POM-based Chiral Hybrids Synthesis via Organoimidization Covalent Modification of Achiral Precursors Chapter 1 Advances in Chemical Engineering Yifei Zhang 1,4 †, Yichao Huang 2+ , Huan Wang 1+ , Jiangwei Zhang 1,2,3 *, Gao Li 1 *, and Yongge Wei 2* 1 State Key Laboratory of Catalysis & Gold Catalysis Research Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), Dalian 116023, PR China. 2 Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China. 3 State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, PR China. 4 College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang, 110034, PR China. † Equal Contribution Correspondence to: Jiangwei Zhang, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, PR China. Email: [email protected]Gao Li, State Key Laboratory of Catalysis & Gold Catalysis Research Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), Dalian 116023, PR China. Email: [email protected]Yongge Wei, Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China Email: [email protected]
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POM-based Chiral Hybrids Synthesis via Organoimidization Covalent Modification
of Achiral Precursors
Chapter 1
Advances in Chemical Engineering
Yifei Zhang 1,4†, Yichao Huang 2+, Huan Wang 1+, Jiangwei Zhang 1,2,3*, Gao Li1*, and Yongge
Wei2*
1State Key Laboratory of Catalysis & Gold Catalysis Research Center, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences (CAS), Dalian 116023, PR China.2Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department
of Chemistry, Tsinghua University, Beijing 100084, PR China.3State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005,
PR China.4College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang, 110034,
PR China.
† Equal Contribution
Correspondence to: Jiangwei Zhang, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen
In this chapter, covalent modification of POMs (polyoxometalates)clusters especially by organo imidization via the well-developed DCC(N,N′-dicyclohexylcarbodiimide)-dehydratingprotocolbyourgroupisbrieflyreviewed.ThefunctionalizationofPOMswithcovalentlyorganicmoietyisoneoftheeffec-tivewaystoincreasethediversePOMsfamilyandincorporateexceptionalproper-tiesthattheseorganicmoleculesattachtoPOMclustersinareliableandpredefinedmanner,whichisexpectedtobeappliedtothedesignofmolecularfunctionalizedmaterials.ThecombinationchiralitywithPOMsisacrucialbutchallengingissuesincePOMsusuallypossesshighsymmetry.Organoimidizationcovalentmodifi-cationofachiralprecursorsLindquist[Mo6O19]2−hasbeenprovedtobeaflexiblestrategyforapplyinginthegenerationofPOM-basedchiralhybrids.TheorganicligandsoffergreatopportunitiestoinstallstereogenicelementstoPOMsstructureasstructuredirecting-agentstoremovesymmetricelementsinPOMclusters,andsomespecialPOMclustersbearingaminogroupon itssurfaceservesasspecialimido ligands.This takes full advantage of the intrinsic hindrance of the bulkyandheavyPOMs,Whichwillgreatlyextendchiralityinpolyoxometalatechemis-trysincetheprimarychiralPOManionsarerare.InconsiderationofthefactthatthereexistplentyofchiralnaturalproductsL-aminoacidsandmanyPOMclustersanchoraminogroupasremotegrouponthesurface.ItcanforeseethatmoreandmorePOM-basedchiralhybridswillbegeneratedbyorganoimidizationcovalentmodificationofachiralprecursorsLindquist[Mo6O19]2−inthefuture.
Graphical abstract
1. Introduction Polyoxometalates (POMs) are an exceptional family of inorganic oxide anions con-sistingofearlytransitionmetalionssuchasMo,W,V,etc.attheirhighestoxidizationstatesbridgingbyoxideanionswithstructuralversatilityandawiderangeofpropertiesincludingcatalysis,medicineandmaterialsscience[1-3].SincethefirstsaltofPOM,ammonium12-molybdophosphate,(NH4)3[PMo12O40]wasreportedbyBerzelius in1826, thechemistryofPOMshasgaineddramaticdevelopment,withthedevelopmentofstructurecharacterizationtechnology,especiallysingleX-raydiffraction,thestructureofPOMsclustercanbeprecisely
Organic–inorganicPOMshybridscanbecategorizedintotwoclassifications[20,27],classificationIandclassificationII (Figure 3),whicharewell-definedaccording to the in-teractionsbetweentheorganicand inorganicmoieties, tobeexact,non-covalentandcova-lentinteractions,respectively.ClassificationIorganic–inorganichybridscanbeassembledbyelectrostaticinteractions,hydrogenbonding,aswellas/orvanderWaalsinteractions.
however,suchorganic-inorganichybridmaterialswillnotonlycombinetheadvantagesofor-ganicmaterials:suchasgoodprocessabilityandfine-regularstructureandelectronicproper-ties,withthoseofinorganicPOMclusters,suchasgoodchemicalstabilityandstrongelectronacceptability, to produce so-called ‘‘value-adding properties’’, but alsomay bring excitingsynergisticeffectsduetothecloseinteractionofdelocalizedorganicpπorbitswiththeinor-ganicPOMcluster’sdπorbits.
Comparedwithsuchorganic-inorganichybrids,POMsareconventionallypreparedbycoordination bonding or electrostatic interaction approaches,which lack predictability andcontrollability.However,theabilityofcovalentlymodifythePOMclustersinareliableandpredefinedmannerholdspromiseforthedevelopmentofmolecularmaterialsthatbridgethegapbetweenmolecularorganicandbulksemiconductingPOMscluster.Furthermore,givingexceptionalphysicalandstructuralpropertiesintrinsictoPOMsuperstructurescanbeachievedbypost-functionalizedthroughcommonorganicreactionsandapplyorganicallyfunctional-izedPOMclustersasbuildingblocks[28-29].
itsthermalandchemicalrobustandeasytobeprepared,whichhastheso-calledLindqviststructure. As it is depicted in Figure 5,theLindqviststructureconsistsofacentraloxyanionsurroundedinanoctahedralcageformedbysixmetalatoms.Allthesixmetalatomsalsohavean octahedral environment.
thePOMreconstructionboth in thepresenceof theN,N′-dicyclohexylcarbodiimide(DCC)dehydratingagent.
2.1. Monosubstituted Derivatives
In the presenceof one equivalent ofDCC, one equivalent of primary amines reactssmoothlywithstoichiometric(n-Bu4N)2
[Mo6O19]underdryN2gasandrefluxingacetonitrile,to give the correspondingmono-substituted imidoderivatives usually in excellent yield ofmorethan90%.Notonlyavarietyofprimaryaminesoccurthisreaction,butalsoitisusuallycompletedinlessthan12hours,andpurecrystallineproductscanbereadilyobtainedwithconvenientbenchmanipulations.Moreover, thisreactioncanbecarriedout in theopenairwithoutnitrogenprotectionifslightlyoveroneequivalentofDCC(1.2equivalent)isadded.(Scheme 1)
Moreover,inourrecentattemptstofunctionalizetheoctamolybdtateswithorganoimidogroups,wediscovered that in thepresenceofDCC, aproton coulddramatically speedupthereactionofα-[Mo8O26]
We conducted our DCC-dehydrating protocol for the direct functionalization of[Mo6O19]
2−withstoichiometricratiosof(n-Bu4N)2[Mo6O19]andthecorrespondingorganoimi-doligands[75-76].Boththecis-andtrans-disubstitutedimidoderivativescouldbeobtainedinreasonableyields,respectively,bycarefulcontroloftherefluxingtime,reactiontemperatureandconcentration.Itshouldbenotedthatthecis-isomerisbyfarthemostcommonstructureamongthedisubstitutedspeciesevenifthekineticallycontrolledtransisomerhasbeenrecent-lyisolated[75-77].Ingeneral,prolongedrefluxingtimeathighconcentrationandhightem-peraturepromotestheformationofthecis-isomer,whilethetrans-isomercanbeisolatedfromarelativeshort-timeatlowconcentrationandlowtemperaturereactionsystem(seeScheme 4 and Figure 5).
Thiscompoundhasafeaturethattheμ2-bridgingoxygenatomsharingbythetwoimido-bearingMoatomsinacis-diimidohexamolybdateissubstitutedwithaμ2-bridgingorganoim-idoligand.Itsuggeststhatsuchoxygenatomsinthecis-diimidohexamolybdateshavebeendoublyactivatedbytheneighboringimidogroupsandbecomemorenegative(nucleophilic)thanotheroxygenatoms,resultingintheeasyelectrophilicattackbyDCC.ItstandsforthefirstexampleofabridgingoxygenatominPOMsreplacedbythebridgingimidogroups.Itbreaksthemyththatonlytheterminaloxogroupscanbedirectlyreplacedwithimidoligandsbefore and brings us awonderful prospect in the chemistry of organoimido derivatives ofPOMs(Figure 7).
In additionofone and ahalf equivalentsofDCC, the reactionofone equivalentof2,6-dimethylanilinewithoneequivalentof(Bu4N)2[MoW5O19]doesoccurinhotacetonitrileundernitrogen,affording themono-imidoderivatives (Bu4N)2 [W5O18(MoNR)]2-. Indeed as expected,theterminaloxygenatombondedtothemolybdenumatomwasselectivelyreplacedbyanimidosubstituent.ThisresultopensageneralroadforthesynthesisoforganoimidoderivativesoflessreactivePOMclusters,namely,byreplacementoneoftheinertterminalmetal-oxogroupswithafunctionalizableMo-Ogroup.
3. POM-Based Chiral Hybrids Synthesis via Organo imidization
The organo imidization strategy is applying in POM-based chiral hybrids synthesisthroughcovalentmodificationofachiralprecursorsPOMcluster[Mo6O19]
For thebiphenyl-typechiralaxlecontainingPOM-basedchiralhybridsdesign.PengdesignedtwopairsofenantiopureC2-symmetric1,1’-binaphthylunitsbasedorganicligand.Throughthepost-functionalization,achiralprecursorsmonosubstitutedorganoimidoderiva-tives[Mo6O18(NR)]
Intheviewofapplyingchiralorganicligandsasstructure-directingagentstotransferchirality to thewholePOM-basedhybrids strategy, chiral species are still consumed.Thisorganoimidization covalentmodificationwouldbemorevaluable if applying intrinsichin-dranceofthebulkyandheavyPOMs.Thenwefurtherpromotedthisstrategy.POM–organichybridchiralmolecularnanorodswereobtainedthroughorganoimidizationofachiralprecur-sorsLindquist[Mo6O19]
We applied anthranilic alkyl ether as imidization reagent,which ismoderately flex-ibleandnon-planeorganicmolecule.ByremovingsymmetricplaneandsymmetriccentersinPOMclustersandtheformationofring,wecouldobtainchiralmetalmacro-cycliccompounds[Mo6O17(C18H20N2O2)]2(±) through organoimidizationcovalentmodification by reacting
2-canbebroken.Inthisway,thewholemetalmacro-cycleturnstobechiral.ThisisanimportantbreakthroughinPOMmetalmacro-cycliccompoundssynthesis(Figure 10). Taking theadvantageofdisubstitutedorganoimidizationstrategy,wealsoattempt todesignmultifunctionalhybridmaterialsbyintroducingaromaticandaliphaticorganicaminemoietiessimultaneously,indeedinsuchcis-disubstitutedimidoderivativesthesymmetrywasreduced to CipointgroupfromreactantsMo6O19
2-clusterwhichhashighsymmetryOh point group.Unfortunatelyspontaneousresolutionandchiralseparationofsuchdisubstitutedimidoderivativeshavenotyetachieved,however this indicated the important issue thatdisubsti-tuted/polysubstitutedorganoimidizationstrategyshouldalsobeaneffectivestrategyinchiralPOM-basedmultifunctionalhybridmaterialsdesign[84](Figure 11).
Considering the polyfunctionalization capacity character of the derivatizational[Mo6O19]
2−, this strategy designing through the post-functionalization monosubstituted or-ganoimido derivatives as building block becomes more accessible and flexible.We haveprovedthattheremainingterminalreactiveoxygenatomsinsuchorganoimidoderivativescanbefurtherfunctionalizedbyotherimidoligands[86].SincethefactthatthereexistplentyofchiralnaturalproductsL-aminoacidsandremoteaminogroupsonthesurfaceinPOMclus-tersincluding{[NH2C(CH2O)3]2V6O13}
WethankthefinancialsupportbytheNationalNaturalScienceFoundationofChina(NSFCNo.21471087,21631007,21606220and21701168),thefundofthe“ThousandYouthTalents Plan”, LiaoningNatural Science Foundation (No. 20170540897) and open projectFoundationofStateKeyLaboratoryofPhysicalChemistryofSolidSurfaces,XiamenUniver-sity(No.201709).