Atwood Leonard J Barbour Praveen K Thallapally and Trevor B Wirsig Received in Cambridge UK 4th November 2004 Accepted 11th November 2004 First published as an Advance Article on the web 30th November 2004 DOI 101039b416752j A wellknown host compoun ID: 53257
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bysublimationundervacuum,eachcapsuleencloses235Aunoccupiedspace.Afterexposureofnewlysublimedcrystalstoairforseveralhours,X-raystructuralanalysisrevealedsignificantresidualelectrondensitywithintheconfinesofthecapsule.ThisobservationimpliesthepresenceofOandNasaresultofsorptionofthesegasesunderambientconditions.Indeed,subsequentsorptionexperimentsinvolvingthesegasesconfirmedthishypothesis.Inspectionofastaticmodelofthecapsuleanditsimmediatemolecularsurroundingsrevealsnoatomic-scaleporesthatbreachthecapsulewalls.Atfacevalue,thisimpliesthatthecapsuleshouldbeimperviouseventosmalllinearmoleculessuchasCO.However,sincethesegasmoleculesclearlydisplayasignificantdegreeofmobilityintheseeminglynonporouscrystallattice,andevenforcetheirwayintothecapsulevoids,wecanconcludethatthestaticmodelderivedfromcrystalstructureanalysisdoesnotcomportaccuratelywithreality.Itisreasonabletoassumethatthe-butylmoietiesattheupperrimsofthecalixarenemoleculesplayakeyroleinfacilitatinggasuptake.Inthesolidstatethearomaticandhydroxylregionsofthemoleculesaregenerallyconsideredtobestructurallyrigid,whilethe-butylgroupscanconceivablyrotateaboutthe)bond.At298Kallfourofthe-butylgroupsofthecalixareneareeachrotationallydisorderedovertworelativelywell-definedpositions.Thisdisorderappearstobeprimarilystatic,althoughasignificantdegreeofdynamicdisplacementofthemethylgroupsisalsoevident(asinferredfromthermalellipsoidmodels).The-butylgroupsformabroadequatorialbandencirclingthecapsulevoidanditthereforeseemsplausiblethattheymightrotatecooperativelytocreatetransientwindowsofopportunityduringwhichgasmoleculesareabletodiffusethrough.InourrecentstudyofCOuptakebyweshowedthatonlyasinglemoleculeofthegascanbeaccommodatedwithineachvoidformedbytwocalixarenemolecules(i.e.host:guest2:1).Foragiventemperaturetheproportionoflatticevoidsoccupiedbytheguestisdependentonpressure;at298K,100%occupancyisonlyreachedunderapproximatelythreeatmospheresofCOAmoleculeofCO.5.2Alongwithamaximumradialdiameterof.3.4A(Fig.3).Incontrast,amoleculeofmethaneismoreisotropicallyshapedwithanaveragediameterof.3.8AThus,accordingpurelytosizeshapeconsiderations(Fig.4),itshouldbepossibletoaccommodateatleasttwoCHwithineachdimericcapsule(host:guest1:1).Baseduponthisassumption,ourpreliminaryresultindicatesthat14%ofthecapsulesare,onaverage,eachoccupiedbytwomoleculesofmethaneat0.54atm.Inrecentyearsthesearchforalternativefueltechnologieshasintensifiedrapidly.Theviablestorageofgasesforuseinfuel-cells,especiallyformobileapplications,hasbeenidentifiedasaparticularlypressingproblemtobesolved.Althoughnewmaterialsconsistingofporousmetalorganicframeworkshavebeensuggestedaspromisinggasstoragesubstrates,purelyorganicmolecularcrystalshavereceivedlittleattention.Thepaucityofdatawithregardtothelatterismostlikelyduetotheirtendencytoclose-pack.Ourresultsdemonstratethatlowdensityorganicsystemsdoindeedmeritconsiderationaspotentialsorbantsforvolatilegases,andthatsuchsorptionprocesscanoccurunderdesirableconditionsclosetostandardtemperatureandpressure.JerryL.Atwood,LeonardJ.Barbour,PraveenK.ThallapallyTrevorB.WirsigDepartmentofChemistry,UniversityofMissouri-Columbia,ColumbiaMO,65211,USA.E-mail:atwoodj@missouri.edu;Fax:5738822754;Tel:5738822730DepartmentofChemistry,UniversityofStellenbosch,Stellenbosch,7602,SouthAfrica.E-mail:ljb@sun.ac.za;Fax:27218083849;27218083335 Fig.2Isothermalsorptionofmethaneby-butylcalix[4]areneat298K. Fig.3SpacefillingmodelsofCH(green)andCO(red/blue)showingapproximatevanderWaalsdimensions. Fig.4Asectionthroughthedimericcapsuleformedbytwofacing,butslightlyoffsetcalixarenemolecules.Forclarity,theindividualcalixarenesarecoloredblueandredwiththesectionedsurfaceshownasayellowplane.Twoidealizedmoleculesofmethane(green)havebeenplacedinsidethedimericcapsulebymeansoftheprogramX-Seedinordertoillustratetheirexcellentsizeshapecompatibilitywithregardtotheavailablespace. Chem.Commun.,2005,5153ThisjournalisTheRoyalSocietyofChemistry2005 Notesandreferences1J.L.Atwood,F.Hamada,K.D.Robinson,G.W.OrrandR.L.Vincent,,1991,,683;G.W.Orr,L.J.BarbourandJ.L.Atwood,,1999,,1049;J.L.Atwood,L.J.Barbour,M.W.HeavenandC.L.Raston,Angew.Chem.Int.Ed.,2003,3254;J.L.Atwood,L.J.Barbour,S.J.Dalgarno,M.J.Hardie,C.L.RastonandH.R.Webb,J.Am.Chem.Soc.,2004,,13170.2J.L.Atwood,L.J.BarbourandA.Jerga,,2002,296,2367.3J.L.Atwood,L.J.Barbour,A.JergaandB.L.Schottel,,2002,298,1000.4J.L.Atwood,L.J.BarbourandA.Jerga,Angew.Chem.Int.Ed.,2004,,2948.5J.L.Atwood,L.J.BarbourandA.Jerga,Chem.Commun.,2002,2952;E.B.Brouwer,G.D.Enright,K.A.Udachin,S.Lang,K.J.Ooms,P.A.HalchukandJ.A.Ripmeester,Chem.Commun.,2003,1416.6J.L.Atwood,L.J.Barbour,G.O.LloydandP.K.Thallapally,Commun.,2004,922.7L.J.Barbour,J.Supramol.Chem.,2001,,189;J.L.AtwoodandL.J.Barbour,Cryst.GrowthDes.,2003,,3.8R.F.Service,Science,2004,305,958;V.C.MenonandS.Komarneni,J.PorousMater.,1998,,43.9M.Eddaoudi,J.Kim,N.Rosi,D.Vodak,J.Wachter,M.OKeeffeandO.M.Yaghi,Science,2002,,469;K.Seki,Chem.Commun.,2001,1496;S.Noro,S.Kitagawa,M.KondoandK.Seki,Angew.Chem.Int.,2000,,2082. ThisjournalisTheRoyalSocietyofChemistry2005Chem.Commun.,2005,5153 AcrystallineorganicsubstrateabsorbsmethaneunderSTPconditions JerryL.Atwood, * a LeonardJ.Barbour, * b PraveenK.Thallapally a andTrevorB.Wirsig a Received(inCambridge,UK)4thNovember2004,Accepted11thNovember2004 FirstpublishedasanAdvanceArticleontheweb30thNovember2004 DOI:10.1039/b416752j Awell-knownhostcompoundreadilyabsorbsmethaneat roomtemperatureandpressuresofoneatmosphereandlower. Ourlong-standinginterest 1 inthehostguestchemistryof calixareneshasrecentlyevolvedintostudiesofthesolidgas sorptionphenomenaexhibitedbythesefascinatingcompounds. Forexample,wehaveshownthatcalix[4]arenehastheabilityto entraphighlyvolatilegasessuchashalons,freonsandmethane withinlatticeinterstices. 2 Althoughthecrystalsareprimarily stabilisedbyvanderWaalsinte ractions,guestssuchasCF 3 Br, CF 4 andCH 4 areretainedattemperaturesofupto260,370and 320 u C,respectively,abovetheirnormalboilingpoints.Inthis system,thehostlatticemustbedisassembledthermallyinorderto releasetheguestmolecules.Whilesuchtenacioushostguest stabilityisusefulforgasseques tration,topicalapplications involvinggas-uptake( e.g. temporarystorageandseparation) generallyrequirereadilyreversibl esorptionreactionssuchasthose observedforactivatedcarbonsandzeolites.Furthermore,to minimizetheexpenditureofenergyduringeithersorptionor releaseofthegas,theidealcrystallinehostmatrixshouldnot undergoasignificantphasetransformationduringtheseprocesses. Althoughaparticularapohostphaseofthewell-knownhost compound p-tert -butylcalix[4]arene, 1 (Scheme1),obtainedby sublimation,undergoesatransitionconcomitantwiththeuptake oforganicvaporssuchasvinylbromideandtoluene, 3 this apparentlydoesnotoccurwhentheguestisagas.Inarecent studywerevealedthat 1 selectivelyincludesCO 2 fromamixture ofH 2 andCO 2 ,andthatthisphenomenoncanbeexploited topurifyeithergas. 4 Whereasitisknownthat 1 willundergo subtle,thermally-inducedphasechanges, 5,6 nodiscernablephase transitionofthehostlatticeoccursduringtheCO 2 sorption process.Untilnowourstudiesofthegasinclusionpropertiesof 1 havebeenconfinedtoalimitedselectionofgases.Hereweshow thatapreviouslydescribed 4 low-densityphaseof 1 iscapableof absorbingmethaneatroomtemperatureandpressuresofone atmosphereorlower. Inordertorecordgassorptionisotherms,weconstructedthe deviceshownschematicallyinFig.1.Theapparatuswasinstalled insideaninsulatedcabinetwh ichcouldbethermostatedata constanttemperature.Apowderedsample(1.403g)ofsublimed 1 wasplacedinchamberB(volume V B 5 12.6cm 3 ).Chambers AandBwereevacuatedthroughvalveV 3 withV 2 openandV 1 closed.ChamberBwasthensealedundervacuumbyclosingV 2 andV 3 .ThepressuresinAandBwererecordedautomaticallyby meansofelectronictransducersP A andP B (WikaECOseries) interfacedtoacomputerequip pedwithananalog-to-digital converter.Aknownamount( P A 5 1503torr)ofmethanewas introducedintochamberA( V A 5 15.4cm 3 )throughvalveV 1 . ValveV 2 wasopenedforapproximatelyonesecondinorder toequalisethemethanepressureinchambersAandB( i.e. P AB 5 [ P A 6 V A ]/[ V A + V B 2 V sample ]).AfterclosingV 2 ,the pressureinBbegantodecreaserapidlyasthegaswasabsorbedby thecalixarenecrystals(Fig.2).Afterapproximately2.5hours,the pressureinchamberBreachedane quilibriumvalueof410torr.At thispressure,themolarratioofmethanetocalixareneiscalculated tobe0.14:1. Althoughtheroomtemperature crystalstructureofthelow- densitypolymorphof 1 hasbeendescribedelsewhere, 5 itisuseful toreaddresssomesalientfeatures ofthisremarkablematerial.The structureconsistsofdimericcapsulescomposedoffacing,but slightlyoffsetcalixarenemolecules.Sincethecrystalsareformed *atwoodj@missouri.edu(JerryL.Atwood) Scheme1 Fig.1 Schematicdiagramofadeviceforthemeasurementofgas sorptionisotherms.Withtheexcep tionofthepressuretransducersP A and P B ,theapparatusconsistsofmodular brasscomponentsmanufacturedby Swagelok H .Thesolidsample(grey)isplacedinchamberB. ljb@sun.ac.za(LeonardJ.Barbour) COMMUNICATIONwww.rsc.org/chemcomm |ChemComm Thisjournalis TheRoyalSocietyofChemistry2005 Chem.Commun. ,2005,5153 | 51