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EtheneConversionataZeolite-SupportedIr(I)Complex.
AComputationalPerspectiveonaSingle-SiteCatalyst
System
[a][a,b][b,c]
,AlexanderGenest,*andNotkerRösch
Applyingaquantummechanics/molecularmechanicsscheme113kJmol1,
involvingDFTcalculations,amodelstudyofmechanismsforviaaMCmechanismisalsonotoperativebecausetheCC
ethenetransformationsatzeolite-supportedIr(I)complexesiscouplingbarrierishigherby30kJmol1(inabsoluteterms)
presentedandtheresultscomparedtothoseofrecentexperi-,when
mentsandpreviousworkontheisostructuralRh(I),thecomputationalresultsallowedto
Startingfromthe2-ligandcomplex[Ir(C2H4)2]+,inthepresencerationalizetheexperimentallydeterminedswitchingfrom
ofH2,theetheneconversionmechanismsstudiedyieldsolelyethenedimerizationtoethaneformationduetothesignifi-
ethanewhilethedimerizationto1-buteneviaeitherthecantlyhighercalculatedbarrier,by~50kJmol1relativetoRh,
Cossee-Arlman(CA)mechanismorthemetallacycle(MC)
Therefore,turningto3-ligandmodels,thecalculationsshowedcatalysissystem,yetitalsohighlightsthepotentialcomplexity
thatthediethylcomplex[Ir(C2H4)(C2H5)2]+stronglyfavorsofsuchsystemsasrevealedbycomparing2-to3-ligandmodels
ethenehydrogenationoverdimerization(viaaCAmechanism),aswellasmodelswithdifferentmetalcenters,RhvsIr,inthe
withcrucialactivationfreeenergiesof27kJmol1andlightofconversionratesviatheenergeticspanconcept.
Introductionwork,areappliedinalkenepolymerization[3]andalkene
epoxidation.[4]Thesecatalystsareexcellentlysuitedforexplor-
Supportedmetalcatalystsarewidelyusedinindustrialingthereactionmechanismbytheory,.,DFTbased
processes.[1]Currently,single-sitecatalystsareintenselydis-modeling,[5]inparticularwhenthesystemisexperimentallywell
cussed,alsoforpracticalapplications.[2]Fromaconceptualpointcharacterized.
ofviewaswellaswithregardtocomputationalmodeling,twoGatesandco-workersreported[6]awellcharacterized
classesofsingle-sitecatalystscometomind:(i)singlemetalzeolite-(faujasite-)supportedRh(I)complex,[Rh(CH)]+that
242
atomsonametalormetaloxidesupport[2b,c,e]and(ii)exhibitsahighselectivityfor1-butene(78%),withethaneas
mononuclearmetalcomplexesoninorganicporoussupportssideproduct(19%),whenthefeedisethenerich(C2H4/H2molar
likezeolites,metal-organicframeworks(MOFs),andmetalratio=4:1).Thecatalystworksundermildreactionconditions,
oxides.[2]Thelatterclassofcatalysts,,usinga
hybridquantummechanics/molecularmechanics(QM/MM)
[a],,[5b]weaddressedtheexperimentallyreportedselectiv-
InstituteofHighPerformanceComputingityforethenedimerizationata[Rh(CH)]+complex,probing
AgencyforScience,TechnologyandResearch242
theCossee-Arlmanmechanism[7](CA)andthemetallacycle
1FusionopolisWay,#16-6Connexis[8]
Singapore138632(Singapore)mechanism(MC),-
[b],öschtionverylikelyoccursfromtheactivecomplex[Rh(CH)]+,
InstituteofMaterialsChemistry243
TechnischeUniversitätWienwiththeMCmechanismpreferredovertheCAmechanism.
Getreidemarkt9/BCWiththeinsitugeneratedRh-metallacyclopentanecomplex
A-1060Vienna(Austria)actingastheactivespecies,wewereabletorationalizethe
E-mail:alexander.******@(preferred)and
[c]ösch[5b]
DepartmentChemieandCatalysisResearchCenterethane.
TechnischeUniversitätMünchenLateron,Gatesandco-workersextendedtheirworktoan
-supportedIr(I)complex,[Ir(CH)]+,[9]which
85747Garching(Germany)242
SupportinginformationforthisarticleisavailableontheWWWundertheyfoundtobenotablylessselectiveforethenedimerization
/(11%),at303Kandatmosphericpressure,suggestingthat
©-
Non-CommercialLicense,whichpermitsuse,distributionandreproduction[10]+
inanymedium,providedtheoriginalworkisproperlycitedandisnotusedstudyalsoreportedthatthecomplex[Ir(C2H4)2],supported
-66orNU-1000,islessselectiveforethene
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ethenedimerizationstudiedhereforM=Irasmetalcenter,usingM-triethene
-Arlmanmechanism(CA,top):ethene
insertionintoametal-ethylbond;metallacyclemechanism(MC,bottom):
directcouplingoftwoethenemoietiesformingametallacyclopentane.
dimerization(<5%).Untilnow,thereexistonlyfewtheoretical
studiesfortheimportantsupportedcatalyticsystem[Ir(CH)]+,
242
addressingstructuralfeaturesofthesupportedcomplexes[11]
andsomestepsofetheneconversion,.,hydrogenationand
dimerization.[10,12]Yet,noneofthesestudiesofferedadetailed
mechanisticcomparisonofbothtypesofprocesses.
Inthepresentcomputationalwork,weexploremechanistic
detailsoftheetheneconversionforazeolite-supportedIr(I)
complex1,[Ir(CH)]+,Scheme2,usingDFTcalculationswith
242
theQM/MMapproach[5b]previouslyappliedtoanalogousRh
,[5b]wealsoexplored
themechanismwhenstartingfromthealternativecomplex14,
[Ir(CH)]+,Schemes3,4,
243
mechanisms,usingstartingcomplexes1or14,withtwoor
threeorganicligandsattheIrmetalcenter(etheneorethyl),
theCossee-Arlmanmechanism,pathwayMC1forthezeolite-supported2-
willbeaddressedas2-ligandmodelsand3-ligandmodels,ligandIr(I)complex1,[Ir(C2H4)2]+.Reactionandactivationfreeenergiesof
respectively.[5]individualstepsaregivenasblackandredvalues,
Finally,
leadingtocomplex3,thebranchingpointforthepathwaystoethaneand
theseIr(I)complexesforthecrucialhydrogenationandCC1-.
couplingstepstothepreviouslyreportedresultsforRh(I),
complexes,[5]togaininsightintohowthechoiceofthemetalMthezeolitesupportofthemetalcomplexisrepresentedbytheOAlO
moiety.
(M=IrorRh)mightaffecttheproductselectivity,ethanevs
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commentonthechallengesposedbythetwoclassesofsingle-
sitecatalystsmentionedabove.
ResultsandDiscussion
2-ligandmodels
Westartbybrieflydiscussingthemechanismsofethene
conversionfromcomplex1,[Ir(CH)]+,,wewill
242
comparethecrucialfreeenergybarriersofhydrogenationand
CCcouplingstepstothepreviouslyelaboratedresults,
obtainedwiththesameQM/MMstrategyfortheisostructural
zeolite-supportedcomplex[Rh(CH)]+.[5b]
242
Etheneconversionmechanismforcomplex1,[Ir(CH)]+.
242
Scheme2presentsthepathwaysofethenehydrogenationand
dimerizationstudied,startingfromcomplex1,[Ir(CH)]+;see
242
alltheschemespresentedinthisstudy,weproviderelativefree
energybarriersofeachstep,togetherwiththecorresponding
showsacompleteenergyprofileincludingalternative,less
favorablepathways.
centerofcomplex1,anendergonicstep,G(1#2)=47kJmol1,
r
leadstocomplex2,[Ir(CH)(H)]+,
2422
oxidativeadditionofH2attheIrcenterof2ishighlyexergonic,
G(2#3)=77kJmol1,-determin-
r
ingsteprequiresovercominganoverallfreeenergybarrierof
G(1#2#3)=51kJmol1,Scheme2andFigure1,thatislower
a
byonly3kJmol1thanintheanalogousstepforthecomplex
[Rh(CH)]+,54kJmol1.[5b]NotethatComplex3actsasa
242
branchingpointonthewaytoethenehydrogenationor
dimerizationviathemetallacyclemechanism,pathwayMC1,
Scheme2andFigure1.
Hydrogenationoftheetheneligandin3yieldstheslightly
morestablecomplex4,[Ir(CH)(CH)(H)]+,G(3#4)=
2425r
6kJmol1,viaamoderatebarrier,G(3#4)=,
a
coordinationofafurtherH2moleculetotheIrcenterof4
producescomplex5,[Ir(CH)(CH)(H)(H)]+,withanendergonic
24252
freeenergychangeG(4#5)=18kJmol1,Scheme2andFig-
r
ethanecomplex6,[Ir(CH)(H)(CH)]+,G(5#6)=25kJmol1,
24226r
featuringamoderateoverallfreeenergybarrier,Ga(4#5#6)=
49kJmol1,,complex6releases
theproductethaneintothegasphase,closingthecatalytic
cycleat3bycoordinatingafurtherfreeetheneligand,Gr(6#
3)=143kJmol1,
etheneuptakeinvolvesfirstatransferintothezeolitecavity,a
-ligandreleasingethaneorbutene,thesetwostepsarereversedin
-,,forhydrocarbons(ethene,ethane,andbutene)
themintothezeolitecavity,about5kJmol1,whichwewill
notaccountforwhendiscussingvariantsofmechanisms.
butene,andthepreferentialethenedimerizationmechanism,Thus,differentfromtheanalogousprocessattheRh
,wewillalsobrieflycomplex,theethenehydrogenationattheIrcomplexavoids
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zeolite-supported2-ligandIr(I)modelcomplex1,[Ir(C2H4)2]+.Freeenergies(kJmol1)withrespecttocomplex1aswellasH2andC2H4inthegasphase,at
-lyingpathwaysareshown;forvariants,
andshowninsketches;:redproductethane;green
productbutene;blackjointsectionofthepathway.
,
energybarrierviaTS2#3,51kJmol1above1,byrestartingoneoftheα-Hmoietiesistransferredtothemetalcenter,
,thispathwayformingthemetallacyclopentenylcomplex33,Gr(1#33)=
beginningatcomplex1utilizestwomoleculesofHonlyonce,25kJmol1,
2
,G(1#33)=176kJmol1,whichmaybe
a
30kJmol1,onedeterminesafreeenergychangeofduetotherelativelylowcoordinationnumberofthemetal
106kJmol1forconvertingonemoleculeofetheneandHtocenterwhenitsoxidationstatechangesfrom+1to+3.
2
ethane,,asquarepyramidalstructure,iscoordinatively
wellwiththecorrespondingexperimentalfreeenergychangeunsaturatedanddoesnotleadtoanyoftheproductsunder
of101kJmol1at298Kand1bar.[13]Notethatforthestudieddiscussion,,werefrainedfromdiscus-
2-ligandIrmodels,wedetermin
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