Lithium bis(trimethylsilyl)amide: Difference between revisions

{{short description|Chemical compound}}

| 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<BR>Hexamethyldisilazane lithium salt

|Section1={{Chembox Identifiers

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

| 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<BR>[[THF]], [[hexane]], [[toluene]]

}}

|Section3={{Chembox Hazards

| MainHazards = flammable, corrosive

}}

|Section8={{Chembox Related

| OtherCompounds = [[Sodium bis(trimethylsilyl)amide]]<BR>[[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)''' ('''li'''thium '''h'''exa'''m'''ethyl'''d'''i'''s'''ilazide - a reference to its [[conjugate acid]] [[Bis(trimethylsilyl)amine|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 compound|cyclic]] [[Trimer (chemistry)|trimer]] in the absence of coordinating species.

LiHMDS is commercially available, but it can also be prepared by the deprotonation of [[bis(trimethylsilyl)amine]] with [[n-butyllithium|''n''-butyllithium]].<ref>{{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}}</ref> This reaction can be performed [[In situ#Chemistry and chemical engineering|''in situ'']].<ref name=Danheiser1996 />

:{{chem2|HN(Si(CH3)3)2 + C4H9Li → LiN(Si(CH3)3)2 + C4H10}}

Once formed, the compound can be purified by [[Sublimation (phase transition)|sublimation]] or [[distillation]].

==Reactions and applications==

===As a base===

LiHMDS is often used in organic chemistry as a strong [[non-nucleophilic base]].<ref name=chemrev /> Its conjugate acid has a [[Acid dissociation constant|p''K''_a]] of ~26,<ref>{{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}}</ref> making it is less basic than other lithium bases, such as [[Lithium diisopropylamide|LDA]] (p''K''_a of conjugate acid ~36). It is relatively more [[steric effects|sterically hindered]] and hence less [[nucleophilic]] than other lithium bases. It can be used to form various [[organolithium]] compounds, including [[acetylide]]s<ref name=chemrev>{{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}}</ref> or lithium [[enolate]]s.<ref name=Danheiser1996>{{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}}</ref>

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 condensation]]s.

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&ndash;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]]:<ref>{{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 }}</ref>

:{{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 [[solvent#Physical properties of common solvents|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 compound]]s.<ref>{{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}}</ref>

===Niche uses===

LiHMDS is volatile and has been discussed for use for [[atomic layer deposition]] of lithium compounds.<ref>{{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}}</ref>

==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 [[ether]]s<ref>{{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}}</ref> and [[amine]]s,<ref name=Lucht1996>{{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}}</ref> the [[monomer]] and [[dimer (chemistry)|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.<ref>{{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}}</ref><ref>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.</ref> In noncoordinating solvents, such as [[aromatic]]s or [[pentane]], the complex [[oligomers]] predominate, including the trimer.<ref name=Lucht1996 /> In the solid state structure is trimeric.<ref>{{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}}</ref>

{|align="center" border="0" width="810"

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|colspan="5" align="right"|[[File:LiHMDS aggregation.png|750px]]

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|width="250" align="center"|[[File:LiHMDS-tmeda complex.png|175px]]<br>LiHMDS adduct with [[TMEDA]]

|width="30"|

|width="250" align="center"|[[File:Li2(Sitms2)2(THF)2.png|200px]]<br>[[THF]] solvated dimer: {{chem2|[(LiHMDS)2(THF)2]}}

|width="30"|

|width="250" align="right"|[[File:The real "Li N(Sitms2 )2".png|200px]]<br>Trimer, solvent free: {{chem2|[(LiHMDS)3]}}

|}

*[[Lithium amide]]

*[[Lithium diisopropylamide]]

*[[Lithium tetramethylpiperidide]]

{{Lithium compounds}}

[[Category:Lithium compounds]]

[[Category:Non-nucleophilic bases]]

[[Category:Organolithium compounds]]

[[Category:Reagents for organic chemistry]]