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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Organometallics

Reactivity of Organometallics L. J. Farrugia MSc Core Course C5 Textbooks:InorganicHousecroft&SharpeCh23theverybasics InorganicChemistryShriver&AtkinsCh16theverybasics This course assumes familiarity with the Level-2 and Level-3 courses on , and this is covered in the above texts Organometallics Elschenbroich & Salzer (library) much more useful Topic 1 - Introduction to Cyclopentadienyl Compounds First part of course covers the role of the cyclopentadienyl (Cp) and its derivatives.CpisoneofthemostimportantinorganometallicsafterCO. Aconsiderablepercentageoforganometalliccompoundscontainthisliganditis alsoagoodligandformaingroupandthe fmetals(lanthanides& ).

H

H H

Fe CO H OC CO Cyclopentadienecomplex(4edonorligand) veryrareasaligand

Na/H2 H - Na+ H acidicH planararomatic (6pi) dienylanion The anion C 5H5 is a very useful synthetic reagent. It is usually treated as equivalent to occupying THREE coordination sites, so that C 5H5 ≡ 3(CO). In electroncountingterms,itcanbetreatedaseitherasa6edonorANIONora5 edonorNEUTRAL.Thelatteristherecommendedapproachbecause itissimpler(donotneedtoworryaboutoxidationlevels). 1.1 Bonding in cyclopentadienyl compounds Cp has 5 π in the 5 outofplane porbitals on theC . These 5 orbitalscombineasa 1+e 1+e 2underfivefoldsymmetry.WithasingleCpring,

1 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics inspectionshowsthepossiblecombinationswiththemetal dorbitalsareshown below

Cporbitals Metalorbitals Symmetryofbond a1 pz dz2 , s σ e1 dxz dyz px py π e2 dx2y2 dxy δ

FortwoCpringsinaM(C 5H5)2theringsmaybeeither staggered or eclipsed -Fe(C 5H5)2iseclipsedbutCo(C 5H5)2andNi(C 5H5)2arestaggered.

Thebondscanalsobedividedintosigma,pianddeltasymmetryasshownin OHP#1 OHP#2showstheformalMOinteractiondiagramforFe(C 5H5)2 Themainpointstoremember (i)the9filledorbitalshave18electronshencetherule! (ii)theLUMOisadoublydegenerate π*orbital,soNi(C 5H5)2isparamagnetic

2 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Thisshowshowthemetalandring πorbitalsmatchbysymmetry.Theresultant MOschemeforferroceneshowsthewaythatthebasisorbitalshavingthesame symmetry can combine to give new orbitals (NOT necessary to remember the MOscheme) Thefillingoftheninebondingorbitalsinferroceneexplainsthehighstabilityof thiscompound.ThemixingofmetalandCporbitalsindicatesastrongcovalent charactertothetransitionmetalcyclopentadienylbond. In Co(C 5H5)2 ONE electron fills the e* 1g while in Ni(C 5H5)2 there are TWO unpairedelectronshencebothcompoundsareparamagnetic

3 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

+ Co(C 5H5)2readilylosesanelectrontogivethecobalticiniumcation[Co(C 5H5)2] an 18e cation. Similarly Ni(C 5H5)2 loses one electron to give a 19e cation, but furtheroxidationresultsindecomposition.

1.2 Structural types of Cp compounds (i) MCp 2

TheseareknownforthemetalsTi,V,Cr,Mn,CoandNi.Staggeredoreclipsed ringsleadsto D5h or D5d symmetrywithvirtuallyNObarriertorotationoftheCp ringaboutthemetalCpaxis.SoallCpprotonsappearasequivalentinthe 1H NMRspectrum V,Cr,Fe,Co andNigivethe"classic"sandwichcompoundsillustrated above. Theexceptionsare

4 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

(a)Titanocene"TiCp 2"

ThisisreallyacomplexmadebyreducingCp 2TiCl 2.Itcontainsbridging * and a TiTi bond gives 16 e Ti atoms. Real "titanocene" TiCp 2 has recentlybeenmadebutisextremelyreactive. (b)MnCp 2 Thisisionicatlowtemperature,withapolymericchainlikestructure

Above159 oCitbecomesisomorphouswithferrocene,somustadoptasandwich structure. Allthesandwichmetallocenesapartfromferroceneareparamagnetic No.unpairedelectrons Cp 2V 3e Cp 2Cr 2e 2+ Cp 2Mn 5e highspinMn Cp 2Co 1e Cp 2Ni 2e (ii) 'bent' metallocenes These have nonparallel rings due to the presence of other ligands. Some examplesare

H Cl W Ti V CO Re H H Cl

18e16e17e18e

5 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

(iii) half-sandwich compounds - piano stool compounds ThesehaveoneCpringandavarietyofotherligands.Someexamplesare

OC V CO Mn Ru Co PPh3 OC CO Cl CO OC CO PPh3 OC CO 4leggedstool3leggedstoolChem3lab2leggedstool The17electronspeciesCpMo(CO) 3andCpFe(CO) 2arenotstableassuch,but dimerise to give the familiar compounds with a MoMo or FeFe metalmetal bond.Thisistypicalbehaviourofoddelectronorganometallicspecies. (iv) other types of bonding mode 5 Sofaronlythesocalledeta5 η C5H5bondingmodehasbeenillustrated,where (inprinciple)allfiveCatomsareequallybondedtothetransitionmetal.However 3 1 thereareotherpossibilities,mostcommonare η C5H5and η C5H5 H M

M

1 3 η C5H5 η C5H5 1edonorlikealkylgroup3edonorlikeallylgroup

H

Mo

ON

5 One good example is Mo(NO)Cp 3 with "normal" η C5H5 bonding modes, electroncountinggivea24electroncompound(!!)simplynotpossible.Infactit isanexampleofacompoundcontainingallthreetypes. 1 η C5H5ligandsareusuallyfluxional.Considerthecompound(C 5H5)4Tiagain 5 cannotbe4 η C5H5bondingmodesbecauseitwouldbea24ecompound.In 5 1 thiscaseithastwo η C5H5andtwo η C5H5ligands.

Ti

6 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Attheverylowesttemperaturemeasured,the 1HNMRspectrumshowsasinglet 5 for the two (equivalent) η C5H5 groups and an AA'BB'C multiplet for the two 1 (equivalent) η C5H5groups(threedifferentHenvironments). Thiscompoundshowstwofluxionalprocesses 1 (i)migrationofmetalatomaroundthe η C5H5groupvia1,2shifts(ringwhizzing) 1 5 (ii)exchangeofthe η C5H5and η C5H5groups Process(i)isaverylowenergyprocess,whichisfrozenoutonlyatthelowest temperatures. Process (ii) occurs at higher temperatures ~ room temperature. Thenetresultof(i)and(ii)isthatall20protonsappearequivalentatthehighest temperaturesmeasured. Process(i)could in principle occuralsoby1,3shiftsorrandomshifts.Howcan wetellwhich?WecanifitispossibletoassigntheprotonsoftheAA'BB'signal

M c c M a M a a a b c

a

b b b b b a

1,2shift Theresultingexchangesare: a→c a→b i.e. both atype protonsexchange b→a b→b i.e. only half of btype protonsexchange c→a So...therate ofbroadeningforthe atype protonsis twice as fast asforthe b type protons 5 1 The actual spectrum of ( η C5H5)( η C5H5)Fe(CO) 2 which exhibits a similar exchangeisshownbelow.

7 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

1.3 Oxidation states in Cp compounds Since Cp is formally charged with a ve charge, each Cp ligand present in a compoundcontributesacharge of+1towardstheformaloxidationstate ofthe metal.Henceametalmustbeina+veoxidationlevel.Theonlyexceptionwould bewith+vechargedligandsNO +istheonlycommonexample.CpNi(NO)hasa zerovalentNiatom. Intermsofitsabilitytostabilizeoxidationstates,Cpiscomfortablewithbothlow andhighoxidationstatesofthemetal(unlikemanypiligandslikeCOwhich areonlyfoundforlowoxidationstates).IngeneralCpisagoodsigmaandpi donor,butapoorpiacceptor.SubstitutingHforMeontheCpringmakesitan evenbettersigmadonorCp*isthecommonsymbolforC 5Me 5.

H2O2

CO Re Re O CO O OC O CO

Re(+1) Re(+7) oxocomplex

8 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Topic 2 - Reactivity of Cyclopentadienyl-type Ligands VeryoftenCpisa spectator ligand(i.e.itdoesnottakepartinthereactionandit isunchangedattheend).Undersomecircumstanceshowevertheringwillreact. Ferrocene Fe(C 5H5)2 has been the most studied (because it is a stable 18 e metallocene) but Ru(C 5H5)2 and Os(C 5H5)2 will give similar reactions they are in the same periodic group ! However, most Cp compoundswillnotsurvivethereactionconditionsdescribedbelow. 2.1 Electrophilic substitution at ring This is a very facile process, occuring some 3 ×10 6 times faster than with .

E E E+

Fe Fe+ H Fe H+

E+isageneralelectrophile,seeE/Sp328330.AconcreteexampleisaFriedel Craftsacylation

O O C C CH3COCl/AlCl3 Fe Fe Fe + O C

Themechanismprobablyinvolves exo attachattheringCatom,followedbya movementoftheprotondowntothemetal.

E + E E + Fe H Fe H Fe

TheintermediatecanbemadebytreatingFeCp 2withverystrong. Thehydridesignalinthe 1HNMRspectrumcomesaroound2.1ppm,whichis typicalofmetalhydrides. This reaction does not work if the electrophile is an oxidising electrophile as + many are such as NO 2 . These reagents oxidise the complex to give the

9 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

+ ferricinium cation [FeCp 2] and the +ve charge now on the rings makes electrophilicattackverydifficult. Another example and indication of the high reactivity of FeCp 2 is that the Vilsmeierreactionworks

O O (i)POCl C 3 H + C Fe Fe MePhN H (ii)H2O

substitutedformamides This reaction only works well with highly activated aromatics such as ()andphenols. Because of the high reactivity of the Cp rings, there is little of the selectivity observedinaromaticcompounds.

4.2 1.4

Et forFriedelCraftsacylation

Fe CH3COCl/AlCl3

1.0 2.2 Reactivity of attached organic groups Organic functional groups attached to Cp undergo many of the most common reactiontypes,e.g.below(nonoxidisingconditions)

O H C H H Fe Fe PhCOMe/ O Ph

aldol condensation H

H Fe R Ph3P=CHR Wittigylid

O O C

CH3 Fe Fe H RCHO/base H R

10 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

2.3 Metallation of ferrocene Further may be introduced by the very useful reaction with butyl lithium

Li CO2H CO2 Fe Fe Fe BuLi

N2O4

NH2OMe

NO2 2BuLi NH2 Fe/HCl Fe Fe

Li PPh2 PPh2

Fe Fe Fe Fe(CO)4 PPh2Cl Fe(CO)5 Li PPh2 PPh2 NotethatitisnotpossibletointroducetheNO 2groupdirectlybynitrationasthe + electrophileNO 2 isoxidising 2.4 Stabilization of ααα-carbonium OAc + R OH

R H H O/H+ H OH R Fe 2 H Fe Fe

carboniumintermediate Thehydrolysisoftheacetosubstitutedferroceneproceeedsseventimesfaster thanthesolvolysisofPh 3COAc(thisisaclassicexampleofS N1 hydrolysisvia thestablecarboniumionPh 3C+the trityl ion).Soweconcludethatthecarbonium ionintermediateismorestablethatthetritylcation. Reactionsinvolvingvinylicsubstituentsalsoproceedthrough αcarboniumions, e.g.thereactionwithCH 3CO 2H

+ CH3 OAc

H OAc CH3 H2O/H+ H Fe Fe Fe

carboniumionintermediate 11 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

ThecarboniumioncanactuallybeisolatedandcrystallisedastheBF 4 orPF 6 salts.Thecrystalstructuresshowclearevidenceofan"interaction"betweenthe exoCCbondandtheFeatom"bends"towardstheFeatom.

Ph overall6edonor Ph Fe + 160deg

Fe crystalstructure There is also NMR evidence for the presence of a πbond between the exoC atomsandtheFeatom.Uses 57 Fe13 Ccouplingconstants,whichwasobserved to be 1.5 Hz in the above cation. Clearly indicates that the bond between the carboniumandtheFeisnearertoa πbond. Type of bond J(57 Fe-13 C) /Hz σFeC ~9 πFeC 1.54.5 2.5 The "indenyl" effect and ring slippage isabicycliccloselyrelatedtocyclopentadiene

base

H H indenylanionInd IndenylformscomplexeswhicharesimilartoCp,butsubstitutionreactionsare muchfaster(upto10 8times!!). Example (Ind)Rh(CO) 2+PPh 3 →(Ind)Rh(CO)(PPh 3)+CO This is an SN2 reaction, i.e. 2nd order and associative. Therefore the rate determiningstepinvolvedadditionofthe Example (Cp)Mn(CO) 3+PBu 3 →noreaction (Ind)Mn(CO) 3+PBu 3 →(Ind)Mn(CO) 2(PBu 3)+CO (Flr)Mn(CO) 3+PBu 3 →(Flr)Mn(CO) 2(PBu 3)+CO Fluorenyl(Flr)is250timesfasterthantheindenylreaction

12 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

FluorenylanionFlr Whatisthereasonforthis?ringslippageshowninthemechanismbelow eta5 eta3 eta3

PPh3

Rh Rh Rh CO OC CO OC CO OC PPh3 18 e 18 e 16 e eta5

- CO

Rh

OC PPh3 18 e The C5 ring is moving from eta5 to eta3 back to eta5 coordination to the rhodium atom. The driving force is that the benzene ring can become fully aromaticwhentheringisslipped.Thisisevenmoresoinfluorenyl. Evidence for ring slippage Theringslippedcomplexisusuallyareactiveintermediateandisnotobserved as a stable compound. However in the following reaction the ring slipped compoundcanbeisolated (Ind)Ir(PMe 2Ph) 2+PMe 2Ph →(Ind)Ir(PMe 2Ph) 3 18e20e??

13 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

TheXraystructureshowsthatonly3CatomsoftheC 5ringarecoordinatedto theiridium

PMe2Ph

[Ir(PMe2Ph)4]+ Ind Ir

PMe2Ph 16e PhMe2P PMe2Ph

18e Theeta3coordinationmeansthatthebondbetweentheIrandtheindenylgroup is weakened and it may be displaced by a further mole of phosphine. The displacement of Cp or Cp related ligands is highly unusual ! 2.6 Heterocycles as Cp analogues TheCHgroupinCpmaybereplacedbyheteroatomstogiveheterocycles N(P,Asetc) ≡CH(isolectronic)

N C4H4N N pyrrolyl C4H4P P phospholyl C4H4As As arsolyl Theseanionsform πcomplexeswhicharedirectlyanalogoustoCp

N heat CpFe(CO)2I + Fe Fe N 2CO OC N OC sigmacomplex Azaferrocene

Ph 150 C P [CpFe(CO)2]2 + Fe

P

Phosphaferrocene

Li P P FeCl2 +2 Fe

P

14 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

The P (and indeed N) atoms have lone pairs and so are capable of acting as ligandsintheirownright.Maybethoughtofasderivitisedphosphineligands.

P P Fe(CO)4 +2Fe(CO)4(THF) Fe Fe Sourceof16e"Fe(CO)4"

P P Fe(CO)4 ThiopheneC 4H4Shasoneextraelectronandsobehavesasasixelectrondonor analogoustobenzene

S S

Cr(CO)3 Cr(CO)3 BorolesC 4H4BRhaveonelesselectronandsobehaveasfourelectrondonors. So compare Cp 2Fe(CO) 4 with (C 4H4BMe) 2Co(CO) 4 isostructural and iso electroniccompounds.

O BMe O C C BMe Fe Fe Co Co

OC C CO OC C CO O O Therearealargenumberofheterocycleswhichcanactasligands(seeE/Sp 376385forotherexamples).ThereplacementofCHbyPforexamplecango thewholeway.

150C [Cp*Fe(CO) ] 2 2 + P4 Fe P P P P P Pentaphosphaferrocene All the P atoms are equivalent and give a singlet at 153 ppm in the 31 P NMR spectrum.

15 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Topic 3 - Stabilisation of Unstable by Complexation Transitionmetalshave the remarkable ability to stabilise unstable, unknown or highly reactiveorganic(andinorganic)molecules.Thisisbecausecoordination toametalchangestheelectrondensityintheligand.Thisfacetoforganometallic chemistrywillbeillustratedbyafewpertinentexamples. 3.1 Unstable molecule - This is a highly strained and unstable molecule. It is nonaromatic, i.e. doesn't obey the (4n+2) πelectron Hückel rule. In fact it is anti-aromatic and is destabilised byasquareplanararrangementofCatoms.Thisiseasilyseenby thinkingaboutthe πorbitalsinthissymmetry( D4h )

bonding vantibonding slightlyantibonding ψ−4 ψ−3,ψ−2 ψ−1

ψ−4

ψ−3,ψ−2

ψ−1 Thefour πelectronsthusgiverisetoanexpectedtripletstate,whichissubjectto JahnTeller distortion. In fact C 4H4 is not square but rectangular with localised doublebondsand D2h symmetry, H H tBu tBu

HH tBu H highlyreactive stablebecauseofstericrestraints In 1955, LonguettHiggins and Orgel predicted theoretically that the aromatic square form of C 4H4 would be stabilised by complexation to a transitionmetal. Thesynthesisofsuchacomplexwassoonaccomplished.

16 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

H C 3 CH3 Cl Cl Cl Ni Ni + Ni(CO)4 Cl Cl Cl H3C CH3 In 1959 the complex of tetramethylcyclobutadiene was synthesised and the Xray crystal structure determined. The Ni(CO) 4 also abstracts the two chlorineatomsandNiCl 2isabyproduct.TheCCdistancesare~1.45Åwhichis intermediatebetweenasingleanddoublebond. The first complex of the parent unsubstituted cyclobutadiene was made in a similarway

Cl

+FeCl +6CO + Fe2(CO)9 2 Fe Cl CO OC CO Thiscompoundisalowmeltingyellowcrystallinecomplexwithasignalat3.91 ppm in the 1H NMR spectrum and two ν(CO) stretches in the IR spectrum, consistentwiththepseudothreefoldsymmetry.Thisreactionalsoproceedsby halogenabstractionanditisageneralreactionforthesynthesisofcomplexesof cyclobutadiene(andsubstituted) Cl

+ 2 2Na [Mo(CO)5] Mo OC CO Cl OC CO 2 The anion [Mo(CO) 5] is a powerful . Another synthetic route for cyclobutadiene complexes is through the dimerisation of . This is a thermallyforbiddenreaction(WoodwardHoffmanrules)butwiththemediationof atransitionmetal,thismaybeovercome R R

CO CO R +2 RC CR R Co Co

3.1.1 Physical properties In complexes the ring is essentially squareplanar. If all distances are approximatelyequal,thisindicatesbonddelocalisationandafullyconjugated π system.Somestructureshowevershowpartialdoublebondlocalisation.Thering hasaromaticproperties,forexamplethe 1HNMRsignalsoftheCHringprotons areintherange46ppm,similartoCpprotons.

17 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

3.1.2 Chemical reactivity

3.1.2.1 Spectator role Thecyclobutadieneligandisoftena"spectator"ligandinmuchthesamewayas Cp

+PPh3

Fe(CO)3 Fe(CO)2(PPh3) hv

O C

Fe Fe

C C O O 18ewithtripleFeFebond Inthelattercomplextheligandactassteric"plugs"preventingaccess tothemetalatom 3.1.2.2 Reactivity at ring ElectrophilicsubstitutionoccursveryreadilyandinaverysimilarfashiontoCp. ForexampletheVilsmeierreactiongivessubstituedringproduct O H

Ph H POCl3 + NC Me O H2O Fe(CO)3 Fe(CO)3 OrganicfunctionalgroupsalsoshowthesamereactivityaswithCprings. O CH2OH CH2Cl H BH4 HCl

Fe(CO) Fe(CO) Fe(CO)3 3 3

LiAlH4

CH3

Fe(CO)3 Likewisethestabilisationofthe αcarboniumionoccursinasimilarfashiontoCp

18 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

CH2+ CH2 CH2Cl SbCl 5 HCl +

Fe(CO) Fe(CO) Fe(CO)3 3 3 OH

CH2OH

Fe(CO)3 3.1.2.3 Release of free cyclobutadiene LowtemperatureoxidationwithmildoxidantssuchasCerium(+4)oxidisestheFe atomandreleasesfreecyclobutadiene.Thisishighlyreactiveanditisprepared in situ with the reactive substrate. This reaction may be used in organic synthesis.AniceexampleisthepreparationofinE/Sp315

2. Unstable molecule - Trimethylene ThismoleculeC 4H6isanofbutadiene.Itisextremelyunstableinthefree statesinceitdoesn'tobeytheconventionalrulesofbonding.

CH2 H2CC

CH2

butadiene trimethylenemethane

19 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Stable complexes of this ligand (and derivatives) can be easily made, in an extensionoftheroutesusedtomakethecyclobutadienecomplexes.

CH2Cl CH2 H2C H2CC +Fe2(CO)9 CH2 CH2Cl Fe (CO)3 FeCl 2isformedasabyproductfromhalideabstraction.ThefourCatomsarenot quite coplanar so the shape is a little like an umbrella, with the exterior CH 2 groupsbendingtowardstheFeatom.Thewholeligandactsasafourelectron donor

The crystal structure of this complex is shown above. The six H atoms are equivalentandthecomplexadoptsastaggeredgeometry,asshownbyaview downthe3foldaxis

HH

OC CO

Fe

H H CO H H Howisthisoddmoleculebondedtothetransitionmetal?Needtoconsiderthe lowestlying πorbitals

esymmetrynonbonding

a1symmetrybonding

20 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

The a 1 is the σdonor orbital and the e set are the πacceptor orbitals. The Fe(CO) 3fragmenthasasimilarset(asfarassymmetryisconcerned)oforbitals whichprovideaperfectmatch

highlying highlyingpiacceptorandlowlyingpidonor sigmaacceptor derivedfromeg sphybrid Thebondingisthusthe"classical" σdonotionfromligandand πbackdonation frommetal.Thisorbitalapproachprovidesadelocalisedviewofthebondingand avoidstheproblemswiththeconventionalviewoflocalisedbonds. ThecoordinationgeometryoftheFeatomsisroughlyoctahedralandthereisa highbarriertorotationaboutthe3foldaxis.Thisbarriercannotbeobservedin theFe(CO) 3complex,becauseall6protonsareequivalentanyway.Inorderto observeanybarrier,weneedtomakethemchemicallyinequivalent.Sochoose anML 3fragmentwhichdoesnothave3foldsymmetry H H

OC PPh3 CH2SiMe3 SiMe3Cl IrCl(CO)(PPh3)+ H2C Ir

CH2Cl H Cl H H H AllsixH'sarenowchemicallyinequivalent(thecompoundischiral)andevenat elevatedtemperatures,thereare6signalsbetween0.1and3.5ppminthe 1H NMRspectrum,showingthatifrotationoccursthatitmusthaveahighbarrier. 3. Unstable molecule - Benzyne Thismoleculeishighlystrainedandhenceveryreactive,butisstableinaninert matrixat8 oK.Thegroupshouldbelinearhencethestrain.Benzynecan bemadeinsitubyabstractionofHClfromchlorobenzene,usingaverystrong basesuchassodamideNaNH 2.Thegeneratedbenzynemaybetrappedusinga suchascyclopentadienebytheDielsAlderreaction

Benzyne

21 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Simple and stable benzyne complexes of transition metals are known, for example

heat

H3C Ta CH4 H3C Ta CH3

H3C H3C

Benzyne has obvious similarities to alkynes in its complexes. Quite a few are knownforclustercompounds Os 3(CO) 12 +C 6H6 →H 2Os 3(CO) 9(C 6H4) Thealkyne"dances"aroundthethreeOsOsedges

4. Unstable molecule - E 2 (E=As,Sb, Bi) analogues of N 2 WhileN 2isthestableformofelemental,thecorrespondingmolecules P2,As 2,Sb 2andBi 2arenotstableatroomtemperature.WhileP 2,As 2,Sb 2have been observed in mass spectra at high temperatures, Bi 2 is wholly unknown. However, when coordinated to transition metals, these molecules may be isolatedatroomtemperature.

(OC)3Co Co(CO)3 (OC)3Co Co(CO)3

CH Bi

C Bi H TheBi 2ligandwithaformalBi ≡BitriplebondisanalogoustoalkynesRC ≡CR

22 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

The"Mercedes"haveasimilarlyE 2group(E=As,Sb,Bi)coordinated tothreeM(CO) 5 groups(M=Mo,W).Theyaremadebythesimplereaction 2 ECl 3+[M 2(CO) 10 ] →E 2[M(CO) 5]3 Thestructuresoftypicalexamplesareshownbelow,bothschematicallyandin 3D.

MercedesBenzene

23 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Topic 4 - Reactivity of Coordinated Olefin Complexes 4.1 Introduction Thecoordinationofligandstometalscan,ingeneral,quitesubstantiallychange not only the stability (as shown in previous topic) but also the reactivity of the ligand.Thereasonforthisisthatcoordinationtoametalchangestheelectron distributionwithintheligand.Asanexampletoshowwhythishappens,consider thecaseofbutadiene

freeligand (OC)3Fe Theligand πorbitals(frontierorbitals)arethemostimportantinbondingtothe metal.Assumingtheconformationfoundincomplexes,weget

ENERGY

() (+) Ψ4 (δ) (+) ()

() (+) Ψ3 LUMO (π) () (+)

() () 2 Ψ HOMO (π) (+) (+)

(+) (+)

Ψ1 (σ) (+) (+)

πorbitalsofbutadiene coefficientsareeither0.37or0.60

24 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Ψ4 0.44 1.48Å Ψ3 0.89 1.34Å Ψ2 πbondorder bondlengths Ψ1

groundstate

Ψ4 0.72 1.39Å Ψ3 0.44 1.45Å Ψ2 πbondorder bondlengths Ψ1

excitedstate Thecoefficientsofeachbasis porbitalontheCatomstellsusqualitativelyabout thebonding/antibondingnatureofeachoftheorbitals • Orbital Ψ1provides πbondingbetweenallatoms,butmoresobetweenthe twoinnerCatoms. • Orbital Ψ2provides πbondingbetweentheinnerandouterCatoms,butis antibondingbetweenthetwoinneratoms. • Orbital Ψ3 provides πbonding between the two inner Catoms, but is antibondingbetweentheinnerandouterCatoms. • Orbital Ψ4isantibondingbetweenallCatomsbutisnotoccupiedorused. Forthe ground state ,thebondingisthesumof Ψ1and Ψ2,whichleadstoa higher πbond order between the inner and outer atoms (as expected from conventionalideas). Forthe excited state ,thepartialpopulationof Ψ3andpartialdepopulationof Ψ 2leadstoaninversionof πbondorderscomparedwiththegroundstate. ThisisthesimpleHückelpictureofbonding.Coordinationofbutadienetoametal fragmentsuchasFe(CO) 3usesthesameorbitalsaspreviouslyshown

highlying highlyingpiacceptorandlowlyingpidonor sigmaacceptor derivedfromeg sphybrid

25 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Theseorbitalsinteractwiththeligandorbitalstogivedelocalisedbondsof σand πsymmetry(the δorbitalistoohighlyingandhasnosymmetrymatchwithany metalorbital).Thesynergicbondingbetweenmetalandligandredistributesthe electronpopulationofthebutadieneinasimilarfashionasthatexplainedabove fortheexcitedstate. Thusqualitativelywecanseethat: (a) changes in the relative populations of the frontier orbitals of the ligand can changethe bonding densityintheligand.Someexamplesshowthe effectson thebondlengths. 1.41 1.45 1.45 1.40

(OC)3Fe Cp2Zr Thearrowsshowthedirectionofflowofelectronsandthenumbersarethebond lengths in Ă units. The Fe fragment is electron rich and the Zr fragment is electron deficient. In the case of the Fe compound there is thus a small populationoftheLUMOofbutadiene,whileintheZrcompoundthereisasmall withdrawalfromtheHOMOofbutadiene. In terms of the bond approach, it is possible to view the bonding as arisingfromtwoextremecanonicalforms

MLn MLn

A B EarlytransitionmetalssuchasZr whichareelectron deficientaremorelikeB, whilelatertransitionmetalssuchasFeareelectronrichandmorelikeA (b) The orbital approach also allows us to rationalise the site of attack of the incomingnucleophile.TheLUMOofbutadienehasmostofthewavefunctionon the outer Catoms and the lone pair of the nucleophile will seek this, hence leading to preferential attack at the outer carbon atom. The reaction of nucleophilewithelectrophileisaLewisacidbaseinteraction. Nu

betteroverlaphere thanattheinnerC atom

26 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

AnumberofMOstudiesofindividualcomplexeshasledtoasetofsimplerules fordeterminingthesiteofnucleophilicattackat ligands in cationic metalcomplexes. 4.2 Green/Mingos/Davies Rules (GMD) for Normally, unsaturated are not at all susceptible to nucleophilic addition. On coordination into a CATIONIC metal complex this changes, and attack bytogive additionproductsiswellknown.Theincreasein susceptibilitytonucleophilicattackarisesbecausethereisanetflowofelectrons from the ligand to the metal (i.e akin to introducing electron withdrawing substituents). An example from organic chemistry of this enhancement is the attack on the bromonium ion by the relatively weak nucleophile Br during the brominationof.

Br

Br

Thenucleophilicadditionstotransitionmetalcomplexesaregenerallyvery regio- specific, i.eonlyoneoutofapossiblenumberofproductsareformed.Forattack on18electronorganometalliccations,wherereactionsarekineticallycontrolled, themostfavourablepositionfornucleophilicattackisgivenbytheGMDrules. Somedefinitionsareneededfirst.Hydrocarbonligandsdescribedas even or odd andas open or closed Even n=2,3,4...... Oddn=3,5,7.... referstothe ηnumber,i.e.howmanyCatomsofligandare bondedtothemetal Open not cyclically conjugated Closed cyclically conjugated Rule1 NucleophilicattackprefersEVENpolyenes withnounpaired electronsintheirHOMO.CycloC4R4onlycommonexample) EVENbeforeC 4R4beforeODD Rule2 OPENbeforeCLOSED Rule3 For even open polyenes, attack occurs at terminal Catom. Foroddopenpolyenes,attackoccursatterminalCatomonly ifML nisstronglyelectronwithdrawing

27 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

eta5 eta5 eta5

MMM

oddclosed oddopen oddopen

MMM

evenclosed evenopen evenopen

eta6 eta4 eta4 Rulesarebestillustratedbyexamples

eta5odd Et

H EtMgCl Fe Fe Fe

Et cyclohexadienyl

H eta6even

Et Even before odd. NOTEthatthenucleophileattacksfromtheEXOposition eta5closed H

H

NaBH4 Rh Rh Rh H

eta5open Open before closed AttackoccursatterminalCatomofconjugatedsystem

28 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Rulesneedtobeappliedsequentially.ConsiderthecaseoftheMo+cation shownbelowwhentreatedwithhydridesource(BH 4 )

eta6evenclosed

BH4

Mo Mo

eta3oddopen eta4evenopen Me Rule1 evenbeforeoddthereforeallylNOTattacked Rule2 openbeforeclosedthereforebutadieneattacked Rule3 terminalCatomthereforeproductthemethylallylcomplex Another way of expressing Rules 1 & 2 is that the order of reactivity of unsaturatedhydrocarbonscoordinatedtocationsisasshownbelow

ItcanbeseenthatCpistheleastreactivethisexplainsit'sstabilityandroleas a"spectator"ligand.Itmaybeusedtodesigncomplexesfornucleophilicaddition toevenligandsandallylandpentadienylligands. CAVEAT Cations which contain only ONE unsaturated hydrocarbon ligand and at least ONEcarbonylligandmayundergonucleophilicattackattheCO.Thismostoften occurs with nucleophiles having heteroatoms as the nucleophilic site (e.g. methoxideOCH 3)andwhenchoiceisbetweenaCOandaneta5ligand. MeO

OMe slowisomerisation

Os Os O Os CO CO OC OC OC CO CO OMe CO When there is more than one unsaturated ligand, the normal MGDrules apply + again,e.g.for[(C 5H5)Mo(CO) 3(C 2H4)] attackoccursattheetheneligand.

29 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Topic 5 - and Complexes Textbooks:E/Spages210220

5.1 Introduction Carbene:CH 2isahighlyreactiveandunstablemolecule.Carbeneitselfcanbe madebythethermaldecompositionofdiazomethane CH 2N2 →N 2+:CH 2 →(CH 2)npolymerisesto"polythene" Chlorocarbeneisalsoeasilymade CHCl 3+strongbase →:CCl 2(reactiveintermediate) may be stabilised by coordination to transition metals, as in this O OC C

Rh Rh +CH2N2 Rh Rh C O C H2 CO example of the formation of a carbene complex directly from carbene. This reactionistheorganometallicanalogueofthereactionofcarbeneswithalkenes togivecyclopropanes. Carbenesarethusfurtherexampleofunstablemoleculeswhicharestabilisedby coordination to transition metals. Relatively recently, stable carbenes at room temperaturehavebeensynthesisedbyArduengocontainNheterocycles.

R R N N

Theterms carbene complex and alkylidene complex andalso carbyne complex and alkylidyne complex areusedinterchangeably. 5.2 Fischer Synthesis of Carbene Complexes Carbenecomplexeswerefirstmadein1964byE.O.Fischerwhosubsequently won a Nobel Prize for his work. The synthesis is based on the nucleophilic additionofheteroatomnucleophilestocoordinatedCO(seelastpartofTopic4).

30 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics CO CO CO OC CO (i)OC CO (ii) OC CO W W W OC CO OC CO OC CO CO C C

R OLi R OCH3

(i)nucleophilicadditiontoCOwithLiR (ii)methylationusingMe3O+BF4 ThelithiumintermediateisunstableandmethylationwithMe +reagentsresultin thermallystableandisolablecarbenecomplexes.Manyexamplesofthistypeare nowknown,most withORorNR 2substituents.These haveseveral forms.

OR OR OR LnM C LnM C LnM C

R R R Thelattercanonicalformbearsapositivechargeonthecarbenecarbon,andso maybeexpectedtobe electrophilic atthisatom. 5.3 Schrock Synthesis of Carbene Complexes Thisistheotherveryimportantroutetocarbenecomplexes,ofquiteadifferent character to those made by Fischer. The reaction was the treatment of the

CMe3 HC Cl LiNp (Np)2ClTa H TaNp5 (Me3CH2)3Ta C Cl H2CMe3

Np=neopentyl=Me3CCH2 CMe4 CMe3 H LiNp C H TaNp3 C Np2Ta CMe3 Cl neopentyl(Np)alkylcomplexoftantalumNp 3TaCl 2withtwomolesofthelithium reagentLiNp.TheoriginalpurposeoftheexperimentwasthesynthesiseTaNp 5 but the actual product was much more interesting (as is often the case in organometallic chemistry). The first stage involves an αdeprotonation step, wherebyaprotonfromoneneopentylgroupistransferredtoanotherneopentyl group.Thisreleasesthevolatilehydrocarbonneopentane(tetramethylmethane) whichisoneofthedrivingforcesforthisreaction.This αdeprotonationofalkyl groupsONLYoccurswithverystericallybulkygroupsandisunusual. A related reaction forming carbenes is that of hydride abstraction from methyl groups

31 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Ph3CBF4 +Ph3CH

Re Re ON ON CH3 CH2 L L ThetworesearchgroupsofFischerandSchrockhavedevelopedthechemistry of these carbenes, which are intrinsically different. The most important differenceslieintheirreactivity. • Fischer carbenes have πdonor heterosubstituents and are electrophilic at thecarbene • Schrockcarbeneshavealkyl(orH)substituents and are nucleophilic atthe carbene 5.4 Evidence for Multiple Bonding 1.XrayStructures.Theseshow (i)planartrigonal sp 2C (ii) M=C bonds is shorter than a single bond, butnotasshortasMCObond

2. 13 CNMRchemicalshiftsareintheregion200400ppmcitedasevidencefor a δ(+)chargeonC 3. MO picture predicts a barrier to free rotation in Fischer carbenes, which is observed in NMR spectra. Also the observed inequivalence of CR 2 groups in Schrock carbenes in the NMR spectra also indicates a very high barrier to rotationaboutM=Cbond

32 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

5.5 Why the distinction between Fischer and Schrock carbenes ? Thetablebelowliststheirgeneralpropertiesanddifferences Property Fischer Schrock Natureofcarbenecarbon Electrophilic Nucleophilic TypicalRgroup πdonor(e.g.OR) Alkyl,H Typicalmetal Mo(0),Fe(0) Tav(V),W(VI) Typicalancilliaryligands Good πacceptorsCO Cl,Cp,alkyl Electroncount 2e 2e Oxidation state change 0 +2 on addition of CR 2 to metal

Actuallynoteasytoprovideaconvincingexplanationforthedifference.TheMO schemesuggeststhatwhilebothligandsareeffective σdonors • theFischercarbeneisa πacid • theSchrockcarbeneisa πbase

33 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

5.6 Reactivity of Fischer carbenes Fischercarbenesarereadilysusceptibletonucleophilicsubstitution

NHEt OCH3 NH2Et (OC)5Cr (OC)5Cr +MeOH

CH3 PhLi CH3

Ph

(OC)5Cr +MeOH

CH3 Ph Mechanisminvolvestheintermediate (OC) Cr 5 OCH3

CH3

Similartoamminolysisofesters

OCH3 OCH3 NHR O NH R O H 2 N O CH HR 3 CH3 CH3 The analogy between metal carbenes and organic ketones is quitestrong. For anotherexample,comparetheirreactionswithPylides: OCH OCH 3 Ph3P=CH2 3

(OC)5W H2C +(CO)5WPPh3 Ph Ph

comparewithWittigreaction

Ph Ph3P=CH2 Ph O +OPPh H2C 3

Ph Ph Another important reaction is with alkenes to give metallocycles, which in turn decompose to give rearranged alkenes. This is the so called " metathesis"reaction,whichisofmajorcommercialimportance. Theresultantolefinhasthe=CPh(OMe)groupfromthecarbenecomplexandthe =CH 2groupfromthestartingCH 2=CH(OR)alkene.Thisrearrangementinvolves breakingaverystrongC=Cdoublebond,andisimpossiblewithoutthepresence oftheorganometaliccarbenecatalyst

34 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

OR

OCH3 (OC) Cr (OC)5Cr 5

Ph C OR

Ph OCH3

OR

OR OCH3

(OC)5Cr + (OC)5Cr Ph H C

Ph OCH3 Carbene complexes are excellent catalysts for the general alkene metathesis reaction R2C=CR' 2+R'' 2C=CR''' 2 →R 2C=CR'' 2+R' 2C=CR''' 2etc 5.7 Reactivity of Schrock carbenes Schrockcarbenesbehavemuchlikeylides

They are easily attacked by electrophiles (i.e. behave as nucleophiles) two examplereactionsare

OneofthemostusefulcarbenecompoundsisTebbe'sreagent.

35 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

ThereagentisreleasedbythetreatmentwithamminebasesNR 3.Itbehavesas acarbenetransferreagent,andhasanumberofspecialiseduses. "Cp 2Ti=CH 2"+RC(O)OR' →R(OR')C=CH 2 ThishastheusefuladvantageoverWittig'sreagentsinthesamereactioninthat itworks!!.Whenusedwithenolisableketonesitdoesnotresultinracemisation.

WhilethedistinctionbetweenFischerandSchrockcarbenesisa usefulone,it should be realised that this distinction is not rigid, but merely represents two extremes.Anexampleofan"inbetween"carbeneisRoper'scarbenebasedon ,whichshowsbothtypesofreactivity,dependingonthesubstrate. SO 2hereisactingasanelectrophile,whileCOisactingasanucleophile.The "inbetween"characterofRoper'scarbeneissensibleintermsofthedescriptions givenintheTable,asthereareboth πdonors(chlorideligand)and πacceptors (nitrosylligand)andthecarbenecarbonhasno πdonorsubstituents.

5.8 Carbyne complexes CarbynecomplexeshaveaCRgroupattachedtoametalatom,withatripleM ≡C bond.Theirchemistryiscloselyrelatedtothoseofcarbenes:

36 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Theevidenceforatriplebondcomesfromcrystalstructures,whichshowavery shortMCbond.MoreovertheMCRangleiscloseto180 oindicativeoflinear sp hybridisation.ThebondingoftheCatomtoametalisquitesimilartothatofCO, withasigmadonationandpibackdonationtothe(unhybridised) porbitalsofthe carbyneatom

The reactions of carbyne complexes show some similarities with carbene complexes. Thus alkyne metathesis with carbyne complexes as catalysts is possible.

37 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

ThisisusuallyonlypossiblewithalkylorarylRgroups. Carbynecomplexesmaybethoughtofas"halfinorganic"analoguesofalkynes Forinstancetheanalogyisobviousbycomparingthesereactions

The PtW 2 compound can also be viewed as a cluster of 3 metals with two bridging CRgroups

In fact, (or alklidynes) are very common as bridging ligands. One particularwellknowseriesofverystablecompoundscanbeeasilymade Co 2(CO) 8+CRX 3(X=Cl,Br,I) →CoX 2+Co 3(3CR)(CO) 9"Fred" A huge variety of derivatives with different R groups have been made and extensive chemistry is known. The structure of the simplest CH derivative is shown.Thethreeatomsandthebridgingcarbonmakeupatetrahedron

38 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

Using the reactions described above it was possible to synthesise a related compound with three different metal atoms. Because the vertices of the tetrahedronwerealldifferent,thecompoundischiralanditispossibletoresolve thesechiralclustersbymakingderivativeswithhomochiralPRR'R''. Theyhaveprovedtobesomeofthemostchiralmoleculesmade(i.e.havethe highestmolarrotationcoefficients).

39