si](C)(C)C.[Li+]

| SMILES_Comment = ionic monomer

| SMILES1 = 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

| SMILES1_Comment = cyclic trimer

| InChI =1S/C6H18NSi2.Li/c1-8(2,3)7-9(4,5)6;/h1-6H3;/q-1;+1

}}

|Section2={{Chembox Properties

| Formula = {{chem2|LiN(Si(CH3)3)2}}

| C=6|H=18|Li=1|N=1|Si=2

| Appearance = White solid

| Density = 0.86 g/cm³ at 25 °C

| MeltingPtC = 71 to 72

| BoilingPtC = 80 to 84

| BoilingPt_notes = (0.001 mm Hg)

| pKa = 26

| Solubility = decomposes

| SolubleOther = Most aprotic solvents
THF, hexane, toluene

}}

|Section3={{Chembox Hazards

| MainHazards = flammable, corrosive

}}

|Section8={{Chembox Related

| OtherCompounds = Sodium bis(trimethylsilyl)amide
Potassium bis(trimethylsilyl)amide}}

}}

Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula {{chem2|LiN(Si(CH3)3)2}}. It is commonly abbreviated as LiHMDS or Li(HMDS) (lithium hexamethyldisilazide - a reference to its 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 bis(trimethylsilyl)amine with n-butyllithium.{{cite book | journal = Inorg. Synth. | doi = 10.1002/9780470132395.ch6 | year = 1966 | last1 = Amonoo-Neizer | first1 = E. H. | last2 = Shaw | first2 = R. A. | last3 = Skovlin | first3 = D. O. | last4 = Smith | first4 = B. C. | title = Inorganic Syntheses | chapter = Lithium Bis(trimethylsilyl)amide and Tris(trimethylsilyl)amine | isbn = 978-0-470-13239-5 | volume = 8 | pages = 19–22}} This reaction can be performed in situ.

:{{chem2|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 non-nucleophilic base. Its conjugate acid has a pK_a of ~26,{{cite journal|last1=Fraser|first1=Robert R.|last2=Mansour|first2=Tarek S.|last3=Savard|first3=Sylvain|title=Acidity measurements on pyridines in tetrahydrofuran using lithiated silylamines|journal=The Journal of Organic Chemistry|date=August 1985|volume=50|issue=17|pages=3232–3234|doi=10.1021/jo00217a050}} making it is less basic than other lithium bases, such as LDA (pK_a of conjugate acid ~36). It is relatively more sterically hindered and hence less nucleophilic than other lithium bases. It can be used to form various organolithium compounds, including acetylides{{cite journal|last1=Wu|first1=George|last2=Huang|first2=Mingsheng|title=Organolithium Reagents in Pharmaceutical Asymmetric Processes|journal=Chemical Reviews|date=July 2006|volume=106|issue=7|pages=2596–2616|doi=10.1021/cr040694k|pmid=16836294}} or lithium enolates.{{OrgSynth| authorlink1-=Rick L. Danheiser |author1 =Danheiser, R. L.|author2=Miller, R. F.|author3=Brisbois, R. G.| year =1990| title =Detrifluoroacetylative Diazo Group Transfer: (E)-1-Diazo-4-phenyl-3-buten-2-one| volume =73| pages =134| collvol =9| collvolpages =197| prep =CV9P0197}}

File:LiHMDS EnolateFormation.png

where Me = {{chem2|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 {{chem2|S(N(Si(CH3)3)2)2}} as a precursor with pre-formed S–N bonds. {{chem2|S(N(Si(CH3)3)2)2}} is prepared by the reaction of lithium bis(trimethylsilyl)amide and sulfur dichloride ({{chem2|SCl2}}).

:{{chem2|2 LiN(Si(CH3)3)2 + SCl2 → S(N(Si(CH3)3)2)2 + 2 LiCl}}

The {{chem2|S(N(Si(CH3)3)2)2}} reacts with the combination of {{chem2|SCl2}} and sulfuryl chloride ({{chem2|SO2Cl2}}) to form {{chem2|S4N4}}, trimethylsilyl chloride, and sulfur dioxide:{{cite book |last1= Maaninen |first1= A. |last2= Shvari |first2= J. |last3= Laitinen |first3= R. S. |last4= Chivers |first4=T |title= Inorganic Syntheses |year= 2002 |volume= 33 |pages= 196–199 |doi= 10.1002/0471224502.ch4 |location= New York |publisher= John Wiley & Sons, Inc. |editor-last= Coucouvanis |editor-first= Dimitri |chapter= Compounds of General Interest|isbn= 9780471208259 }}

:{{chem2|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.

:{{chem2|MX_{n} + 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 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.{{cite book | author = Michael Lappert, Andrey Protchenko, Philip Power, Alexandra Seeber | title = Metal Amide Chemistry | publisher = Wiley-VCH | location = Weinheim | year = 2009 | isbn = 978-0-470-72184-1 | doi = 10.1002/9780470740385}}

=Niche uses=

LiHMDS is volatile and has been discussed for use for atomic layer deposition of lithium compounds.{{cite journal|doi=10.1149/2.052203jes|title=Lithium Phosphate Thin Films Grown by Atomic Layer Deposition|journal=Journal of the Electrochemical Society|volume=159|issue=3|pages=A259–A263|year=2012|last1=Hämäläinen|first1=Jani|last2=Holopainen|first2=Jani|last3=Munnik|first3=Frans|last4=Hatanpää|first4=Timo|last5=Heikkilä|first5=Mikko|last6=Ritala|first6=Mikko|last7=Leskelä|first7=Markku}}

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{{cite journal|last=Lucht|first=Brett L.|author2=Collum, David B. |title=Ethereal Solvation of Lithium Hexamethyldisilazide: Unexpected Relationships of Solvation Number, Solvation Energy, and Aggregation State|journal=Journal of the American Chemical Society|year=1995|volume=117|issue=39|pages=9863–9874|doi=10.1021/ja00144a012|bibcode=1995JAChS.117.9863L }} and amines,{{cite journal|last=Lucht|first=Brett L.|author2=Collum, David B. |title=Lithium Ion Solvation: Amine and Unsaturated Hydrocarbon Solvates of Lithium Hexamethyldisilazide (LiHMDS)|journal=Journal of the American Chemical Society|year=1996|volume=118|issue=9|pages=2217–2225|doi=10.1021/ja953029p|bibcode=1996JAChS.118.2217L }} the monomer and dimer 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.{{cite journal|author1=Neufeld, R.|author2=Michel, R. |author3=Herbst-Irmer, R. |author4=Schöne, R. |author5=Stalke, D. |journal=Chem. Eur. J.|year=2016| volume=22|pages=12340–12346|title=Introducing a Hydrogen-Bond Donor into a Weakly Nucleophilic Brønsted Base: Alkali Metal Hexamethyldisilazides (MHMDS, M = Li, Na, K, Rb and Cs) with Ammonia |issue=35 |doi=10.1002/chem.201600833 |pmid=27457218}}Neufeld, R.: [https://ediss.uni-goettingen.de/bitstream/handle/11858/00-1735-0000-0028-8713-A/Roman_Neufeld_Diss-ECC-MW-Determination.pdf?sequence=1 DOSY External Calibration Curve Molecular Weight Determination as a Valuable Methodology in Characterizing Reactive Intermediates in Solution.] In: eDiss, Georg-August-Universität Göttingen. 2016. In noncoordinating solvents, such as aromatics or pentane, the complex oligomers predominate, including the trimer. In the solid state structure is trimeric.{{cite journal | title = The crystal structure of N-lithiohexamethyldisilazane, [LiN(SiMe₃)₂]₃ | first1 = Robin D. | last1 = Rogers | first2 = Jerry L. | last2 = Atwood | first3 = Rainer | last3 = Grüning | journal = J. Organomet. Chem. | volume = 157 | issue = 2 | year = 1978 | pages = 229–237 | doi = 10.1016/S0022-328X(00)92291-5}}