Immobilization of a Ligand-Preserved Giant Palladium Cluster on a

Apr 17, 1999 - A 1,10-phenanthroline (phen) ligand-preserved five-shell giant palladium cluster, Pd5(phen), could be uniformly immobilized on a neutra...
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Langmuir 1999, 15, 3557-3562

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Immobilization of a Ligand-Preserved Giant Palladium Cluster on a Metal Oxide Surface and Its Nobel Heterogeneous Catalysis for Oxidation of Allylic Alcohols in the Presence of Molecular Oxygen Kohki Ebitani, Yoko Fujie, and Kiyotomi Kaneda* Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Received December 15, 1998. In Final Form: March 1, 1999 A 1,10-phenanthroline (phen) ligand-preserved five-shell giant palladium cluster, Pd5(phen), could be uniformly immobilized on a neutral TiO2 surface while keeping its original cluster size of ca. 30 Å and local ordering around Pd atoms. The Pd5(phen)/TiO2 acted as an efficient and reusable heterogeneous catalyst for the oxidations of primary allylic alcohols into the corresponding R,β-unsaturated aldehydes in the presence of molecular oxygen as well as the corresponding homogeneous Pd5(phen) cluster. Roles of the phen ligand in the oxidation catalysis of the giant Pd cluster are discussed in relation to electronic and geometric effects of the Pd atoms on the Pd cluster surface.

Introduction One of the current and great interests in designing highperformance practical cataysts is the heterogenization of the metal complexes by fixing on organic and inorganic surfaces.1 In spite of many studies on the fixation of metal clusters having relatively small metal atoms, the immobilization of the large metal clusters has been scarecely reported. Recently, studies on syntheses and catalyses of large metal clusters which contain more than 100 metal atoms have become the faciliting fields because they are subjects for investigation not only of quantum size effect in their properties but also of model compounds for heterogeneous metal catalysts.2-6 Schmid et al. have synthesized giant Pd clusters with seven and eight shells, Pd7/8 cluster, supported on titanium dioxide (TiO2) and mesoporous metal oxides such as MCM-41, yielding heterogeneous catalysts for selective hydrogenation of alkynes and oxidation of CO.7 In their * To whom correspondence should be addressed. Telephone and fax: +81-6-6850-6260. E-mail: [email protected]. (1) (a) Tailored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel Publishing: Dordrecht, The Netherlands, 1986. (b) Bergbreiter, D. E. CHEMTECH 1987, 17, 686. (c) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (d) Gates, B. C. Chem. Rev. 1995, 95, 511. (e) Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996; Vol. 2. (2) (a) Shriver, D. F.; Kaesz, H. D.; Asams, R. D. The Chemistry of Metal Cluster Compounds; VCH Publishers, Inc.: New York, 1990. (b) Kno¨zinger, H. Cluster Models for Surface and Bulk Phenomena; Plenum: New York, 1992. (c) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (d) Schmid, G. Aspects of Homogeneous Catalysis; Ugo, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; Vol. 7, p 1. (e) Schmid, G. Polyhedron 1988, 7, 2321. (f) Schmid, G.; Morun, B.; Malm, J.-O. Angew. Chem., Int. Ed. Engl. 1989, 28, 778. (3) Schmid, G. Chem. Rev. 1992, 92, 1709. (4) Vargaftik, M. N.; Zagorodnikov, V. P.; Stolarov, I. P.; Moiseev, I. I.; Kochubey, D. I.; Likholobov, V. A.; Chuvilin, A. L.; Zamaraev, K. I. J. Mol. Catal. 1989, 53, 315. (5) (a) Vargaftik, M. N.; Moiseev, I. I.; Kochubey, D. I.; Zamaraev, K. I. Faraday Discuss. 1991, 92, 13. (b) Volkov, V. V.; Van Tendeloo, G.; Tsirkov, G. A.; Cherkashina, N. V.; Vargaftik, M. N.; Moiseev, I. I.; Novotortsev, V. M.; Kvit, A. V.; Chuvilin, A. L. J. Cryst. Growth 1996, 163, 377. (6) Catalysis of the Pd561 cluster: (a) Moiseev, I. I.; Vargaftik, M. N.; Chernysheva, T. V.; Stromnova, T. A.; Gekhman, A. E.; Tsirkov, G. A.; Makhkina, A. M. J. Mol. Catal. A 1996, 108, 77. (b) Moiseev, I. I. Catal. Oxid. 1995, 203.

studies, the atomic ordering of the Pd atoms at the surface Pd7/8 cluster has been well observed by means of a highresolution transmission electron microscopy (HR-TEM) technique, but it seems that the dispersion state of the Pd clusters on the surface of the metal oxides was not definitively evaluated.7 Determination of the dispersion state of the supported metal particles is crucial to the study of catalysis of metal particles because their catalytic activities are strongly influenced by the particle size.8 In the present study, we report on heterogenizatin of the giant Pd cluster with five shells, Pd561phen60(OAc)180,4-6 by fixing on the surface of metal oxides, i.e., TiO2. The structure and the dispersion state of the immobilized ligand-preserved giant Pd cluster on the surface of neutral TiO2 were characterized by means of field emission scanning electron microscopy (FE-SEM), HR-TEM, and Pd K-edge X-ray absorption fine structure (XAFS) techniques. It was revealed that the Pd561phen60(OAc)180 cluster, Pd5(phen), could be uniformly dispersed on the TiO2 surface while keeping its original structure. Further, the Pd5(phen)/TiO2 exhibited a nobel and reusable catalysis for selective oxidation of primary allylic alcohols in the presence of molecular oxygen.9-12 The specific catalytic activity for the oxidation of primary allylic (7) Schmid et al. have also reported the synthesis and catalysis of the phen ligand-stabilized palladium 7/8 shell giant clusters. See: (a) Schmid, G.; Harms, M.; Malm, J.-O.; Bovin, J.-O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046. (b) Schmid, G.; Emde, S.; Maihack, V.; Meyer-Zaika, W.; Peschel, St. J. Mol. Catal. A 1996, 107, 95. (c) Junges, U.; Schu¨th, F.; Schmid, G.; Uchida, Y.; Schlo¨gl, R. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1631. (8) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26. (9) The preliminary results on alcohol oxidations using the Pd5(phen) giant cluster catalysts have been reported. Kaneda, K.; Fujie, Y.; Ebitani, K. Tetrahedron Lett. 1997, 38, 9023. (10) Kaneda, K.; Fujii, M.; Morioka, K. J. Org. Chem. 1996, 61, 4502. (11) Kaneda, K.; Yamashita, T.; Matsushita, T.; Ebiani, K. J. Org. Chem. 1998, 63, 1750. (12) Examples for palladium-catalyzed oxidation in the presence of molecular oxygen: (a) Blackburn, T. F.; Schwartz, J. J. Chem. Soc., Chem. Commun. 1977, 157. (b) Hronec, M.; Cvengrosova´, Z.; Kizlink, J. J. Mol. Catal. 1993, 83,75. (c) Go´ez-Bongoa, E.; Noheda, P.; Echavarren, A. M. Tetrahedron Lett. 1994, 35, 7097. (d) Noronha, G.; Henry, P. M. J. Mol. Catal. A 1997, 120, 75. (e) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185. (f) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahedron Lett. 1998, 39, 6011.

10.1021/la981720a CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999

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alcohols could be ascribed to the multiple interaction between the alcohol and the paired Pd cationic sites at the surface of the dispersed giant Pd cluster surrounded by the phen ligands and OAc anions. Experimental Section General Procedures. All organic reagents were purified by the standard procedures used before.13 1,10-phenanthroline (phen) and its derivatives were obtained from Nacalai Tesque Co. Ltd. and used after recrystallization. Pd(OAc)2 was purchased from N. E. Chemcat Co. Ltd. The five kinds of TiO2 were supplied from the Catalysis Society of Japan as reference catalysts of JRC-TIO-1-5.14 The surface acid-base properties of the TiO2 samples were estimated by volumetric methods of adsorbents such as NH3 and CO2.14 The amounts of surface acid sites per gram are as follows: TiO-1 (51) > TiO-4 (40) > TiO-3 (41) > TiO-2 (14) > TiO-5 (2.8), where the number is in parentheses show the BET surface areas in m2/g. The order of the basicity per gram was foumd to be as follows: TiO-3 > TiO-1 > TiO-4 > TiO-2 > TiO-5. JRC-TIO-2, as a typical sample of TiO2, contained 98.5 wt % TiO2 together with 0.4 wt % Nb2O5 and 0.18 wt % P2O5 as its main impurities, and its pore volume was 3.3 × 10-2 cm3 g-1. The main phase was anatase, and the diameter of the crystallite was 38 nm. The amount of gas absorption was volumetrically measured by a gas buret directly connected to the rotary vacuum pump. GC analysis was performed with a Shimadzu GC-8A gas chromatograph with a thermal condutivity detector and a 2 m column (KOCL-3000T and Silicon US W-98). Infrared spectra were recorded on a Jasco FT/IR 410. 1H NMR spectra were obtained on JEOL GSX-270 (270 MHz) or JNM-AL400 (400 MHz) spectrometers with tetramethylsilane as the internal standard. Elemental analysis was performed on a Perkin-Elmer 2400 II CHNS. Synthesis and Immobilization of a Giant Pd Cluster. The phen ligand-preserced five-shell giant Pd cluster, Pd5(phen), was prepared according to the method in the literature.5 First, hydrogen was absorbed in an acetic acid solution of Pd(OAc)2 (0.073 g, 0.44 mmol) (H2/Pd ) 1.3 mol/mol) at room temperature in the presence of phen (0.22 mmol) to yield a Pd hydride cluster, [Pd4phen(OAc)2H4]n (n ) 100). Subsequently, the solution of the Pd hydride cluster was treated with oxygen (O2/Pd ) 0.5 mol/ mol), and then 50 mL of benzene was added to the solution of Pd clusters. After decantation, a black precipitate was washed with benzene to yield the giant Pd5(phen) cluster, Pd561phen60(OAc)180.5,6 Anal. Calcd for Pd561C1080H960N120O360: C, 16.4; H, 1.3; N, 2.1. Found: C, 16.1; H, 1.3; N, 2.6. Its ideal structure is an icosahedron five shell of 28 Å diameter.3,4 A typical example for the immobilization of Pd5(phen) on the metal oxides is described for Pd5(phen)/TiO2. TiO2 (JRC-TIO-2, 0.30 g) was added into the 5 mL acetonitrile solution of the Pd5(phen) cluster (0.0155 g) at room temperature (5 wt % as Pd cluster) and then stirred for 2 h. The solid compound of 0.294 g was obtained after filtration, washing with acetonitrile, and drying at room temperature. The treatment of Pd5(phen)/TiO2 (0.106 g) with an atmospheric hydrogen at room temperature in an acetonitrile solvent for 2.5 h resulted in an absorption of hydrogen (H2/Pd ) 1.6 mol/mol). Analysis of the acetonitrile filtrate by liquid chromatography showed that most of the phen ligand was released from the solid sample. Characteriation. FE-SEM studies were performed using a Hitachi S-5000L microscope (22.0 kV). The images of the clusters were recorded without sputtering of the samples. HR-TEM micrographs were obtained with a Hitachi Hf-2000 FE-TEM equipped with Kevex σ energy-dispersive X-ray detector (EDX) operated at 200 kV. The specimens were prepared by dipping a copper grid into sample powders. The palladium K-edge X-ray absorption data were measured at room temperature in a transmission mode at the EXAFS facilitates installed on the BL-10B line of the Photon Factory at (13) Purification of Laboratory Chemicals, 3rd ed.; Perrin, D. D., Armarego, W. L. F., Eds.; Pergamon Press: Oxford, U.K., 1988. (14) The physicochemical properties of JRC-TIO samples were available in: Catal. Catal. (J. Catal. Soc. Jpn.) 1996, 38, 370.

Ebitani et al. High Energy Accelerator Research Organization, Tsukuba, Japan, witha ring energy of 2.5 GeV and a storage position current of 390-210 mA, using a double-crystal Si(311) monochromator. X-ray absorption spectra were recorded by scanning from 330 eV to 1630 eV above the Pd K-edge at 24.353 keV. The energy step of the measurement in the XANES region of (30 eV from the absorption edge was 1.0 eV. The oscillation part of the absorption as a function of the X-ray photon energy was extracted as described in the literature.15 The EXAFS data were normalized by fitting the background absorption coefficient around the energy region higher than the edge about 35-50 eV with the smoothed absorption of an isolated atom (McMaster type function, CE-2.75). Fourier transformation (FT) of k3 -weighted normalized EXAFS data was performed over the 3.5 < k/Å < 12 range to obtained the radial structure function.16,17 CN (the coordination number of scatterers), R (the distance between an absorbing atom oand scatterer), and the Debye-Waller factor were estimated by curvefitting analysis with the inverse FT with the 1.8 < R/Å < 2.9 range assuming single scattering.18 In order to obtain the degree of uncertainty in the determination of the structural parameters from EXAFS data, the analysis was performed on a palladium foil as reference materials of known structure (Pd-Pd shell: coordination number ) 12, bond length ) 2.75 Å). Data reductions were performed with the FACOM M-780 computer system of the Data Processing Center of Kyoto University. Catalytic Reaction. A typical procedure for the oxidation is as follows. Acetic acid (15 mL) and cinnamyl alcohol (0.61 g, 4.5 mmol) were added to a reaction vessel containing the giant Pd5(phen) cluster/TiO2 (0.90 g, 0.30 mmol of Pd atom). Then, the heterogeneous mixture was stirred at 60 °C for 3 h under an O2 atmosphere. The catalyst was separated by filtration, and the GC analysis showed a quantitative yield of cinnamaldehyde. After the usual workup, the reaction mixture was subjected to column chromatography on silica gel (hexane/ethyl acetate, 10:1) to yield a pure cinnamaldehyde (0.49 g, 83%).

Results Characterization. The FE-SEM image of the immobilized Pd5(phen) cluster on a TiO2 surface showed the homogeneous distribution of Pd particles with less than ca. 30 Å diameter on the whole surface, which can be clearly seen in Figure 1. The observed particle size was smaller than those of Pd7/8 clusters, i.e., 31.5 and 36 Å,7 and very close to that of the five-shell Pd(phen) cluster of 28 Å.4-6 A homogeneous distribution of the Pd particles with 25-30 Å diameter on the TiO2 surface could be also observed by using the TEM micrograph. Among the various JRC-TIO samples as a support for the immobilization of Pd5(phen), it was found that JRC-TIO-2 was the most suitable TiO2 for giving the homogeneous distribution of the Pd5(phen) cluster.14 The JRC-TIO-5 sample was also a good support for the uniform distribution of the Pd5(phen) cluster. The above two TiO2 samples are characterized by low acidity and basicity in the surface properties compared with other TiO2 samples, as evidenced by volumetric absorption of NH3 and CO2 on the TiO2 samples.19,20 Acetonitrile was the best solvent while theuse (15) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1988, 84, 2987. (16) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (17) Sayer, D. E.; Stern, E. A.; Lytle, F. W. Phys. Rev. Lett. 1971, 27, 1204. (18) Bart, J. C. J. In Advances in Catalysis; Eley, D. D., Pines, H., Weiz, P. B., Eds.; Academic Press: San Diego, 1986; Vol. 34, p 203. (19) The amounts of NH3 desorption from TiO-2 and TiO-5 were 0.087 and 0.059 mmol/g, respectively. The amounts of CO2 absorption on TiO-2 and TiO-5 were 1.2 and 0.4 mmol/g, respectively. These values were smaller than those of other TiO2 samples by 1 order of magnitude. See ref 14. (20) A reviewer has suggested that the Pd cluster may be immobilized through an ionoic bonding between the cluster and support surface. We think that the acetate anion ligands of the Pd5(phen) cluster are substituted for several protons of the hydroxyl groups at the TiO2 surface to give the immobilized Pd5(phen) cluster on TiO2.

Immobilization of Ligand-Preserved Giant Palladium Cluster

Figure 1. FE-SEM images of (a) Pd5(phen)/TiO2, and (b) Pd5(phen)/TiO2 treated with H2. The white point shows the Pd cluster particles. In both measurements, JRC-TIO-2 was used as a support for the immobilization of the giant Pd5(phen) cluster in acetonitrile.

of acetic acid and benzene resulted in the heterogeneous distribution of the Pd cluster on the TiO2 surface. In the Fourier transforms (FTs) of k3-weighted EXAFS of the Pd5(phen) cluster on TiO2, the peak around 2.4 Å was dominant, as displayed in Figure 2a. The curve-fitting analysis using a k range of 7 < k < 12 Å-1 in its inverse FT showed that the main peak was characterized to a single Pd-Pd shell in a metallic Pd and also that CN and R of the shell were 6.3 and 2.74 Å, respectively. The results are summarized in Table 1. The above CN and R values were unchanged whether the Pd5(phen) cluster was located on the TiO2 surface or not. The R value agreed well with that of the bulk metallic Pd (R ) 2.75 Å), while the CN value became smaller than that of the Pd bulk (CN ) 12), which is due to the smaller particle size of the Pd clusters. The above results support that the Pd5(phen) cluster was homogeneously immobilized on the TiO2 surface while keeping the particle size and a local structure around Pd atoms of the Pd5(phen) cluster itselt the same. Valgaftik et al. have assigned four different Pd-Pd shells in the FTs of the Pd5(phen) cluster in the R range of 2-4 Å by using the peak discrimination method.4 However, we could not succeed in a curve fitting of the small peaks observed at 3-4 Å by the Pd-Pd shell parameters probably because of a limitation of EXAFS measurement where the long-

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range structures far from an absorbing atom were not observable.21 Further, no Pd-ligand shells of Pd-C and/ or Pd-N were detectable, which may be due to an insufficient back-scattering of photoelectrons by these low-Z atoms. A treatment of Pd5(phen)/TiO2 with hydrogen in acetonitrile released the phen ligands from the Pd5 cluster surface. Interestingly, the FE-SEM image in Figure 1b clearly showed that the Pd cluster particles were homogeneously distributed on the surface of the TiO2. XAFS experiments also support that the particle size of Pd clusters did not change during the above treatment; CN and R values for the Pd-Pd shell were constant even after the hydrogen treatment of the Pd5(phen)/TiO2 sample, as listed in Table 1. The HR-TEM image of the H2-exposed Pd cluster on TiO2 is depicted in Figure 3. The EDX analysis confirmed that particles having about 30 Å of vertical diameter on the surface were composed of palladiums. The Pd particles had Pd atoms with the structure of cubic close packing (fcc) type of a (111) layer on the faces of a Pd5(phen) cluster particle with icosahedral shape. The observed Pd-Pd distances were 2.58 and 4.2 Å, which are close to those of fcc bulk metallic Pd of 2.60 and 3.66 Å. The 3.5 Å lattice fringes indexed to an anatase(101) lattice plane could be observed in the TiO2 support.22 Oxidation Catalysis. The immobilized Pd5(phen) on TiO2 was found to catalyze the oxidative dehydrogenation of cinnamyl alcohol to cinnamaldehyde in the presence of molecular oxygen, and its catalytic activity was comparable to the corresponding homogeneous Pd5(phen) complex. The use of typical Pd compounds of Pd(OAc)2 and PdCl2 resulted in low yields of cinnamaldehyde under our reaction conditions, respectively. Further, supported Pd catalysts such as Pd/carbon, Pd/Al2O3, and Pd/SiO2 were also poor catalyts for the above oxidation. The Pd5(phen) cluster immobilized on other metal oxides such as SiO2, Al2O3, and MgO in acetonitrile also showed a quantitative yield of cinnamaldehyde. However, the use of FSM-16, active carbon, and hydrotalcite resulted in low yields of the aldehyde (68-76%). Oxidations of various allylic alcohols with Pd5(phen) and Pd5(phen)/TiO2 catalysts are summarized in Table 2. Many kinds of aromatic and aliphatic primary allylic alcohols were efficiently oxidized to give the corresponding R,β-unsaturated aldehydes in high yields, respectively. Generally, it is well-known that many metal catalysts can promote oxidations of both primary and secondary allylic alcohols as well as benzyl ones.23 Notably, two Pd5(phen) and Pd(phen)/TiO2 catalysts showed low catalytic activities for oxidations of seconary allylic alcohols and benzyl alcohols. After oxidation of cinnamyl alcohol, Pd5(phen)/TiO2 was separated from the reaction mixture by a simple filtration and subjected further to use as a catalyst. Pd5(phen)/ TiO2 could be reused without an appreciable loss of the catalytic activity and selectivity for oxidation; cinnamaldehyde was obtained over 90% yields during several runs (21) Fairbanks, M. C.; Benfield, R. E.; Newport, R. J.; Schmid, G. Solid State Commun. 1990, 73, 431. (22) Braunschweig, E. J.; Logan, A. D.; Datye, A. K.; Smith, D. J. J. Catal. 1989, 118, 227. (23) Generally, many metal catalysts could promote oxidations of both allylic and benzyl alcohols. For examples, see: (a) Harding, K. E.; May, L. M.; Dick, K. F. J. Org. Chem. 1975, 40, 1664. (b) Barton, D. H. R.; Kitchin, J. P.; Motherwell, W. B. J. Chem. Soc., Chem. Commun. 1978, 1099. (c) Hirano, M.; Morimoto, T.; Itoh, K. Bull. Chem. Soc. Jpn. 1988, 61, 3749. (d) Muzart, J.; Ajjou, A. N’A.; Ait-Mohand, S. Tetrahedron Lett. 1994, 35, 1989. (e) Lenz, R.; Ley, S. V. J. Chem. Soc., Perkin Trans. 1997, 1, 3291. (f) Marko´, I. E.; Giles, P. R.; Tsukazaki, M.; Chelle´Regnaut, I.; Urch, C. J.; Brown, S. M. J. Am. Chem. Soc. 1997, 119, 12661.

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Ebitani et al.

Figure 2. (a) FT magnitude of k3-weighted EXAFS on Pd5(phen)/TiO2 prepared in an acetonitrile solvent and (b) inverse FT of the main peak appearing at 1.8-2.9 Å in a. The dotted line in b shows the result of a curve-fitting analysis using Pd-Pd shell parameters in the range of 7-12 Å-1. Table 1. EXAFS Data of the Coordination number (CN), Interatomic Distance (R), and Debye-Waller Factor (∆σ2)a of the Palladium Samples sample

CNPd-Pd

RPd-Pd/Å

∆σ2/Å2

Pd5(phen) Pd5(phen)/TiO2b Pd5(phen)/TiO2c Pd5(phen)/TiO2d Pd5(phen)/TiO2e Pd foil

7.4 6.3 9.1 6.8 7.2 (12)

2.745 2.738 2.743 2.734 2.746 (2.75)

0.002 59 0.002 51 0.002 23 0.001 46 0.002 27 (0)

a ∆σ2 is the difference between Debye-Waller factors of the sampleand those of the reference sample of the Pd foil. b Immobilized Pd561phen60(OAc)180 cluster (5 wt %) on TiO2 in an acetonitrile solvent. c Immobilized Pd5(phen) on TiO2 after H2 treatment of that in a at room temperature. d Immobilized Pd5(phen) on TiO2 after reaction of that in a with cinnamyl alcohol. For reaction conditions, see Table 2. e Immobilized Pd561phen60(OAc)180 cluster on TiO2 in an acetic acid solvent.

with spent catalysts. The spent Pd5(phen)/TiO2 sample was analyzed by XAFS. The structural parameters of the Pd-Pd shell determined by the curve-fitting analysis in Table 1 showed the same CN and R values as those of the fresh Pd5(phen)/TiO2 catalyst; the Pd5(phen) cluster on the TiO2 surface was stable under the above oxidation conditions. When the H2-exposed Pd5(phen)/TiO2 was used as a catalyst in the oxidation of cinnamyl alcohol, the yield of cinnamaldehyde was drastically decreased to 20%. Vide supra, H2 treatment of Pd5(phen)/TiO2 released phen ligands to give a naked Pd cluster on TiO2 without an appreciable change of the Pd particle size. These phenomena indicate that the phen ligand on the surface of the Pd cluster might play an important role in the selective oxidation of allylic alcohols.24 In addition, Pd/carbon having Pd particles of average 20 Å diameter25 showed only 8% yield of cinnamaldehyde under the above oxidation conditions. (24) In the oxidation of cinnamyl alcohol, methyl substituents of the phen ligand strongly affected the yield of cinnamaldehyde. The following reactivities of ligands were observed: phen (94) > 2,9-dimethyl-phen (85) > 4,7-dimethyl-phen (71) > 5,6-dimethyl-phen (62) > 5-methylphen (53). The values in parentheses are the yields of cinnamaldehyde. The indicated order cannot be simply explained by pKa values of the phen compounds. The order of pKa is as follows: 4,7-dimethyl-phen > 2,9-dimethyl-phen > 5,6-dimethyl-phen > 5-methyl-phen > phen. (25) Galezzot, P.; De Mesanstourne, R.; Christidis, Y.; Mattioda, G.; Schouteeten, A. J. Catal. 1992, 133, 479.

Figure 3. HR-TEM image of Pd5(phen)/TiO2 treated with H2.

Discussion It is well-known that both oxidations of benzyl and secondary alcohols are catalyzed by Pd(II) compounds in the presence of molecular oxygen. However, the Pd(II) compounds are not excellent catalysts for the oxidation of allylic alcohols because of an irreversible coordination of allylic alcohols to divalent Pd species.12 We found that the homogeneous and heterogeneous Pd5(phen) clusters efficiently catalyzed the oxidation of allylic alcohols to give the corresponding R,β-unsaturated carbonyl compounds. To our knowledge, there are few reports on the selective oxidation of allylic alcohols into the corresponding aldehydes using palladium catalysts.12c Therefore, we will

Immobilization of Ligand-Preserved Giant Palladium Cluster

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Table 2. Oxidation of Various Allylic Alcohols with Pd561 and TiO2-Immobilized Pd561 Clusters in the Presence of Molecular Oxygena

a Substrate, 1.5 mmol; solvent, AcOH 5.0 mL; O atmosphere; 60 °C. b Homogeneous system: catalyst Pd 2 561phen60(OAc)180 0.0075 g [0.05 mmol of Pd atom]. c Heterogeneous system: Pd561phen60(OAc)180/TiO2 0.30 g [0.10mmol of Pd]. d N2 was used instead of O2. e Reuse 1. f Reuse 2. g 5 mL of benzene was used as a solvent. h A mixture of 4.5 mL of benzene and 0.5 mL of AcOH was used as a solvent.

discuss the nobel catalysis of the Pd5(phen) clusters in the oxidation of allylic alcohols by two categories of the local electronic and geometric structures of Pd atoms on the surface of the giant Pd cluster. Electronic Effect In principle, the edge position in the XAFS corresponds to the core electron binding energy of the initial electronic state.16 The giant Pd cluster samples have the same edge position as that of Pd foil within the experimental error of (1 eV, which is lower than that of a reference Pd(II) sample of the Pd oxide (PdO). It seems that the average oxidation state of the Pd atoms in the Pd5(phen) giant cluster is lower than 2 and close to zero valence. From the formula of the giant Pd cluster of Pd561phen60(OAc)180, the average oxidation number of each Pd atom is calcualte as 0.32, assuming that the negative charge of 180 OAc anions was compensated equally by all 561 Pd atoms. Because the inner part of the Pd5(phen) cluster is exclusively composed of metallic Pd (Pd0) as evidenced by the XAFS analysis, the Pd atoms at the outer surface of the Pd cluster should be responsible for their positive charges neutralizing the 180 OAc anions. It can be thought that there is a heterogeneous distribution of the oxidation state between outer surface Pd atoms and inner Pd atoms in the giant Pd cluster particle. According to the full-shell cluster model, the five-shell cluster consisting of 561 atoms has 252 atoms on the outer surface of the Pd cluster, which corresponds to 45% of the total Pd atoms.2d Thus, the oxidation state of the surface Pd atoms in the giant Pd cluster would become 0.71 on average. In the liquid-phase oxidations of alcohols using supported metal catalysts such as Pt/carbon and Pd/carbon, molecular oxygen irreversibly deactivates the catalyst surfaces to form inactive surface metal oxides.12b,25-27 Low catalytic activities of Pd/carbon or Pd/Al2O3 in our oxidation of cinnamyl alcohol might be ascribed to such an irreversible poisoning of molecular oxygen onto the metallic surface of the small Pd particles. On the contrary, in the case of the giant Pd cluster, such an oxidation of (26) Dijkgraaf, P. J. M.; Rijk, M. J. M.; Meuldijk, J.; Van Der Wiele, K. J. Catal. 1988, 112, 329. (27) Nicoletti, J. W.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 759.

Pd particles by molecular oxygen would be presumably prevented because of the slightly cationic Pd atoms on the cluster surface. This phenomenon could also be related to a high reactivity of hydride species on the Pd atoms toward molecular oxygen to give H2O. In relation to this, 3-phenyl1-propanol and 3-phenyl-1-propanal of the byproducts from the hydrogenation and isomerization of cinnamyl alcohols, which is induced by the remaining hydride species on the Pd cluster, could hardly be detected in this oxidation sytem using the Pd5(phen) cluster. Conclusively, the Pd atoms on the surface of the giant Pd cluster existed as cationic species with an oxidation state of smaller than +2, whereas the electronic state of the Pd atoms in an inner part of the giant cluster was exclusively zero valence. Such Pd cationic species on the surface of the cluster stabilized by the phen ligand and OAc anions would lead to high catalytic activity for the oxidation of allylic alcohols with molecular oxygen. Geometric Effect An idealized structure model of the giant Pd5(phen) cluster is illustrated in Figure 4, based on the literature by Moiseev et al.4 On the surface of the Pd5(phen) cluster consisting of 252 Pd atoms, the 60 phen ligands are coordinated to the 60 Pd cations probably at the edge and/ or corner of the icosahedron. There might be a low accessibility of a substrate to Pd cations neighboring at Pd species bound to a phen ligand. Therefore, the number of Pd cations attacked by substrates would be extremely limited in the surface of the Pd cluster. Here, we propose that an ensemble site of the Pd cations acts as active sites for the oxidation of allylic alcohols. That is, a multiple interaction between the Pd cations and allylic alcohols plays an important role in the selective oxidation, where a matching of the Pd-Pd shell and allylic alcohols would become a decisive factor. Vide supra, the distances between both the Pd cation and the neighboring Pd cation was 2.74 Å obtained by EXAFS and was 2.6 Å by HR-TEM, which fit well with ca 2.8 Å distance between a terminal β-carbon atoms of a CdC bond and an oxygen atom of the OH function of primary allylic alcohols,12d as visualized

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to the Pd hydride species regenerates the active Pd cation together with the formation of H2O. Low reactivity of secondary allylic alcohols might be explained by a steric crowdedness around active Pd cations on the cluster surface. Conclusions

Figure 4. Idealized structure model of a Pd561phen60(OAc)180 cluster (from ref 4). Acetate anions in the outer sphere are omitted.

Figure 5. Proposed multiple interaction of paired Pd cations and primary allylic alcohol.

in Figure 5. The importance of such a multiple interaction in the selective oxidation of allylic alcohols by Pd catalysts has recently been emphasized.12d A possible mechanism of the present oxidation of allylic alcohols could be considered as follows. First, a primary allylic alcohol is coordinated to the Pd cation through the CdC bond and dehydrogenation occurs by a neighboring Pd cation to form R,β-unsaturated aldehyde and hydride species on the Pd atom. Then, attack of molecular oxygen

On the basis of the FE-SEM, HR-TEM, and XAFS results, it was revealed that the phen ligand-preserved giant five-shell Pd cluster can be uniformly immobilized on the surface of neutral TiO2 without destruction of the structure of the precursor Pd cluster. The immobilized giant Pd clusters efficiently catalyzed the oxidative dehydrogenation of primary allylic alcohols in the presence of molecular oxygen. A nobel catalysis of the giant Pd clusters could be attributed to the pair of Pd cations with an oxidation state of smaller than +2, stabilized by phen ligands and OAc anions on the surface of the Pd cluster; the oxidation occurs via multiple interaction between the Pd-Pd paired sites and primary allylic alcohols. Both oxidation of the surface Pd atoms by an oxygen molecule and an irreversible coordination of allylic alcohols to the Pd atoms could be prevented because the giant Pd cluster consists of slightly cationic Pd species. The conventional preparation method of the metal particles such as impregnation with metal salts followed by a calcination and reduction with hydrogen at high temperatures often leads to a broad distribution of the relatively large metal particles on many metal oxide supports.28 The use of giant metal clusters as precursors of the metal particles is a novel method homogeneously to distribute metal particles on the surface of metal oxides, which gives a clue to a clear understanding of the catalysis of supported metal particles.29 Acknowledgment. This work is partly supported by the Grant-in-Aid for Scientific Research from Ministry of Education,Science,SportsandCultureofJapan(09750856). The X-ray absorption experiments have been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 97G006). The authors thank Prof. Masaharu Nomura of KEK-PF for helpful technical advice and Assoc. Prof. Tsunehiro Tanakak at Kyoto University and Dr. Hisao Yoshida at Nagoya University for their help in XAFS measurements and data reduction. We are also grateful. to the Department of Chemical Science and Engineering, Faculty of Engineering Science, Osaka University, for scientific support by “GasHydrate Analyzing System (GHAS)”. LA981720A (28) Che, M.; Bennett, C.O. In Advances in Catalysis; Eley, D. D.; Pines, H., Weiz, P. B., Eds.; Academic Press: San Diego, 1986; Vol. 36, p 55. (29) Nashner, M. S.; Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1998, 120, 8093.