J . Phys. Chem. 1993,97, 656-660
656
Formation of a Stabilized Coordinatively Unsaturated Metal Carbonyl Cluster, HtRw(C0) 12, by Photochemical Reaction of H2Ry(C0)13 Adsorbed on the Surface of Silica Sadaaki Yamamto,**+Kiyotaka Asakura,* Kudo Mochida,QAtsubiko Nit@+ and Haruo Kuroda# Central Research Institute, Mitsui Toatsu Chemicals, Inc., 1190 Kasama-rho. Sakae-ku, Yokohama 247, Japan, Department of Chemistry, Faculty of Science, Gakushuuin University, Mejiro. Toshima-ku, Tokyo 171, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received: June 30, 1992; In Final Form: October 6, 1992
The photochemical formation of coordinatively unsaturated metal carbonyl clusters on the surface of metal oxide supports has the potential of leading to interesting new materials that can be used as selective catalysts. In line with our work on this theme, the photochemical reaction of dihydrotetraruthenium tridecacarbonyl, H2Ru4(CO) 13, adsorbed on the surface of silica was investigated by DRS UV-visible, FT-IR, and EXAFS spectroscopies. When irradiated by UV-visible light, H2Ru4(CO) 13 loses one carbon monoxide ligand, producing selectively a coordinatively unsaturated H~Ruq(C0)12on the surface of silica. The initial photoproduct, H2Ruq(C0)12, is interacting weakly with surface hydroxyl groups, which are the only functionality available to act as a stabilizing ligand, and the dissociated carbon monoxide is lost from the surface. This process prevents secondary reactions, thus leading to the selective formation and stabilization of the coordinatively unsaturated HzRud(C0) 12, which is unstable in solutions and undergoes secondary reactions.
Iatroduction Metal carbonyls have been used extensively for preparation of well-defined catalytically active metal carbonyl clusters and small metal clusters on metal oxide supports. Traditionally, the treatment of metal carbonyl clusters on supports to form catalytically active materials has relied on thermally induced reactions.’ The photochemistryof metal carbonyls adsorbed on solid surfaceshas recently attracted attention due to the possibility of forming active species more selectively on the surface of the support and to the possibility of using low temperature preparations of such surface species.2-lo As an alternative to the traditional thermal procedures, the photochemical approach may have several advantagesin the preparation of such surface species. One clear advantage is that extremely small metal clusters can be produced directly by photodecarbonylation without the sinteringeffect that produces larger metal particlesduringthermal treatments. In addition, the photochemical process can produce unique species through controlled decarbonylation that cannot be prepared by thermal treatments. For example, we have recently demonstrated that the photochemistryof triiron dodecacarbonyl, Fep(CO)lz, on the surface of silica results in the formation of hydride anion carbonyl, H F ~ ~ ( C O ) I I -which ’, could not be obtained by thermal treatment due to declu~terification.~Motivated by such encouragingfindings, we have been investigating the transformation of more complex polynuclear metal carbonyls to catalytically active species on metal oxide support~.~-I~ Coordinatively unsaturated metal carbonyls are one of the catalytically active species that have attracted much attention. Coordinativelyunsaturated metal carbonyls are known to act as catalysts in homogeneous solution-phaseor gas-phase processes such as isomerization, hydrosilation, hydrogenation, water gas shift reaction, dehydrogenation, and carbonylation.1 The catalytically active coordinatively unsaturated metal carbonyls can be easily prepared by photoexcitation of metal carbonylsfollowed by efficient decarbonylation.’I The coordinatively unsaturated species are generally so reactive that in solution they react with each other or with impurities in the solution. Further, they react 7 Mitsui Toatsu Chemicals, t The University of Tokyo.
Inc.
I Gakushuuin University.
0022-3654/58/2097-0656S04.00/0
efficiently with released carbon monoxide to regeneratethe parent metal carbonyl. Because of these fast secondary reactions in solution, the lifetime that coordinatively unsaturated species function as active sites is very short. In addition, it is difficult topreparea pure coordinativelyunsaturatedspecies,whichmakes it difficult to identify the active species. Formation of a coordinatively unsaturated species on the surface of a support may be one approach for preparing catalytically active, coordinatively unsaturated species, while avoiding the usual problems encountered when solutions are used. Recent studies on the photochemistry of mononuclear metal carbonyls on solid surfaces suggests that it is possible to use this approach to design catalysts composed of a coordinatively unsaturated species that retain their inherent reactivity.’” Studies on the photochemistry of polynuclear metal carbonyls, M,(CO), ( x 1 4), on oxide supports are relatively few compared with that of mononuclear metal carbonyls. Doi et al. have shown that photoirradiation of H4Ru4(C0)12 adsorbed on the surface of silica results in a coordinatively unsaturated species that retains the original metal framework, although its exact structure was not clear.I2 In our previous study, we found that the mixed metal carbonyl cluster, HzFeOs3(C0)13,adsorbed on the surface of silica undergoes clean decarbonylation to yield selectively the coordinatively unsaturated HzFeOs3(CO)12which could not be formed on the surface of silica by thermal treatment due to rapid declusterification.lo In this report, we describe our investigation of the photochemistryof H2Ru,(CO) 13 adsorbed on the surface of silica. This study was aimed at further showing the potential of photochemical approaches for the preparation of coordinatively unsaturated carbonyl species on oxide supports. Experimental Section
The starting metal carbonyl H2Rud(C0)13 was prepared according to the method given in the 1iterat~re.I~ We used two types of silica as the support: Aerosil 380 (Nippon Aerosil), which has surface hydroxyl groups, and RY200 (Nippon Aerosil), in which all surface silanol groups have been replaced by methyl groups. We refer to the first typeof silica as “hydroxy silica” and the second type as “methyl silica”. Before use, the silica powders were well dried by heating them at an elevated temperature in Ca 1993 American Chemical Society
Formation of Stabilized H ~ R u ~ ( C O ) I Z
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 651
vacuo (1 X lo4 Torr) for 12 h. The hydroxy silica was heated to 723 K, and the methyl silica was heated to 473 K. n-Hexane (Merck Uvasol), which was used in the sample preparations, was dried over CaC12 and then distilled on Na and LiAlH4 prior to use. H2Ru4(CO)13 was impregnated onto the silicas according to the following procedure. Dried silica powder was put into an n-hexane solution of H ~ R u ~ ( C Oand ) I ~stirred for about 30 min under the oxygen- and water-free atmosphere in a drybox (DRILAB equipped with HE-493 DRI-TRAIN, Vacuum Atmospheres Co.). After the mixing process was completed, the suspension was transferred to a vessel connected to a vacuum line and the n-hexane was removed by trap-to-trap distillation. The nominal metal loading was 1-2 wt % for each sample used in the present study. Hereafter, we denote the sample where HzRu4(CO)l3 was adsorbed on the hydroxy silica as 'HzRu4(CO)13/Si02(hydroxy)" and the sample on methyl silica as ' H ~ R U ~ ( C O ) I ~ / SiO2(methyl)". Irradiation of H2Ru4(CO)13/Si02(hydroxy)and H2Ru4(CO)13/SiO2(methyl) with UV-visible light was done while the samples were contained in an ultrahigh vacuum chamber (backpressure ca. 5 X Torr). The chamber is equipped with a quartz window, two CaF2 windows, and a quadrupole mass filter. The samples were irradiated by using an ultrahigh pressure Hg lamp (WACOM BLM-SOOD, 500 W) as the light source. The light was passed through one of threedifferent filter combinations that transmitted light at 313,365, and 405-578 nm. The filter combination providing the 313-nm light was made with aqueous solutions of K2Cr04 (0.27 g/L) and Na2COp (1 g/L)I5 and a glass filter (UV-D33S, Toshiba). The 365-nmlight wasobtained by using a water filter in combination with two glass filters (UVD33Sand UV-35, Toshiba). The405-578-nm lightwasobtained by using an aqueous solution of CuSO4 l 4 (transmittance at 366 nm = 0.82; optical path length = 5 cm) and a cutoff filter (L39, Toshiba). The optical path length of the quartz cell used to hold the aqueous filter solutions was 5 cm. The gas evolved from the sample during irradiation was analyzed in situ by a quadrupole mass analyzer attached to the chamber. UV-visiblediffuse-reflectance spectra were measured with a Hitach U3400 spectrophotometer equipped for diffuse reflectance measurement. The measurement of FT-IR spectra was done in situ with a Jasco FT/IR-8300 FT-IR spectrometer while the samples were held in the chamber. EXAFSdata wereobtained by use of the EXAFS spectrometer on line BL-1OB in the Photon Factory at the National Laboratory for High-Energy Physics in Tsukuba, Japan (PF-KEK). The synchrotronradiation was monochromatized by a Si(3 11) channel cut monochromator. The energy resolution was estimated to be 8 eV. The X-ray was monitored by ionization chambers filled with Ar. Ru powder was used for angle calibration. The analysis of the EXAFS data was carried out by using the 'EXAFS2" program. The EXAFS oscillations were extracted from wt by using the cubic spline method. The Fourier transformation of the Ru K-edge EXAFS oscillations was carried out over the range 30-150 nm-l. The Fourier transformed peaks were filtered over the range 0.1-0.3 nm and were inversely Fourier transformed to k space. The coordination number Ni, interatomic distance ri, and Debye-Waller factor u, of the ith shell were determined by a curve fitting analysis using the following equation
x ( k ) = x N i F i ( k ) / k r : exp(-2a:k2)
sin (2kri
+ cPi(k))
300
400
500
600
Wavelength/ nm
Figure 1. UV-visible diffuse-reflectancespectral changesaccompanying 313-nm irradiation of H2Rw(C0)13adsorbed on the surface of hydroxy silica versus irradiation time (min): (a) 0; (b) 140. 0.1000
0 .oooo
zpoo.0
2100 .o
2000 .o
*.vmnu.b.r
(cd]
lWO.0
lwo.o
Figure 2. FT-IR absorption spectral changes accompanying 313-nm irradiation of H2Rur(CO)ls adsorbed on the surface of hydroxy silica versus irradiation time (min): (1) 0; (2) 40; (3) 160. FT-IR absorption spectra (insert) of (- -) H~RQ(CO)I~ in a polystyrene film at 77 K and (-) of H2Rw(C0)12 obtained by irradiation of H2Rw(C0)13 in a polystyrene film at 77 K are also shown3
-0.0
2100 .o
po00.0 Wavenhmr (ern-')
1wo .o
1mo .o
Figure 3. FT-IR absorption spectral changes accompanying 313-nm irradiation of H2Rw(C0)13adsorbed on the surface of methyl silica versus irradiation time (min): (1) 0; (2) 60;(3) 120. IT-IR absorption I ~ is also shown (4). spectrum of H ~ R u ~ ( C Oin) KBr
differences between Fi and * i of Rh-0 and Rw-0 are not significant. However, thereare two types of Ru-CO coordination (Le. terminal and bridge CO), each with their own multiple scattering. Thus, the results on the Rw-0 interaction are less reliable than those on the Ru-C and Ru-Ru interactions. We checked the reliability of the Fi(k) and I # J ( ~ )functions by comparing the curve fitting results for H2Ru4(CO)l3 with the structural parameters derived from an X-ray diffraction analysis.
where Fi(k) and @@) are the amplitude function and the phase shift for the ith shell, respectively. F, and for Ru-Ru and RkYdtS Ru-C were empirically derived from Ru foil and H ~ R U ~ ( C O ) ~ ~ , respectively. There is no good Ru reference compound for the UV-Visible Diffuse Reflectance Speetm. Figure 1 shows the Ru-0 interaction, where Fi(k) and @,(k) involve a large multiple UV-visible diffuse reflectance spectral change of H2R~(C0)13/ scattering from the carbon atom between the ruthenium and Si02(hydroxy) upon 3 13-nm light irradiation. Irradiation at 366 oxygen. We used Rh2(C0)4C1~as a reference because the nm or broad band (405-585 nm) irradiation produced similar
Yamamoto et al.
658 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
TABLE I: IR and UV-Visible Absorption Spectral Data for Relevrnt Tetranuclear Metal Carbonyls temperIR spies aturd vco(terminal)/cm-l vco(bridge)/cm-I H2Ru4(CO)13/Si02(hydroxy) RT 21 11 (vw), 2082 (s), 2071 (s), 2058 (s), 2028 (m) 1860 (vw,br) 1882 (vw, br) HzRu(C0) I 3/ 3-mcthylpentane RT 2078 (s), 2067 (vs), 2054 (s), 2034 (m),2025 (s), 2008 (w) photoproduct/Si02(hydroxy) RT 21 10 (vw). 2082 (s), 2070 (s), 2057 (m, sh), 2030 (m) 1860 (vw, br) H2Ru4(C0)12/polystyrene 77 K 2098 (vw), 2081 (s), 2067 (vs), 2054 (m, sh), 2024 (m) 1865 (vw, br) 2081 (m), 2067 (s), 2030 (sh), 2021 (m), 2000 (sh)b RT HIRu~(CO)I~/S~OZ
UV-vis X/nm 365,410,550 357,440,530 365,450 350,440,600 260 (m), 310 (w), 363 ( s ) ~
* RT = room temperature. Reference 12. Reference 16. spectral changes to those seen for the irradiation done at 3 13 nm. The diffuse reflectance spectrum of the non-irradiated sample of H2Ru4(CO)I 3/SiO~(hydroxy)contains a series of poorly resolved bands at 365,410, and 550 nm. These bands agree well with the absorption bands of HzRu4(C0)13 in organic solvent and can be assigned to the electronic transitions localized primarily within the metal framework.15 Irradiationsdoneat 3 13or 366 nm involve irradiation of this transition. After irradiation, these bands still exist in the spectrum of the product, although the peak intensities decrease slightly. m-IR Absorptioa Spectra. The FT-IR absorption spectral change of H2Ru4(CO)13/Si02(hydroxy) and H2Ru(C0)13/Si02(methyl) upon 313-nm irradiation are shown in Figures 2 and 3, respectively. Thespbctrumof non-irradiated HZRu4(C0)13/Si02(hydroxy) is composed of the bands associated with terminal CO groups at 21 11 (w), 2082 (s), 2071 (s), 2058 (s), and 2028 (m, br) cm-I and an additional broad weak band and with a peak maximum around 1860 (w, br) cm-1 that is associated with the carbon monoxide ligand that bridges two ruthenium atoms. This spectrum agrees well with the spectrum obtained for the carbonyl in 3-methylpentane, as shown in Table I. The sharp resolution ofthe peaksin theFT-IRspectraofH2Ru4(CO)13/SiOz(hydroxy) is similar to that of the metal carbonyl cluster in solution. This indicates that the carbonyl cluster molecules are in a molecularly dispersed state on the surface of hydroxy silica. On the other hand, the FT-IR spectrum of HzRu4(C0)13/Si02(methyl) was significantly different from that of H~Ru4(C0)13dissolved in organic solvents and resembled the spectrum of H2Ru4(C0)13 powder dispersed in a KBr disk, as shown in Figure 3. This suggests that the H ~ R u ~ ( C Oon) Imethyl ~ silica aggregated to form small crystallites or particles. The FT-IR spectral changes during irradiation of H2Ru4(CO)13/Si02(hydroxy) exhibit clear isosbestic points. On the contrary, irradiation of HzRu4(C0)13/ SiOz(methy1) shows a different spectral change than those observed for H2Ru4(CO)13/Si02(hydroxy). Furthermore, no isosbestic points were observed in the spectral changes for the material on methyl silica. The photoproduct on methyl silica shows a broad band with a peak maximum around 2040 cm-I, as shown in Figure 3. Cur Analysis. Analysis of the gas evolved during the irradiation of H2Ru4(CO)13/SiO2(hydroxy) showed that carbon monoxide was evolved. No indication could be found for the evolution of hydrogen. Both carbon monoxide and hydrogen were evolved during irradiation of H2Ru4(CO) 1$3iOz(methyl). EXAFS Analysis. To obtain direct structural information on the photoproduct, we studied the Ru K-edge EXAFS of H2Ru4(CO)l,/SiOz(hydroxy) before and after the irradiation. The Fourier transform of the Ru K-edge EXAFS oscillation is shown in Figure 4. The Fourier transform of the light irradiated sample is a little different, but the main feature remained unchanged. The Fourier transforms for both samples showed two main peaks. The first peaks can be attributed to Ru-C, and the second one to Ru-Ru and R w O ( C 0 ) . We carried out a curve-fitting analysis of the EXAFS data for the Ru-Ru peak in the Fourier transform. The results are given in Table 11. The Ru-Ru and Ru-C bond distancesin H2Ru4(CO)1$302(hydroxy) were found to be almost the same as in H ~ R u ~ ( C Oboth ) I ~before and after
0
1
2 3 . 4 Di stancelA
5
6
Figure 4. Fourier transform of Ru K-edgeEXAFS oscillationsof H2Ru4(CO)i, adsorbed on the surface of hydroxy silica: (a) irradiated, (b)
non-irradiated. The irradiation time is 120 min.
TABLE Ik Curve-Fitting Results of Ruthenium K-Edge EXAFS for H914(CO)l,/SiOz(hydroxy) and the Pbotoproduct/SiOz(bydroxy) Ru-C Ru-RU Ru.**O sample N r/A N r/A N r/A H2Ru(CO)l3/Si02(hydroxy)
photoproduct/Si02(hydroxy)
4 3.2
1.90 1.3 1.90 0.95
2.81 2.80
2.54 2.57
3.04 3.03
the light irradiation. The Ru-Ru bond distance for both samples (2.80 A) is different from the Ru-Ru distance (2.70 A) in Ru metal particles. The obtained values of Ru-Ru and Ru-C coordination numbers were a little smaller in irradiated H2Ru4(CO),,/Si02(hydroxy) compared with non-irradiated H2Ru4(CO)13/Si02(hydroxy). The difference can be attributed to the error in the normalizing factor accompanying the small absorption edge. Therefore, the bond length and the coordination number are regarded as being almost the same within experimental error.
Discussion As shown in Figure 2, clear isosbestic points are visible in the spectral change of H2Ru4(CO) 13/SiO2(hydroxy). This indicates that the reaction is clean and that there are no secondary products. The gas evolution analysis provides some insight into the nature of the reaction that takes place. No indication could be found for the evolution of hydrogen. Thus, the gas evolution results indicate that a clean reaction involving the loss of one carbonyl group per a cluster occurred. The bands in the UV-visible diffuse-reflectance spectrum of HzRu4(C0)13/Si02(hydroxy)shown in Figure 1 agree well with the absorption bands of H2Ru4(CO)l3 in 3-methylpentane (see Table I). These can be assigned to the electronic transitions associated with the Ru4 skeleton of the H ~ R U ~ ( C O cluster.ls )I~ The fact that the UV-visible diffuse-reflectance spectrum of the photoproduct retains the same broad bands indicates that the Ru4 skeleton is still intact in the product. The EXAFS data clearly show the presence of Ru-Ru bonding in the photoproduct
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 659
Formation of Stabilized H ~ R U ~ ( C O ) I ~
at essentially the same distance as that in non-irradiated HzHzRu4(CO)13/SiO2(hydroxy)system. From the resemblance of the IR spectrum, as well as other spectral data supporting the Ru4(CO)13/Si02(hydroxy). The fact that the Ru-Ru bond retention of the original metal skeleton as mentioned above, it is distance of the cluster before and after light irradiation are reasonable to conclude that the photoproduct on hydroxy silica different from that in Ru metal particles indicates the Ru metal is H2Rud(C0)12. Thus, the photochemical process which takes particles were not formed by light irradiation. These findings place in the H2Ru4(CO)13/Si02(hydroxy)systemcan be assumed rule out the formation of metal aggregates or surface attached mononuclear carbonyls resulting from declusterification. Furto be the following: thermore, these rule out the formation of clusters with higher H,Ru4(CO),,(ads) H,Ru,(CO),,(ads) CO(gas) nuclearity, thus supporting the conclusion that the original R u ~ metal skeleton is retained in the photoproduct. We propose that the mechanism leading to selectiveformation The results described above indicate that the reaction involves of the reactive cluster H z R u ~ ( C O )involves I~ a rather weak the loss of carbon monoxide ligand and that the Ru4 skeleton of interaction between the cluster and the surface of silica. On the photoproduct is intact. Thus, it can be concluded that the hydroxy silica where H2Ru4(CO)13 is molecularly dispersed, the photochemical process which takes place in the H ~ R u ~ ( C O ) I ~ / initial photochemical process involves the dissociation of one Si02(hydroxy) system is m e t a l 4 0 bond dissociation, yielding carbon monoxide ligand, the same process as that occurring in a product having a R u skeleton ~ whose structure is almost the solution15or in matrices at 77 K.19 The initial photoproduct is same as that of the original metal carbonyl molecule. Three the coordinatively unsaturated HzRu~(CO)IZ. The H2R~q(C0)12 possibilities need to be considered for the structure of the clustersthus formed are isolated from each other and are stabilized photoproduct which retains the original Ru4 metal skeleton. The by the interaction with the surface of silica, presumably with first possibility is H.+Ru4(C0)12 formed by reaction of the surface hydroxyl groups. The evolved carbon monoxide is photoexcited H ~ R u ~ ( C Owith ) I ~surface hydroxy groups. Such efficiently removed from the gas-solid interface. These two a reaction was reported by Foley et al. who showed that H2effects, interaction with the surface and loss of carbon monoxide, Rur(C0)12(PPhs) is formed when HzRu4(C0)13 is irradiated in help to deter such secondary reactions as the recombination the presence of PPh3 in a hydrocarbon solution and that H4reaction with carbon monoxide, reactions between H2Ru4(C0)12 Ru4(C0)12is formed when the irradiation is done in the presence clusters, or reaction of the photoproduct with starting cluster. of h~dr0gen.l~ Thus, one possible structure for the photoproduct Thus, irradiation of the H2Ru4(C0)13/SiOz(hydroxy) system could be HdRud(C0) I 2 produced by the following photochemical leads to the selective formation of HzRu4(C0)12 on silicareaction involving surface hydroxyl groups: (hydroxy). This proposal is supported by the dependence of the H ~ R u ~ . H2Ru4(CO),, 2HO-Si= (CO)13 reactivity on the degree of dispersion. The resemblance 2 0 - S ~ C O H4Ru4(CO)12 of the IR spectrum of H2Ru4(CO)13/Si02(methyl)to that c f H2Ru4(C0)13powder in KBr indicates that the clusters on methyl However, the IR spectrum of H4Ru4(C0)12/Si02 shown in Table silica are in an aggregated state. Isosbestic points were not I is different from that obtained for the photoproduct on hydroxy observed in the FT-IR spectral changes for H z R u ~ ( C O ) I ~ / S ~ O ~ silica. Thus, H ~ R U ~ ( C Ocan ) I Zbe ruled out as the photoproduct (methyl) upon irradiation. This means that the reaction is not in the H2Ru4(C0)13/Si02(hydroxy) system. clean and that there are secondary products. The broad band The second possibility that must be considered is the formation centered at 2040 cm-l can be assigned to carbon monoxide of a surface bound hydride cluster, HRu4(C0)13-. Hydride adsorbed on Ru microcrystallites.20 The gas analysis results carbonyl clusters are known to be formed by thermal treatment indicate that hydrogen as well as carbon monoxide are evolved. of the metal carbonyl cluster on basic oxides via a proton-transfer Thus, the reaction on methyl silica differs from the reaction on reaction.17J8 It is conceivable, therefore, that the same protonhydroxy silica and is significantly more complex. From the gas transfer reaction is induced in the cluster on hydroxy silica by analysis results and the IR spectrum of the photoproduct, it is UV-visible light irradiation, which results in the formation of likely that decomposition occurs on methyl silica. The initial HRud(CO)13-. HRu4(CO)l3- has an equal number of carbon photoprocess on methyl silica is probably the same as that on monoxide ligands to that of the parent H ~ R u ~ ( C O )Evolution I~. silica(hydroxy) and in solution and matrices, that is, loss of one of carbon monoxide, however, implies that each parent metal carbon monoxide to form H ~ R u ~ ( C O ) IThe Z . evolved carbon carbonyl lost one carbon monoxide ligand upon UV-visible light monoxide can be efficiently removed from the cluster molecules irradiation. Therefore, the product should have fewer carbon on the outermost surface of aggregated particles. The unstable monoxide ligands than the parent carbonyl cluster. This experH2R~q(CO)lzformed;however, neither can bestabilized by nearby imental finding rules out the second possibility that the product hydroxyl groups, nor recombine with carbon monoxide, so is the hydride anion HRud(C0)13-. decomposition results. Thus, the photoprocess of H~Ru4(C0)13 The third possibility that can be considered is the formation on methyl silica leads ultimately to decomposition. Hydroxy of the electronically deficient or coordinatively unsaturated H2groups are known to act as weak ligands,2thus lending support Ru4(C0)12 formed by the loss of one carbon monoxide ligand. to the scheme proposed above to account for the stabilization of The coordinatively unsaturated cluster H~Ru4(C0)12is proposed the coordinatively unsaturated cluster on silica(hydroxy). to be the initial photoproduct when the starting carbonyl cluster is irradiated in solution.15However, because of its high reactivity, Conclusions it has not been isolated in a crystalline state nor have its IR and The tetraruthenium hydride carbonyl cluster HzRu4(C0)13 UV-visible spectra been reported. Recently, we found that such adsorbed on the surface of silica having surface hydroxyl groups an unsaturated species is formed when H ~ R u ~ ( C Oisolated ) I ~ in undergoes a photoinduced CO dissociation reaction when irraa polystyrenefilm or a 3-methylpentane glass at 77 K is irradiated diated with light that excites the metal-centered electronic with UV light.I9 This reaction involves the photochemical absorption bands of the cluster. This reaction yields selectively dissociation of one carbon monoxide ligand. Furthermore, the the coordinatively unsaturated H ~ R u ~ ( C Oon) Ithe ~ surface of isolated HzRuq(C0)12is soreactive that it re-forms H2Ru4(CO)13 silica. In solution, the same primary photochemical process when warmed to 170 K. The re-formation of the starting cluster involving loss of one carbon monoxide ligand occurs, but the occurs because of the facile recombination of the coordinatively unstable photoproduct easily undergoes secondary reactions. On unsaturated cluster with carbon monoxide that remains at nearby the other hand, with HzRud(C0) 13/Si02(hydroxy) the primary . sites. The IR spectrum of H2Ru4(C0)12 in a polystyrene film, photoproduct achieves some degree of stability through weak inserted in Figure 2, is similar to that of the photoproduct in the
-
+
+
-
+
+
+
660 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 interaction with surface hydroxy group. The dissociated carbon monoxide iseficiently lost from the surface. Thisprocess prevents secondary reactions, thus explaining why the coordinatively unsaturated H2Ru4(C0)12, which is unstable in solution, is efficiently formed when HzRu4(CO)13/Si02(hydroxy)is irradiated with UV-visible light. The results of this study, as well asour previousstudyon themixedmetalclusterH2FeOs3(CO)13,10 suggest that unstable coordinatively unsaturated species which retain their original metal skeleton but loose one carbon monoxide ligand can be selectively prepared on the surface of silica when supported tetranuclear metal carbonyl clusters are photoirradiated.
References and Notes (1) Gates. B. C.; Gwzi, L.;Knlkinger, H. Metal Clusters in Catalysis; Elsevia: Amsterdam, 1986. ( 2 ) Jackson, R. L.;Tmheim, M.R.J. Am. Chem. Soc. 1982,104,6590. (3) Simon, R. C.; Gafney, H. D.;Morse,D.L. Inorg. Chem. 1985.21, 2565. (4) Darsillo, M.S.;Gafny, H. D.;Paqutte, M.S.Inorg. Chem. 1988,27, 2815.
Yamamoto et al. (5) Wada, Y.; Nakaoka, C.; Morikawa, A. Chem. Lcrr. 1988,25. (6) Wada, Y.; Nakaoka, C.; Morikawa, A. J. Chem. Soc.. Chem. Commun. 1990,319. (7) Yamamoto. S.;Lewis,R. M.; Hotta, H.; Kurcda, H. Inorg. Chem. 1989,28,309 1. (8) Yamamoto, S.;Lewis, R. M.;Hotta, H.;Kuroda, H. Vacuum 1990, 41, 65. (9) Yamamoto, S.;Lewis, R. M.; Nabata, Y.; Hotta, H.; Kurodn, H. Inorg. Chem. 1990, 29,4342.
(IO) Yamamoto,S.;Miyamoto,Y.;Koizumi.M.;Lewb,R.M.;Morioka, Y.; h k u r a , K.; Kuroda, H.J. Phys. Chem. 1992, Sa, 6367. (1 1) Gwffroy, G. L.;Wrighton, M.S.OrganometallicPhotochemistry; Academic Prms: New York, 1979. (12) h i , Y.; Yano, K. Inorg. Chlm. Acra 1983, 76, L71. (13) Johnson, B. F. G.; Johnston, R. D.;Lewis, J.; Robinson, B. H.; Wilkinson, G. J. Chem. Soc. A 1!W, 2856. (14) M u m , S. L. Handbook of Photochemistry; Dekker: New York, 1973. (15) Foley, H. C.; Gwffroy, G. L. J . Am. Chem. Soc. 1981,103,7176. (16) Uchiyama, S.; Gates, B. C. J. Carol. 1988, 110, 388. (17) Budge, J. R.; Lucke, B. F.; Gates, B. C.; Toran, J. J. Catal. 19885, 91, 1985. (18) Choplin, A.; Huang, L.;Basset. J. hi.;Mathieu, R.; Siriwardane, U.; Shore,S. G. Organometallics 1%. 5, 1547. (19) Yamamoto. S.;Asakura, K.; Mochida, K.; Nitta, A.; Kuroda,H. 1. Phys. Chem.. companion paper in this issue. (20) Kuznetsov, V. L.;Bell, A.; Yennakov, Y. I. J . Carol. 1980,155,374.
