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Tuning the Catalytic Activity of Cyclodextrin-Modified Palladium Nanoparticles through Host-Guest Binding Interactions Jian Liu, Julio Alvarez, Winston Ong, Esteban Roma´n, and Angel E. Kaifer* Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431 Received September 4, 2001 Water-soluble Pd nanoparticles (diameter 3.5 ( 1.0 nm) modified with covalently-attached cyclodextrin (CD) receptors were prepared by the reduction of PdCl42- in dimethylformamide solution containing perthiolated β-CD. These Pd nanoparticles behave as active catalysts for the hydrogenation of alkenes in aqueous media. For instance, the turnover frequency for the hydrogenation of 1-butenyltrimethylammonium bromide (1) was found to be 320 (moles of 1) (moles of Pd)-1 h-1 at 25 °C and 1.0 atm of H2(g). The addition of cationic ferrocene derivatives, such as ferrocenylmethyltrimethylammonium bromide (3), to the reaction medium decreases the catalytic activity of the Pd nanoparticles. Other CD substrates, such as anionic ferrocene derivatives or neutral adamantanol, are much less effective as inhibitors of the catalytic hydrogenation process.
The preparation and characterization of stable metal and semiconductor nanoparticles by solution (wet) methods is a very active field of chemical research.1 From the standpoint of catalysis, metal nanoparticles with diameters smaller than 5 nm are very attractive because of the large fraction of metal atoms that reside on their surfaces, thus affording very efficient use of the metal. The stabilization of metal nanoparticles in solution requires their partial or complete coverage with organic structures. However, to exhibit catalytic activity, metal surface sites must remain accessible to the substrate molecules. In recent and elegant work, Crooks and co-workers have prepared catalytically active metal nanoparticles encapsulated inside dendrimers.2 The dendrimer effectively stabilizes the nanoparticles, without passivating their surfaces, and its branched structure acts as a “molecular filter” imparting selectivity to the catalyst assembly. We have previously reported the surface modification of gold,3 platinum,4 and palladium4 nanoparticles with cyclodextrin (CD) receptors,5 which remain capable of binding appropriate solution guests. Our reported CD-capped Pt and Pd nanoparticles (diameter: 13-16 nm) were watersoluble and exhibited catalytic activity for the hydrogenation of allylamine.4 In this report, we focus our attention on smaller CD-capped Pd nanoparticles (diameter: ∼3 nm) and demonstrate that their activity as hydrogenation catalysts can be “tuned” by host-guest binding interactions between the surface-immobilized CD receptors and appropriate guests in the solution. The CD-capped Pd nanoparticles were prepared by BH4reduction of PdCl42- in dimethylformamide solution also containing β-SH-CD (see Chart 1). The resulting watersoluble Pd nanoparticles were characterized by transmission electron microscopy (TEM), 1H NMR, and UV-vis spectroscopic data (see Supporting Information for details). (1) Templeton, A. C.; Wuelfing, A. C.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (2) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (3) (a) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (b) Liu, J.; Ong, W.; Roma´n, E.; Lynn, M. J.; Kaifer, A. E. Langmuir 2000, 16, 3000. (4) Alvarez, J.; Liu, J.; Roma´n, E.; Kaifer, A. E. Chem. Commun. 2000, 1151. (5) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875.
Chart 1. Structures of Compounds Used in This Work
From the analysis of TEM images, their diameter was determined to be 3.5 ( 1.0 nm. Recovery and quantitation of the β-SH-CD released upon redissolution of the nanoparticles in acid media allowed us to determine that 46 ( 6% of their surface is covered by CD hosts. This is an encouraging finding as it means that a reasonable fraction of the Pd surface may be catalytically active. We tested this possibility with a simple reaction, such as alkene hydrogenation. Figure 1A shows the time evolution of the concentration of alkene 1 in D2O solution, at 25 °C, under 1 atm of H2(g) and in the presence of 8 µg/mL of β-SH-CD-capped Pd nanoparticles. The data clearly show that the reaction takes place under conditions of saturation kinetics; that is, the reaction exhibited zero order with respect to the substrate.6 Furthermore, the reaction rate increases linearly with the concentration of Pd nanoparticles (see Figure 1B), providing convincing evidence that the reaction is catalyzed on the Pd surface.7 No hydrogenation of 1 was detected in the absence of Pd nanoparticles after several hours of exposure to 1 atm of H2(g). The turnover frequency (TOF) obtained from the slope of the straight line in Figure 1A was 320 (moles of 1) (moles of Pd)-1 h-1. (6) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992; Chapter 6. (7) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385.
10.1021/la015563i CCC: $20.00 © 2001 American Chemical Society Published on Web 09/28/2001
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Figure 2. Dependence of the catalytic rate of hydrogenation of compounds 1 (b) and 2 ([) on the added concentration of 3. [Substrate]0 ) 3.0 mM, [Pd] ) 8 µg/mL, 25 °C, 1 atm H2(g). Scheme 1. Deactivation of Active Catalytic Sites by Binding of 3 to the CD Hosts on the Pd Nanoparticles
Figure 1. (A) Time evolution of the concentration of compound 1 (3.0 mM initially) in D2O, at 25 °C under 1 atm of H2(g), in the presence of CD-capped Pd nanoparticles (8 µg/mL). (B) Rate of hydrogenation of 1 as a function of the nanoparticle concentration. Table 1. Effect of Various Additives on the Rates of Hydrogenation [as TOFs in (moles of substrate) (moles of Pd)-1 h-1] in D2O at 25 °C under 1.0 atm of H2(g) and in the Presence of 8 µg/mL β-SH-CD-Capped Pd Nanoparticles entry
additive (concentration)
TOF
1 2 3 4 5 6 7
Substrate: Compound 1 (3.0 mM) none 3 (0.5 mM) 3 (3.0 mM) Me4N+Br- (3.0 mM) Et4N+Br- (3.0 mM) adamantanol (0.5 mM) 4 (0.5 mM)
320 131 112 311 290 192 230
8 9
Substrate: Compound 2 (3.0 mM) none 3 (3.0 mM)
380 312
Since the CD-capped Pd nanoparticles behave as efficient hydrogenation catalysts in aqueous media, we decided to investigate whether their catalytic activity could be modulated through binding of guests to the surfaceimmobilized CD hosts. For instance, it is well-known that ferrocene derivatives form stable inclusion complexes with β-CD.8 We have shown that ferrocene derivatives are bound to β-CD hosts covalently attached to the surfaces of gold nanoparticles.3b 1H NMR spectroscopic data (Supporting Information) verified that compound 3 also binds to the CD hosts on the surface of these Pd nanoparticles. Interestingly, addition of millimolar concentrations of 3 to the reaction mixture has a substantial effect on the rate of hydrogenation of 1 (Table 1, entries 1-3). However, addition of similar concentrations of tetramethylammonium bromide or tetraethylammonium bromide has a much smaller effect on the rate of hydrogenation of 1 (entries 3-5). Similarly, the negatively charged ferrocene derivative 4 and adamantanol (a neutral β-CD guest5) are considerably less effective than 3 as catalyst inhibitors (entries 2, 6, and 7). All these data support the idea that the two properties that combine in 3 to make it an effective inhibitor of the catalytic activity of the CD-capped Pd nanoparticles are (1) its ability to act as a guest with the CD cavities and (2) its positive charge. (8) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62 and references therein.
Further evidence for the strong correlation between inhibition by 3 and its binding ability with the CDs that decorate the Pd nanoparticles was obtained by investigating the concentration dependence of the reaction rate. The data obtained are plotted in Figure 2 (filled circles) and show the saturation behavior that is clearly associated to the binding isotherm of the ferrocene derivatives in the binding sites (CD cavities) on the surface of the nanoparticles. Figure 2 also compares the ability of compound 3 to inhibit the hydrogenation of substrates 1 and 2. Clearly, 3 is a much more effective inhibitor with 1 than with 2. This is probably due to the ferrocene group present in 2, which increases its affinity for the CD-modified nanoparticles. Thus, 3 must compete with the substrate itself (2) for the available binding sites, and its overall inhibition effect is strongly curtailed. A surprising aspect of the data shown in Table 1 is that in the absence of any additives the TOF value observed for compound 2 is slightly larger than that observed for compound 1. Compound 2 (or its reaction product) could inhibit its own hydrogenation reaction as a result of its cationic nature and the ferrocene unit present in its structure. This is clearly not the case. Perhaps, the binding and preconcentration of compound 2 on the nanoparticle surfaces compensate for the self-inhibition effect, affording a small degree of selectivity to the catalytic process. The results presented in this work clearly show that (1) β-SH-CD-capped Pd nanoparticles are active catalysts for
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the hydrogenation of water-soluble alkenes 1 and 2, (2) the catalytic activity can be substantially decreased by addition of the cationic ferrocene derivative 3, and (3) the inhibitor character of 3 is due to its ability to create Coulombic barriers for the approach of the positively charged substrates (see Scheme 1), thus decreasing the surface density of catalytic active sites. These Pd nanoparticles serve as heterogeneous catalysts while their small sizes and solubility properties approach those of homogeneous catalysts. This work affords an interesting and novel example of “tunable catalyst” design at the molecular level that relies on well-established properties of hosts covalently attached to the catalytic surface. While the specific approach followed here may or may not prove to be suitable for practical applications, our data show that it is possible to
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manipulate the surface of catalytically active metal nanoparticles with binding sites that can be used to modulate the catalytic activity on demand. Acknowledgment. The authors are grateful to the National Science Foundation for the generous support of this research work (to A.E.K., DMR-0072034). E.R. thanks the University of Miami for a Maytag graduate fellowship. Supporting Information Available: Experimental details for the preparation of β-SH-CD-capped Pd nanoparticles and characterization data (TEM and 1H NMR spectroscopic data). 1H NMR evidence for binding of ferrocene derivatives to the particle-immobilized CDs and experimental details on the kinetic measurements (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA015563I
