X-ray Photoelectron Spectroscopy and the Auger Parameter As Tools

Jan 12, 2009 - Stephanie MacQuarrie , Bendaoud Nohair , J. Hugh Horton , Serge Kaliaguine and Cathleen M. Crudden. The Journal of Physical Chemistry C...
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J. Phys. Chem. C 2009, 113, 1901–1907

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X-ray Photoelectron Spectroscopy and the Auger Parameter As Tools for Characterization of Silica-Supported Pd Catalysts for the Suzuki-Miyaura Reaction Kevin McEleney, Cathleen M. Crudden, and J. Hugh Horton* Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6 ReceiVed: October 6, 2008; ReVised Manuscript ReceiVed: December 3, 2008

Palladium has been immobilized on thiol-modified mesoporous and amorphous silicates and used as a catalyst for the Suzuki-Miyaura reaction. Heterogeneous catalysts are difficult to characterize due to the difficulty of applying traditional solution state characterization methods like NMR to solid samples. As such, the catalysts are often poorly understood. In an effort to better understand the oxidation state and electronic environment of both the reactive palladium center and the thiol ligands through which it is bound in the catalyst, we characterized a wide range of both Suzuki-Miyaura catalysts and Pd compounds by both X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger spectroscopy (XAES) focusing on Pd, S, and Si. The use of XAES data allows determination of the Auger parameter for Pd and S, which provides information not only on oxidation state but also on the polarizability of the surrounding ligands in the catalyst. Changes in XPS binding energy and XAES kinetic energy are discussed particularly in terms of the nature of the reactive Pd center before and after use, and the effects on the S ligand as a function of Pd loading. 1. Introduction The use of palladium catalysts for formation of C-C and C-X bonds has become commonplace in organic chemistry.1 This is due to the mild conditions and functional group tolerance of this process and to the simplicity of the process relative to conventional organic methods. Although soluble palladium complexes are commonly utilized as catalysts, the palladium is often retained in the product as an unwanted contaminant, even after attempted purification.2 Converting a homogeneous complex to a heterogeneous one offers several advantages, including easier recovery of the catalyst, recyclability and the capability to be used in continuous flow or batch reactors.3 A variety of strategies have been used to convert homogeneous catalysts to heterogeneous materials, the most straightforward of these being adsorption of the metal onto a solid support, such as the ubiquitous palladium on carbon.4 However, this method relies on the physisorption to keep the metal bound to the solid and often results in a large amount of leached metal contaminating the product. In addition, detailed studies have shown that such catalysts generally operate by a homogeneous mechanism, via dissolved Pd.4,5 A second strategy is to covalently attach a well-defined ligand to the surface and then bind a metal to it.6 In this vein, typical ligands for transition metal catalysis including phosphines, carbenes, amines, sulfides, thiols, and pyridines have been immobilized on inorganic supports.7 Although potentially more effective than simple adsorption, this method requires the preparation of a bifunctional ligand with remote functionality for attachment to the inorganic support, greatly increasing the cost and difficulty of preparing the supported catalyst. To overcome this issue, simple commercially available organosilanes, such as mercaptopropyltrimethoxysilane, have been used in place of traditional transition metal ligands as described herein.8-10 Mesoporous silicates have attracted much attention as potential supports for such heterogenized catalysts with many * To whom correspondence should be addressed. Tel.: (613)-533-2379. Fax: (613)-533-6669. E-mail: [email protected].

groups having described supported palladium catalysts on silicates.8-15 In addition to catalysts prepared by directly adsorbing Pd to mesoporous supports by the Ying group and others,14 the groups of Shimizu,9 Davis,8 and Crudden10 have described thiol-functionalized mesoporous silicates as supports for Pd catalysts. These materials have been shown to be catalytically active for the Suzuki-Miyaura and Mizoroki-Heck reactions demonstrating low levels of leached palladium following use. Davis8 and Crudden10 showed that leached Pd species are catalytically active, although high levels of recapture after reaction renders these species potentially interesting catalysts for industrial purposes where minimizing Pd contamination is critial.2 Interestingly, catalytic activity is largely retained after recovery of the supported Pd species, although in some cases material degredation by aqueous base resulted in complete loss of activity.16 Additionally, for catalysts based on amorphous materials, a slow loss of activity was observed that could not be related to a degredation of material structure or entrapment of catalytically active Pd.10c This loss of activity is likely related to aglomeration of the Pd into larger particles by an Ostwaldripening-type process.9,10c Despite these extensive studies, the oxidation state of the catalyst before and after use and the nature of the palladiumsulfur interaction within the catalyst are not fully understood. EXAFS spectroscopy has been employed to obtain information about changes in oxidation state after catalysis. In this detailed study, Shimizu et al. saw evidence of a Pd-S bond in the FSMSH-Pd catalyst and demonstrated varying amounts of reduction of palladium (II) to palladium nanoparticles in the used catalysts.9 Palladium-based coupling catalysts have also been studied using X-ray photoelectron spectroscopy (XPS).17-21 A general trend from these studies is that catalysts which have Pd 3d5/2 spectra of lower binding energy, and hence presumably lower Pd oxidation state, tend to have higher activity. For example, Paul and Clark18 studied a series of eight different N-N, N-O, and N-S chelating pyridine-type ligands in homogeneous catalysts and found binding energies ranging from

10.1021/jp808837k CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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McEleney et al.

333.45 to 336.35 eV. They ascribed the increased activity of the low binding energy Pd species (most of which also tended to have higher co-ordination numbers around the Pd center) to a larger electron density about the Pd, facilitating oxidative addition. Pd nanoparticle catalysts on polyaniline supports17,20 and thiol Pd catalysts supported on mesoporous silicate MCM-4119 were both found to be more active in the reduced form, as expected based on the mechanism. However, based on XANES studies, Shimizu et al. proposed that the oxidation state of Pd is primarily +2 in thiol-modified FSM-based Pd catalysts, with some Pd(0) observed.9 Larger amounts of Pd(0) were observed on thiol-modified silica compared with the mesoporous analog, and significant deposits of Pd(0) nanoparticles were observed in unfunctionalized supports.9 While XPS of the Pd 3d5/2 state alone may provide valuable information on the oxidation state of the reactive Pd center, there is much less information in the literature on the characterization of the support and ligand portions of the catalysts using photoelectron spectroscopy. The binding energy of any XPS line alone is also not a definitive indication of the oxidation state of an element. For example, Pd foils are often studied as they are used as catalysts for methane oxidation.22 Prins et al.23 reported XPS data on foils, films, and nanostructured palladium on silicon oxide and found that as the nanodots or nanopits of palladium became smaller the binding energy of Pd increased relative to that of the foil. They attributed this not to a change in oxidation state but to a valence electronic effect due to the isolation of Pd nanoparticles on an insulating surface. Herein, we report a complete study of Pd catalysts immobilized on thiol-functionalized silica supports of various types using X-ray induced Auger spectroscopy (XAES) along with XPS. Combining the XPS and XAES data into a twodimensional chemical state plot (Wagner plot) yields important information about the chemical environment in which an element resides than either technique alone. Summing the XPS binding energy (BE) and XAES kinetic energy (KE) together provides the Auger parameter, R′24

R′ ) KE(Auger) + BE(photoelectron)

(1)

The Auger parameter has been shown to be a measure of the extra-atomic relaxation or screening effect of the surrounding medium on the final ion state. When the Auger transition involves only core levels, the change in Auger parameter is equal to twice the change in extra-atomic relaxation, Rea25

∆R′ ) 2∆Rea

(2)

This is particularly helpful in interpreting binding energy shifts between samples because binding energy shifts between two species, ∆EB, depends not only on the difference in groundstate energy between two species, ∆ε, but also on the extraatomic relaxation26

∆EB ) ∆ε - ∆Rea

(3)

Differences in oxidation state control the value of ∆ε and often result in “diagnostic” binding energy ranges associated with a particular oxidation state of a given element. Nonetheless, the extra-atomic relaxation term can be significant and thus the polarizability of the surrounding chemical species can have a strong, and sometimes counterintuitive, effect on the observed binding energy shift if only oxidation state is considered. However, in systems where the Auger transition involves the valence band, such as here with Pd, the assumptions underlying eq 2 break down. Nonetheless, the Auger parameter is still a useful qualitative measure of the polarizability of the medium

in which the core hole is located and an indicator of the extent to which the true oxidation state of Pd differs between samples. In this paper, we present a comprehensive study of a series of silicate-supported palladium catalysts used in both the Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. The electronic structure of both the reactive Pd center and the S portion of the thiol ligand by which it is tethered to the silica support are studied using XPS and XAES. Using Wagner plots of binding energy vs Auger kinetic energy, we compared these data to those from a series of standard compounds containing both Pd and S, as well as compendia of XPS and Auger parameter data on these elements previously published in the literature. We show that combined XAES and XPS data, especially when compared to values from a wide range of Pd and S compounds, are effective in ascertaining the electronic structure of both the Pd and S sites in the catalyst as a function of Pd loading and catalyst activity. 2. Experimental Section Reagents were purchased or obtained from the following suppliers. Pluronic 123 (P123) was donated by BASF. Mercaptopropyl functionalized amorphous silica, tetraethylorthosilicate (TEOS), mercaptopropyl trimethoxysilane (MPTMS), bis(dibenzylideneacetone) palladium, palladium sulfide, and DMF were obtained from Aldrich. Palladium acetate was obtained from Pressure Chemical Company. Tetrakis(triphenylphosphino) palladium (0) was obtained from Strem. In a typical procedure for synthesis of SBA-15,27 4.0 g of the triblock copolymer P123 was dissolved in 150 mL of water and 20 mL of conc HCl and stirred at 40 °C for 16 h. A total of 9 mL of TEOS was then added and stirred for 24 h at 40 °C. The resulting powder was then hydrothermally treated at 80 °C for 48 h under static conditions. The solid was recovered by filtration and washed with water. The surfactant P123 was then removed by Soxhlet extraction with ethanol for 5 days. The solid was then filtered and dried under vacuum. To prepare grafted SBA-15-SH(g), 900 mg of SBA-15 was suspended in dry toluene (50 mL) in a flask flushed with argon. Then, 3.6 mL of MPTMS was added via syringe and the mixture refluxed for 18 h. The powder was recovered by filtration and washed with three portions of ethyl ether. The recovered powder was then washed via Soxhlet extraction with ethanol for 24 h. To prepare co-condensed (cc) catalyst SBA-15-SH(cc), 4.0 g of triblockcopolymer P123 was dissolved in 150 mL of water and 20 mL of conc HCl and stirred at 40 °C for 16 h. Then, 8.6 mL of TEOS was added and stirred for 5 h at 40 °C. Finally, 0.5 mL of MPTMS was added and the reaction mixture stirred for 19 h at 40 °C. The resulting powder was then hydrothermally treated at 80 °C for 48 h with no stirring. The solid was recovered by filtration and washed with water. The surfactant P123 was then removed by Soxhlet extraction with ethanol for 5 days. The solid was then filtered and dried under vacuum. Aluminum-doped SBA-1516 was made according to the procedure of Li et al.28 In a typical procedure, 4.0 g of P123 surfactant was dispersed in water. Once a clear solution was obtained, the solution was rendered acidic, and 5.0 g of NaCl was added. After a further 16 h of stirring at room temperature, TEOS was added dropwise to the reaction solution. The silica was allowed to condense partially for 6 h before addition of 0.15 g of NaAlO2. Condensation of the silica about the surfactant proceeded for another 18 h before the as synthesized material was hydrothermally treated at 100 °C for 48 h under static conditions. Removal of the surfactant was achieved by calcining the material at 550 °C for 6 h in air, yielding a highly ordered

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J. Phys. Chem. C, Vol. 113, No. 5, 2009 1903

Figure 1. Representative XPS and Auger specra for C, Pd, S, and Si collected from an SBA-15-SH(g)-Pd sample before use. (a) C 1s, (b) Si 2p, (c) Pd 3d, (d) Pd MNN, (e) S 2p, and (f) S KLL.

mesoporous aluminosilicate (Al-SBA-15). Grafting of the thiol functionality to the surface of the material was achieved by adding 4 mL of mercaptopropyl trimethoxysilane, MPTMS, to a dispersion of 0.85 g of Al-SBA-15 in toluene, and heating to reflux for 18 h. To dope the above supports with palladium, Pd(OAc)2 corresponding to 50% of the sulfur loading determined by elemental analysis was first dissolved in 75 mL of THF. This was stirred under an argon atmosphere for 15 min to ensure complete dissolution of the palladium species. The SBA-15 or amorphous silica source was then added to the solution and stirred under argon for 1 h at room temperature. The catalyst was recovered using a sintered glass funnel, scraped into a vial, and dried overnight under high vacuum. The filtrate was collected and sent for ICPMS analysis to determine palladium content. The residual palladium was found to be 166.5 eV.29 The Pd-containg thiolmodified molecular seives described herein have S 2p binding energies between 162.7 and 163.9 eV, placing them into a range between that observed for S0 and S4+ compounds. Functionalized amorphous SiO2 in which no Pd was present, has a sulfur binding energy of 164.6 eV, which is significantly higher than materials containing Pd. It should be noted that there is some disparity in the literature on the placement of the sulfur oxidation states along the binding energy axis. Unlike Yu et al.,29 Bensebaa et al.30 place alkyl thiols (S0) between 163.5 and 163.8 eV, similar in binding energy to our sulfur supported palladium catalysts. They also define thiolates chemisorbed on metals (S-) at 161.8 to 162.6 eV and oxidized sulfur at binding energies >167 eV. No corresponding Auger data was given for these groupings, so these ranges are noted as black bars along the binding energy axis in Figure 4. An expansion of the Wagner plot in Figure 4 for the binding energy region corresponding to our catalysts is shown in Figure 5. As with the Pd data, this plot shows that there is a clear differentiation of the sulfur 2p binding energy data between the catalysts before and after use with pristine catalysts having binding energies above 163.5 eV and used catalysts below this value. The Auger parameters for both pristine and used catalysts lay between 2275.7 and 2277.4 eV and did not show a clear differentiation between the two types. Other Elements. We recorded the Si 2p XP peak for the silicate materials studied (see the Supporting Information for complete tabular information). The as-synthesized catalysts had binding energies between 103.6 and 103.8 eV with the exception of an SBA-15-SH catalyst prepared by co-condensation of the thiol and silicate precursor Si(OEt)4, which had a lower binding energy of 103.1 eV. Thsee values are similar to those in the literature for SiO2 (103.2 to 103.9 eV).26,31 After use, the catalysts demonstrated lower Si 2p binding energies of 103.1 to 103.3 eV. The only other element observed in the catalysts, besides carbon, oxygen and those noted above, was potassium. The K

Suzuki-Miyaura Reaction

Figure 4. Wagner chemical state plot for S 2p XPS and S KLL XAES data. Our data are represented by open shapes. Data collected from the literature is represented by closed shapes. Specific compounds discussed in detail in the text are labeled on the figure. The region denoted in the dashed lines is expanded in Figure 3. The binding energy ranges for various sulfur chemical states given in ref 21 are shown as lines above the x axis.

Figure 5. Expansion of the Wagner chemical state plot for S in the binding energy region of our catalysts. Specific compounds discussed in detail in the text are labeled on the figure.

1s peak was observed near the C 1s spectral window at 293.5 eV and was only observed in the used catalysts and not in any other samples. The potassium likely arises from the base utilized in the Suzuki-Miyaura couplings, potassium carbonate. 4. Discussion Prior to use, the catalysts studied herein have palladium 3d5/2 binding energies in the range generally associated with Pd (II) species, but clearly lie in a different region of the Wagner plot than Pd(OAc)2, with a significantly lower binding energy and similar Auger kinetic energy. This observation provides strong evidence that the Pd(OAc)2 is not merely adsorbed onto the catalyst but that the ligand environment around palladium has changed. Furthermore, the Auger kinetic energy and Pd 3d5/2 binding energy for PdS lie in between the values observed for the catalysts, suggesting that some degree of Pd-S bonding takes place. However, the lack of any systematic pattern in binding energy or Auger parameter upon changing the nature of the silicate support indicates that this part of the catalyst has little effect on the local electronic structure of Pd and also

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1905 supports the notion that it is the binding to the thiol ligand which controls the adsorption of Pd into the catalyst. Figure 3 clearly indicates that following their application through one or more cycles of the Suzuki-Miyaura reaction, all but one of the catalysts cluster into a separate group from the as-synthesized catalysts, indicating the palladium species has changed. The binding energy shift is to lower energy, consistent with reduction of the Pd center. Based on the most commonly accepted Suzuki-Miyaura reaction mechanism, Pd is only active in the Pd(0) oxidation state.32 We have previously reported the observation of structures that appear to be Pd nanoparticles in TEM images of silicates after use in the Suzuki-Miyaura reaction.10 Metallic Pd and various Pd alloys lie in the 334.5 to 335.5 eV binding energy range, considerably lower than those observed here for the used catalysts. The relatively diffuse electronic structure of a metal allows the charge generated by the core-electron loss to be shared over a relatively large number of atoms. This extra-atomic relaxation stabilizes the core-hole and leads to a lowering of binding energy. In nanoparticles, the relatively compact ligand environment means this extra atomic relaxation is reduced and the binding energy of the metal species increases. For example, increases in binding energy of up to 1.6 eV have been observed for small metal clusters relative to bulk metal.33 Since there is no effort to control exposure to air, it is possible that the catalysts have partially oxidized prior to measurement on XPS. However, given the 0.6 eV higher binding energy measured for palladium oxide and a lack of observable sulfur oxidation in the XP spectra, it is unlikely that large quantities of the palladium is oxidized and rather we are observing an extra-atomic relaxation effect. Furthermore, analysis of palladium on carbon and palladium powder reveal a chemical state of palladium that is halfway between our catalysts and palladium metal (335.6 and 335.7 eV, respectively). Previous TEM data on the silica-supported catalysts studied herein indicates that the used catalysts display nanoparticles on the order of 5-7 nm in diameter, or about the same as the diamater of the pores.10 By contrast, Pd on carbon and Pd powder catalysts are known to contain larger particles between 10 and 20 nm in diameter, as there is no pore structure to constrain nanoparticle growth.4 These larger palladium nanoparticles presumably have stronger extra-atomic relaxation contributions that make them behave more like the bulk metal.23 The second palladium state observed in these materials is a high binding energy state with binding energy and Auger kinetic energy lying in the same region as the PdO sample, and presumably arises from a palladium oxide-like state resulting from the air oxidation of the surface of the nanoparticles. This is a commonly observed contaminant on commerical Pd/C and other commercial Pd sources,34 and can be the major species in some cases.9 Interestingly, there is very little XPS and XAES data in the literature for Pd(0) species, other than metallic Pd or various Pd alloys noted above. A reasonable comparison might be to what are formally Pd (0) complexes. In order to achieve this, we examined tetrakis(triphenylphosphine) palladium (Pd(PPh3)4) and bis(dibenzylideneacetone) palladium (Pd(dba)2). Our measurements show that these species have binding energies only slightly lower than those observed for the reacted catalysts. Since both of these compounds are susceptible to oxidation in air, considerable care was taken in order to minimize any oxidation of these samples during transfer to the XP spectrometer system; solid state NMR of Pd(PPh3)4 was also carried out and a single 31 P chemical shift at 14.1 ppm was observed, consistent with an intact phosphine ligand and nonoxidized Pd(0) species.

1906 J. Phys. Chem. C, Vol. 113, No. 5, 2009 Importantly, no free triphenylphosphine was observed in the solid state spectrum. It should also be noted that both Pd(PPh3)4 and Pd(dba)2 have very different binding energies and Auger parameters compared to PdO, which we also measured (Figure 3). Furthermore, the shift observed in the used catalysts as compared to the as-synthesized ones is to lower binding energy and higher Auger kinetic energy in all cases, but the Auger parameter is roughly constant. As discussed in the introduction section, this suggests that the extra-atomic relaxation contribution to the binding energy shift is roughly unchanged, indicating that the local environment around the Pd species also remains unchanged, but that the oxidation state of the Pd has decreased. These observations then are most consistent with reduction of Pd (II) in the unused catalyst to a Pd (0) species, probably in the form of nanoparticles constrained in diameter by the pore size (167 eV) peaks were observed. Finally, we also examined the Si 2p binding energy to assess any changes in the silicate, expecting that this peak would remain unchanged over our catalytic manipulations. The Si 2p binding energy for the as-synthesized catalysts was consistently higher (103.8 eV) than that found for the used catalysts (103.3 eV). This difference may be due to the presence of potassium in the used catalysts and its absence in the unused catalysts. As potassium carbonate is used as the base for the Suzuki-Miyaura reaction in a 20:1 DMF:water solvent, it is reasonable to assume that potassium hydroxide is formed during the reaction. Potassium hydroxide is known to restructure silicates by converting bridging oxygen Si-O-Si bonds to nonbridging species Si-O-X (X ) K or H).37 The overall effect of converting bridging to nonbridging oxygen bonds is to increase the electron density at silicon and thus decrease the binding energy of the silicon atoms. The relatively minor shift in Si 2p binding energy is likely due to low potassium incorporation into the materials. 5. Conclusions We have described the use of XPS and related XAES techniques as a tool for the characterization of heterogeneous catalysts. Here, we focus on catalysts for the industrially relevant Suzuki-Miyaura cross-coupling reaction. In particular, we studied heterogeneous catalysts in which Pd is coordinated to a thiol ligand on an amorphous or mesoporous silica support. By using both XPS and XAES to determine Auger parameters for various elements in the catalysts, we can determine information on not only the oxidation state of the catalytic metal center and the ligands to which it is coordinated in the catalyst, but also on the polarizability of the environment in which these species are located, more than can be determined from XPS binding energies alone. The differentiation of oxidation state by binding energy alone is here shown to be potentially misleading as to the true electronic state of the active catalyst species. Effects such as extra-atomic relaxation, core-hole screening and polarization need to be considered. As such, species in the bulk metallic form can have quite different binding energies than monatomic species of the same oxidation state. We have shown that a change in chemical environment for both palladium and sulfur occurs upon addition of palladium to the thiol-functionalized silicate. Additionally, further changes to the electronic environment after the catalyst has been used in a Suzuki-Miyaura coupling reaction, imply reduction of the reactive Pd center and a subsequent small increase in electron density around the S ligand.

Suzuki-Miyaura Reaction Acknowledgment. The Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged for financial support of this research in terms of operating grants to J.H.H. and C.M.C. K.M. also acknowledges NSERC for support in terms of PGSM and PGSD scholarship, and Queen’s University for support in terms of a Queen’s Graduate Award. The Canada Foundation for Innovation and the Ontario Innovation Trust are acknowledged for infrastructure support to C.M.C. Supporting Information Available: Numerical tables of XPS binding energy, XAES kinetic energies, and Auger parameters corresponding to Figures 2-5. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-I., de Meijere, A., Eds.; Wiley-VCH: New York, 2002. (b) Metal-Catalyzed Cross Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998. (2) (a) Garrett, C. E.; Prasad, K. AdV. Synth. Catal. 2004, 346, 889. (b) Ko¨nigsberger, K.; Chen, G. P.; Wu, R. R.; Girgis, M. J.; Prasad, K.; Repic, O.; Blacklock, T. J. Org. Proc. Res. DeVel. 2003, 7, 733. (3) (a) Jas, G.; Kirschning, A. Chem. Eur. J. 2003, 9, 5708. (b) Shore, G.; Morin, S.; Organ, M. G. Angew. Chem., Int. Ed. 2006, 45, 2761. (c) Nikbin, N.; Ladlow, M.; Ley, S. V. Org. Proc. Res. DeVel. 2007, 11, 458. (d) Greenway, G. M.; Haswell, S. J.; Morgan, D. O.; Skelton, V.; Styring, P. Sens. Actuators B 2000, 63, 153. (4) Yin, L.; Liebscher, J. Chem. ReV. 2007, 107, 133. (5) (a) Kohler, K.; Kleist, W.; Prockl, S. S. Inorg. Chem. 2007, 46, 1876. (b) Zhao, F. Y.; Shirai, M.; Ikushima, Y.; Arai, M. J. Mol. Catal. A. Chem. 2002, 180, 211. (bb) Dams, M.; Drijkoningen, L.; Pauwels, B.; Van Tendeloo, G.; De Vos, D. E.; Jacobs, P. A. J. Catal. 2002, 209, 225. (c) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. AdV. Synth. Catal. 2006, 348, 609. (6) (a) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. ReV. 2002, 102, 3615. (b) Weck, M.; Jones, C. W. Inorg. Chem. 2007, 46, 1865. (7) (a) Fan, Q.-H.; Li, Y.-M.; Chan, A. S. C. Chem. ReV. 2002, 102, 3385. (b) Leadbeater, N. E.; Marco, M. Chem. ReV. 2002, 102, 3217. (8) Ji, Y.; Jain, S.; Davis, R. J. J. Phys. Chem. B 2005, 109, 17232. (9) Shimizu, K.; Koizumi, S.; Hatamachi, T.; Yoshida, H.; Komai, S.; Kodama, T.; Kityama, T. J. Catal. 2004, 228, 141. (10) (a) Crudden, C. M.; Sateesh, M.; Lewis, R. J. Am. Chem. Soc. 2005, 127, 10045. (b) Crudden, C. M.; McEleney, K.; MacQuarrie, S. L.; Blanc, A.; Sateesh, M.; Webb, J. D. Pure Appl. Chem. 2007, 79, 247. (c) Webb, J. D.; MacQuarrie, S.; McEleney, K.; Crudden, C. M. J. Catal. 2007, 252, 97. (d) Nohair, B.; MacQuarrie, S; Crudden, C. M.; Kaliaguine, S. J. Phys. Chem. C 2008, 112, 6065. (11) Thomas, J. M.; Raja, R. J. Organomet. Chem. 2004, 689, 4110.

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