243
Inorg. Chem. 1992, 31, 243-252
signal at g = 2.02 ([3Fe-4S]+). This indicates that the [4Fe4SI3+ center is only slowly oxidized by 0 into [3Fe-4S]+ and that, were it an intermediate in scheme C, it should have been observable. Further evidence for the formation of a 3Fe center with these type I1 oxidants has been recently provided by oxidizing [Fe,S4(Stibt),lz- (by O2 or [Fe(CN),]'-) in the presence of an equimolar amount of CoC12. A [3Fe,Co-4SI2+ species is then formed,32which suggests that the Co2+has been incorporated into the vacant site of a Fe3S4core. When a H202oxidation (scheme D, Figure 3) is performed in DMF/H,O (4/1), no formation of either [4Fe-4SI3+,[3Fe4S]+, or Fe3+is observed, and only the immediate formation of the same unidentified radical is observed (g = 2.05 1, 2.005, 1.984) up to 60 K (it power-saturates easily a t low temperatures and remains stable for approximately 24 h). We were unable to determine whether [4Fe-4SI3+and [3Fe-4S]+ are intermediates of reaction or not, since oxidation by H202of the monooxidized cluster [Fe,S,(Stibt),]- under the same conditions immediately produced the same radical. The mechanism of action of H202as an oxidant has been extensively investigated:"*33 and it is assumed therefrom that H202oxidizes coordinated thiols via nucleophilic attack of the sulfurs on the 0-0 bond. The radical observed here could indeed correspond to an oxidized state of the Stibt ligands. Indeed, an EPR signal with similar gvalues (2.09, 2.01, 1.98) has been reported in the [Fe(CN),I3- oxidation of A. uinelandii Fd16 and tentatively attributed to a cysteinyl disulfide radical. The latter has been identified among the products of radiation damage in y-irradiated cysteine h y d r ~ h l o r i d e .Its ~ ~EPR spectrum is also observable at liquid-N, temperature, with g values of 2.06,2.02, and 2.00, not unlike those observed here. In conclusion, chemical oxidation of an Fe-S cluster with bulky thiolates such as [Fe,S,(Stibt),l2- can lead to different products when the oxidizing power and the physical properties of the reaction medium are varied: ~~
~~
Roth, E. K. H.; Greneche, J. M.; Jordanov, J. J . Chem. Soc., Chem. Commun. 1991, 105. (a) Allison, W. S.Acc. Chem. Res. 1976, 9, 293. (b) Herting, D. L.; Sloan, C. P.; Cabral, A. W.; Krueger, J. H. Inorg. Chem. 1978, 17, 1649. (c) Adzamli, I. K.; Libson, K.; Lydon, J. D.; Elder, R. C.; Deutsch, E. Inorg. Chem. 1979,18,303. (d) Heeg, M. J.; Elder, R. C.; Deutsch, E. Inorg. Chem. 1979, 18, 2036. Gordy, W. In Theory and Applications of Electron Spin Resonance; Wiley: New York,1980; Chapters 6 and 7.
(A) Moderate oxidants (Ag', 12, [Fe(CN),13-, [C(C6H5)3]+) in aprotic media (CH2C12, CH,CN) produce a one-electron metal-based oxidation, and [4Fe4Sl3+is quantitatively generated (type I oxidants). An increase of basicity of the solvent will also lead to an increased instability of the Fe-S core.25 (B) When the same moderate oxidants (Ag', [Fe(CN),]*) are used in a medium where H 2 0 is also present, an overall twoelectron oxidation occurs, a 4Fe 3Fe core conversion is then observed, and [3Fe4S]+ is generated (type I1 oxidants). The latter is unstable, which possibly accounts for its low concentration (less than 15%). (C) With a stronger oxidant (oxygen) and in aprotic media 3Fe conversion is (CH2C12,CH3CN, DMF), the same 4Fe observed (type I1 oxidant). The presence of H20is not required, and the mechanism of oxidative conversion is possibly different from case B). (D) Oxidation by an even stronger oxidant (H20,) leads to an immediate oxidation into a radical of possibly organic nature (type I11 oxidant). It appears thus that the site of oxidation on the [Fe$,(Stibt),12cluster can be controlled by a combination of factors, principally the oxidizing strength of the oxidant and the presence of H20in the reaction medium. Thex observations may help to explain why, upon oxidation of a 4Fe protein by [Fe(CN)6]3-, there is either formation of a [4Fe-4SI3+center, conversion into a 3Fe center, or appearance of an organic radical. In the case of proteins, the medium, as well as the oxidizing strength of [Fe(CN),I3-, is constant. Modulation of the reaction outcome can therefore be brought about by (1) the Fe-S cluster redox potential (the higher it is, the more stable the core with respect to oxidative degradation and the less likely a 4Fe 3Fe conversion) and (2) the local environment of the 4Fe cluster, that is its degree of hydrophobicity and polarity (the more accessible the 4Fe cluster to water, the more easily it will destabilize and convert into a 3Fe center or even a more oxidized radical). Indeed, when characteristic features of the cluster-binding cavities in HP and in bacterial ferredoxins2 are compared, it appears that nonpolar side chains surrounding the ferredoxin clusters differ from those found in contact with the HP cluster. Thus, the ferredoxin cavities contain mostly aliphatic chains, while the HP environment includes many aromatic side chains, and whereas the latter will effectively prevent access of solvent molecules, the ferredoxin clusters will be only moderately protected.
-
-
-
Contribution from the School of Chemical Sciences, University of Illinois at Urbana-Champaign, 505 South Mathews Avenue, Urbana, Illinois 61801
Carbon-13 Nuclear Magnetic Resonance Spectroscopic Study of Fe,(CO) 12 and RU,(CO)~~ Supported on Metal Oxide Surfaces? Linda Reven and Eric Oldfield* Received April 23, 1991
"C-enriched Fe3(CO)12and R U ~ ( C Osupported ) ~ ~ on a variety of metal oxide surfaces (y-Al2O3, Si02,MgO, and HNa-Y zeolite)
have been examined by carbon-13 "magic-angle"sample-spinning (MAS) nuclear magnetic rewnance (NMR) spectroscopy. Our results show that the y-A1203-supportedFe3(C0)12cluster is fluxional at room temperature, but low-temperature spectra give no evidence in favor of the unusual bridging carbonyl proposed previously by some workers from infrared (IR) spectroscopy. On HNa-Y, Fe3(CO)12 forms a highly mobile chemisorbed species, and the I3Cchemical shift is consistent with formationof the dianion and Si02surfaces, our results are in agreement with IR studies and show that, under [Fe3(CO)II]2-.For R U ~ ( C Oon) ~y-A1203 ~ anaerobic conditions, highly mobile physisorbcd clusters are initially formed. Upon exposure to air or upon thermal activation, divalent tricarbonyl species are generated. The resonances of these chemisorbed species are enhanced by cross polarization and display large chemical shift anisotropies,due to restricted mobility. On MgO, R U ~ ( C Obehaves ) ~ ~ quite differently, the 13CNMR results being consistent with the formation of highly fluxional hexaruthenium carbonyl clusters.
Introduction
Metal carbonyl clusters supported on high surface area metal oxides are of interest in relation to the clustersurface analogy 'This work was supported by the Solid-state Chemistry Program of the
US.National Science Foundation (Grant DMR 88-14789).
0020-1669/92/133 1-0243.$03.00/0
between catalytic transition-metal surfaces and molecular metal clusters,' as well as being good candidates for extending the study of molecular mobility to species chemisorbed on surfaces.'-' Not (1)
Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.;Pretzer, W. R. Chem. Rev. 1979, 79, 91.
0 1992 American Chemical Society
244 Inorganic Chemistry, Vol. 31, No. 2, 1992 only do metal oxide supported metal carbonyl clusters serve as precursors in the preparation of highly dispersed metal catalysts, but they can also be novel catalysts by themselves, and many of these materials exhibit a high selectivity for specific reactions. For example, one of the heterogeneous catalysts in this study, Fe3(C0)12supported on alumina, is a catalyst for the reduction of nitrobenzene to aniline, as well as a precursor for small metal aggregates which catalyze the Fisher-Tropsch reaction6 One goal in the preparation of highly dispersed metal catalysts is to achieve dispersion of species of well-defined nuclearity and composition. However, the chemisorption of metal carbonyl clusters is a complicated process, the conditions under which clusters remain intact and the nature of surface bonding being poorly understood. Since many structural questions remain after characterization with more traditional methods, such as infrared and ultraviolet-visible spectroscopies and temperature-programas solid-state nuclear med desorption, other techniques-such magnetic resonance ( N M R ) spectroscopy and extended X-ray adsorption fine structure (EXAFS) spectroscopy-are currently under development. For the study of dynamic processes on surfaces, there has not been an experimental probe comparable to NMR spectroscopy to determine the nature of ligand mobility in metal clusters. Surface diffusion of gases adsorbed on transition-metal surfaces can be studied by field ion spectroscopy, but this technique cannot provide the type of detailed structural information obtainable (at least in principle) from N M R spectroscopy.] In the first solid-state I3C N M R studies of metal clusters adsorbed on metalaxide surface^,^^* only the highly mobile physisorbed species were observed. In physisorption, there are only weak van der Waals type interactions between the carbonyls and the metal oxide surface, the surface species maintaining the same basic structures and chemical shifts as in solution. In chemisorption, a covalent bond is formed between the surface and the adsorbate, and the mobility of the adsorbate becomes restricted, giving rise to an anisotropic chemical shift which broadens the resonance. Fortunately, solid-state NMR techniques, such as "magic-angle" sample spinning (MAS) and cross polarization (CP), have been used to successfully detect these less mobile chemisorbed surface species, and the reader is referred to a variety of interesting recent publications which discuss the NMR spectroscopy of supported metal carbonyls and other organometallic species, as well as model compounds, for an up-to-date account of progress in this area? In this study, we describe the I3C MAS N M R behavior of Fe3(C0),2 and R u ~ ( C O ) adsorbed ,~ onto a variety of metal oxide surfaces. The surface chemistry and mobility of the supported (physisorbed or chemisorbed) metal carbonyl clusters are compared with their spectroscopic behavior in solution, and in addition, our results are compared with those obtained by previous workers who used other spectroscopic techniques.
(2) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Metal Complexes; Elsevier: Amsterdam, 198 1.
(3) Hartley, F. R. Supported Metal Complexes; Reidel: Dordrecht, The
Netherlands, 1985.
(4) Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knozinger, H.,
Eds.; Elsevier: Amsterdam, 1986. (5) Surface Organometallic Chemistry: Molecular Approach to Surface Catalysis; Basset, J. M., et al., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988. (6) NGuini Effa, J. B.; Djebailli, B.; Lieto, J.; Aune, J. P. J . Chem. SOC., Chem. Commun. 1983, 408. (7) Ben-Taarit, Y.; Wicker, G . ;Naccache, C. In Magnetic Resonance in Colloid and Interface Science; Fraissard, J. P., Resing, H. A,, Eds.; Reidel: Dordrecht, The Netherlands, 1980; p 497. (8) Derouane, E. G.; Nagy, J. B.; Vgrine, J. C. J . Catal. 1977, 46, 434. (9) Hasselbring, L.; Puga, J.; Dybowski, C.; Gates, B. C. J. Phys. Chem. 1988, 92, 3934. Lamb, H.H.; Hasselbring, L. C.; Dybowski, C.; Gates, B. C. J. Mol. Catal. 1989,56, 36. Toscano, P. J.; Marks, T. J. Langmuir 1986,2,820. Heddon, D.; Marks, T. J. J . Am. Chem. SOC.1988, 110, 1647. Finch, W. C.; Gillespie, R. D.; Heddon, D.; Marks, T. J. J . Am. Chem. SOC.1990, 112, 6221. Uchiyama, S.; Gates, B. C. J. Catal. 1988, 110, 388. Puga, J.; Fehlner, T. P.; Gates, B. C.; Braga, D.; Grepioni, F. Znorg. Chem. 1990, 29, 2376.
Reven and Oldfield Experimental Section Chemical Aspects. Fe3(C0)12(Aldrich, Milwaukee, WI) was enriched in "C by exchange with 99%-"C "CO (Monsanto Research Corp., Miamisburg, OH) in heptane, and the enriched sample was then recrystallized from toluene. Three enrichment cycles with "CO, each 7 days long at room temperature, were used to obtain the high-enrichment levels (typically -60% "C) needed for surface studies. R U , ( C O ) ~ ~ (Strem Chemicals, Newbury Port, MA) was dissolved in heptane and I3CO exchange carried out at 80 "C for 8 h, the product being recrystallized from 1,2-dichloroethane. Samples were examined by solution I3C NMR spectroscopy to verify both purity and uniformity of enrichment. Overall enrichment levels were determined by electron impact mass spectroscopy. Support Preparations. y-A1203(surface area ca. 200 m2/g) was derived from Catapal boehmite (Conoco Chemicals, Houston, TX) by calcining at 650 OC for 8 h. Silica (surface area ca. 220 m2/g) was CabO-Sil M5 (Catwt Corporation, Tuscola, IL) which was dried at 100 "C under vacuum to remove adsorbed water. MgO (surface area ca.180 under vacuum at 400 OC m2/g) was derived by decomposing MD(OH)~ for 2 h. Linde HNa-Y zeolite (SK40, Alfa, Danvers, MA) was used without pretreatment. Surface area measurements of the supports were performed on an AUTOSORB-1 BET instrument (Quantachrome Corp., Syosset, NY). [Me4NIHFe3(CO),,]was prepared from Fe3(C0)12as described by Hieber and Brendel.lo A 1.68-g sample of KOH and 15 mL of CH30H were combined in a Schlenk tube, which was then degassed by using three freezepumpthaw cycles. An 0.8155-g amount of Fe3(CO),2was added to the KOH/CH,OH solution under an argon atmosphere, and the mixture was stirred. After a few minutes, the solution turned reddish brown and was then transferred, along with 35 mL of degassed glacial acetic acid, into a Schlenk filter flask, using a gastight syringe. The solution was filtered into a Schlenk round-bottomed flask containing a degassed solution of 0.5 g of [(CH,),N]I in 30 mL of distilled water. The bright red crystals which formed were filtered out, washed with water, and then dried under vacuum. [Me.,N][HFe,(CO),,] was enriched in "C by stirring a dichlorcethane solution under W O at room temperature for 23 days, and the product was then recrystallized from 1-propanol. Fe3(C0),2/y-A1203 samples were prepared using Schlenk techniques by impregnating hydroxylated y-A1203(surface area ca. 200 m2/g) with a hexane solution of 60% "C-enriched Fe3(CO)12,to give loadings of between 2 and 5 wt % metal carbonyl. Hexane was dried over CaH, and then distilled just prior to use. When a solution of Fe3(C0)12in hexane was added to the y-A1203,the solution decolorized, and the alumina became dark red. The sample was washed several times with hexane, the solvent syringed off, and the sample dried under a stream of argon. Infrared analysis of a representative sample (Nujol mull) revealed strong absorptions at -2006 and 1986 cm-I and a weaker, broad absorption at -1645 cm-I (data not shown), in accord with the results of Hanson et a1.23
Fe3(CO)12/HNa-Y zeolite was prepared basically according to Iwamoto et al." except that the %enriched Fe3(C0)12was first dissolved in hexane and the solution then mixed with the zeolite in order to achieve a higher loading. A 43.7-mg sample of ['3C]Fe3(C0)12in degassed hexane was added to 0.6747 g of Linde HNa-Y zeolite under an argon atmosphere. Fe3(C0),, did not immediately adsorb from solution onto the zeolite, as occurred with y-A1203. The hexane was thus pumped off, and the finely mixed zeolite and Fe3(C0)12were placed under an argon atmosphere, then warmed to 70 OC for 8 h, and evacuated at the same temperature for 1 h to remove any unreacted Fe3(C0)12. (~-H)RU,(CO)~,,(SC~H~) was prepared from R U ~ ( C Oand ) ~ ethane~ thiol according to the procedure described by Crooks et all2 A 0.2-mL portion of ethanethiol (Aldrich, Milwaukee, WI) was added to a solution ) ~ ~ argon; the mixture of 100 mL of benzene and 1.O g of R U ~ ( C Ounder was then refluxed for 15 min. After cooling, the solvent was pumped off, leaing a dark orange oil, which was crystallized from pentane, producing dark orange crystals, which were characterized by infrared spectroscopy. For I3C enrichment, a solution of (~-H)RU,(CO)~O(SC~H~) in hexane was warmed to 40 OC and stirred under 13C0for 5 days. [RU(CO)~CI,]~ was prepared from 75% I3C-enrichedR U ~ ( C O ) : ~ A. ~ ~ 0.57-g sample of R u ~ ( C Owas ) ~ ~dissolved in 40 mL of chloroform in a glass-lined autoclave. The autoclave was charged with 5 atm of N2 and heated to 110 OC in an oil bath for 8 h. The autoclave was vented and
(IO) Hieber, W.; Brendel, G. Z. Z . Anorg. Allg. Chem. 1957, 289, 324. (1 1) Iwamoto, M.; Nakamura, S.; Kusano, H.; Kagawa, S. J . Phys. Chem. 1986, 90, 5244. (12) Crooks, G. R.; Johnson, B. F. G.; Lewis, J.; Williams, I. G. J . Chem. SOC.A 1969, 797. (13) Mantovani, A,; Cenini, S . Inorg. Synth. 1976, 16, 51.
Inorganic Chemistry. Vol. 31, No. 2. 1992 245
Supported Metal Carbonyls Tmbk I. Comparison between Methods Used To -re Metal Carbonyls
surface area.
wt %
metal this -2.4 work 14 0.7. 3.3 0.63-2.5 38 NR
39
this 2.8 work 40 2-4
this 4.9 work 2.2 48 16
0.95
this 2-5 work 20 5 4 23
NR
22
NR
this 6.1 work II 3.7
A Hugues model
Supported
200
inair for 8 h: no -ptreaticnt Degussa PI I O no pretreatment Kaiser 201 heated in vacuo at 100-800 ‘C, calcined in air at 400 OC NR RudC0)dSiO~ CabO-Si1 MS, 20 hat I00 OC in vacua, carbonyl-activatedat 80 OC for 2 h Acrosil 0 Dcgussa, 16 hat 25 or 225 ‘C in vacuo, thermally activated at 80 OC for several hours
I00 NR‘
NR
B: lwasawa model 220 200
RudC0)idMgO MgO from Mg(OH)a 400 ‘C in vacuo for 2 h MgO from Mg(OH)2, 400 OC in vacuo, several 0, cycles at 400 OC. rehydrated, heated in vacuo at 25,200, and 500 *C MgO heated in vacuo at 100 ‘C
200
29
R g w 1. Proporcd structures of chemisorbed Fe,(COj,> on yAI1O1: (A) as pmpoaed by Hugues et ai.:” (B) as proposed by Iwasawa et aLu
F~W~IT-AIPI Catapal B y-A120,
200
spectra were oblaincd under cross-polarization conditions. in order to avoid the broad Kcl-F probe background or to obtain qualitative information on the prcscncc of protons.
I80
Degum -t-Al,O,, 500 ‘C O2cycle, rehydrated at 25 OC Catapal SB, 400 ‘C O2cycle, hydrated ill . I. n. n O -C -. -pAl,O, heated in dry sir at 300 ‘C for 2 h
NR
Fc,(C0)12/HNa-Y Zmlite SK-40 zmlite. as received
NR
SK-40 -lite.
NR
400 *C O2cycle. rehydrated
100
150
at 25 OC
.Not nportcd or measured cooled. and the suspension of [Ru(CO),C1J2 was coUected on a filter and washed with chloroform. Ro,(CO),Jr-AI,O, was prepared from 75% ‘3Cmriched Ru,(CO),, and yA120, (surface area ca. 200 mz/g).” A 6.1” sample of Ru,(CO),, in hexane was added to 0.25 g of yAI,O, under argon and stirred. The alumina turned light yellow and was then washed with hexane to remove any possible excess Ru,(CO),,. The solvent was pumpcd off, and the sample was stored under argon. Ru,(CO)&.i% was prepared from 75% ilcCnriched R U ~ ( C O )and ,~ silica (surface area ca. 220 m’/g).” A 20-mg sample of R U ~ ( C O )inI ~ hexane was added to 0.7 g of SiO,, and the mixture was then stirred under argon. Excess solvent was remwed, and the sample was dried under vacuum. The silica was light yelloworange. The sample was thermally activated by heating to 80 OC for 2 h, under vacuum. The silica turned light tan after thermal activation. Ry(CO)12/Mg0 was prepared from 75% ‘)Georiched RU,(CO)~~ and MgO (surface area a.180 m’/g).I6 A 34.2“ sample of Ru?(CO),, in hexane was added to 0.7 g of MgO. under argon. After stimng, the solvent decolorized and the magnesia turned dark orange. The sample was then washed several times with hexane, dried under vacuum, and stored under argon. N M R Spectroscopy. Solid-state ‘IC NMR spectra were obtained using a ‘home-built” 360-MHz (8.45-T) NMR spectrometer. Solution spectra were obtained on a 500-MHz ( I 1.7-T) instrument.” For MAS NMR spectr-py, the samples were packed into 7-mm cylindrical rotors (Doty Scientific, Columbia, SC) in a glovebag, under a nitrogen atmosphere. Sample weights were typically 400 mg. All spectra were obtained using 90° p u k excitation (typically 5-10 11s). with high power ( 4 W O W) proton decoupling during data aquisition. In some cases, (14) Zsochina, A.: Gugliclminati, E.;Bassi, A,; Camia. M. J. Corol. 1982,
74. 225. (15) Thcolicr, A.; Choplin. A.; DOmclas, L.;Bassct, J. M.; Zandcrighi, G.; Ugo. R.; Psam, R.; Saurissesu, C. Polyhedron 1983,2, 119. 116) , ~Picrant-. ~ ~~~, , R.: Valacene.. E. 0.: . Nadauist..A. F.:. Dver.~P.N. 1. . Mol. Carol. 1989. fi, 189: (17) Smith, K.A.; Kirkpatrick. R. J.;Oldlield, E.;Henderson, D. M. Am. Mincrol. 1983.68, 1206. ~~~~~~~
.
RdtS We have obtained ”C magic-angle sample-spinning NMR spcnra of a range of supported (and unsupported) metal carbonyls, and Table I shows a comparison betwem the suppon and sample preparation protocols we have used (for the supported carbonyls) with those reported by other workers. Figures 2-9 show the experimental ”C NMR spectra we have record& Figure 2, Fe,(CO),,/~-AI,O,; Figure 3, [Me,NIIHFc,(CO)ll]; Figure 4, Fe,(CO),,/HNa-Y mlite: Figure 5, Ruj(CO),,/y-Al,O,; Figure 6, Ru,(CO)12/Si02; Figure 7, H R U ~ ( C O ) , ~ ( S C H , C H JFigure ; 8, [ R U ( C O ) ~ C I ~ ]Figure ,; 9,Ru,(CO),,/MgO. A compilation of the chemical shift tensor elements for iron and ruthenium carbonyl clusters is given in Table 11, and carbon-13 isotropic chemical shifts for some ruthenium carbonyl clusters are presented in Table 111. These will be introduced a t appropriate places in the sections which follow.
Diseussioa Fe,(CO),, Adsorbed 00 Y-AI~O,. The reaction of Fe,(CO),, with silica and hydroxylated alumina and magnesia surfam has bccn studied by infrared, W-visible, and ESR spccl”pies.’8-z1 On silica, Fe,(CO),, is only physisorbed and can be extracted quantitatively with n-hexane. However, when a hexane solution of Fe,(CO),, is put into contact with hydroxylated alumina or magnesia. chemisorption takes place; the green cluster in solution transfen lo the alumina or magnesia support. forming a dark pink surfacc spxies. which cannot bc extracted with n-hexane. Hugues and coworkers propose that there is a nuclmphilic surfacc attack on a CO ligand, with simultaneous elimination of CO,. to form the monoanionic cluster IHFe,(CO),
LAzSr, L.;Matusck, K.; Mink, J.: D o h , S.: C u d . L.:Vi-omSz, A,: Mark% L.:Reiff, W. M. J. Coral. 19Ll4.87. 163. Hugucs, F.: Smith, A. K.; Ben-Taarit, V.; Basset, J. M.; Commcreuc, D.; Chauvin, Y. J. Chem. Soe.. Chem. Commun. 1980,68. Hugucs, F.; Basact, J. M.; &n-Taarit, Y.; Choplin, A,; Primel, M.; Rojas, D.; Smith, A. K. 1. Am. Chem. Soe.1982,104.7020. Hugues, F.; Daimon, J. A,; Bufsicrc, P.; Smith, A. K.; Bawl. J. M.; Olivicr, D. J. Phys. Chem. 1982, 86, 5136. Iwasawa, Y.; Yamada, M.; Ogasawara. S.;Sato. Y.; K u d . H. Chm. Len. 1983, 621. Hanson, B. E.; Bergmister, J. J., 111: Petty, I. T.; Cannaway. M. C. I m r ~Chem. . 1986.25,3089. Hansan, E.E. In Adwnees in Dynamic Srereochemisrry;Gielen, M. F., Ed.;Freund Publishing House Ltd.: London, 1985; Voi. I, p 89.
246 Inorganic Chemistry, Vol. 31, No. 2, 1992
Reven and Oldfield
Table 11. Carbon-13 Chemical Shift Tensors for Iron and Ruthenium Carbonyl Clusters
4:
compd Fe3(C0)
site
12
static Ru3(CO)12
axial
equatorial static
[MedN][HFedCO)iiI
axial
equatorial bridging
Fe3(C0)12/y-A1203 Fe3(C0)12/HNa-Y (~-H)R~~(CO)IO(SC~HS)
[RU(CO),Cl212 R'3(C0)
12/Y-A1203
R U ~ ( C O ) ~ ~(in / Sair) ~O~
611:
622,b
6337b
A&=
PPm
PPm
PPm
PPm
PPm
Idd
203 210 213 225 212 210 189 20 1 220 210 277 22 1 230 205 199 193 185 182 194 193
342 344 348 328 325 346 330 348 382 369 298 405 232 382 36 1 364 338 337 358 365
322 328 311 321 316 338 312 319 333 334 287 259 234 347 317 307 312 291 310 312
-54 -42 -20 26 -6 -54 -74 -6 3 -52 -73 193 36 22 1 -74 -8 1 -92 -96 -8 5 -8 1 -9 1
386 378 350 298 327 396 395 397 410 425 99 296 12 438 420 428 421 400 417 429
0.08 0.06 0.16 0.04 0.04 0.03 0.07 0.1 1 0.18 0.12 0.13 0.78 0.22 0.13 0.16 0.20 0.09 0.17 0.17 0.19
Isotropic chemical shift, in ppm downfield from an external standard of tetramethylsilane. Chemical shift tensor element component obtained by using the graphical method of Herzfeld and Berger.32 611 is the most deshielded tensor element; 633, the most shielded. High-frequency, low-field, paramagnetic or deshielded values are positive (IUPAC 6 scale). 'Anisotropy of the shift tensor, A6 = b j 3 - 1/2(611 + aZ2). dAsymmetryparameter of the chemical shift tensor. The absolute value can vary from 0 to 1. Table 111. Carbon-13 Isotropic Chemical Shifts for Ruthenium
Carbonyl Clusters sDecies 6;.DDm Ru(CO),(OM
