二茂铁较准电化学电位

2854chloromanganatesystems,sincethetetrachloroferratesaltsareknowntoundergotransitionssimilartothetetrachloro-manganates,butathighertemperature(87K).ConclusionInorg.Chem.1980,19,2854-2855layercouplingbecauseofthecationiccharacteroftheguestspecies.Anisotropyinthe2-DHamiltonianisthusrevealed,withtheOccurrenceofweakferromagnetismasaconsequence.RegistryNo.MnPS3,43000-56-2;I,74346-92-2;11,74346-93-3;ThisworkhasshownthatMnPS3behavesasatwo-di-mensionalantiferromagnet.Intercalationreducestheintra-111,74346-94-4;IV,14346-95-5.CorrespondenceFerroceneasanInternalStandardforElectrochemicalMeasurementsSir:Electrochemistryisanincreasinglypopulartechniqueforthecharacterizationofnewcompounds.Thebasicthermo-dynamicquantitythatisassignedtoanelectrodeprocessisthestandardorformalreductionpotential(EoorEf).Inaqueoussolutionthemeasurementofreductionpotentialsisfacilitatedbytheuseofreliableanduniversallyacceptedreferenceelectrodessuchasthenormalhydrogenelectrode(“E)orthesaturatedcalomelelectrode(SCE).’Inmanyinstanceselectrochemicalmeasurementsinwaterareimpos-sibleduetoinsolubilityorinstabilityofthecompound.Un-fortunately,nouniversalreferenceelectrodeexistsfornona-queoussolvent~.~JSinceavarietyofreferenceelectrodesystemshavebeenemployed,theliteraturecontainsnumerousreductionpotentialswhichcannotberelatedtoeachotherandwhichare,occa-sionally,difficulttoreproduce.Forexample,twocommonmethodsforreportingnonaqueouspotentialsarevs.theSCEwhichinevitablyincludesunknownliquidjunctionpotentials2andvs.pseudoreferenceelectrodessuchasasilverwire.Webelievethatthesituationdescribedaboveneednotexist,andonthebasisppfconceptsanddataalreadyintheliterature,wesuggestasimplemethodforreportingreductionpotentialsinnonaqueoussolvents.InternalStandard.Duringthecourseofourstudiesofcopper-macrocyclicligandcomplexeswehavefoundthatapracticalapproachtothisproblemistheuseofthepotentialoftheoxidationofferroceneasaninternal~tandard.~-*OAftertheelectrochemistryattributedtothecompoundofinteresthasbeenidentified,ferroceneisaddedtotheworkingcom-partmentofthecell.Theelectrochemicalexperimentisre-peated,andthepositionofthewavescanbedirectlycomparedtothepotentialoftheferrocenium/ferrocene(Fc+/Fc)couple.Figurelaillustratesthismethodbyshowingacyclicvoltam-mogramoftris(acetylacetonato)ruthenium(III)inCH3CN.,,,,I,,Y,‘8,1’,,,,I,,,,,Figure1.Platinumbuttoncyclicvoltammetryat50mV/sof0.005MRu(acac),inCH3CNwith0.1Mtetrabutylammoniumperchlorate:b,c,andd,ferroceneadded;aandb,vs.Ag/AgN03(0.01M),TBAP(0.1M),CH,CN;c,vs.SCE;d,vs.Cuwire.Figure1bshowsthesamevoltammogramafteradditionofasimilaramountofferrocene.UseoftheformulaEf=(E+~~(~~~~~~))/2allowscalculationofformalpotentialspdrtheprocessesinFigure1of0.602and-1.157Vvs.Ef(Fc+/Fc).Animportantadvantageofthismethodisthatchoiceofthereferenceelectrodeusedtosatisfythethree-electrodepo-tentiostat(aAg/Ag+electrodeinFigurela,b)isreducedtoamatterofexperimentalconvenience.PartscanddofFigure1wererecordedunderconditionsidenticalwiththosefor-6-14-12-10-oa-06-c4-02o02040608IOIL1416E(volts)FigurelbexceptthatanSCEwasusedasareferenceelectrodeforFigureIC,whileacopperwirewasusedforFigureId.shifted,butformalpotentialsrelativetoEf(Fc+/Fc)remainunchanged.Formalpotentialsobtainedinthemannerde-scribedaboveshouldbereproduciblebecausevariablessuchastypeofreferenceelectrode,liquidjunctionpotentials,andreferenceelectrodedegradationareeliminated.”TheFc+/Fccouple(Eo=0.400Vvs.“E)’*maybe(1)Willard,H.H.;Merritt,L.L.;Dean,J.A.“InstrumentalMethodsofAnalysis”;VanNostrand-Reinhold:NewYork,1965;pp531-540.(2)Mann,C.K.;Barnes,K.K.‘‘ElectrochemicalReactioninNonaqueousSystems”;MarcelDekker:NewYork,1970.(3)Mann,C.K.Electroanal.Chem.1969,3,57-134.(4)Gagnt,R.R.;Allison,J.L.;Gall,R.S.;Koval,C.A.J.Am.Chem.(5)Gagn&,R.R.;Koval,c.A,;Smith,T.J.J.AM.Chem.soc.i977,99,8367-8368.FromthevaluesonthepotentialaxisthewavesappeartobeSOC.1977,99,7170-7178.(6)Gagnt,R.R.;Allison,J.L.;Lisensky,G.C.Inorg.Chem.1978,17,(7)Gagnt,R.R.;Spiro,C.L.J.Am.Chem.Soc.1980,102,1443-1444.(8)Gagnt,R.R.;Cimolino,M.C.;Koval,C.A,;Smith,T.J.J.Am.Chem.SOC.1979,101,4571-4580.(9)Gagnb,R.R.;Allison,J.L.;Ingle,D.M.Inorg.Chem.1979,101,2767-2774.3563-3571.(11)Ferroceneaswellasothermaterialshasbeenusedasanexternalstandardtodeterminesolventandjunctionpotentialcorrectionfactors.Oneshortcomingofthisexternalapproachisjunctionpotentialvaria-bilityfromexperimenttoexperiment.(12)Koepp,H.M.;Wendt,H.;Strehlow,H.Z.Elektrochem.1960,64,(10)Gagnt,R.R.;Kreh,R.P.J.Am.Chem.Soc.1979,101,6911-6921.483-491.0020-1669/80/1319-2854$01.00/001980AmericanChemicalSocietyAdministrator高亮Administrator高亮Administrator高亮Administrator高亮Inorg.Chem.1980,inappropriateasaninternalstandardforsomesystemsduetooverlappingwaves.Inthesecasesothercompoundssuchascobaltocene(EO=-0.918Vvs.NHE)12oranyofavarietyofaromaticcompounds,comprisingavirtualcontinuumofreductionpotentials,canbesubstituted.13PotentialscanstillberelatedtoEf(Fc+/Fc)throughasecondexperiment.Employingaredoxcoupleasaninternalstandardinelec-trochemicalexperimentscanbecomparedtotheuseofinternalstandardsinnuclearmagneticresonancespectroscopy.Forexample,inprotonNMRspectroscopytetramethylsilaneiscommonlyusedtoreferencechemicalshiftsofotherprotonsbutissometimesreplacedbychloroformorothermaterialstoavoidoverlappingsignals.AswithNMRstandards,anidealelectrochemicalinternalstandardshouldnotinteractwithotherspeciesinsolution.Ferroceneadequatelyfillsthiscriterion.Useofferroceneasaninternalstandarddemandstheob-servationofreversibleelectrochemistryfortheFc+/Fccouple.Reversiblebehaviorhasbeenobservedinawidevarietyofsolventswiththefollowinglimitations:(a)inwaterandal-coholstheinstabilityoftheferroceniumioncausesreductionpotentialsmeasuredbypotentiometrytodifferfromvaluesmeasuredbyfastertechniques;(b)cyclicvoltammetryatslowscanrates(10-50mVs-l)resultsinreversiblebehavior,whilefasterscanrates(>1Vs-l)revealquasi-reversiblebehav-ior.l4-I6Cyclicvoltammetricpeakseparations,AEp,serveasonecriterionforelectrochemicalreversibility.Forexample,aone-electronprocess(n=1)isexpectedtohaveAEp=59mV.Highsolutionresistance,especiallyinnonaqueoussolvents,canresultinlargepeakseparations,complicatingwaveshapeanaly~is.l~.~*StateoftheartelectrochemicalinstrumentationprovidesacapabilityforcompensatingforIRdropinanat-tempttoovercomesolutionresistancecontributionstotheshapesofcyclicvoltammetricwaves.Unfortunatelysomeinstrumentationpermitsovercompensation,sometimesresultinginmeaninglessresults.Forexample,wehavebeenabletoobtainpeakseparationsof40-50mVforboththeFc+/FcandR~(acac)~/Ru(acac)~-couplesbyusinginstrumentalIRcompensation.InpracticewedonotuseinstrumentalIRcompensation,butsimplycompareAE,forthecoupleofin-terestwiththatoftheFc+/Fccoupleunderthesamecondi-tions.Itshouldbeemphasizedthat,whilewaveshapecom-parisonsmaybeuseful,reproducibilityofreductionpotentialmeasurementsisthekeyadvantageoftheferrocenereferencingsystem.ThermodynamicSignificance.Thenotionofferroceneasaninternalreferencecouplestemsinpartfromattemptsbyotherstocompareredoxpotentialsobtainedindifferentsol-vents.Thepotentialscaleinwaterisestablishedbyassigningthepotentialofthehydrogenhalf-reactionaszero.Sincetheactivityoftheprotonisnotnecessarilythesameinallsolvents,asimilarconventionestablishesadifferentscaleineachsolvent.Theproblemsassociatedwithmeasuringandcom-paringreductionpotentialsonthesamescaleinawidevarietyofsolventshasreceivedconsiderableattentionthroughat-temptstoestimatefreeenergiesoftransferofsingleions,AGt,.lg(13)Bauer,D.;Beck,J.P.Bull.SOC.Chim.Fr.1973,1252-1259.(14)Alexander,R.;Parker,A.J.;Sharp,J.H.;Waghorne,W.E.J.Am.Chem.SOC.1972,94,1148-1158.(15)Diggle,J.W.;Parker,A.J.Electrochim.Acta1973,18,975-979.(16)Delahay,P.J.Am.Chem.Sor.1953,75,1430-1435.(17)Adams,R.N.“ElectrochemistryatSolidElectrodes”;MarcelDekker:NewYork,1969;pp143-162.(18)Nicholson,R.S.Anal.Chem.1965,37,667-671,1351-1355.(19)Forexamplesee:(a)Popovych,0.“TreatiseonAnalyticalChemistry”,2nded.;Kolthoff,I.M.,Elving,P.J.,Eds.;Wiley:NewYork,1978;Vol.1,pp711-771;(b)Popovych,0.CRCCrit.Rev.Anal.Chem.1970,1,73-117;(c)Bauer,D.;Breant,M.Electroanal.Chem.1976,8,282-348;(d)Strehlow,H.“ChemistryofNon-AqueousSolvents”;Lagowski,J.J.,Ed.;AcademicPress:NewYork,1966;pp129-171.0020-1669/80/1319-285.5$01.00/0<19,2855-28572855Theseestimatesrequiretheuseofanextrathermodynamicassumption.One-suchassumptionisthattheactivityofaunivalent,large,symmetricalion,withthechargedeeplyburied,isthesameastheactivityofanunchargedmoleculeofthesamesizeandstructureinallsolvents.Strehlowpro-posedthatFc+/Fcfitstherequirementsandassuchisasolvent-independentredoxcouple.12ThusEo(Fc+/Fc)inanysolventisassumedtobethesameasmeasuredinwater,i.e.,0.400Vvs.NHE,andpotentialsvs.theFc+/Fccouplearedirectlycomparablebetweensolvents.Alternatively,tabula-tionsofAG*(Ag+)basedonotherassumption^'^^^^canbeusedtocalculatethepotentialofAg/Ag+referenceelectrodes(Pleskovelectrpdes)vs.NHEinwater.21Itisnotobviouswhichoftheextrathermodynamicas-sumptionsusedtointerrelateelectrochemicalpotentialsmeasuredinvarioussolventsismorevalid,andwedonotpresumeinprincipletofavoranyoneassumptionoveranother.Inpractice,reproducibilityofpotentialsvs.theAg/Ag+Pleskovelectrodeareadverselyaffectedbytheinsolubilityofsilversaltsinsomesolventslikedichloromethane,3byreactionsofAg+withsomesolventssuchasacetoneandN,N-di-methylf~rmamide,~orbytraceimpuritiessuchaswaterwhichinteractstronglywithAg+.14Likewise,referenceelectrodescontainingknownconcentrationsofFcandFc+areunstableduetodecompositionofferroceniumion.14J5Instead,weprefertheinternaluseofferrocenebecauseitisconvenient,isreproducible,providesabasisforjudgingelectrochemicalreversibility,andallowsforaquantitativecomparisonofre-ductionpotentialsmeasuredinvarioussolvents.Acknowledgment.WeappreciatefinancialassistancefromtheNationalScienceFoundationandaSloanFellowshiptoR.R.G.RegistryNo.Fe,102-54-5.(20)Cox,B.G.;Hedwig,G.R.;Parker,A.J.;Watts,D.W.Aust.J.Chem.1974,27,477-501.Kolthoff,I.M.;Chantooni,M.K.,Jr.J.Phys.Chem.1972,76,2024-2034.(21)&HE(Ag+,s)=E0”E(Ag+,HzO)+(AG,,(Ag+,H20.-S)+RTIn[aAg+l)/nFwhereEo”e(Ag+,HzO)=0.799VandSisthesolventofinterest.I5(22)ContributionNo.6139.DivisionofChemistryandChemicalCaliforniaInstituteofTechnologyPasadena,California91125RobertR.GagnE*EngineeringZZCarlA.KovalGeorgeC.LisenskyReceivedDecember6,1979TransferabilityofLigandFieldParametersandNonlinearLigationinChromium(II1)ComplexesSir:Inapreviouspaper’wehavedemonstratedthattransferableligandfieldparametersofanadditivefieldquitesatisfactorilycouldaccountforourexperimentalspectralresultsforaseriesoftruns-tetraammine-andtetrakis(pyridine)chromium(III)complexes.Weusedtheangularoverlapmodeltodescribethechromium(II1)-pyridinebondandobtainedanegativeA’.p,,forpyridine.ThislatterresulthasactedasanimpetusforarecentpaperinthisjournalbySmith2inwhichthisresultisquestioned,partlybyreviewingourpaperinafashionwhichwefindunsatisfactory.Thepresentpaperisanattempttoclarifyandextendcertainpointsinourpreviouspaperandalsotodiscuss(1)Glerup,J.;Monsted,0.;Schaffer,C.E.Inorg.Chem.1976,15,1399.(2)Smith,D.W.Inorg.Chem.1978,17,3153.j,1980AmericanChemicalSocietyAdministrator高亮Administrator高亮