Langmuir 1993,9, 2963-2964
2963
Oxide-Supported Triruthenium Ketenylidene Cluster: Evidence for Metal-Metal Bonds from Laser Raman Spectroscopy Feng-Shou Xiao' Department of Chemistry, Jilin University, Changchun 130023, China
Masaru Ichikawa Catalysis Research Center, Hokkaido University, Sapporo 060, Japan Received October 5, 1992. In Final Form: April 27, 199P
The Raman spectra of MgO and Si02 supported on triruthenium ketenylidene clusters exhibited bands at 156,205, and 317 cm-l and 174,211,and 322 cm-', respectively, which are possibly assigned to Ru-Ru and Ru3-C stretching modes of the oxide-supported triruthenium ketenylidene species. In combination with infrared spectra, the Raman spectra are strong evidence for the presence of the triruthenium ketenylidene clusters grafted on the surface of MgO and Si02.
Introduction Metal clusters grafted on oxide supports are very important in the design of active metal centers in heterogeneous catalysis, because they provide highly dispersed metal particles in a uniform size distribution, with a discrete metal composition of the precursor metal frameworks. These catalysts exhibit good catalytic performance in some catalytic reactions and offer higher activities and selectivities, compared with the conventional Catalysts prepared by coimpregnation and ion exchange of metal Ketenylidene ligands in trinuclear clusters [PPNl2[M3(CO)&CO)I (M = Fe, Ru, Os;PPN = bis(tripheny1phosphine)nitrogen( 1+)) are possible intermediates in CO hydrogenation over supported-metal catalysts, and transformation of the ketenylidene cluster by the reaction with nucleophiles and electrophiles has been extensively in~estigated.~ we reported the nature of triruIn previous thenium ketenylidene on oxides such as MgO, SiOz, and si02-&03, forming different surface species on the various supports. Notably, all these supported clusters have been characterized by comparison of their infrared spectra in the carbonyl region with the spectra of analogous molecular clusters in CH2C12; there is still a shortage of direct evidence for the presence of triruthenium ketenylidene clusters grafted on oxides. Further structural characterization is needed. A distinctive characteristic of a metal cluster is its metalmetal bonds. Raman spectroscopy has been used to Abstract published in Advance ACSAbstracts, August 15,1993. (1) Ichikawa, M. In Tailored Metal Catalysts;Iwasawa,Y., Reidel, D., Eds.; Dordrecht, 1985. (2) Gates, B. C., Guczi, L., Knozinger, H., Eds. In Metal CEuster in Catalysis; Amsterdam, 1986. (3) Choplin, A.; Huang, L.; Theolier, A.; Gallezot, P.; Basset, J. M.; Siriwardane, V.; Shore, S. G.;Maithien, R. J. Am. Chem. SOC.1986,108, 4224. (4) Shriver, D. F.; Sailor, M. J. Acc. Chem. Res. 1988, 21, 374, and
references therein. (5) Xiao, F.-S.;Fukuoka, A.; Henderson, W.; Shriver, D. F.; Ichikawa, M. Cutal. Lett. 1990, 6, 361. (6) Xiao, F A ;Fukuoka,A.; Ichikawa, M.;Guo, X.-X.(CuihuaXuebao 1990, 11, 311. (7) Xiao, F A . ; Guo, X.-X.; Fukuoka, A.; Ichikawa, M.; Shriver, D. F. Cuihua Xuebao 1992, 13, 174; 1992,13, 264; 1992,13, 269. (8) Xiao, F.-S.; Xu, R.-R.; Guo, X.-X.; Fukuoka, A.; Ichikawa, M.; Henderson, W.; Shriver, D. F. J . Mol. Catal. (Chinu)1991,5, 301.
demonstrate metal-metal bonding in a wide range of molecular metal clusters! and the Raman technique is potentially valuable for determining the structures of supported-metal clusters; in particular, it determines whether the structures of metal clusters can exist on the surface of oxide supports. On the other hand, it is wellknown that there are a number of experimental difficulties because of the high fluorescence, low sensitivity, and decomposition of metal clustem2 Since the metal-metal vibrations fortunately have rather high Raman scattering cross sections, scientific researchers can use a low power laser line and sample spinning cell techniques to obtain the suitable Raman spectra of metal-metal stretching vibration. Up to now, Deeba et al.l0 reported the bands at 160 and 119 cm-' in Raman spectra of anchored triosmium carbonyl cluster on alumina, assigned to the Os-Os stretching mode; Basset et d.l1reported the band at 155 cm-l in Raman spectra assigned to the Os-Os stretching mode of triosmium carbonyl cluster grafted on silica; Theolier et al.12 reported the bands a t 201 and 162 cm-l in Raman spectra assigned to Ru-Ru stretching vibration of a grafted cluster, HRw(CO)lo(OSi);and Kirlin et al.13 reported the bands at 90 and 119 cm-' assigned to the Re-Re stretching vibration of [H&e3(C0)121-/MgO. In this communication, is presented the evidence for RuRu bonds of triruthenium ketenylidene cluster on MgO and Si02, indicating that the structure of a triruthenium framework really exists on the surface of the MgO and Si02 at room temperature.
Experimental Section All manipulationswere performed in an inert atmosphere by the use of a glovebox for sample preparation. Cluster [PPNh[Rus(CO)&CO)] was synthesizedaccording to previous report." CHzClS, usedas asolvent,waa storedunderN d t e r beingrefluxed with and distilled from P4O10. Si02 (Aerosil300, Nippon Aerosil (9) Kettle, S. F. A. Top. Curr. Chem. 1977, 71, 111.
(10) Deeba, M.;Streusand, B. J.;Schrader, G.L.; Gates,B. C. J. Catal. 1981, 69, 218. (11) Choplii, A.; Leconte, M.; Basset, J. M.; Shore, S.;Hsu, W. L. J. Mol. Cutal. 1983,21,389. (12) Theolier,A.; Choplin, A.; D'ornelas, L.; Basset, J. M.;Sourieseau, C.; Zauderighi, G.M.; Ugo, R.; Psaro, R. Polyhedron 1983,2, 119. (13) Kirlin,P.S.;DeThomas,F.A.;Bailey,W. J.;Gold,H.S.;Dybowski, C.; Gates, B. C. J. Phys. Chem. 1986,90,4882. (14) Sailor, M. J.: Brock, C. P.; Shriver, D. F. J . Am. Chem. SOC.1987, 109,6015.
Q743-7463/93/ 24Q9-2963$Q4.QQ/Q 0 1993 American Chemical Society
2964 Langmuir, Vol. 9,No. 11,1993
raman shift
cm-'
Figure 1. Raman spectra of [PPNIz[Ru~CO)Q(CCO)I on (a) MgO and (b) Si02 a t room temperature. Co.) and MgO (Merck, GR) were used as supports, respectively. Prior to use, the oxides such as Si02 and MgO were pretreated in flowing oxygen at 673K overnight and then evacuated at 573 K for 2 h. Supported triruthenium ketenylidene clusters were prepared by contacting CHzCl2 solution containing [PPN] ~[RQ(CO)Q(CCO)]under Nz with Si02 and MgO. After being stirred for 2 h the solvent was evaporated from the slurry under vacuum. Raman spectra were obtained by krypton laser excitation (6471 or 6764 A) fiitered by a premonochromator to remove the laser plasma line. A standard Spex solid sample rotator was used to prevent thermal or photochemicaldecomposition,and the Raman scattered light was analyzed with a Spex 1403 double monochromator. The laser power was about 30 mW, and about 50 scans had to be averaged to produce a suitable signal-to-noise ratio. The spectra were recorded with a resolution of about 4 cm-l and an accuracy of 2 cm-l.
Results and Discussion Figure 1 shows the Raman spectra of the triruthenium ketenylidene cluster on MgO and Si02 in the region from 30 to 400 cm-'. For the MgO-supported triruthenium ketenylidene cluster, we observed three bands at 156,205,and 317 cm-l, with broad and shoulder bands appearing at lower frequencies. At frequencies less than 400 cm-', several vibrations are expected to be active in the Raman spectra of metal clusters, which are related to the Ru-Ru, Ru-C, and Ru-0 stretching modes. Therefore, i t is proposed
Xiao and Ichikawa
that the bands at 156 and 205 cm-' are assigned to Ru-Ru stretching modes, and the band at 317 cm-' is assigned to the Ru3-C stretching mode, respectively. These assignmenta are in good agreement with those of the parent compound [PPN]~[RU~(CO)~(CCO)I in C H Z C ~ ~ On .'~ comparison of the MgO-supported triruthenium ketenylidene cluster with the triruthenium ketenylidene cluster in CHzClz, it is very interesting to note that the peak position of Raman spectra between the MgO-supported triruthenium ketenylidene cluster and the triruthenium ketenylidene cluster in CHzClz is almost the same. The difference of the above mentioned two spectra is only for band intensity, which is really contributed by a small amount of the cluster on the surface of MgO support. The above results demonstrate that the symmetry between the MgO-supported the triruthenium ketenylidene cluster and the triruthenium ketenylidene cluster in CHzClz is the same, indicating that the cluster physisorbs on the surface of MgO support. For SiOz-supported triruthenium ketenylidene cluster, the Raman spectrum gives rise to bands at 174,211,and 322 cm-', with weak and broad bands at 40-110 cm-l. The bandsat 174and211cm-1areassignedtoRu-Rustretching modes and the band at 322 cm-l is assigned to the Ru3-C stretching mode, which are well consistent with those of the monohydride triruthenium ketenylidene cluster [PPNI [HRu3(CO)g(CCO)I in CHzC1z.lS The weak and broad bands at lower frequencies are probably assigned to the Ru-C bending modes, in agreement with those of Rus(C0)1~.'~Notably, we observed that there is approximately a 20-cm-l shift for Raman spectra in the Ru-Ru stretching mode between MgO- and SiOz-supported triruthenium ketenylidene clusters, which may be explained by the different surface species, possibly assigned to [Ru3(CO)g(CCO)I"/MgO and [HRu3(CO)g(CCO)l-/SiOz. Such a conclusion has been suggested in the previous reports? used by IR spectroscopy.
Conclusion In summary, the Raman results provide the vibration spectra indicating ruthenium-ruthenium and triruthenium-carbon bonds in the MgO- and SiOz-supported triruthenium ketenylidene cluster. In combination with the previously reported infrared spectra, the Raman spectra are strong evidence for the presence of the framework of triruthenium ketenylidene clusters grafted on the surface of MgO and SiOz. Acknowledgment. The authors greatly thank Professor D. F. Shriver's group for the synthesis of [PPNIz[RLQ(CO)~(CCO)]and the National Natural Science Foundation of China for Young Scientists. (15)Sailor, M. J.; Went, M. J.; Shriver, D. F. Inorg. Chem. 1988,27, 2666. (16) Quicksall,C. Q.;Spiro, T. G. Znorg. Chem. 1968, 7, 2365.
