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Article
SyntheticandStructuralStudiesofEthylZincβ-Amidoenoates
andβ-Ketoiminates
,,*
DepartmentofChemistry,UniversityCollegeLondon,20GordonStreet,LondonWC1H0AJ,UK;
.******@(.);j.******@(.);caroline.******@(.)
*Correspondence:.******@
Abstract:Asetofheterolepticethylzincβ-amidoenoates(1,2)andβ-ketoiminates(3)oftheform
[LZnEt]2withvaryingstericbulkhavebeensynthesisedviathereactionofdiethylzincwithβ-
aminoenoateligandsHL1andHL2andβ-
via1Hand13CNMR,massspectrometryandsingle-crystalX-raydiffraction,whichunambigu-
dimerisationof1and2throughcoordinationofthecentralzincatomtothemethinecarbonofthe
secondmonomer,
areexploredviathermalgravimetricanalysis(TGA),toinvestigatetheirpotentialassingle-source
precursorstozincoxide,whichshowsthat3hasasignificantlylowerdecompositiontemperatureas
3
comparedtoitsbis-ligatedcounterpart[Zn(L)2],whichgives3promiseasasingle-sourceprecursor
tozincoxide.
Keywords:precursor;dimerisation;materials;zincoxide;chemicalvapourdeposition
Citation:Bhide,.;Manzi,.;
Knapp,.;Carmalt,
β-Amidoenoatesandβ-(ZnO)iswidelyusedasatransparentconductingoxide(TCO)material.
Molecules2021,26,://TCOscombinehighelectricalconductivitywithopticaltransparency,makingthemanes-
/-doped
tinoxide(ITO)andfluorine-dopedtinoxide(FTO)arethemostindustriallyproducedfor
AcademicEditor:,thescarcityandsubsequentrisingcostofindiumrequiresashift
towardsTCOmaterialsincorporatingmoreabundantelements,suchasZnO[1,2].
DepositionofZnOthinfilmshasbeenundertakenviaseveralmethodssuchassputter-
Received:29April2021
ing[3],pulsedvapourdeposition[4],spraypyrolysis[5]andchemicalvapourdeposition
Accepted:20May2021
Published:25May2021(CVD)[6–8].Theadvantageofusingsolution-basedmethodssuchasaerosol-assistedCVD
isthatprecursorsonlyhavetobesoluble,whichthereforeallowsforagreaterrangeofpre-
Publisher’sNote:
withregardtojurisdictionalclaimsindepositionmethods[9,10].Aninsitureactionwithanoxygensource,commonlymethanol,
publishedmapsandinstitutionalaffil-
,makingitsuseandhandlingdifficult,especially
forscale--sourceprecursors(SSPs)forZnOthinfilms
hasledtoseveralcomplexesbeingreportedintheliterature,includingzincacetate[11],
zincβ-diketonates[12],zincβ-ketoiminates[13–15],zincβ-amidoenoates[16],zincox-
anes[17]andzincalkoxides[18],
Copyright:©2021bytheauthors.
LicenseeMDPI,Basel,-ligatedspecies,
Thisarticleisanopenaccessarticletheyarestillair-andmoisture-sensitive,theyarerelativelystablecomparedtopyrophoric
—thethermaldecompositiontemperaturesof
conditionsoftheCreativeCommonsthesecomplexestendtobehigher,aswellashavinggreatercarboncontaminationinthe
Attribution(CCBY)license(https://resultantdepositsupondecomposition[19,20].
,weexplorethesynthesisofasetofheterolepticethylzincβ-amidoenoates
/).andβ-
Molecules2021,26,:///molecules26113165
Molecules2021,26,xFORPEERREVIEW2of10
depositsupondecomposition[19,20].
Inthiswork,weexplorethesynthesisofasetofheterolepticethylzinc
Molecules2021,26,3165β-amidoenoatesandβ-
hypothesisethattheheterolepticnatureofthesecomplexeswillallowforboththehigh
reactivityfromtheethylgroupwhilealsomaintainingthestabilityofferedfroma
groupwhilealsomaintainingthestabilityofferedfromachelatingligand.
-aminoenoatesDiscussionligandsMeC(NHiPr)CHC(O)OEt(HL1)and
i2i1i
MeC(NH(CHTheβ-aminoenoates2)2Pr)CHC(O)OEtligandsMeC(NH(HL)werePr)CHC(O)OEtsynthesised(HLvi)anda1:1MeC(NH(CHacid-catalysed2)2Pr)
condensationCHC(O)OEt(HLreactions2)weresynthesisedbetweenviaethyl1:1acid-catalysedacetoacetatecondensationandisopropylaminereactionsbetweenand
isopentylamine,ethylacetoacetaterespectively,andisopropylamineusingK-10andmontmorilloniteisopentylamine,respectively,-10mont-The
-ketoimineforthepreparationligands,MeC(NHofβ-ketoimineiPr)CHC(O)Meligands,
(HLMeC(NH3)wasiPr)CHC(O)Mesynthesisedvia(HLa3)simplewassynthesised1:1condensationviaasimplereaction1:1condensationofacetylacetonereactionandof
isopropylamine,acetylacetoneandandisopropylamine,isolatedasanandorangeisolatedoilasofanahighorangepurityoilofandahighwithpurityagoodandyieldwith
(Figureagoodyield1).(Figure1).
.
Theethylzinczincββ-amidoenoate-amidoenoatecomplexescomplexes1and1and2,and2,andtheethyltheethylzinczincβ-ketoiminateβ-ketoiminatecom-
3,
complexplexwere3,synthesisedweresynthesisedvia1:1reactionsvia1:1ofreactionsdiethylzincofwithdiethylzinctherespectivewiththeβ-aminoenoaterespective
orβ-,allthreecompoundscrys-
β-aminoenoateorβ-,all
tallisedoutofconcentratedtoluenesolutionsafterbeingstoredat−18◦Cfor24h(3)and
threecompoundscrystallisedoutofconcentratedtoluenesolutionsafterbeingstoredat
48h(1,2).Compounds1,2and3wereallcharacterisedby1Hand13CNMRspectroscopy,
−18°Cfor24h(3)and48h(1,2).Compounds1,2and3wereallcharacterisedby1Hand
aswellassingle-crystalX-raydiffraction(XRD),whichconfirmedthestructuresasdimeric
13CNMRspectroscopy,aswellassingle-crystalX-raydiffraction(XRD),which
speciesinthesolidstate(Figure2).Peaksobservedfortheβ-amidoenoateprotonsinthe
confirmedthestructuresasdimericspeciesinthesolidstate(Figure2).Peaksobserved
1HNMRspectrumof1and2wereallshiftedascomparedtothefreeligands,confirming
fortheβ-amidoenoateprotonsinthe1HNMRspectrumof1and2wereallshiftedas
coordinationtothezinccentre,andwealsosawtriplet–quartetpeakscorrespondingtothe
comparedtothefreeligands,confirmingcoordinationtothezinccentre,andwealsosaw
boundethylgroupsappearingasbroadsignals.
triplet–quartetpeakscorrespondingtotheboundethylgroupsappearingasbroad
Theethylzincβ-amidoenoates1and2bothcrystallisedoutascentrosymmetriccrystals
signals.
inthetriclinicspacegroupP1,eachwithtwozinccentres;oneofwhichwasuniqueand
Theethylzincβ-amidoenoates1and2bothcrystallisedoutascentrosymmetric
1
crystalsdimeriseinanalogously,thetriclinicwithspacedativegroupinteractionsP,eachwithbetweentwothezinczinccentres;atomononeoneofmonomericwhichwas
uniqueunitandandtheonemethineofwhichcarbonwasonsymmetricallytheligandofthegeneratedsecondmonomericaboutaninversionunit().ThisBoth
thedimerisationcomplexesisdimeriseuncommonanalogously,inzincalkylwithcomplexes,dativeinteractionswithmostbetweendimerisationthezincoccurringatomon
onemonomericunitandthemethinecarbonontheligandofthesecondmonomericunit
througha[Zn–L]2(L=donoratom)ringinteraction(Figure3,left)[21–24].Abis(N-
(Figureheterocyclic3).carbene)Thisdimerisationzincethylcomplexisuncommonpreviouslyinreportedzincalkyldimerisescomplexes,inasimilarwithfashionmost
dimerisationto1and2,thoughoccurringthiswasthroughdueato[Zn–L]theunusual2(L=donordeprotonationatom)ringofinteractionthemethylene(Figureproton3,left)of
[21–24].theligandAbisbackbone(N-heterocyclicbydiethylzinc,carbene)andzinctheethylformationcomplexofapreviouslyformalZn–Creportedbonddimerises(Figure3,
inright)asimilar[25].fashionto1and2,thoughthiswasduetotheunusualdeprotonationofthe
methyleneprotonoftheligandbackbonebydiethylzinc,andtheformationofaformal
Zn–Cbond(Figure3,right)[25].
MoleculesMolecules20212021,26,26,x,3165FORPEERREVIEW3of3of1010
Molecules2021,26,xFORPEERREVIEW3of10
(above1(above)and)and2(below2(below)with)withthermalthermalellipsoidsellipsoidsdrawndrawnat50%at50%
(aboveomittedomitted)andforfor2clarity.()withthermalellipsoidsdrawnat50%
probabilityandwithhydrogenatomsomittedforclarity.
[Zn–L]2ring;Lrepresentsadonoratom
(left).Bis(N-heterocyclicofofethylethylzinczinccomplexcarbene)complexdimerisationzincdimerisationethylcomplexthroughthroughareported[Zn–L]a[Zn–L]2ring;by2ring;
dimerisationaofbidentateabidentateligandto1ligandand(left2).(right(leftBis().N).-heterocyclicBis(N-heterocycliccarbene)carbene)
todimerisation1and2(rightto).1and2(right).
Molecules2021,26,31654of10
Inboth1and2(Figure2),twocoordinationsitesaroundZn1areoccupiedbythe
coordinationoftheappropriatebidentateligandL1/L2whichbondsthroughtheO10
012
andN1atoms,andthebondingofL/Lformsapuckeredsix-memberedZnOC3Nring.
Zn1formsadativeinteractionwithC3onthesymmetryequivalentmonomerunit,and
assuch,1and2existasdimersinthesolidstatewithtwodativeinteractionsbetween
twomonomerunits,
compound1asanexample,thelongestinteractionmadebyZn1istheZn1–C3interaction
betweenZn1ononemonomerunittoC3ontheother((3)Å).TheZn1–C10bond
(3)Å,whichistypicalofaZn––
bondstoO10andN10oftheligandL1,(2)Å(2)Å,
respectively(Table1).(13)◦for
N10–Zn1–C10,whichisover40◦largerthantheN10–Zn1–O10angle((9)◦).Thelarge
N10–Zn1–C10bondanglereducesthestericinteractionbetweentheterminalethylgroup
,theO10–Zn1–N10angleissmallduetothesteric
–Zn1–C3,O10–Zn1–
C10,N10–Zn1–C3andC3–Zn1–(9)◦,(11)◦,
(9)◦(12)◦,
◦areexpectedincomplexesofthistypeandresultfromtheinflexibilityofthebound
chelateligandandtherequirementtoreducethestericinteractionbetweentheiPrgroups
tobothcomplexeshavingverysimilarstructuresinthesolidstate.
(Å)andbondangles(◦)forcompounds1,2and3.
BondLengths/Å123
Zn1-(2)(14)(10)
Zn1-O2--(10)
Zn1-(2)(18)(11)
Zn1-(3)(2)(14)
O1-(4)(3)(17)
BondAngles/◦
O1-Zn1-(9)(7)(4)
O1-Zn1-(11)(9)(5)
N1-Zn1-(13)(10)(5)
ThebondanglesaroundthefourcoordinateZncentresin1a