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Notes Cyclodextrin-Capped Palladium Nanoparticles as Catalysts for the Suzuki Reaction
Scheme 1. Idealized Representation of CD-Capped Pd Nanoparticles
Lidia Strimbu, Jian Liu, and Angel E. Kaifer* Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431 Received September 13, 2002. In Final Form: November 12, 2002
Introduction Metal nanoparticles capped or modified with monolayers of protecting organic molecules constitute a very active topic of research in chemistry.1 While these organic-metal composite systems may exhibit a range of interesting properties, their catalytic properties are starting to attract increasing attention.2 It is often stated that metal nanoparticles with diameters smaller than 5 nm offer great promise as catalysts due to the fact that the ratio of surfaceto-bulk metal atoms increases as the particle diameter decreases and, thus, a larger fraction of metal atoms would be actually utilized in smaller particles. However, several studies have shown that the dependence of catalytic activity on particle diameter is more complicated, since catalytic active sites are usually associated with specific crystallite faces or defects.3 Our group is specifically interested in the catalytic properties of Pd nanoparticles derivatized with surfaceattached perthiolated cyclodextrin (CD) receptors (Scheme 1).4,5 Ideally, the molecular recognition properties of the cyclodextrin hosts could be used to improve some properties of Pd heterogeneous catalysts, such as their stability or selectivity. We have recently shown that CD-capped Pd nanoparticles are effective catalysts for the hydrogenation of water-soluble alkenes.4,5 Furthermore, we have shown that the catalytic activity of β-CD-capped Pd nanoparticles (∼3 nm in diameter) for the hydrogenation of 1-butenyltrimethylammonium can be modulated by host-guest interactions between the surface-immobilized CDs and a cationic ferrocene derivative.5 This work illustrates one of the many possible ways in which the presence of molecular receptors covalently attached to the catalytic surface may change the properties of the (1) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. C. Acc. Chem Res. 2000, 33, 27. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27. (c) Caruso, F. Adv. Mater. 2001, 13, 11. (d) El-Sayed, M. A. Acc. Chem. Res. 2001, 74, 257. (2) (a) Li, Y.; Petroski, J. M.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 10956. (b) Pathak, S.; Greci, M. T.; Kwong, R. C.; Mercado, K.; Prakash, S. G. K.; Olah, G. A.; Thompson, M. E. Chem. Mater. 2000, 12, 1985. (c) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (d) Kralik, M.; Biffis, A. J. Mol. Catal. A 2001, 177, 113. (e) Bo¨nnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455. (3) (a) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921. (b) Le Bars, J.; Specht, U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999, 15, 7621. (4) Alvarez, J.; Liu, J.; Roma´n, E.; Kaifer, A. E. Chem. Commun. 2000, 1151. (5) Liu, J.; Alvarez, J.; Ong, W.; Roma´n, E.; Kaifer, A. E. Langmuir 2001, 17, 6762.
catalyst. We are currently exploring the potential applications of these novel catalysts to other types of chemical reactions. Suzuki cross-coupling reactions,6 in which an arylboronic acid and an aryl halide combine to yield a biaryl derivative, are usually catalyzed by soluble Pd complexes. The reaction is typically run in organic solvents in the presence of a base. Although homogeneous Pd catalysts are usually effective in this reaction, their use often brings about the typical problems associated with all homogeneous catalysts, that is, separation of the catalyst from the reaction products and catalyst recovery. Because of the synthetic importance of the Suzuki reaction, a few reports on the use of Pd nanoparticle catalysts in this reaction are available.2b,3a,7 Here, we explore the application of CD-capped Pd nanoparticles as catalysts for the Suzuki cross-coupling reaction. Experimental Section Chemicals. Perthiolated-β-CD was synthesized as reported before.8 4-Bromoacetophenone, ferrocene, 4-iodoanisole, 4-iodophenol, Na2PdCl4, phenylboronic acid, and sodium borohydride were obtained from Acros. 1-Bromo-4-nitrobenzene, tert-butyllithium, 1-iodo-4-nitrobenzene, and tributyltinchloride were purchased from Aldrich. All other chemical and solvents were obtained from either company. Iodoferrocene was prepared following a reported procedure9 with minor modifications [both the intermediate (tri-n-butylstannyl)ferrocene and iodoferrocene were purified by column chromatography on silica gel (hexane/ 1%Et3N)]. (6) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168. (7) (a) Reetz, M. T.; Breinbauer, R.; Wanninger, K. Tetrahedron Lett. 1996, 37, 4499. (b) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385. (c) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (d) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (e) Moreno-Man˜as, M.; Pleixats, R.; Villarroya, S. Organometallics 2001, 20, 4524. (f) Kogan, V.; Aizenshtat, Z.; Popovitz-Biro, R.; Neumann, R. Org. Lett. 2002, 4, 3529. (8) Rojas, M. T.; Ko¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (9) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502.
10.1021/la026550n CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002
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Notes Table 1. Results from Suzuki Cross-Coupling Reactions Catalyzed by β-CD-Capped Pd Nanoparticles entry 1 2 3 4 5 6
catalyst (mol %)
Ra
Xa
yieldb (%)
reaction time (h)
turnover freq (h-1)
1.0 1.0 1.0 2.0 1.0 2.0
NO2 OCH3 OH NO2 C(dO)CH3 C(dO)CH3
I I I Brc Brc Brc
98 95 83 77d 88 89
2 2 2 5 7 3.5
48 46 41 7.8 13 12
a See Scheme 2. b Isolated yield. c Catalyst precipitation was observed. d Determined by NMR.
Figure 1. (A) TEM image of the β-CD-capped Pd nanoparticles used in this work. (B) Size distribution obtained by measuring a sample of 100 nanoparticles from representative TEM pictures. Scheme 2. Conditions and Reactants for the Suzuki Coupling Reactions Investigated in This Work
Preparation of CD-Capped Pd Nanoparticles. The method utilized in this work has already been reported.5 The nanoparticles were fully characterized by 1H NMR and UV-vis spectroscopy and TEM. Their diameter was found to be 3.5 ( 1.0 nm (see Figure 1). Using 1H NMR spectroscopy, we determined that 46 ( 6% of the particle surface was covered by thiolated β-CD. General Procedure for Suzuki Reactions. A solution of the catalyst and the base was first prepared by dissolving 1.12 mg (1.0 mol %) of the CD-capped Pd nanoparticles and 2.4 mmol of Na2CO3 in 10 mL of H2O. This solution was deoxygenated with pure nitrogen gas for 30 min. Acetonitrile (10 mL) was added to a separate two-necked flask and deoxygenated for 30 min. At this point, 1 mmol of the arylhalide and 1.1 mmol of the arylboronic acid were added to the flask along with the aqueous solution containing the Pd catalyst and the base. The reaction mixture was refluxed for 2 h under nitrogen. After the reaction mixture was cooled, the acetonitrile was evaporated under vacuum and the products were extracted from the aqueous layer with chloroform. After solvent removal and drying, the isolated products were collected and analyzed.
Results and Discussion The Suzuki cross-coupling reactions between several aryl halides and phenylboronic acid were carried out as summarized in Scheme 2. A significant problem quickly encountered in our experiments is that the CD-capped Pd nanoparticles are very soluble in water but rather insoluble in most organic solvents. On the other hand, the reactants and products of these reactions tend to be rather insoluble in aqueous media. We compromised between these two opposing solubility requirements and used a mixed solvent (acetonitrile/H2O, 1:1 v/v) for the reactions. The results obtained in these experiments are shown in Table 1. In general terms, the reactions proceeded rather smoothly under the experimental conditions chosen. As is usually
the case, iodoaromatics were found to be more active substrates than bromoaromatics. Thus, lower yields were obtained with the latter. Furthermore, in the reactions with bromoaromatics we observed some precipitation of the Pd catalyst, probably resulting from the prolonged exposure of the nanoparticles to solvent refluxing temperatures in basic media. Also, the cross-coupling reaction was favored by the electron withdrawing character of the para substituent (R) in the aryl halide reactant, as evidenced by the data in entries 1-3 of the table. These results are encouraging and demonstrate that CD-capped Pd nanoparticles are effective as heterogeneous nanocatalysts1d in a reaction that is generally considered to require homogeneous catalysis. Although it was possible to recover and reuse the nanocatalyst, some loss of activity was noted in the limited number of experiments performed to address this issue. The turnover frequency (TOF) values were calculated as (moles of product)‚(moles of catalyst)-1‚ (reaction time in hours)-1. For the sake of simplicity in the calculations we considered that the catalyst was pure Pd, neglecting the small fraction of organic material (∼10%) on the nanoparticle surfaces. The reported TOF values are thus slightly underestimated. Our TOF values are slightly lower than those observed by El-Sayed and co-workers for the coupling reaction of iodobenzene and phenylboronic acid catalyzed by Pd nanoparticles stabilized by poly(N-vinyl-2pyrrolidine).3a This Pd nanoparticle system utilizes a polymer as the stabilizer, while the Pd nanoparticles used in this work are stabilized by covalently attached CDs, which cover and render about 50% of the surface inaccessible for catalysis. This factor and the approximations involved in our calculations of TOF values may explain the moderately lower TOF values observed in our experiments. The binding ability of the particle-immobilized CD receptors was not utilized in these experiments. In fact, the immobilized CDs simply serve a role as suitable particle stabilizers. To take advantage more fully of the presence of the CDs on the surface of the nanoparticles, we set up to run Suzuki reactions on a substrate that can form stable inclusion complexes with the surface-immobilized CD receptors. From our previous work with CDcapped metal nanoparticles, ferrocene derivatives are known to be excellent guests for the particle-attached β-CD receptors.10 Therefore, we prepared iodoferrocene to investigate its Suzuki cross-coupling reaction with phenylboronic acid to yield phenylferrocene (see Scheme 3). McCleland and co-workers have reported a modified Suzuki reaction for the preparation of arylferrocenes.11 They found that stronger bases, such as Ba(OH)2, and oxygen exclusion from the reaction medium increased the (10) Liu, J.; Alvarez, J.; Ong, W.; Roma´n, E.; Kaifer, A. E. J. Am. Chem. Soc. 2001, 123, 11148. (11) Imrie, C.; Loubser, C.; Engelbrecht, P.; McCleland, C. W. J. Chem. Soc., Perkin Trans. 1 1999, 2513.
Notes Scheme 3. Suzuki Cross-Coupling Reaction between Iodoferrocene and Phenylboronic Acid
product yields to optimum levels around 50-60%. We run the reaction of iodoferrocene with a 5-fold excess phenylboronic acid in a mixed solvent (EtOH-H2O, 2:3 v/v), using Ba(OH)2 as the base and 1 mol % CD-capped Pd nanoparticles as the catalyst. After refluxing for 5 h under nitrogen, the phenylferrocene product was purified using column chromatography to give an isolated yield of 70%. Although some precipitation of the Pd catalyst was observed also in this case, the product yield compares favorably to the data obtained by McCleland and coworkers using a homogeneous catalyst.11 To verify that binding of iodoferrocene to the CD cavities on the Pd nanoparticles is a key factor for the increased product yield observed with our catalyst, we run a control reaction with 3 equiv of ferrocene (calculated from the amount of iodoferrocene used as reactant) under otherwise identical reaction conditions. In this control reaction, the yield of phenylferrocene was considerably lower (35%), which is consistent with the anticipated competition between ferrocene and iodoferrocene for the limited CD binding sites on the nanocatalyst surface. These findings suggest that the β-CD-capped Pd nanoparticles might find specific applications in which the binding ability of the CD receptors on their surfaces plays a beneficial role in the overall catalytic efficiency.
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In conclusion, the results presented here demonstrate that CD-capped Pd nanoparticles are effective catalysts for Suzuki cross-coupling reactions between aryl halides and phenylboronic acid. The reactivity trends observed with these nanoscopic heterogeneous catalysts are similar to those found with more traditional Pd-based homogeneous catalysts. We also found experimental conditions in which the sluggish reaction between iodoferrocene and phenylboronic acid was improved by the use of CD-capped Pd nanoparticles compared to the same reaction homogeneously catalyzed by Pd molecular compounds. In general terms, the key difficulty for the general application of CD-capped Pd nanoparticles as catalysts in Suzuki cross-coupling reactions is the solubility mismatch between these hydrophilic, water-soluble nanoparticles and the more hydrophobic reactants and products that are usually involved in these reactions. Although mixed solvents constitute a reasonable compromise between these opposing solubility requirements, the formation constants of CD inclusion complexes are adversely affected by the presence of nonaqueous solvents. This effect tends to limit the possible ways in which one can take advantage of the presence of the CDs on the surface of the catalyst to enhance its selectivity. To overcome this problem, we have begun work on the synthetic modification of the cyclodextrins as well as on two-phase catalytic schemes. Acknowledgment. The authors are grateful to the NSF for the generous support of this work (to A.E.K., DMR-0072034). LA026550N
