Relative Catalytic Activities of Carbon Nanotube-Supported Metallic

Hydrogen gas was obtained from Oxarc (Spokane, WA). Multiwalled carbon nanotubes (1.0 g, 10−30 nm in diameter) with a purity of 95% were obtained fr...
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J. Phys. Chem. C 2009, 113, 1520–1525

Relative Catalytic Activities of Carbon Nanotube-Supported Metallic Nanoparticles for Room-Temperature Hydrogenation of Benzene Byunghoon Yoon, Horng-Bin Pan, and Chien M. Wai* Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844 ReceiVed: October 22, 2008; ReVised Manuscript ReceiVed: December 2, 2008

Carbon nanotube-supported metallic nanoparticles including Pt, Rh, and bimetallic Pd-Rh are effective catalysts for the hydrogenation of neat benzene at room temperature, which cannot be achieved by carbon-based Pd and Rh catalysts available commercially. The rate of the hydrogenation reaction is zero-order with respect to benzene and first-order with respect to hydrogen and the catalyst. CNT-supported Pd and Au nanoparticles show negligible activity for room-temperature hydrogenation of benzene. The bimetallic Pd-Rh/CNT nanoparticle catalyst exhibits a strong synergistic effect relative to the individual single metal nanoparticles for catalytic hydrogenation of benzene, toluene, and 1-phenyl-1-cyclohexene. Introduction Catalysis using nanoscale materials has been one of the most active research areas in recent years because of its relevance to chemical,1 pharmaceutical,2 and energy related applications.3 Recently, several excellent review articles have shown that nanocatalysts with high dispersion and narrow size distributions stabilized by appropriate supports or capping materials can work under milder conditions with higher activity and selectivity as compared to conventional heterogeneous catalysts.1,4 It is known that transition metal nanoparticles are effective catalysts, and the catalytic activity depends on their shape, size, and surface structure of the solid supports.1,4 Carbon nanotubes (CNTs) due to their unique properties and surface structures are attractive carbonaceous materials as solid supports for heterogeneous catalysts.5-8 CNT-supported metallic nanoparticles exhibit remarkably high catalytic activities for hydrogenation of aromatic compounds.5 Effective dispersion of the nanocatalysts in organic solvents is one obvious reason favoring the CNTsupported metallic nanoparticles for catalytic hydrogenation reactions. In addition, carbon nanotubes may also play a role in the hydrogenation processes. A recent report by Kim et al. shows that when hydrogen gas is in contact with multiwalled carbon nanotubes (MWCNTs) attached with Ni nanoparticles, the formation of atomic hydrogen is possible, as indicated by the presence of C-H stretching vibrations in FTIR spectra.9 Although the causes are not well understood, experimental evidence supports the fact that CNT-supported metallic nanoparticles represent a new class of recyclable and efficient catalysts for chemical syntheses particularly for catalytic hydrogenation reactions. Developing effective catalysts for hydrogenation of arenes is of considerable importance because of increasing industrial demands including production of low-aromatic diesel fuels.10 Hydrogenation of arenes is conventionally performed with heterogeneous metallic catalysts often at elevated temperatures.4e Because CNT-supported metallic nanoparticle catalysts are small in size, they can be uniformly dispersed in reaction media by mechanical stirring, resulting in enhanced catalytic efficiencies for organic reactions. This situation is similar to the homogenization of heterogeneous catalysis approach reported previ* Corresponding author. E-mail: [email protected].

ously using microemulsion-stabilized metallic nanoparticles for catalyzing chemical reactions in organic solvents.11 However, the recycling of microemulsion-stabilized catalytic metal nanoparticles is difficult to accomplish. Using CNT-supported metallic nanoparticle catalysts for organic reactions, the catalysts can be easily recovered by sedimentation or by addition of a small amount of methanol, which would rapidly remove the CNT-supported catalyst from organic solvents such as hexane or benzene to the alcohol phase. The catalysts after washing can be reused according to a previous communication.5 Catalytic hydrogenation of benzene under mild conditions is also interesting in terms of energy and environmental considerations. Benzene is a potential organic system for storage of hydrogen gas for fuel cell applications.12 Recently, Hogen et al. reported that hydrogenation of benzene in 2-propanol catalyzed by metallic Rh nanoclusters derived from [Rh(η5C5Me5)Cl2]2 could be achieved in the temperature range 50-100 °C with 50 atmospheric pressure of hydrogen.13 We have succeeded in hydrogenation of neat benzene at room temperature with 1-20 atm of hydrogen pressure using different CNTsupported metallic nanoparticle catalysts, which cannot be accomplished using commercially available carbon-based catalysts. Particularly, carbon nanotube-supported bimetallic Pd-Rh nanoparticles exhibit an unusually high activity for hydrogenation of neat benzene. The rates of the hydrogenation reaction at room temperature catalyzed by different metallic nanoparticles have been evaluated, and the relative catalytic activities of different CNT-supported metallic nanoparticle catalysts are given in this Article. Experimental Section Reagents and Synthesis of CNT-Supported Metal Nanoparticles. Sodium tetrachloropalladate(II) (Na2PdCl4), sodium tetrachloroplatinate(II) hydrate (Na2PtCl4 · xH2O), sodium borohydride (NaBH4), and sodium bis(2-hexyl)sulfosuccinate (AOT) were purchased from Sigma-Aldrich. Rhodium(III) chloride hydrate (RhCl3 · xH2O) and hydrogen tetrachloroaurate(III) hydrate (HAuCl4 · xH2O) were purchased from Strem Chemicals Inc. Other reagents including benzene, toluene, and 1-phenylcyclohexene were also purchased from Sigma-Aldrich. Hydrogen gas was obtained from Oxarc (Spokane, WA). Multiwalled carbon nanotubes (1.0 g, 10-30 nm in diameter) with a purity

10.1021/jp809366w CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

Catalytic Activities of CNT-Supported Nanoparticles

Figure 1. MWCNT-supported hybrid metal nanoparticles synthesized by water-in-hexane microemulsion method: (a) Pd-Rh/CNT, (b) Pt/ CNT, (c) Rh/CNT, (d) Pd/CNT, and (e) Au/CNT (every scale bar ) 20 nm). The nanoparticle size distribution was measured using the imaging analysis software Matrox Inspector.

of 95% were obtained from Nanostructured & Amorphous Materials Inc. and pretreated by sonication in 14 M of nitric acid for 1 h and then refluxed for 12 h in a mixture of nitric acid (50 mL, 14 M) and sulfuric acid (98%, 50 mL).14 The darkbrown suspension mixture was then diluted with distilled water to 400 mL and stirred for several hours, cooled to room temperature, and filtered. The recovered black solid was washed several times with distilled water and finally washed with ethanol. The functionalized MWCNTs were then dried in an oven at 100 °C for 4 h. No metal and organic impurities were found in the functionalized MWCNTs by energy dispersive X-ray spectrometric (EDX) analysis. For synthesizing the CNT-supported metallic nanocatalysts, the water-in-hexane microemulsion method was employed as reported previously in the literature.5,7,8 The microemulsiontemplated synthesis was carried out at room temperature at a W value (molar ratio of water to the surfactant, AOT) of 12 with 0.1 M of relevant metal ions in the aqueous phase. Hydrogen gas at a pressure of >1 atm was bubbled through the aqueous phase to reduce the metal ions in the water core of the microemulsion. The system was stirred vigorously during the reduction stage in the presence of the carboxylic acidfunctionalized MWCNTs. Metal nanoparticles formed in the water core of the microemulsion can be effectively transferred to the surfaces of the functionalized MWCNTs. After the hydrogen reduction, the CNT-supported metal nanoparticles would precipitate to the bottom of the flask without stirring. The hexane solution containing AOT was carefully removed, and the surfactant AOT could be recovered later if needed. The CNT precipitates were washed with ethanol several times and then dried in an oven for hydrogenation experiments. Characterization of the Nanoparticle Catalysts. Transmission electron microscope (TEM) measurements of the Pd-Rh, Pt, Rh, Pd, and Au nanoparticles deposited on the surfaces of the functionalized MWCNTs were carried out using a JEM1200EX with a field emission source, and the accelerating voltage was 120 kV. For the TEM sample preparation, the CNTsupported metallic nanoparticles were dispersed in ethanol with 2-3 min sonication. One drop of the suspension solution was then placed on a piece of carbon-coated Formvar copper grid purchased from Ted Pella (Redding, CA). TEM images of different CNT-supported metallic nanoparticles are shown in Figure 1. The metallic and bimetallic nanoparticles were found to be evenly distributed on the CNT surfaces. The particle size and standard deviation were measured by pixel counting at least 150 particles from enlarged TEM images using an imaging analysis software, Matrox Inspector, purchased from Matrox Electronic Systems Ltd. A short description of determining the size and standard deviation of nanoparticles using the imaging analysis software, Matrox Inspector, is described in the Supporting Information. The average particle size distributions

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1521 determined by this method are given as follows: Pd/CNT (3.4 ( 0.8 nm), Pt/CNT (3.8 ( 0.7 nm), Pd-Rh/CNT (4.6 ( 1.0 nm), Rh/CNT (5.6 ( 1.3 nm), and Au/CNT (7.2 ( 1.8 nm). X-ray photoelectron spectrometric (XPS) experiments were performed on a KRATOS Analytical AXIS-165 electron spectrometer. Monochromatic radiation from an Al KR (BE ) 1486.6 eV) X-ray source was used for excitation. The pressure of the analyzer chamber was maintained at a base pressure of 5 × 10-10 Torr during the measurement. X-ray photoelectron spectroscopy (XPS) data of the CNT-supported Pd-Rh nanoparticles showed about a 1:1 ratio of Pd:Rh content in the catalyst with 6.3 ( 0.1 wt % Pd and 6.9 ( 0.4 wt % Rh. XPS spectra of the Pd-Rh catalyst (Figure 2) revealed the presence of Pd(3d5/2) and Pd(3d3/2) peaks at 335.4 and 340.7 eV, respectively, and Rh(3d5/2) and Rh(3d3/2) peaks at 307.2 and 311.9 eV, respectively. Energy dispersive X-ray spectrometric (EDX) experiments were carried out on an AMRAY 1830, HITACHI S-2300 scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDX) results were consistent with the XPS data for the Pd:Rh content in the CNT-supported Pd-Rh nanoparticles. The X-ray diffraction (XRD) analyses were performed on a Siemens D5000 powder X-ray diffractometer with Cu KR radiation at 40 kV and 30 mA. The X-ray diffraction (XRD) pattern (Figure 2) showed a single peak for the Pd-Rh/CNTs with a 2θ value between that of the Pd (2θ ) 40.20°) and Rh (2θ ) 41.15°) single metal nanoparticles. The Au/CNT catalyst revealed the presence of Au (4f7/2) and Au (4f5/2) peaks at 84.0 and 87.8 eV (Figure 3d) of binding energies, respectively, based on the XPS data. The XRD pattern (Figure 3d) of the Au/CNT catalyst showed well-defined peaks of the Au nanoparticles with 2θ values ) 38.3°, 44.5°, 64.8°, 77.8°, consistent with the metallic Au data known in the literature.15 Other characterization data for Pt/CNT8 (Figure 3a), Rh/CNT5 (Figure 3b), and Pd/CNT8 (Figure 3c) were reported previously in the literature. The bonding of metal nanoparticles to functionalized CNTs (acid washed) is discussed by other reports in the literature.16 According to these reports, CNTsupported metal nanoparticles are bound to the surface of functionalized CNTs via chemical bonding with the carboxylate as COO(M) or the O atom of the ester-like form as C(dO)CO(M). Procedures for Catalytic Hydrogenation and Product Analysis. The experimental setup for the catalytic hydrogen of benzene and benzene derivatives is shown in the Supporting Information. A typical procedure for the hydrogenation experiments involved placing 44.76 mmol of neat benzene (or benzene derivatives) and 5 mg of CNT-supported metallic nanoparticle catalysts in a 20 mL cylindrical glass vial (2.5 cm in diameter and 6.5 cm in height) with a magnetic stirring bar. The beaker with the reactant and the catalyst was then placed in a homemade stainless steel high pressure reactor (130 mL) for hydrogenation reaction. The reaction cell was first flushed with hydrogen gas for 1 min to replace the air. The outlet valve was then closed to maintain 10 atm of hydrogen pressure in the system controlled by a large hydrogen gas reservoir connected to the reaction cell. The reaction mixture was stirred at a speed of about 600 rpm. At different time intervals after the reaction, aliquots of samples were taken from the liquid phase for proton NMR or GC/MS analyses. Products of hydrogenation of benzene and toluene were analyzed by a 1H NMR (300 MHz, CDCl3, Bruker Avance, AMX300). A GC (HP 6890)/mass spectrometer (JEOL JMSAX505HA) was used for the analysis of products of hydrogenation of 1-phenyl-1-cyclohexene with ultra high purity helium carrier gas.

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Yoon et al.

Figure 2. XPS and XRD data of CNT-supported Pd-Rh bimetallic nanoparticles. The arrows in the expanded XRD are the literature 2θ values for metallic Pd and Rh.

TABLE 1: Rate of Reaction and TOF Value for Room-Temperature Hydrogenation of Benzene Catalyzed by Different CNT-Supported Metallic Nanoparticle Catalysts

catalyst

r (mmol h-1)

TOF (h-1)

Pd-Rh/CNT Pt/CNT Rh/CNT Au/CNT Pd/CNT

1.87 ( 0.26 1.14 ( 0.37 0.99 ( 0.23 0.05 ( 0.02 0.04 ( 0.01

592.6 495.2 118.1 39 6.2

Results and Discussion Catalytic Hydrogenation of Neat Benzene at Room Temperature. The results of room-temperature hydrogenation of neat benzene catalyzed by different CNT-supported metallic nanoparticle catalysts are given in Figure 4. The commercially available carbon-based Pd or Rh catalysts do not show any catalytic activity in 24 h under our experimental conditions. If only CNTs were added to benzene with 10 atm of H2, no hydrogenation of benzene was observed at room temperature or at 50 °C. CNT-supported palladium (Pd/CNT) and gold (Au/CNT) nanoparticles show negligible catalytic activity for hydrogenation of benzene at room temperature. At a higher temperature (e.g., 50 °C), conversion of benzene to cyclohaxane (48.8% after 24 h with 10 atm H2) was observed using the Pd/CNT catalyst. Rh/CNT and Pt/CNT show good catalytic activities for hydrogenation of benzene at room temperature with 50% or higher conversions to cyclohexane in 24 h. Cyclohexane is the only product detected in the hydrogenation of benzene. Recently, Kakade et al. have also reported that Rh/CNT prepared by a

simple microwave treatment method shows a remarkable catalytic activity for hydrogenation of arenes in hexane under 20 atm of hydrogen gas and at 40 °C.17 An interesting observation in our study is that the bimetallic Pd-Rh/CNT nanoparticle catalyst (0.005 g in 3.5 g of benzene) shows a much higher catalytic activity relative to the single metallic Rh/CNT or the Pd/CNT catalyst. The bimetallic catalyst is able to convert 98% of benzene to cyclohexane after 24 h of reaction under 10 atmospheric pressure of hydrogen gas at room temperature. The turnover number is as high as 13,909 for the CNT-supported bimetallic nanoparticles catalyst. According to the results given in Figure 4, the catalytic activity of the CNT-supported metallic nanoparticles increases in the order Pd/CNT < Au/CNT < Rh/ CNT < Pt/CNT < Pd-Rh/CNT. For the CNT-supported single metallic nanoparticle catalysts, this order follows generally the same trend as the typical catalytic activities of transition metals known for hydrogenation of benzene, that is, Co < Pd < Ni < Pt < Ru < Rh.4e,18 The reason for this order is not known in the literature, but the solvent has been shown to play a role for the hydrogenation of monocyclic arene in the conventional heterogeneous catalytic system using transition metals as catalysts. The difference in enthalpy of vaporization among the transition metals has also been related to their difference in catalytic activity.4e,18 Carbon nanotube-supported bimetallic nanoparticles sometimes show enhanced catalytic activities (positive synergistic effect) according to the literature.5,19 For example, high activities of MWCNT-supported bimetallic Pt-Pd crystallites for catalytic hydrogenation of aromatic rings in the vapor phase were reported recently, and the enhanced activity was attributed to the ensemble effect of Pt48Pd25 alloy located on the outer surface of the MWCNTs.19 The conversion of benzene to cyclohexane increases linearly with time in the range shown in Figure 4, suggesting the catalytic

Catalytic Activities of CNT-Supported Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1523

Figure 3. XPS and XRD data of different CNT-supported monometallic nanoparticles: (a) Pt/CNT, (b) Rh/CNT, (c) Pd/CNT, and (d) Au/CNT.

hydrogenation process can be fitted into a zero-order reaction under our experimental conditions. Kehoe and Butt reported previously that the kinetics of hydrogenation of neat benzene

catalyzed by nickel at 60 °C followed a zero-order reaction under a constant hydrogen pressure.20 Based on the results obtained from our study, the rate of reaction (r in mmol per hour) and

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Yoon et al. TABLE 2: Rate of Reaction and TOF Value for Room-Temperature Hydrogenation of Neat Toluene and 1-Phenyl-1-cyclohexene Catalyzed by Different CNT-Supported Metallic Nanoparticle Catalysts (Hydrogen Pressure ) 10 atm)

reactant

catalyst

r (mmol h-1)

TOF (h-1)

1-phenyl-1-cyclohexene

Pd-Rh/CNT Rh/CNT Pd/CNT Pd-Rh/CNT Rh/CNT Pd/CNT

1.92 ( 0.09 1.38 ( 0.33 1.07 ( 0.06 1.40 ( 0.31 0.78 ( 0.13 N.R.

608.5 163.6 173.9 444.0 92.2 N.R.

toluene Figure 4. Rate of hydrogenation of benzene to cyclohexane catalyzed by different CNT-supported metallic nanoparticles at room temperature (20 °C) and 10 atm H2 pressure.

TOF (turnover frequency in h-1) value for different CNTsupported metal nanoparticle catalysts are given in Table 1. Because the conversion of benzene to cyclohexane is a linear function of time (zero-order reaction), the calculated TOF number is independent of the percent conversion shown in Figure 4. The TOF numbers given in Table 1 thus represent the order of relative catalytic activity of different CNT-supported metallic nanoparticles for hydrogenation of benzene at room temperature. The TOF number for conversion of benzene to cyclohexane catalyzed by the Pd-Rh bimetallic nanoparticles is about 5 times higher than that of Rh nanoparticle and nearly 2 orders of magnitude higher than that of Pd nanoparticles. To obtain the rate law of benzene hydrogenation with respect to hydrogen pressure, we used 5 mg of Pd-Rh/CNT as the catalyst at 20 °C under 1, 10, and 20 atm pressure of hydrogen gas. The observed rates of reaction were 0.25 ( 0.04, 1.87 ( 0.26, and 4.07 ( 0.41 mmol h-1 for 1, 10, and 20 atm of hydrogen gas, respectively. The results suggest a first-order reaction with respect to hydrogen gas within the pressure range of 1-20 atm. To evaluate the reaction order of the catalyst, the hydrogenation reaction was performed at 20 °C under 10 atm of hydrogen gas with 5 or 10 mg of the Pd-Rh/CNT. The rates of reaction were 1.87 ( 0.26 and 4.11 ( 0.37 mmol h-1, respectively, suggesting a first-order reaction with respect to the amount of the catalyst. Therefore, the rate law can be expressed as r ) k[C6H6]0[catalyst]1[PH2]1. Choukroun et al. used rhodium nanoparticles embedded in polyvinylpyrrolidone (PVP) as a catalyst for benzene hydrogenation under biphasic conditions (liq/liq, in 5 mL of H2O at 30 °C) and reported the same rate law.21 The activity of the recycled Pd-Rh/CNT catalyst was also tested. The activity of the catalyst remained virtually unchanged after six times of recycling and repeated use. TEM micrographs of the recycled Pd-Rh/CNT catalysts showed little change of the particle density on the CNT surfaces, and no observable detachment or aggregation of Pd-Rh nanoparticles during the catalytic reaction was observed after six times of repeated use. This result shows the Pd-Rh/CNT catalyst is able to preserve its remarkable catalytic activity during multiple cycles of hydrogenation. Catalytic Hydrogenation of Toluene and 1-Phenyl-1cyclohexene. To test the activity of the bimetallic Pd-Rh/CNT catalyst for substituted benzene derivatives, room-temperature

hydrogenation of neat toluene and neat 1-phenyl-1-cyclohexene (25.3 mmol) was also studied, and the results are given in Table 2. The procedure for the toluene and the 1-phenyl-1-cyclohexene hydrogenation experiments is similar to that described above for the benzene experiments. According to our results, hydrogenation of these two benzene derivatives also follows zeroorder kinetics at a fixed hydrogen pressure and a given amount of the catalyst. Hydrogenation of neat toluene to methylcyclohexane occurs at room temperature, but the rate is slower than the hydrogenation of neat benzene catalyzed by the same Pd-Rh/CNT catalyst (rbenzene:rtoluene ) 1:0.75). When a cyclohexene group is attached to the benzene ring as in the case of 1-phenyl-1-cyclohexene, catalytic hydrogenation occurs only at the cyclohexene ring, leading to the formation of phenylcyclohexane at room temperature. The absence of cyclohexylcyclohexane at room temperature (20 °C) in this system was confirmed by proton NMR and GC/MS (see Supporting Information). At a higher temperature, for example, 60 °C, a small amount of cyclohexylcyclohexane (about 2.3%) was observed using the Pd-Rh/CNT catalyst in 15 h. This phenomenon of protecting the aromatic ring during catalytic hydrogenation of benzene derivatives with unsaturated functional groups has been reported previously. For example, under mild conditions (20-60 °C, e10 atm H2), metal nanoparticlescatalyzed hydrogenation of styrene,21,22 phenylacetylene,21 trans-stilbene,11,23 4-methoxycinnamic acid,23 and methyl transcinnamate11 occurs primarily to the unsaturated functional groups, and the benzene ring remains basically untouched. Recently, Leger et al.22 reported that no hydrogenation of the benzene ring was observed in the catalytic hydrogenation of styrene with rhodium nanoparticles in ionic liquids under mild conditions (20 °C, Pt > Rh > Au > Pd. The Pd-Rh bimetallic nanoparticle catalyst shows a strong positive synergistic effect relative to the individual single metal nanoparticle catalysts for the hydrogenation reactions described in this Article. It is conceivable that other CNT-supported bimetallic or multimetallic nanoparticles could also enhance catalytic activities of the nanoparticle catalysts for various chemical reactions. The microemulsion technique utilized in this study for synthesizing the catalysts appears to provide an effective nanoreactor for preparing bimetallic or multimetallic nanoparticles for catalysis applications. Acknowledgment. This work was supported by a grant from DOD-AFOSR (FA9550-06-1-0526). Supporting Information Available: Experimental apparatus for catalytic hydrogenation of benzene; procedure for determining the particle size and standard deviation; NMR spectra of hydrogenation of benzene, toluene, and 1-phenylcyclohexene; GC/MS, but with an increase of the hydrogenation pressure up to 30 bar for the hydrogenation of 1-phenylcyclohexene at 20 and 60 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (c) Richards, R. Surface and Nanomolecular Catalysis; CRC/Taylor & Francis: Boca Raton, FL, 2006. (2) Heitbaum, M.; Glorius, F.; Escher, I. Angew. Chem., Int. Ed. 2006, 45, 4732. (3) (a) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (b) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220. (4) (a) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (b) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (c) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455. (d) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. Acc. Chem. Res. 2003, 36, 20. (e) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A 2003, 191, 187.

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