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rsc.
org/chemcommJournalNameCOMMUNICATIONThisjournalisTheRoyalSocietyofChemistry20xxJ.
Name.
,2013,00,1-3|1PleasedonotadjustmarginsPleasedonotadjustmarginsDepartmentofBiologyandChemistry,CityUniversityofHongKong,TatCheeAvenue,Kowloon,HongKong,China.
Email:mcwchan@cityu.
edu.
hk;kaichung@cityu.
edu.
hkElectronicSupplementaryInformation(ESI)available:Experimentalproceduresandcharacterizationdata,selectedNMRspectra,detailsforDFTcalculations,andcrystaldataforTi2H,Zr2HandHf2Me(CCDC14861411486143respectively).
SeeDOI:10.
1039/x0xx00000xReceived00thJanuary20xx,Accepted00thJanuary20xxDOI:10.
1039/x0xx00000xwww.
rsc.
org/Multifacetedchelatingμ-(η3:η3-antifacial)-(cis-C4R2H2)coordinationmotifinbinuclearcomplexesCham-ChuenLiu,MichaelC.
W.
Chan,*Po-KamLo,Kai-ChungLau*andShek-ManYiuThenovelμ-C4R2H2corestructure(formedbyanunprecedentedregioselective,redox-neutralC(sp2)–C(sp2)couplingprocess)inbinucleargroup4complexesdisplaysadaptablecoordinationandaccommodatesdifferentmetalsizes,andissufficientlyrobusttopromoteinterestingcatalyticreactivityatthebimetalliccenters.
Thesearchfornovelorganometallicligandsystemscontinuestomotivateresearchendeavors.
Forbinuclearcomplexes,unsaturatedcarbon-basedbridgingligandsthatcanexhibitvariablehapticity(reducedtofacilitatesubstratebindingorincreasedtosupporthithertounstablespecies)andredox-activebehavior(topromoteandtoleratedifferentoxidationstatesandelectronicrequirements),andcanpotentiallymediateattractivereactivityandcatalytictransformations(e.
g.
metal-metalsynergisticandcooperativeeffects1),areofinterest.
Inthiswork,theunusualdelocalizedμ-(C4R2H2)ligandcoreisformallydianionicanda6-electrondonor.
Relatedauxiliariessuchasthe2,2-bipyridyldianionin[μ-(η4:η4)-(bipyridyl)-Y2],2μ-(trans-butatriene)in[μ-(C4H4)-Ti2],3andtheisomerictrimethylenemethane,TMMin[μ-(η3:η3)-(C(CH2)3)-Ln2]4,5areknownintheliterature.
However,theseligandsdonotcontainadditionalchelatingsitesandarepronetoundergotransformation(e.
g.
byelectrophilicattack)andsubsequentdisplacement,andthisisreflectedbyadearthofcatalyticapplications.
Here,wedemonstratethatbyexploitingthechelateeffectandattendantstericprotection,thebis-(phenolate-pyridine)-(μ-C4R2H2)frameworkcansupportinterestingcatalyticreactivityatthebimetallicsites.
Ourongoingstudiesconcerninggroup4organometallicsindicatedthat,comparedwithchelatingσ-arylligands,6,7theσ-alkenylmoietycanoffergreaterflexibilityandπ-donation.
ThemetalationofthevinylligandH2LH(R=H)withM(CH2Ph)4inpentane/Et2Oyieldedblack-purple(Ti)anddeeporange(ZrandHf)crystallineproducts,whileforH2LMe(R=Me),intenselydarkturquoise(Zr)anddarkindigo(Hf)crystallinesolidswereobtained(Scheme1).
8Thesecolorsarereminiscentofd1compounds(e.
g.
ZrCl3existsasblue-blacksolid),butallproductswerefoundtobediamagnetic.
ThesurprisingnatureoftheproductswereelucidatedbyX-raycrystallographyforTi2H,Zr2HandHf2Me(Figure1).
Thedimeric,racemicmolecularstructuresdepictabis-[O,N]-chelatingμ-(η3:η3-antifacial)-(cis-C4R2H2)frameworkthatcoilsaroundandbindsthetwometalsonoppositefacesofthedelocalizedC4moiety.
WhiletheM–C(η3)bondlengths9arevaried(av.
2.
427,2.
566,2.
565respectively),itisparticularlystrikingthattheM-C4-Mcoordinationisdissimilar(seeTable1forillustration);forexample,theM1atominbothTi2HandZr2HisclosesttoC4followedbyC3,andweaklyboundbyC2,buttheHf1atominHf2MeisclosesttoC2andweaklycoordinatedtoC1andC3.
Conversely,thegeometryoftheC4coreisconsistentandrelativelyindependentofthemetal;theC–Cdistances(1.
409(3)–1.
446(5))maybeconsideredequivalentwithinexperimentalerror,andtheC–C–Cangles(av.
128.
1–133.
0°)andtorsionangle(24.
6–30.
3°)showonlyminordifferences.
TheM–C(η3)distancesinTi2H,Zr2HandHf2Me(2.
335(2)–2.
700(2))aregenerallylongerthantheM–C(sp2)bondsinmononuclearσ-arylcongeners(M=Ti–Hf:2.
15–2.
35),7butshorterthanthoseintheμ-TMMrelatives[(C5Me5)2Ln]2[μ-(η3:η3)-(C(CH2)3)]containinglargerLn3+ions(La,Pr,Sm;2.
546(4)–2.
870(5)).
4Incontrast,theC–CdistancesreportedfortheTMMligand(1.
406(9)–1.
432(6))areremarkablysimilartothosefortheμ-C4R2H2core.
Scheme1SynthesisofbinuclearcomplexesM2R.
ONMNOHR+M(CH2Ph)42PhCH3H2LRPhPhONMPhPhHRHRR=H:Ti2H,Zr2H,Hf2HR=Me:Zr2Me,Hf2Mepentane/Et2OPage1of4ChemCommCOMMUNICATIONJournalName2|J.
Name.
,2012,00,1-3ThisjournalisTheRoyalSocietyofChemistry20xxPleasedonotadjustmarginsPleasedonotadjustmargins(a)(b)(c)Figure1Perspectiveviews(30%probabilityellipsoids;keyatomslabeledandHshownforμ-C4unitonly)ofTi2H(a),Zr2H(b)andHf2Me(c).
Table1Selectedbondlengths(;squareparenthesessignifynon-bondeddistances)andangles(°)forTi2H,Zr2HandHf2MeTi2HZr2HHf2MeC1–C2()1.
438(3)1.
434(4)1.
435(4)C2–C3()1.
409(3)1.
412(4)1.
442(5)C3–C4()1.
432(3)1.
428(4)1.
446(5)M1–C1,M2–C4()[3.
161(2),3.
140(2)][3.
187(2),3.
270(3)]2.
640(3),2.
571(3)M1–C2,M2–C3()2.
553(2),2.
552(2)2.
665(3),2.
700(2)2.
431(3),2.
413(3)M1–C3,M2–C2()2.
378(2),2.
396(2)2.
533(2),2.
532(3)2.
651(3),2.
684(3)M1–C4,M2–C1()2.
335(2),2.
349(2)2.
461(3),2.
507(2)[3.
437(3),3.
460(3)]C1–C2–C3()128.
9(2)129.
6(2)132.
7(3)C2–C3–C4()129.
2(2)128.
1(2)133.
0(3)C4torsionangle()30.
3(4)27.
3(5)24.
6(6)M1···M2()[4.
453(1)][4.
731(1)][4.
847(1)]Theμ-C4R2H2moietycanexistascisortransaswellassynoranti(Rgroupsonsameoroppositesides)isomers.
Intriguingly,onlythecis-synstructurewasisolatedandcharacterizedineachcase,andthisregioselectivityiscorroboratedbycleanstoichiometricNMRreactions(≥90%conversion;FiguresS1–S4)forallcomplexes.
10Theseobservationsaresuggestiveofaconcerted,associativedimerizationmechanismfortheformationoftheC4R2H2bridgeinthesecomplexes,andinvestigationsbyDFTcalculationshavebeenperformed(seeESIfordetails).
Startingfromthemonomeric(σ-vinyl)-metalatedbis(benzyl)species,contrastinganglesofapproachandinteractionbetweentwoM-(σ-vinyl)moieties11leadstodistinctdimerizationpathwaysandtheformationofdifferentμ-C4R2H2isomers.
TakingZr2Hasarepresentationexample(FigureS5),thebarrier(ΔG298=31.
4kcal/mol)fortheformationofthecis-syndimerissubstantiallylowerthanthat(ΔG298=69.
7kcal/mol)forthecis-antiisomer,thusindicatingthattheformeriskineticallyfavored.
Forallcomplexes,thepathwaytothecis-syndimerrequiressignificantlyloweractivationenergythanthecis-antiform(TableS1;considerationswerealsogiventotrans-syn[evenhigherbarrierswerepredicted]andtrans-anti[notransitionstatestructurewaslocated]pathways),consistentwiththeobservedstrongpreferenceforcis-synstructures.
12Additionaldiscussionandobservationsconcerningthedimerizationreactionispertinent.
(1)WhenthevinylunitinH2LR(Scheme1)wasreplacedby1-cyclohexene,reactionwithM(CH2Ph)4affordedthecorrespondingmononuclear[O,N-(σ-cyclohexenyl)]bis(benzyl)complexes(TiCyandZrCy;seeESI).
Therefore,asexpectedfortheaboveassociativedimerizationpathway,suchatransformationwouldbehinderedbyasubstituentattheterminalC(σ-alkenyl)atom.
(2)Coatesreportedthatmetalationofpyridylamidoandphenolate-amineligandsbearingapendantvinylgroup,withgroup4alkylprecursors,bothproceededbyintramolecularvinylinsertionandalkylmigrationtoaffordC(sp3)-chelatingcatalystsforisoselectiveα-olefinpolymerization.
13Incontrast,thevinyl-containingligandsheremanifestlypromotefaciledimerizationoftheresultantmetalatedspecies.
(3)Thisregioselectivetransformationisaformallyredox-neutralC(sp2)–C(sp2)couplingprocess,whichishighlyunusualforearlytransitionmetals.
Relateddimerizationreactionsthatarereductiveinnaturearewell-established,2,14andreportsofnon-redoxpathways(toaffordLn2,15Fe2,16andPt217complexes)areuncommon.
Page2of4ChemCommJournalNameCOMMUNICATIONThisjournalisTheRoyalSocietyofChemistry20xxJ.
Name.
,2013,00,1-3|3PleasedonotadjustmarginsPleasedonotadjustmarginsNMRcharacterizationdatafocusingontheμ-C4R2H2moietyarelistedinTable2(seeFiguresS3andS4for1HNMRspectra),andcomparisonswithη3-allylderivativesmaybedrawn.
18ForM2Hcomplexesbearingthenon-coplanarμ-C4H4fragment,JHHcouplingsof4–6Hzbetweenthesemagneticallyinequivalentnucleiareconsistentwithacis-conformation.
19RelativetoM2H,theupfieldNMRshiftforCβintheM2Meseriesmaybeattributedtothedissimilarμ-C4coordinationapparentfromthemolecularstructures.
Bearinginminddifferencesinelectrophilicity,itissurprisingthatZr2HandHf2H(plusZr2MeandHf2Me)exhibitcloselymatchedδCresonancesforthemetal-boundC(sp2)atomsCαandCβ.
7Inthiscontext,thesubstantiallydownfield-shiftedNMRpeaksfortheμ-C4H4fragmentinTi2H,andparticularlythearomatic-likeresonancesforCβandHβ,arenoteworthy.
20WhilethehigherelectrophilicityofTiisacontributingfactor,theseresultsimplythatthiscomplexiselectronicallydifferentandmaypotentiallypointtoaredoxnon-innocentcapabilityfortheμ-C4H4group(especiallyforreduction-pronemetalcenters).
Fromaqualitativeviewpoint,acombinationofμ-cis-but-2-ene-1,4-diyl(d0,d0)andμ-cis-butadiene(d1,d1)resonanceformsmaybeemployedtodescribetheμ-(C4R2H2)-M2bonding.
21Molecularorbital(MO)interactionshavebeenanalyzedbyDFTcalculations,andfourMOsinvolvingtheπorbitalsoftheC4bridge(πCαCβCγCδ)andmetaldorbitalshavebeenfound(TablesS4andS5forM2HandM2Merespectively).
Thelowest-energyMOshowsbondingacrosstheC4unitandisdominatedbytheπCαCβCγCδorbital.
Thesecond-lowest-energyMOdisplaysbondingfortheCαCβandCγCδbonds(butantibondingwithrespecttoCβCγ),andin-phaseoverlaptakesplacebetweendxyorbitalsandπCαCβ(andπCγCδ).
ThenextMO,whichistheHOMO(Table3forTi2H),showsbondingforCβCγ(butantibondingwithrespecttoCαCβandCγCδ),andout-of-phaseoverlapbetweendxyorbitalsandπCαCβ(andπCγCδ)isevident.
TheLUMOisantibondingforallthreeCCbonds,andcontainssignificantcontributionsfrombothdxyanddxzorbitalsfortheM2Hseries.
Ontheotherhand,thedxyanddyzorbitalscontributetotheLUMOfortheM2Mecomplexesbecauseofthedissimilarμ-C4coordination.
ComplexesZr2MeandHf2Mearehighlycoloredanddisplayintensechargetransferabsorptionbandsatλmax600and578nm(ε~1.
4*103dm3mol–1cm–1)respectively(FigureS6).
AnadmixtureofπC4→d(M)/π*pyligand-to-metalandintraligandchargetransfer(LMCTandILCT)istentativelyassigned,inagreementwiththecalculatedMOs(TableS5),andtheblueshiftfromZrtoHfisentirelyconsistentwithaLMCTtransitionford0centers.
TheZr2HandHf2Hderivativesexhibitabsorptionshouldersatλmax400–500nmthataretentativelyassignedtoπC4→π*py(ILCT)withminorLMCTcharacter.
ForTi2H,theextremelydiffusenatureoftheabsorption(λ350–700nm)ispatentlydifferentfromtheTiCyderivativeandothercomplexes.
TheresultsfromtheX-raydataandDFTcalculationsallowustoevaluatetheμ-C4R2H2structureandM-C4-Mcoordinationasafunctionofgroup4metals.
Theversatilityofthedelocalizedμ-C4R2H2moietyisillustratedbyitsabilitytomaintainstructuralconsistencywhilebindingmetalsofdifferentsizesthroughvariableM–Cdistancesandhapticity.
Table2NMRaandIRbdataM2RCω(ppm)Cα(ppm)[1JCH(Hz)]Hα(ppm)Cβ(ppm)[1JCH(Hz)]Hβ(ppm)νCC(cm–1)Ti2H163.
8495.
79[154]5.
27126.
65[163]7.
121592Zr2H159.
7585.
37[158]5.
36115.
63[159]5.
561593Hf2H160.
5385.
15[157]5.
04115.
04[162]5.
611594Zr2Me151.
5892.
78–107.
22[151]5.
101591Hf2Me151.
0694.
03–104.
46[154]5.
131591aC6D6,400(1H)/101(13C)MHz,295K(assignedusing[1H,13C]-HSQC,-HMBC,135-DEPT,COSYandNOESYexperiments).
bThinfilmonKBr(νCC=1607and1606cm–1forH2LH,Merespectively).
Table3CompositionofHOMOandLUMO(%;isovalue=0.
03)forTi2HHOMOLUMOCompositiondxydxzπCαCβCγCδdxydxzπCαCβCγCδTi2H10.
22.
130.
214.
538.
17.
4Scheme2ReactionofZr2Hwith[Ph3C][B(C6F5)4]inC6D5Br.
Next,weendeavoredtodemonstratethattheμ-C4R2H2coreisresistanttoelectrophilicattackandsufficientlyrobusttosupportcatalyticreactivity.
Forexample,thetreatmentofZr2Hwithtwoequivalentsoftritylborate[Ph3C][B(C6F5)4]inC6D5Brproceededcleanlytogeneratethecorrespondingdication(containingaη2-CH2PhmoietyateachZrcenter)22andtwomoleculesofPh3CCH2Ph(Scheme2).
Theformationofthelatter,andthesuccessfulZr2H+2[Ph3C][B(C6F5)4]2[B(C6F5)4]ONZrONZr++-+2Ph3CCH2PhC6D5BrHHHHPage3of4ChemCommCOMMUNICATIONJournalName4|J.
Name.
,2012,00,1-3ThisjournalisTheRoyalSocietyofChemistry20xxPleasedonotadjustmarginsPleasedonotadjustmarginsNMRcharacterizationofthedicationicspecies(FigureS7),confirmthatelectrophilicattackbyPh3C+doesnotoccurattheμ-C4moietyandleadstobenzylabstractionfromtheZrsites.
Inthisregard,thereactivenatureoftheμ-TMMmoietyin[(C5Me5)2Sm]2[μ-(η3:η3)-C(CH2)3],whichundergoesfacileC–Hactivationoftoluene,isnoted.
4Preliminaryethylenepolymerizationtestsinconjunctionwithtritylborateascocatalysthavebeenundertaken,23andcomparisonswiththeσ-cyclohexenylanaloguesTiCyandZrCyhavebeenmade.
Uponactivation,theZr2Hcomplexfunctionsasawell-definedcatalystandproducespolyethylene(Mn=2.
4*105)withverynarrowMw/Mn(2.
0),whiletheactivity(22g(polymer)(mmolcatalyst)–1h–1atm–1)ismodestbutneverthelesssuperiortoZrCy(ccommodated,andstabilityagainstelectrophilicattackisconferredbythechelateeffectandtheassociatedstericprotection,thusengenderingcatalyticreactivity(andpotentialcooperativity)atthebimetallicsites.
Furtherworktobroadenthescopeofcatalyticapplicationsandprobethepossibilityofbimetalliccooperativity,andsyntheticstudiestoextendtoothermetalsystems,areinprogress.
TheworkdescribedinthispaperwasfullysupportedbyagrantfromtheResearchGrantsCounciloftheHongKongSAR,China(CityU11300414).
WearegratefultoDr.
Man-KitTseforassistanceinNMRexperiments.
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9AM–Ccutoffdistanceof≤2.
7toassignbondingisapplied.
10NMRreactions(C6D6,–30to20C)betweenM(CH2Ph)4andH2LRresultedinhighconversions(≥90%)tothecis-(syn-R2)products,andthesoleidentifiablebyproductwastoluene.
11Two-center-two-electron(σ-vinyl)-bridgingbimetallicinter-mediateswerealsoconsidered,butsubsequentlydiscountedduetoexcessivesterichindrance.
12Forcis-synstructures,thecalculatedactivationenergyfortheformationofTi2Me(ΔG298=40.
1kcal/mol)washighestamongstthecomplexeshere,consistentwithourinabilitytosynthesizethiscomplex.
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21NaturalresonancetheorycalculationsperformedonmodelcomplexesofM2Hrevealedthattheresonanceweightingof(d0,d0):(d1,d1)metalcentersincreasesfromTitoZrandHf,thusimplyingsignificantlymored1,d1characterfortheTi2Hcomplex(TablesS2andS3).
22ThestructureofthedicationwasoptimizedbyDFTcalculations.
ComparedwiththemolecularstructureofZr2H,changesevidentintheenergy-minimizedcalculatedstructureincludethehapticityoftheμ-C4H4unit(Zr–Cαcoordinationissignificantlystronger)andslightcontractionoftheZr···Zrdistance(4.
52;FigureS8).
23Conditions:1.
5molofcatalyst,iBu3Al/[Ph3C][B(C6F5)4]/catalyst(100/6/1equiv.
respectively),40mLoftoluene,1atmethylenefeed,20oC,10minutes.
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