Solid-state NMR studies of supported organometallics

Journal of Molecular Catalysis, 86 (1994) 447-477

447

Elsevier Science B.V., Amsterdam M220

Solid-state NMR studies of supported organometallics L. Reven* Department of Chemistry, McGill University, 801 Shrbrooke St. West, Montreal, Quebec H3A 2K6 (Canada) (Received January 1,1993; accepted March 1,1993)

Abstract Solid-state nuclear magnetic resonance spectroscopy has been increasingly applied for characterization of supported organometallic complexes. Whereas early studies focused on highly mobile physisorbed species, the development of high-resolution solid-state techniques has extended NMR studies to less mobile chemisorbed complexes. In addition to identification of surface species, solid-state NMR has yielded information concerning mobility, the nature of the bonding to the surface, and even the active sites in catalytic reactions of supported organometellic complexes, When coupled with other characterization methods, NMR has proven to be an effective probe of surface organometallic structure. Solid-state NMR studies of the following systems are reviewed: ligand attached metal complexes, supported metal carbonyls and olefms, supported organoactinides and zeolite encapsulated organometallics.

Key words: nuclear magnetic resonance; organometsllics; surface chemistry

Introduction The developmentof molecularmodels for heterogeneouscatalysts,as well as design of more uniform, selectivecatalysts,are among the principle objectives of the field of surface organometallicchemistry.For both of these goals, structuraldeterminationat an atomic level of the intermediatespecies and product surface complexes is essential. A wide variety of characterization methods, such as temperatureprogrammeddecomposition or optical and vibrational spectroscopy [11, have been combined to provide quantitativeand some qualitativeinformationabout the surfacecomplexes.However,the number of techniqueswhich provide direct information about molecularstructure of organometallicssupportedon high surfaceareaoxidesare limitedsincemany of the surfacescience techniquesdevelopedfor metal singlecrystal studiesare destructiveor not applicable to polycrystallinesamples. Recently, more suitable, non-invasiveprobes of local structuresuch as EXAFS (extended X-ray adsorption fine structure) [ 21 and CP MAS NMR (cross polarizationmagic*Corresponding author.

0304~5102/94/$07.00 0 1994- Elsevier Science B.V. All rights reserved. .SSDZ0304-5102(93)E0153-8

surface conformation, mobility surface species, mobility

surface conformation, bonding sites

(model compounds ) Au, cross polarization times. & Aa

1J(1”Pt,3’P)* Si

Pd (II) phosphine complexes/silica

3lP

%3i, 1%

‘3C

(l,l’-ferrocenediyl)dimethylsilane/silica

LRu(CO),, L,Ru,(CO),/silica L=PPh,(CH,)Si(OEt),

=P,

"S1,

Pt(C204)L&lica L= (OMe)3Si(CH,),PEt,

3lP 4, ‘Y!

4

surface structure

surface structure, bonding sites

surface conformation, detection of side reaction

‘J( 1wPt,31P)*Si

Pt (II) phosphine complexes/ polystyrene-divinylbenxene

3’P

4

surface conformation, detection of side reactions

8,

‘J(‘vt,*‘P),

Pt(II), Ni(II), Pd(I1) phosphine complexes/ silica, glass

3lP

Ia&

Information obtained

NMR parameters measured

Supported complex studied

Nuclear species

Metal complexes attached via functionalixed ligands

TABLE 1

15

14

697

4 5

Ref.

L. Reven /J. Mol. Catal. 86 (1994) 447-477

449

angle-spinningnuclear magnetic resonance) [3] have been applied to these systems.Whereas EXAFS yields structuraldetailsof the metal-supportinterface and metal-metal bonding, NMR also providesdynamicalinformationimportant to understandingthe chemical reactivitiesof supportedorganometallies. The potential of NMR to yield the same type of structuraland dynamical information as it has in molecularchemistryhas motivateda variety of solidstate NMR studies of organometallicssupportedon functionalizedpolymers, metal oxides and xeolites. Early NMR investigationsof supported organometallicswere limited to highly mobile, physisorbed mononuclear metal complexes, principally metal carbonylsoccludedin zeolites,since their solution-likespectracan be observed by widelineNMR. High-resolutionsolid-stateNMR techniqueswere first applied to supportedorganometallicsby Fyfe and coworkerswho employed high power decoupling,cross polarization and magic-anglespinning to obtain 31P NMR spectra of transition metal complexes immobilized on functionalized solid supports [ 4-71. More recently,high-resolutiontechniqueshave allowed structuraland dynamicalcharacterizationof less-mobilechemisorbedspecies formed by the reaction of metal carbonyls with metal oxide surfaces.In addition to structuralcharacterizationof the adsorbedcomplex, the chemicalreactivities of supported metal hydrocarbyl complexes have been followed using =C CP MAS NMR. After a brief summaryof the parametersand experimentalmethods relevant to surface analysis,a reviewof the solid-state NMR studies is presented for the following systems: (i) metal complexes attachedto functionalizedsupports, (ii) metal carbonyls adsorbed on metal oxides, (iii) the surface chemistry of metal hydrocarbylcomplexes and (iv) organometallicsencagedin zeolites.The studiesin these four areaswhich are reviewedin this articleare listed in Tables l-4.

Spin interactions in the solid state

For a nucleuswith a spin I= l/2, the main interactionsin the solid-state are [3-101: (i) Zeeman interaction of the nuclear magnetic moment with the applied magneticfield. (ii) Dipole-dipole interactionbetween nuclearspins. (iii) Chemicalshift arisingfrom the shieldingof the nucleusby surrounding electrons. (iv) Spin-spin coupling (scalar coupling) between neighboringnuclei. The general Hamiltonian for spin I= l/2 containing terms for the four interactionslisted above is:

22 24 25 26,27

surface structure, mobility mobility mobility surface structure, mobility

Si, Aa (temperature dependence) Si, Au (temperature dependence ) relaxation time (temperature dependence ) line widths (temperature dependence) S,, Au (temperature dependence, model compounds)

Mo(CO)&al~ina

Mo(CO)&alumina

Mo(CO)&-alumina

MO (CO ),/y-alumina

Os,(CO),,/silica

1%

W

‘SC

W

W

20

4

21

Ref.

Information obtained

surface species, mobility

Mo(CO)&alumina

‘SC

NMR parameters measured

decomposition products

Supported complex studied

Nuclear species

Metal carbonyls supported on metal oxides

TABLE 2

36

surface species, mobility

& ACT(temperature dependence, model compounds)

KFe&fn (CO) ,Jcarbon

1%

35

surface structure, mobility

Si, Au (temperature dependence, model compounds)

Ru3(CO) ,,/y-alumina, silica, MgO

35

surface structure, mobility

& Aa (temperature dependence, model compounds)

13C

31

surface structure, reaction pathway

Si (isotopic labeling)

CpFe(C0)23CHS, CpFe(‘sCO)zCH,/y-alumina Cp= $-CSHB

13C

30

surface structure

Si, ‘J( ‘03Rh,13C), relaxation time

Rh(CO)&lz/alumina

‘3C

26,29

surface structure

4

HzcOs(COL,H~Os,(CO),,/MgO

1%

49

41

42-46

47,48

surface species formed

surface species formed, reaction pathway, catalytically active sites

surface species formed, catalytically active site

Si (isotopic labeling, paramagnetic probes, field dependence, model compounds) $ (isotopic labeling)

ZrNp/silica Np=CH,C(CH,),

Cp’AcR/ahunina, silica, MgO, MgCl Ac=Th,U,Cp’=$(CH3),Cb; R= CH3, CH&H3

CpxZr( 13CH3)x/alumina, methylaluminoxane, Cp = $-C&H6

1%

i3C

‘SC

W

surface species, ligand exchange at surface

Max ( C3HB),, Zr (C,H,)Jahrmina,

13C

[RhClL],/y-alumina, L = 1,5 cyclooctadiene, ( C2H1)2, (CO Jz

silica

NMRparameters measured

39

Supported complex studied

Nuclear snecies surface species formed

on metal oxides Ref.

su~mrted

Information obtained

Metal olefins

TABLE 3

Rh(1) (CO)*, Rh,(CO)ix/NaY

Ir, (CO)is/NaY zeolite

Fe3 (CO )JNaY

‘3C

13C

‘SC

zeolite

MO (CO)s/NaY zeolite

“Na zeolite

Ni (II) carbonyl phosphine complexes/NaY zeolite

Si, Au (temperature dependence)

4

4

4

4

60

complex synthesized within zeolite cages

35

59

complexes synthesized within zeolite cages

complex generated within zeolite cages, mobility

58

57

surface structure, bonding sites

complexes synthesized within zeolite cages

56

31P

4

decomposition products

NaX, NaY zeolite

MO (CO)s, W (CO),/

54,55

51-53

Ref.

surface species formed, mobility

1%

& AU (temperature dependence)

NaX Zeohte

Cr (CO),,, MO(CO),/

170

‘3C

t

surface species formed, bonding to surface, mobility, decomposition prod&e

Si (temperature dependence)

Ni(COL, Fe(CO)3,WCOh, Mo(COh/

13C

Nay, HY Zeolite

Information obtained

NMR parameters measured

Supported complex studied

Nuclear species

Organometallics encaged in zeolites

TABLE 4

:

$

a Q,

; h E

p a Z 3 .

454

L. Reven /J. Mol. CataL 86 (1994) 447-477

(1) where the last three terms are tensoral quantities due to orientation dependence in the solid-state. From this expression, it is seen that the Zeeman and chemical shift interactions are dependent on the magnitude of the applied magnetic field, whereas the dipolar and spin-spin coupling are field-independent. Since NMR is an inherently insensitive technique, the application of large magnetic fields for higher resolution is advantageous for surface studies. However, the lines will also be broadened since the anisotropic part of the chemical shift, an important interaction in the solid-state, will also increase. In the case of abundant spins with large magnetic moments (‘H, ‘?F), homonuclear dipolar interactions will dominate, leading to severe line broadening and making it exceedingly difficult to obtain high resolution spectra. For rare spins, such as 13C, heteronuclear dipolar interactions will be important and high power proton decoupling and cross polarization, as described below, are necessary to obtain high-resolution spectra. Since the spin-spin or scalar coupling is usually much smaller than the other interactions in the solid-state, its effect on the spectra is usually minimal. Exceptions include abundant spin 1=1/2 metals such as lg6Pt or lo3Rh bonded to 31P or 13C,which have large scalar couplings that are extremely useful for structural determination of organometallic complexes. Chemical shift anisotropy (CSA)

For a rare spin I= l/2, such as 13C,the dominant interaction may be the chemical shift anisotropy (CSA) [ 111. In solids, where the molecular axes are fixed with respect to the applied magnetic field, the chemical shift becomes orientation dependent. The spectra of polycrystallme materials consist of broad resonances known as CSA powder patterns due to a distribution of chemical shifts. The chemical shift interaction in solids is given by a second rank tensor, a, which is described by three principal components (a,,, rx22,CJ~~), when diagonalized by a correct choice of coordinate system. In solution, the nuclei experience an averaged environment due to rapid molecular tumbling, the anisotropic contribution vanishes, and the chemical shift is given by the average of the principal elements, known as the isotropic shift, Si: (2)

h=jhl+~22+~33)

In solids, the anisotropic contribution, which is related to the width of the powder pattern, is defined as: da= q, - f

(61

+a22

1

(3)

In the case of an axially symmetric site, as is the situation for carbonyl

L. Reven /J. Mol. Catd. 86 (1994) 447-477

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ligands, two of the principal elements become equal so that two unique elements remain, g1 = all - az2and ali= a33.An additional parameterdescribing the chemical shift in solids is the shieldingasymmetryfactor q, a measureof the deviation from axial symmetry: 9=

I

61 033

-

022 -

4

I

whereby convention, q3 > az2> all. Considerable information can be acquired from the chemical shift anisotropy: (i) Surface species assignments.In addition to the isotropic shift value, different functional groups have characteristicCSA values. (ii) Structuralinformation. Molecular configuration and bonding arrangements will be reflectedin the chemical shift anisotropy. (iii) Molecular dynamics. CSA interactionswill be averagedin the presence of molecular motion on the order of dram,,where vL is the Larmour or resonancefrequencyat a particularfield strength. Magic-angle spinning Althoughvaluablestructuraland dynamicinformationcan be derivedfrom measurementsof the chemical shift anisotropy, excessivelylarge CSA values will lead to severelyoverlappingpowder patterns and the inability to distinguish between chemically distinct sites. However, the chemical shift anisotropy can be reduced by macroscopically spinning the sample at an angle of 54.7” with respect to the external magneticfield since the dipolar interaction and the anisotropic contribution to the shielding contain the angular term (3cos2#- 1). The result of magic-angle spinning (MAS) is a solution-like spectrumof narrow resonances at the isotropic chemical shifts. However,for complete removal of the anisotropic contribution, the spinning rate, v~,must be greaterthan da, the chemical shift anisotropy. Otherwise,incompletetime averagingof the chemical shift anisotropy will result in the CSA powder pattern being broken up into a series of spinning sidebands located at multiples of the spinning frequency,v~,on either side of the isotropic peak for each site. These sidebandswill vary in intensity dependingon the ratio Y,/&v~ where vLis the Larmor frequencyand da is givenin ppm. If v,/~Qv~> 1 the sidebands will be 10% or less of the centerband (isotropic peak) intensity.The isotropic peaks, or centerbands, can be identified by varying the spinning speed and observingwhich peaks do not change position. The CSA information can be recoveredfrom slow-MAS spectrasince the relativeintensitiesof the spinning sidebands are related to the principal elements of the chemical shift tensor [ 121. Thus, in the slow spinning regime, V,
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L. Reven /J. Mol. Catal. 86 (1994) 447-477

numberof spinningsidebandsis desirableto obtain accuratevaluesof the CSA parameters.

Proton decoupling and cross polarization

For materials containing protons, heteronucleardipole-dipole interactions can be a significantsource of line broadening.In the case of 13C,with a natural abundance of 1% as compared to 100% abundant protons, the spectrum can be completely obscured by proton dipolar broadening.For 31P,also 100% abundant,both homonuclear31P-31Pand heteronuclear1H-31Pdipolar interactionswill contribute to the line width. As in solution NMR, these interactions can be removedby proton decoupling,althoughin solids, the magnitude of the decouplingfield requiredcan be quite large. One method of decoupling is to apply a second r.f. field and periodically flip the proton spins with a series of pulses [lo]. This will invert and time-averageaway the field from the proton spins felt by the carbon spins. Another method is to spin lock the proton spins by applyinga continuousirradiationalong an axis transverse to the magnetization.This will effectively scramblethe proton spins so that the net dipolar field felt by the carbon spins is greatlyreduced. The presence of strong heteronucleardipolar coupling can also be exploited for signal enhancement through the technique of cross polarization [ 131. In cross polarization, the large magnetizationof the protons is transferred to the rare spins, usually carbons, by simultaneouslyirradiatingboth spins with r.f. power so as to match the nuclear spin energy levels and allow the magnetizationof the abundantspinsto flow to the rarespin reservoir.This matching condition is met by adjusting the two r.f. field strengths so that yc.EIlc=yn~lH,where Hl is the magnitudeof the r.f. field and y is the gyromagnetic ratio of the nucleus.After creatingcarbon magnetizationby simultaneous application of the two r.f. fields, the 13Cr.f. field is abruptlyturned off, and the ‘H r.f. field is left on to obtain a high resolutionproton decoupled 13C spectrum.In the case of carbon, cross polarizationis very advantageoussince the experimentwill depend on the short proton relaxationtimes rather than those of carbon which can be very long in the solid state. The theoreticalmaximumenhancementobtainabledependson the ratio of the gyromagneticratios of the two spins involved,a factor of 4 in the case of ‘H-13C cross polarization. Cross polarizationis only efficient in the case of less mobile chemisorbedspecies with long relaxationtimes. More mobile physisorbedspecieswith shorter Tl valuesmay be better detectedby high power decouplingalone. The combinationof cross polarization(CP ), magic-anglespinning(MAS) and highpower decouplinghaspermittedhigh-resolutionspectraof lessmobile organometallicsurface species, as will be discussed in detail in the following sections. Despite the high surface areas of the commonly employed inorganic supports, carbon-13 isotropic enrichment of the precursor organometallic

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complex is often necessary.In addition, special&d equipmentis requiredfor MAS NMR of air sensitive samples, such as probes designed to spin sealed samplesor spinningrotors sealedwith o-rings. For NMR studiesof molecular motion, variable temperatureMAS NMR experimentscan be quite difficult. The small activation energiesfor mobility of many adsorbed organometallics are such that the slow motion regimeis often well below the lowertemperature limit of current MAS NMR technology. These experimentalobstacles have limited the number of MAS NMR studies of supported organometallics,despite evidence for the incisive type of structural information which can be obtained

Metal complexes attached via functionalized ligands

The fmt solid-state NMR studies of supportedorganometallicsthat employed high-resolutiontechniques focused on transition metal complexes anchored to solid-supportsby functionalixedligands. Pt, Pd, Ru, and Ni complexes in the solid-stateand covalentlyattachedto silica by phosphine ligands were characterixedusing 31P,?3i and 13CCP.MAS NMR. Fyfe and coworkers exploited the largescalarcouplingbetween 18sPtand 31Pfor structuralstudies of platinum(II) complexes immobilizedon functionalixedpolymers and silica surfaces.The magnitudeof ‘J (‘%Pt, 31P) is particularlysensitiveto the tnuLs ligand in squareplanar platinum(II) complexes and the ‘lP resonancesof the cis and tmns isomers are easily distinguishedby the magnitudesof the scalar couplingswhich are ca. 3500 Hz for cis complexes and 2500 Hx for tmm complexes. Measurement of V( ??t,3lP) for cis- [ PtC12(P&)2 ] (PR, = PPh,(CH,),Si(OEt),) attached to silica and glass surfaces revealed that the cis geometry is retained in the immobii complex [ 431. As evidenced in the 31Pspectra shown in Fig. 1, the complex remains intact after attachment to the surface. However, the relative broadness of the 31Presonances reflect a highly disorderedenvironmenton the surface.The relativeefficienciesof differentsyntheticroutesto immobii metal complexeswas also evaluatedby 31PNMR. Two alternativepreparationsare commonly used: (i) The support is first functionalixedwith phosphine groups and then the metalcomplexis attached to the phosphinatedsurfacevia ligandexchange. (ii) The functionalixedmetal complex is first prepared and then attached to the surface. In both preparations, the 31Pspectra revealed the production of phosphines and phosphine oxide in addition to the bound complex. However, the first route is much less efficient,producinga largeramountof phosphineoxide and a mixtureof products. Apparently,the initial functionalixationof the surface through the reaction of (EtO),Si(CH,),PPh, with the surface hydroxyl groups results in substantial oxidation of the tertiary phosphine. 31PNMR spectraconfirmedthat this side reactioncould be substantiallyreducedby first

L. Reven /J. Mol. Cati. 86 (1994) 447-477

do

30

0

k

-30

-60

PPM Fig. 1. (A) “P CP MAS NMR spectrum of ck-{PtCl,(Ph,PCH2CHzSi{OEt}s)2}. (B) 31P CP MAS NMR spectrum after immobilization on silica gel. Reproduced with permission from ref. 5.

immobilizing an organic group with active chlorine and then capping remaining surface hydroxyls with chlorotrimethylsilane before generating the phosphine ligand. Fyfe and coworkers also used 31P NMR to characterize polymer-immobilized complexes of platinum (II) [ 6,7]. Two different preparative routes were again evaluated, one in which polystyrene-divinylbenzene is functionalized by a chemical reaction to attach the phosphorous group, and a second method in which a functionalized monomer is synthesized and then copolymerized with styrene and divinylbenzene. Both methods were observed to produce bound phosphine oxide which was then reduced to tertiary phosphine before the Pt complex was attached. The 31P NMR showed these preparations to be only partially successful with uncomplexed, immobilized tertiary phosphine remaining in addition to the production of a mixture of tram and cis isomers of the bound metal complex. Other researchers have also used 31P NMR to study Pd and Pt immobilized complexes and came to similar conclusions in regard to the efficiency of the different preparative routes and the side products produced. However, contrary to the earlier results, Komoroski and coworkers concluded that palladium complexes preferentially bind to the surface with the tram configuration [ 141. The conflicting 31Pchemical shift results were attributed to the different preparative routes used and the method employed by Fyfe was repeated and found to produce significant impurities. Although the large scalar couplings are not present for structural information as in the case of the bound platinum com-

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plexes, a study of model squareplanar palladiumcomplexes showed that the 31Pisotropic shift is a very sensitiveindicator of molecularstrain in the solid state. From comparison with these model palladium complexes, it was concludedthat the complexesarebound in an unstrainedfashion,with interligand distancessuitablefor trunscomplexationon the surface.A reductionin da, the chemical shift anisotropy and/or cross polarization efficiency indicates the presenceof surfacemobility. However,in the case of the palladiumcomplexes, the static 31Pspectrapowder patterns displayedthe full CSA width as well as efficient cross polarizationwhich showed that the complexes are bound in an unstrainedbut rigid manner. A 31P static and magic-angle spinning NMR study of the silica-bound triosmium clusters H20s3(CO) iOL’ and 0s3 (CO) ilL’, {L’ = PPhz[ CH2],Si (OEt ),}, displayed spectra consistent their precursors [ 151. However, in contrast to infraredresults,the NMR data of the third clusterstudied, H20s3(C0)9L’, showed that this complex does not undergoa simple immobilization. Both the isotropic shift, & and the chemical shift anisotropy, da, measuredfrom MAS and static 31Pspectra respectively,were smallerthan in the precursor complex. Based on EXAFS data of these clusters,the authors suggestthat H20s3( CO)9L’ furtherreactswith the surfacehydroxylgroupsto form a complex bound by an oxygen ligand. In other investigations of immobilized complexes, ?3i and 13CNMR yielded very specific information in regards to the bonds which anchor the metal complex to the surface [16-181. The large body of chemical shift data availablefrom studies of derivatizedsilicas was found to be useful for identifyiig the types of anchoring ligands. For example, from ?3i NMR, Prignano determinedthat in silica-boundPt (C,O,)L,, [L = ( 0Me)3Si (CH,),PEtJ ,40% of the silylphosphineligands had two Si-0-Si linkagesand 40% had one Si0-Si linkageto the surface [ 161.Maciel and coworkersemployed2gSi,31P,and 13C CP MAS NMR to characterize the attached ruthenium complexes LRu(CO)~~~~L~RU~(CO)~,L=PP~~(CH~)~S~(OE~)~ [17].Theobservation of 13Cpeaks for OEt carbons ruledout complete hydrolysisand a largefraction of SiOEt bonds were found to remain intact in the bound complex. By comparison with underivatizedsilica, the 2gSiNMR spectra of the silica function-

R’\si/ OR

R’

I RO-

SiI 0

I

OR /\ 0

0

Ila

R’

I

1

RO-Si-&Si-OR I 0

I 0

Ilb

Fig. 2. Proposed structures for surface linkages of the silica-bound complexes LRu(CO)~ and L3Ru3(CO)9, L=PPh2(CH2)2Si(OEt)3.

Adapted with permission from ref. 17.

460

L. Reven /J. Mol. Catal. 86 (1994) 447-477

alized with the ruthenium complexes were determined to be consistent with a mixture of structures I and II shown in Fig. 2 for the anchoring bonds to the surface. Since there was no %i resonance for intact -Si(OEt)3 groups on [ SiOz]-LaRu3 ( CO)e, it was concluded that all three anchoring ligands of the trinuclear Ru complex are involved in the covalent attachment to the surface. The surface structure of silica functionalized with (l,l’-ferrocenediyl)dimethylsilane was also characterized by this group using 13C and “Si NMR and confirmed to be surface-O- (dimethylsilyl) ferrocene [ 181. In summary, high-resolution solid-state NMR has been demonstrated to be a valuable characterization tool for metal complexes immobilized on functionalized supports. In addition to determination of the geometry of the metal complex and the bonding to the surface, side reactions were detected which assisted in the development of cleaner preparative routes.

Metal carbonyls supported on metal oxides Metal carbonyls adsorbed onto metal oxides have been extensively studied as precursors for supported metal catalysts. The carbonyl ligands are easily stripped off, leaving no organic impurities and a wide range of mono- and bimetallic carbonyl clusters of varying nuclearity are available as precursors. There have been relatively few 13Csolid-state NMR studies of metal carbonyl clusters due to the excessively long relaxation times and the large CSA parameters of the carbonyl ligands which result in severe overlapping of the resonances [ 191. However, isotropic enrichment by exchange with 13C0 is usually a facile process and adsorption onto the support normally results in reduced relaxation times due to dynamic processes. For chemisorbed metal carbonyls, cross polarization can often be employed, due to the presence of protons on hydroxylated metal oxide surfaces in order to circumvent the obstacle of the long carbon relaxation times. The large CSA’s of the carbonyl ligands, which are almost always axially symmetric, are sensitive probes for the presence of molecular motion, which results in partial or complete averaging [ 191. SlowMAS experiments will resolve the isotropic shifts of the individual sites as well as yield the CSA parameters for structural information [ 121. On a less positive side, many of these systems are extremely air sensitive and require special techniques in order to carry out variable-temperature MAS experiments. In many cases, the activation energy is so small that the molecular motion of the supported metal carbonyl cannot be frozen out within the experimental temperature range of MAS NMR. Despite these obstacles, a number of 13Chighresolution studies of mononuclear metal carbonyls and metal carbonyl clusters have been carried out which have identified the surface species formed, their dynamic behavior, and the nature of the bonding to the surface.

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L. Reven / J. MoL CataL 86 (1994) 447-477

Molybdenum subcarbonyls on alumina surfaces

The Mo(CO)~/~-alumina system has been investigated by several research groups, both by wideline NMR and by high-resolutionmethods [20251. This system has also been extensively studied by other techniques such as infrared spectroscopy and temperature-programmeddecomposition. The reaction of molybdenumhexacarbonylwith y-aluminahas been determinedto occur throughthe following steps: Mo(C0)

CPhya

+= +co

Mo(CO)+

s

Mo(CO)~,~

(5)

The initialphysisorptionof molybdenumhexacarbonylresultsin a single narrow resonance at 201 ppm. Shirley and coworkers used wideline NMR to follow the decarbonylation of molybdenum hexacarbonyl on alumina [20]. However,the resonancesfrom the more stronglyadsorbed,less mobile subcarbony1 species were not detected. Hanson et al. applied variable temperature CP MAS to this system, [21] and after thermal activation, observed the appearance of resonancesat 203 and 223 ppm which were assignedto MO(CO ) 3 and MO( CO)5 respectivelyby comparison with model compounds. The presence of fluxionalbehavior in these subcarbonylswas also suggested.MO(CO), is proposed to bond to the surfacein an octrahedralgeometrywith the surface bond servingas the sixth ligand.The resonancesfor the ciaand trans carbonyl ligandsof MO(CO), were not detected in these spectrawhich were acquiredat a relativelylow applied field ( 13CvL= 15 MHz). The absence of separateresonances for the cis and bans carbonyl ligands was attributedto either axialequatorial CO exchange or an accidental degeneracy of the peak positions. However,theseresonanceswereobservedin a subsequent13CMAS NMR study of MO(CO), by Walter et al. who also proposed a dynamical model [ 221. As shown in Fig. 3, the higher applied fields in this work ( 13C~~=96.5 MHz) improvedthe resolutionenoughto allow observationof separatecis and trans resonancesat 206 and 210 ppm. Slow MAS experimentsrevealedthat the cis carbonyls have an anomalously amall chemical shift anisotropy of da=180 ppm, as compared to the usual value of ca. 400 ppm for a rigid terminal carbony1 ligand. A static low temperaturespectrum at 10K consisted of a single broad powder pattern, displayingthe full rigid-lattice anisotropy, indicating that this surfacespecieshas a very low activationenergyto motion. As depicted in Fig. 4, an anisotropic motion, in which this octahedralspecies undergoesa free rotation about the MO-surfacebond, was found to be consistent with the observedmagnitudesof the CSA values. The 13Crelaxationbehavior of MO(CO),/y-alumina preparedunder various conditions was examinedwith the objective of determiningthe source of the shortenedrelaxationtimes of the supportedcomplex, howeverthe results were inconclusive [ 24 J. Calculationsof the correlationtimes for MO( CO)3,d, and Mo(CO)h,adsfrom the line widths as a function of applied field strength and spinning rate supported the suggestionthat the broad line observed for

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300

100

200 PPM

FROM

TMS

Fig. 3. (A) 90.5 MHz magic-angle spinning “C NMR spectrum of Mo (CO)&-alumina activated at 100°C for 1 h; spun at 2.2~ lOa Hz, 1580 acquisitions, 5 s recycle. (B) Simulation of MO(CO), (ads) resonances. Reproduced with permission from ref. 22.

Fig. 4. Model proposed to account for the motionally averaged 13C shielding tensors of Mo(CO),(ads). Reproduced with permission from ref. 22.

MO (CO) 3,ads is due to slow molecular motion rather than a distribution of overlapping isotropic chemical shifts for different adsorption sites [ 251. Osmium carbonyl clusters on silica and m4xgnesM The surface organometallic chemistry of osmium carbonyl clusters is relatively well understood. The high kinetic stability of osmium clusters has facilitated characterization of intermediate species by a wide range of chemical and spectroscopic methods. However, structural questions remain unanswered for many of these systems. An extensive 13C MAS NMR investigation of OS, (CO) 12supported on silica was used to resolve several proposed structural

L. Reven /J. Mol. Catal. 86 (1994) 447-477

463

2

3

F’ig. 5. Structures proposed for 0s3(CO),, from ref. 26.

chemisorbed on silica. Reproduced with permission

models,displayedin Fig. 5, whichcould not be definitelydistinguishedbetween by EXAFS or infrared spectroscopy [26]. High resolution 13CMAS NMR spectrum of OS,(CO),Jsilica after thermal activation, shown in Fig. 6, displayed considerablefine structureof the centerbands (isotropic shifts) which was compared to the model complexes HOs, (CO) 1o(OSiEt,), 0s3(CO) 10( OCH3)2, and HOs, (CO) 1,,(O&H). Based on the isotropic chemical shifts and relativeintensities,structureI of Fig. 5 was determinedto be correct. Since the CSA parametersrangedfrom 150 to 250 ppm, a factor of two smaller than the CSA of 350 ppm observed in OS,(CO) 12,it was suggestedthat the chemisorbedclusterundergoesrestrictedmotion.This was confmed by a MAS NMR spectrumtaken at - 105’ C which displayedthe full rigid-latticeanisotropy [27]. Several dynamical models were considered; free rotation of the triosmiumcluster about the 0-Si bond, localized carbonyl exchange,and rotational jumps. Free rotation of the cluster, as in the case of chemisorbed MO(CO), discussed above, would produce reduced anisotropies given by the following expression: da’ = 4 (3 cos2e- 1)da

(6)

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L. Reven JJ. Mol. Catal. 86 (1994) 447-477

wherethe reduction factor is determinedby the anglebetween the individual C-O bonds and the rotationaxis.However,the calculatedvaluesdifferedgreatly from the experimentalresults. Likewise, CO exchangebetween inequivalent sites was ruled out based on the temperatureinvarianceof the peak positions. The CSA parametersfor a two-site rotation were calculatedfrom the average of the CSA tensors for pairs of interconvertingorientationsfor a range of angles.The highlyasymmetricexchanged-averagedtensorscalculatedsupported a model of rapid ( Z+32 kHz), largeangle (QV-120” ) rotationaljumps [ 271. Hydrido-osmiumcarbonyls on magnesiahave been investigatedwith infrared spectroscopy and high resolution NMR [ 28,291. Although the 13CCP MAS NMR spectrumfor H,Os, (CO) JMgO was poorly resolved,the chemical shift of the broad resonancewas consistentwith the formation of the monoanion [H30s4(CO),,] -, which has only a weak ionic interaction with the surface and is easily extracted by cation methathesis [ 281. In contrast, the species generatedby chemisorptionof H,Os(CO), was not extractable,indicative of a strong chemical bond to the surface.The 13CCP MAS NMR spectrum of H,Os(CO),/MgO consisted of two peaks at 182 and 203 ppm with an intensity ratio of 1:3, markedly different from the single 196 ppm resonance spectrumofthe modelcompound [N(PPh,),] [H,Os(CO),]. The less intense resonanceat 203 ppm was assignedto a carbonyl ligandof the surfacespecies

Fig. 6. SO.5 MHz ‘% MAS NMR spctrmn of &(CO),,/SiO,. Inset shows expanded view of centerband resonances. Reproduced withpermission from ref. 26.

L. Reven

/J. Mol.

CataL.

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465

[HOa(CO) which forms a tight ion pair bond with a surface Lewis acid site. The 182ppm peak was attributedto the other three carbonyl ligandssince the ionic bonding to the surface will diminish the electron density on the osmiumand alongwithit, the &-p,back donationto the carbonylligands,thereby lowering the shift by decreasing the paramagnetic contribution to the “C chemical shifts of these ligands. In a relatedstudy,the anionic surfacespeciesformedby the deprotonation of H,Os,(CO),, on highly dehydroxylated MgO were identified as [H,Os,(CO),,]- and [H,OS,(CO),,]~- by 13CCP MAS NMR in addition to infraredand W-Via spectroscopy [29 1. The resolutionwas very poor, due to relativelylow 13Cenrichment level (37% ) and low applied field ( 13CvL= 25 MHz). However,the chemical shifts of the two resonancesat 175 and 185-196 ppm indicate that the carbons of the surface species are deshielded as compared to the neutralcluster H,Os, (CO) 12which is consistent with the formation of anionic clusters. Rhodium dicarbonyl on alumina There has appearedone 13CNMR study of a supportedrhodiumcomplex [ 301. Rhp(CO),Cl,/alumina was preparedas a homogeneousmodel systemof dispersed Al-0-Rh ( CO)z units for comparison with the dicarbonyl sites detected on CO-treated reduced alumina-supportedRh catalysts. The presence of isolated Ru (CO)x units in supported Rh catalysts has been proposed, due to the observationof IR bands whose positions do not depend on CO coverage. The 13CMAS NMR spectrumof the Rh, (CO)&l,/alumina model systemconsisted of a doublet centered at 136.9 ppm with a relaxationtime, Tl = 966 ma. The 60 Hz splitting of the observed doublet is due to scalar coupling between 13Cand lmRh (I= l/2,100%abundant). The ‘J ( 13C,10sRh) couplingconstant and isotropic shifts are within range of the values observed for terminal CO ligands in rhodium clusters. The absence of an EPR signal, along with the presence of an NMR resonance,show the Rh (CO), species to be diamagnetic since relaxation of a carbon directly bonded to a paramagnetic site would broaden the signalbeyond detection. Earlier 13CNMR studiesof “CO on Hareduced Rh/alumina identified dicarbonyl sites with a similar chemical shift (6= 177 ppm) but with a relaxation time a 106 times shorter. A model was proposed for the Ru (CO) 2 sites formed on CO treated reducedRh catalystsin which the dicarbonyl units are not directly bonded to each other, but instead aggregatenear the minority of paramagneticRh centers which have been detected by low temperatureEPR. This clusteringmodel was invoked to explain the enhanced 13Crelaxationof the dicarbonyls on the reduced Rh catalyst as compared to the Rha( CO),C!lz/aluminamodel surface. Supported iron and ruthenium carbonyls In a 13CCP MAS NMR studyof the surfacechemistryof CpFe( CO)&H3, Cp = $‘-C6H6,on y-alumina,site selective13Cenrichmentproved highlyuseful

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for determiningthe surface reaction pathway [31]. Methyl transfer to an aluminasite, nucleophilicadditionto a carbonylligandor protonolysiswereruled out as possible reaction pathways since the experimental13CNMR spectra were not consistent with the surface structuresexpected from such mechanisms.A new, highlyshiftedpeak at 333.9 ppm appearsin the spectrumof the isotopomer CpFe( 13CO)zCH3,in addition to the resonances for Cp and terminal CO carbons. The spectrumof the other isotopomer, CpFe( C0)213CH3, contained only a methyl resonance (6~45.2 ppm) which is shifted paramagnetically as compared to the methyl carbon peak in neat CpFe(CO)&H3 (6= -21.8 ppm). The shifts of this new peak at 333.9 ppm and the methyl group, which are close to those reported for CpFe(CO) (L ) C (H) R+ carbene complexes and methylcarbenerespectively,supported a structurecontaining a carbenoid-likecarbon, as shown in Fig. 7, which forms via a surfaceinduced, migratoryCO insertion. Infrared studies suggest the generation of the anionic hydride, [ HFe3(CO) il] - when Fe, ( CO)12is chemisorbedonto aluminaand magnesia surfaces [ 32-341. In one proposed structure,Fig. 8A , the cluster bonds to a surface Lewis acid site via a bridging CO ligand [ 321. Other work employing EXAFS in additionto IR and W spectroscopy [ 331 concludedthat the triiron clusterbonds to the surface via an oxo-bridge across one of the Fe-Fe bonds ratherthan a bridgingcarbonyl, as shown in Fig. 8B. Since the 13Cresonances of bridgingand terminal carbonyls are easily differentiatedby the largerisotropic shift and smalleranisotropy of the bridgingcarbonyl, [ 191 these structures shouldbe distinguishableby NMR. In fact, the 13CCP MAS spectrumof the model compound [Me,N] [HFe3(CO),,] revealedthat the bridging carbony1has an anomalouslysmall CSA, da=90 ppm [35]. The 13CMAS spectrum of Fe3(CO)12/y-A1203,Figure 9A, shows that the supported cluster undergoes some type of motional averaging,but the signal is much broader than those observed for highly mobile physisorbed species in which the CSA is completelyaveraged.The residualbroadeningwas suggestedto be due to the presenceof subcarbonylsor anisotropicmotion and/or incompletescrambling of the CO ligands [35]. As seen in Fig. 9B, 13CMAS spectrumat -85°C displays a series of broad spinning sidebands as the complex becomes rigid, but no resonancefor a bridgingcarbonyl is evident. This resulttends to favor the oxo-bridgedstructureas the surfacespecies, althoughit is noted that another /

CH3

Fig. 7. Proposed structure for CpFe (CO)&Ha

Cp = $-CBH6, on y-alumina. Adapted from ref. 31.

L. Reven /J. Mol. Catal. 86 (1994) 447-477 A:

B:

Hugues

467

model

lwasawa

model 0

0

Fig. 8. Proposed structures of chemisorbed Fe3 (CO) 12on y-alumina. Reproduced with permission from ref. 35.

-85%

I

. I

’

400

’

’

I

’

’

ppn

from

Fig. 9. 13CMAS NMB

’

I 0

200

’

’

TMS

spectra of Fe3 (CO) 12on y-alumina at (A) room temperature; (B ) - 85 ’ C. Reproduced with permission from ref. 35.

466

L. Reven /J. Mol. Catal. 86 (1994) 447-477

IR study showedthe appearanceof the band assignedto the bridgingcarbonyl to be very sensitiveto reaction conditions [ 341. The generationof highlymobile hexarutheniumcarbonylclusterswas detected by 13CNMR when Ru3(CO),, is chemisorbed on hydroxylated MgO 1351.The two narrow resonances,which only began to broaden at - 122”C, wereassignedtotheanionicclusters [R~(CO),8]2-and [Ru,(CO),,]~-based on the isotropic shifts of these anions in solution. The formation of these high nuclearityclusters is not surprisingsince the reaction of Ru3(CO ) 12with the basic hydroxylgroupson MgO parallelsthe chemistryin basic solutionin which theanionic hydride [HRu,(CO),,] - is initiallyformed,and afterlongertimes, is produced. The highersurface loadings and longer reaction M~WI,~~times for these samples were believed to be responsible for the formation of the hexarutheniumclustersas the major speciesratherthan [HRu, (CO),,] -. This behaviorcontrastswith the surfacechemistryof Ru3(CO) 12on y-alumina and silica in which rutheniumdicarbonylspecies are formedwhich display 13C spectrathat are enhancedby cross polarization and have large chemical shift anisotropies,indicatingrestrictedmotion of a speciescovalentlybonded to the surface [ 351. Only one 13CNMR study of a supportedbimetallic carbonyl cluster has been reported,KFe2Mn(CO) i2on a carbon support [ 361. In the 13CMAS spectrum of the solid precursorcomplex, separateresonancesfor the two bridging and terminal CO groups are visible. However, supported KFe2Mn(CO)lz exhibits a spectrumconsistingtwo broad resonancesat 214 and 205 ppm which was attributedto motional averaging.The resonanceswere assignedto mobile KFe2Mn(CO) 12and the decompos,itionproduct Mn2(CO) 1o,accordingto the shifts calculatedfrom the solid-statespectrumof the pure compounds.A variable temperaturestudy down to 123 K was not low enough to reach the slow exchangeregime,and relativelysmall activation energiesof 0.6 kcal/mol and 0.5 kcal/mol were calculated from the line widths. It was noted that these values are much smallerthan those measuredfor the fluxional iron carbonyl cluster,Fe, (CO) 12,in either in solution [ 371 or in the solid-state [ 381. Supported metal olefirw The interaction of MO,(C,H,), and Zr (C,H, )4 with alumina and silica was investigatedby FT-IR and 13CCP MAS in orderto determinewhetherthe molybdenummetal-metal bond is retained,as had been suggestedby earlier work [39]. A spectrum of narrow lines similar to that of neat Mo~(C~H~)~ demonstratedthat a fluxional molybdenum-molybdenumspecies is present. The 13Cspectrum of Zr(C3HS)4exhibited a solution-like spectrum reflective of a simple x-bonded di-ally1structurein which the ally1groups are not perturbedby interactionswith the surface,in contrast to the a-ally1MO,(C,H,), surfacespecies. EXAFS and 13CNMR were combined to characterizethe surfacespecies formed by chemisorption of the rhodium complexes [ RhC1L12(L = 1,5 cy-

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clooctadiene, (C&H,),, (CO),) onto partiallydehydroxylatedy-alumina [40]. These two techniques are complementaryin that whereasNMR provides information about the organic ligands, EXAFS probes the local bonding environment of the metal. 13CCPMAS spectra of the rhodiumolefm complexes in solution, as neat solids, and chemisorbedwere compared to the olefins alone, physisorbedon alumina.Since the carbon shifts of the neat and chemisorbed rhodiumcomplexes were found to be similarbut distinct from those observed for the physisorbedolefins, it was concludedthat all of the olefin and carbonyl ligandsremainedattachedto rhodium after chemisorption.In addition, when the chemisorbed rhodium ethylene species, prepared from [ RhCl ( C2H4)2]2, was exposed to cyclooctadiene, new carbon signals appeared,consistent with the chemisorbed rhodium cyclooctadiene species. Apparently the olefm exchange behavior observed for the precursorcomplexes in solution is retained in the chemisorbedspecies. Recently, the structureof a grafted,highlyelectron deficient, Zr (IV) surface complex, formed by the reaction of tetrakis(neopentyl ) zirconium,ZrNp,, with the surface hydroxyl groups of silica has been definitivelycharacterized by 13CCP MAS NMR in conjunction with IR spectroscopy [ 411: ZrNp,(I) +Si-OH+ (Si-O)ZrNp,(JI) +NpH

(7)

(Si-O)ZrNp, + $0, +Si-0-Zr (ONp), (III)

(8)

In the first reaction step, only the methyl carbon signalwas observed for II since the CH2 signal of the neopentyl ligand, which is also very weak in ZrNp4,cannot be observed due to the long relaxationtimes. The quaternary carbon peak was broadened beyond detection as well. However,after reaction with dry oxygen, new resonances appear which, by comparison with neopentanol and neopentane,were assignedto the methyl,quaternaryand methylene carbons of a well-defined surface neopentoxy complex, Si-OZr [O-CH2C ( CH3)3]3,a highlyelectron deficientdocomplex, apparentlystabilizedby the silica surface. Marks and coworkershave carriedout a seriesof 13CCP MAS NMR studies of the surface chemistry of supported organoactinides (Th, U) as model systems for supported early transition metal olefm polymerizationcatalysts [ 42-451. A reviewof this work has also recently appeared [ 461. Site-selective isotopic labeling,paramagneticprobes, model compoundsand variableapplied fields wereused to show that on dehydroxylatedaluminaand MgCI,the organothorium complexes Cp’,ThR, (Cp’ = f- ( CH3)&; R= CH3,CH,CH,) form a cation-likesurfacespecies,structureI of Fig. 10, via the transferof a methide anion to a Lewis acid site. The Al-CH3 site assignmentwas confirmedby comparison with the model compounds poly(methylaluminoxane) and [ ( CH3) Ah.&] 2, and the observation that the resonance became narrower at higherappliedfieldsas expectedfor residualdipolarbroadeningof a spinbonded to a quadrupolarnucleus ( 2’A1,I= 5/2), an interactionwhich is not removable

470

L. Reven /J. Mol. Catul. 86 (1994) 447-477

fH3

WzTh,

0

I I

II

Ill

Fig. 10. Proposed structures for the supported organothorium complexes Cp’,Th& (CH,)&; R=CHS, CH&H,). Adapted with permission from ref. 45.

(Cp’ =$‘-

by magic-angle spinning. In order to rule out a bridging alkyl structure, structure II in Fig. 10, the reaction of the U (IV) complexes, which are expected to have a surface chemistry similar to Th (IV), were also examined. A large paramagnetic shift of the methyl resonance from the 5f unpaired electrons of U (IV) would be expected for such a complex. In neat Cp’,U(CH,), the U-CHB 13C resonance had an extremely large paramagnetic shift of 1430 ppm. However upon reacting Cp’JJ(CH,), with dehydroxylated alumina, the Al-CH3 carbon resonances appeared at similar shifts as in the supported thorium species and proximity of the metal cation species to the Al-CH3 moieties could be definitely ruled out. 13CNMR studies of the reaction of the supported Th complex with ethylene revealed the Th-CH3 bond to be the active site for ethylene polymerization via olefm insertion. The Th-CH3 carbon peak diminished with ethylene dosing whereas the Al-CH3 carbon resonance was unaffected. 13C NMR also showed that this reaction is highly dependent on the support. Only a small fraction of the thorium surface species react with ethylene on dehydroxylated alumina, whereas over 50% of these sites were active on the MgCl, support. On more basic supports such as silica and magnesia, ~-0x0 species are formed, structure III, which are not active catalysts for olefin polymerization. An extension of these experiments to organozirconium complexes on alumina, showed these surface reactions to be general&able to commercial olefin polymerization catalysts, which consist of supported early transition metals [47,43]. The 13C CP MAS spectra demonstrated that the reaction of CpzZr ( ‘3CH3)2 (Cp = $-C5H5) on dehydroxylated alumina also results in the transfer of a methyl ligand to the Al surface sites and the generation of the cationic species Cp,Zr ( 13CH3)+ [ 471. However, in contrast to the supported organoactinide complexes, the organozirconium complexes exhibited significant catalytic activity on partially dehydroxylated alumina. On methylaluminoxane, [Al ( CH3) 0 ]n or MAO, which, when reacted with group 4 metallocene dialkyls is an extremely reactive olefin polymerization catalyst, the cation-like zirconocene alkyl surface species was also found to be the catalytically active surface species [ 481. As in the case of the supported organoactinide catalysts, dosing with ethylene resulted in complete diminution of the Zr-CH3 signal, and this was taken as proof that the mechanism of polymerization is sequential olefin insertion into the Zr-CH3 bond of the cation.

L. Reven /J. Mol. Catal. 86 (1994) 447-477

471

Overall,these 13CNMR studies on the organoactinidemodel systems illustratethe largelyuntappedabilityof high resolutionNMR to provide specific mechanistic information, in addition to characterization of adsorbate structures.

Organometallics encaged in zeolites

As indicated by the number of monographs and reviews [49,50] which have been published on the subject, solid-state NMR is one of the primary characterizationmethodsof zeolitestructures.Althoughadsorbedorganicsubstrates have been routinely investigated,far fewer NMR studies of organometallic guest molecules have appeared. Due to the enormous surface areas involved, the intrazeolitechemistryof metal complexes is a problem which is quite amenable to NMR analysis. The dynamic and structuralproperties of zeolite encapsulatedmetal complexes are of interestboth in regardto producing tailoredcatalystsas well as observingnovel organometallicchemistrysince the zeolites can act as a solvent or a ligand to stabilize a complex. The NMR studieswhich havebeen conductedon these systemsarepromisingin that they have given insight into the nature of the metal-ligand bond to the surface, dynamicbehavior,and intrazeolitestructureas well as confirm the intrazeolite synthesisof organometalliccomplexes. Mononuclear metal carbonyls and phosphines in zeolites

Mononuclearmetal carbonylsadsorbed into zeoliteswere among the first supported organometallicsto be characterizedby 13CNMR. However due to the high mobility of these physisorbed complexes, solution-like spectra are observedwithout magic-anglespinning.With the exception of Ni (CO)*, [ 511 themetalcarbonylsFe(CO),, [52] Cr(CO)6andMo(CO)6 [53] adsorbedinto NaY and HY zeolites have chemical shifts very close to those observed in solution. In the smallercages of HY zeolite, Ni (CO), has a lower chemical shift of 184.7 ppm as compared to its shift of 192 ppm in solution or 193.4 ppm in the largercages of NaY zeolite [ 511. Due to steric constraintsin the small HY cages, Ni (CO), is distorted from a tetrahedralgeometryto squareplanar, increasingthe interactionof the metal center with the acidic sites on the zeolite surface.This reducesthe electron density on the metal, and the back-bonding into the x* orbital of CO, which decreasesthe paramagneticcontribution to the 13Cchemical shift. Interestingbehavior of the chemicalshift was observed for thermal versus photochemical decomposition of Fe(CO)5 in HY zeolite [ 521. In the case of thermal decomposition, the 13Ccarbonyl resonance progressivelydiminishesand movesto higherchemicalshiftswhereasphotochemical decomposition results in a decreasingshift. As for Ni (CO )4, this trend is explainedby the influence of the surface-metal interaction on the metal carbony1back-bonding. Whereas removalof the CO ligandsby heatingincreases

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L. Reven /J. Mol. Catal. 86 (1994) 447-477

the electron density on the metal and the paramagneticcontribution to the carbonyl shift, photochemical decomposition resultsin an increasedsurfacemetal interaction which reduces the back-bonding and paramagneticcontribution. Irradiationweakensthe Fe-axial CO bond by populatingthe 4” orbital and the selectivedepartureof the axial carbonylthen exposesthe metal center to the zeolite surface. In another static NMR study, the temperaturedependenceof the chemical shift anisotropy powder patterns observed for the reaction of i3C and “0 enrichedchromiumand molybdenumcarbonyls with NaX xeoliteswas examined for structuraland dynamic information [ 54,551. The 13Cand “0 NMR spectra showed somewhat distorted axially symmetricpowder patterns with CSA parameters smaller than expected for static carbonyls. The resonance narrowedand became symmetric at high temperature,but the broad powder patterns returnedupon cooling, indicatingthat some sort of isotropic motion commences at highertemperatures.An analysisof the line shapes concluded that the room temperaturespectra are consistent with an anisotropic motion of the tricarbonylfragmentinvolvingrapidlyexchangingcarbonylligandswith a C-metal-C angle of 75‘, which much smaller than observed in (arene)chromium tricarbonylmodel compounds. Highresolutiontechniqueshavebeen usedto observethe less mobilemetal carbonyl species formed in zeolites. The static subcarbonyls formed when MO(CO), and W (CO), are decomposed on NaX and NaY zeolites were detected with magic-angle spinningbut were not clearly assigned [ 56 1. The adsorption and synthesisof nickel carbonyl phosphine complexes in Y zeolites was monitoredby 31PMAS NMR in addition to EXAFS and IR spectroscopy [ 571. In contrast to the IR spectra, it was observedthat the 31PNMR shift is relativelyunaffectedwhen the complexes were adsorbedinto the xeolite,making it a reliableidentificationtoo&both in the case of adsorptionof smallnickel phosphine complexes from solution, and the intrazeolite synthesis of complexes with bulkier phosphine ligands.When Ni (CO), (PPh,CHMe,) is synthesized in the supercage of NaY from Ni(CO), and PPh2CHMe2,the 31P MAS-NMR spectrum showed complete conversion of the phosphine to the nickel-phosphinecomplex which, when washed,only displacesNi (CO), since is Ni(CO)3 (PPh,CHMe,) too large to fit throughthe supercagewindows. In the synthesis of other nickel-phosphine complexes, the 31P spectrum also showed the presence of unreactedphosphines in addition to the metal-phosphine complexes. Recently, the intraseolitestructureof organometallicshave been investigatedby detectingthe influenceof the adsorbedmetal complexeson the NMR spectrumof the xeolitematrixnuclei. Ozin and coworkersused 23NaMAS and DOR (double-rotation) NMR [58], a technique for narrowingthe lines of quadrupolenuclei, to determinethe bonding sites of molybdenumand tungsten hexacarbonylsin NaY zeolite. Sample-spinningtechniques allow some resolution of the four different types of extra frameworksodium cations lo-

L. Reven /J. Mol. Catal. 86 (1994) 447-477

413

cated in the sodalite and a-cages of NaY zeolite.The peak ascribedto the type II Na+ site, located above six membered rings in the a-cage, shifts with increasing metal carbonyl loading. Together with evidence from infrared spectroscopy, this observation supported a structuralmodel in which the metal hexacarbonyl is anchored by the truns carbonyl ligands to two site II Na+ cations. When feasible as in the case of cation-exchangedzeolites, combined high resolutionNMR studiesof both the organometallicitself and of the support should be a highly effective probe of local structurefor supported metal complexes. Metal carbonyl clusters in zeolites

In an extensive study employing IR, W, and 13CMAS NMR spectroscopy, Gelin and coworkers monitored the intraxeolitechemistry of rhodium and iridium complexes [ 59,601. When rhodium exchanged zeolite is treated with low pressures of carbon monoxide at room temperature,infrared spectroscopy indicatesthat a rhodium(I )-dicarbonyl speciesis formed. In contrast to the metal carbonyls discussed above, static 13CNMR spectra gave a broad resonance indicating the immobility of the rhodium species, despite the fact that Fe(CO),, Cr (CO), and MO(CO), are bulkier complexes. Magic-angle spinning gave a sharp peak at 183 ppm which, by comparison with model XRh(C0)2 compounds, was concluded to be consistent with a zeolite-ORh (I) (CO), chemisorbedspecies. When this dicarbonylcomplex is subjected to a CO:H20 mixtureunder mild heating,a new species forms which givesrise to two sharp resonances under magic-angle spinning which have the same chemical shifts as the terminal and bridging carbonyls of Rh, ( CO)12in solution. This indicatesthat while the rhodium clusteris not highly mobile, there are only weak van der Waals interactions with the zeolite surface, as in the case of the physisorbed mononuclear metal carbonyls. The intrazeolitesynthesis of an iridium carbonyl cluster Ir,(CO),, was also confirmed by its 13C MAS NMR spectrumwhich contained a singleresonanceat 158 ppm, close to the value of 156 ppm which has been measuredfor the pure solid 1611.From infraredstudies,it was found that in the absenceof waterthis clustercontained both linear and bridgingCO ligands,whereasthe presence of water led to the same symmetryas in solution, wherethe Ir, clusteronly has linear CO groups [ 601. The intraxeolitegenerationof this particularcluster is interestingsince it will only form under much more severeconditions in solution,indicatingthe reactivityof zeolitestowards inert organometallics. The generation of a highly mobile metal carbonyl cluster, the dianion [Fe, (CO) 11]‘--, was observedby 13CMAS NMR when Fe, (CO) 12is sublimed into hydratedNa-Y xeolite [ 351. This reaction proceeds more slowly than the reaction of mono or diiron carbonyls with Na-Y xeolite since the windows to the supercagesof this zeolite arejust largeenoughto allow the triiron carbonyl clustersto enter. The 13CMAS spectrumat room temperature,Fig. llA, contains a singlesharp resonanceat 230 ppm with a smallpeak at 211 ppm due to

L. Reven /J. Mol. Catal. 86 (1994) 447-477

A

22% o+= 9OOHz

.ppm

from

TMS

Fig. 11. “C MAS NMR spectra of Fe3 ( C0)12/HNa-Y zeolite at (A) room temperature, spun at 2.6X 1OaHz; (B) room temperature, spun at 900 rpm; (C) -85”C, spun at 2.9x 1O’Hz. Reproduced with permission from ref. 35.

residualphysisorbedFe3(CO) 12.Spinningat lower speeds,Fig. llB, gave rise to a small set of spinningsidebandsindicatingthat the chemical shift anisotropy is not completely averagedout and the motion is more restrictedthan in the case of the mononuclearmetal carbonyls.Iwamotoand coworkersobserved the formation of the anionic hydride, [ HFe3(CO) ii] - by IR spectroscopy in NaY zeolite, [62] which has an isotropic shift of 221 ppm, [63] somewhat lower than the observed value of 230 ppm. The effect of counteranions and solvent polarityupon the bridgingcarbonyl of [ HFe, (CO) 11]- has been studied by solution 13CNMR [63] and the CO-acid interactionsresult in an averageshift of about 224 ppm. The observed shift of 230 ppm was concluded to be too largeto be due to strong acid-base interactionsof [ HFe3(CO) 11]- with the zeolite. Insteadthe spectrumwas assignedto the formation of an alternative structure,the dianion [Fe3(CO),,]“- which was proposed to form when the zeolite is hydratedby the following steps [ 641:

Fe3KW12

Na-Y/HsO,

[HFe,(CO),,]-

26°C ’

WFe3Whl-+C02

Na-y’H20’60”C~ [Fe3(C0),,]2-

(9) (10)

L. Reven /J. Mol. Catal. 86 (1994) 447-477

475

The dianionic cluster, [Fe,(CO),,] 2-, which has an unusual solid-state structure containing an edge bridging carbonyl and a highly unsymmetrical face-bridging carbonyl, has an extremely small barrier to scrambling of the CO ligands in solution [65]. This behavior is in contrast to [ HFe3 (CO) 11]-, where acid-base interactions cause the fluxional processes to cease at higher temperatures [ 631. As seen in Fig. llC, similar behavior is observed for the supported clusters. Whereas the 13C resonance for [HFe3 (CO),,] - on y-alumina begin to broaden at - 60 ’ C and sidebands appear at - 80 ’ C, the sharp resonance of the iron carbonyl species in the xeolite only begins to broaden at - 130 oC. This fluxional behavior along with the isotropic shift supported the formation of the dianionic iron carbonyl cluster, [Fe, (CO) il] 2-, as the intraxeolite species.

Conclusions

Solid-state NMR has furthered the goals of surface organometallic chemistry by allowing direct comparison of the dynamic and chemical behavior of metal complexes in solution or the solid state with their behavior on surfaces. Fluxional and chemical processes of metal complexes, which have been monitored by NMR in solution, can be observed to occur in supported organometaIlics using high-resolution solid-state techniques. Examples, described in this review, of such comparative studies include the dynamic behavior observed for the supported metal carbonyl clusters and the ligand exchange reactions of chemisorbed rhodium olefins. As exemplified by the 13C NMR study of supported rhodium carbonyl, carefully chosen model surfaces can resolve specific structural questions in real catalysts. When a number of NMR experiments are carried out, as in the investigations of supported organoactinides which employed isotopically labeled precursors, paramagnetic probes, and variable field strengths, more difficult problems, such as identification of a catalytically active site, can be solved. In particular, these studies of the supported organoactinides both illustrate the power of NMR as a site selective probe to understand a catalytic process on a molecular level and demonstrate the need for more systematic NMR studies which examine both a range of model systems and the real catalysts. The combination of NMR with complementary experimental techniques, in particular EXAFS and infrared spectroscopy, provide more complete characterizations by probing both the organic ligands and the metal centers of the supported organometallics. Characterization of the binding sites of the organometallic complex on the support remains a difficult problem. However recent results employing more sophisticated solid-state NMR techniques, as in the case of the 23Na NMR studies of metal carbonyls on zeolite supports, are encouraging. In spite of the experimental difficulties which have impeded surface studies, NMR, as one of the more important probes of molecular structure, promises to contribute further to the field of surface organometallic chemistry.

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