Langmuir 2004, 20, 11293-11296
11293
Colloidal Gold Nanoparticles as Catalyst for Carbon-Carbon Bond Formation: Application to Aerobic Homocoupling of Phenylboronic Acid in Water Hironori Tsunoyama,† Hidehiro Sakurai,† Nobuyuki Ichikuni,‡ Yuichi Negishi,†,§ and Tatsuya Tsukuda*,†,§,| Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, Department of Materials Technology, Faculty of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan, Department of Photoscience, School of Advanced Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan, and CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 3320012, Japan Received August 31, 2004. In Final Form: October 12, 2004 Gold nanoparticles (N-CdO sites are coordinated to the NP surface. Further structural investigation was conducted only for Au:PVP(K-30) as a representative of the NP samples. The UV-vis absorption spectrum (Figure 2a) exhibits a small peak at ∼2.5 eV (∼500 nm), which is appreciably blue shifted from the well-known surface plasmon (SP) (7) See the Supporting Information. (8) (a) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (b) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596. (c) Han, M. Y.; Quek, C. H.; Huang, W.; Chew, C. H.; Gan, L. M. Chem. Mater. 1999, 11, 1144. (d) Henglein, A. Langmuir 1999, 15, 6738. (e) Porta, F.; Prati, L.; Rossi, M.; Coluccia, S.; Martra, G. Catal. Today 2000, 61, 165. (f) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 2002, 18, 2888. (g) Carotenuto, G.; Nicolais, L. J. Mater. Chem. 2003, 13, 1038. (h) Tan, Y.; Dai, X.; Li, Y.; Zhu, D. J. Mater. Chem. 2003, 13, 1069.
Figure 1. Typical TEM images and particle size distributions of (a) Au:PVP(K-15), (b) Au:PVP(K-30), and (c) Au:PVP(K-90).
band of Au NPs (> 2 nm) located at ∼2.3 eV.7,9 The blue shift is ascribed to the reduction of particle size down to the sub-2-nm region, based on the observation made by Whetten and co-workers in their systematic study on optical properties of alkanethiolate-protected Au (Au:SR) NPs.9 The SP band appears in the 1.3-nm Au:PVP NPs, whereas it is absent in the Au:SR NPs smaller than ∼2 nm.9 Such difference in the SP band intensity is probably due to that in chemical interaction between the protective agent and the particle surface. The XRD pattern (Figure 2b) reveals a broad (111) peak associated with small facecentered cubic (fcc) gold crystals.10 The density of the d-electrons at the Fermi level was probed by XANES.11 As shown in the Au L3-edge XANES spectrum (Figure 2c), the intensity of the whiteline is smaller than that of bulk gold, suggesting the Au NPs gain 5d electrons through the interaction with PVP. The Au-Au coordination number and the Au-Au bond distance are determined to be 5.3 ( 1.0 and 2.79 Å, respectively, from the EXAFS analysis (Figure 2d).7 The Au-Au bond in Au:PVP(K-30) (9) (a) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (b) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. C.; First, P. N.; Gutie´rrez-Wing, C.; Ascensio, J.; JoseYacama´n, M. J. J. Phys. Chem. B 1997, 101, 7885. (10) The average size of the gold crystallites for Au:PVP(K-30) is estimated to be ∼1.7 nm by using Scherrer’s equation. (11) Zhang, P.; Sham, T. K. Appl. Phys. Lett. 2002, 81, 736.
Letters
Langmuir, Vol. 20, No. 26, 2004 11295 Table 1. Homocoupling of Phenylboronic Acid in Water Catalyzed by Au:PVP NPsa
yield (%)b entry
catalyst
1c
1 2 3 4 5 6f
Au:PVP(K-15) Au:PVP(K-30) Au:PVP(K-90) recovered from no. 2e recovered from no. 4e Au:PVP(K-30)
16 3 trace 26 43 >99
2
3
d (nm)d
62 72 64 61 49 trace
22 23 35 13 8 trace
3.1 ( 0.7 2.9 ( 0.4 3.1 ( 0.5 3.4 ( 0.5 3.4 ( 0.6
a The reactions were carried out at room temperature under air for 24 h. b Estimated from NMR analysis. c Detected as anhydride. d Particle diameter after the reaction determined by TEM measurement. e The Au:PVP NPs were recovered by centrifugal ultrafiltration with a filter of 10 kDa cutoff. f The reaction was carried out in deaerated water.
Scheme 1. Idealized Scheme of Homocoupling of 1 Catalyzed by Au:PVP NPs
Figure 2. (a) Optical absorption spectrum, (b) XRD profile, (c) Au L3-edge XANES, (d) FT-EXAFS, and (e) XPS of Au:PVP(K-30).
is contracted by 3.4% as compared with that in the bulk gold (2.88 Å), as expected from a simple liquid drop model.12 The XPS spectrum (Figure 2e) exhibits core-level peaks associated with C(1s), N(1s), O(1s), and Au(4f). The Au(4f7/2) binding energy of 83.5 eV agrees well with those of typical Au(0) NPs.13 Absence of the Cl signal indicates that the NP surface is not contaminated by ionic species such as AuIIICl4- and AuICl2-.14 We infer from these characterizations that fcc gold crystallites with sizes comparable to Au55-Au147 are weakly trapped within the PVP matrix. Homocoupling of Phenylboronic Acid Catalyzed by Au:PVP NPs. The catalytic activity of the Au:PVP NPs was examined toward homocoupling of phenylboronic acid 1 in water under aerobic conditions. After the reaction at ambient temperature for 24 h, we obtained biphenyl 2 as a major product along with phenol 3 as a minor product (Table 1, entries 1-3). Note that no effort was made in the present study to maximize the yield of 2 by optimizing the experimental conditions, such as reaction temperature, concentration of the base, and reaction time. The conversion of 1 into 2 or 3 is higher in the order of Au:PVP(K-15) < Au:PVP(K-30) < Au:PVP(K-90) (entries 1-3). This trend suggests that the access of 1 to the NP surface is hindered by a higher degree of coordination of the smaller PVP. The TEM measurement and optical spectroscopy7 revealed that the average sizes of the Au NPs increase up to ∼3 nm during the reaction (entries 1-3). It is stressed here, however, that the resulting Au:PVP NPs retain their catalytic activities, as exemplified for Au:PVP(K-30) NPs (entries 4 and 5). The lower yields of 2 by larger particles (12) Balerna, A.; Mobilio, S. Phys. Rev. B 1986, 34, 2293. (13) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (14) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277.
may be due to decrease in the exposed surface areas of the Au:PVP NPs and/or reduction of catalytic activity of the larger particles. Quantitative evaluation of the recycle experiments is formidable because of the particle growth, the side reaction for the production of 3, and a lack of kinetic data and is beyond the scope of the present paper. The most remarkable finding regarding the reaction mechanism is that molecular oxygen dissolved in water plays a vital role in the Au NP catalyzed homocoupling reactions. When the concentration of dissolved oxygen is reduced from 8.7 (entry 2) to 300 nm. The results suggest that the dissolution of the Au NPs into a putative Au ionic species is not intimately involved in the homocoupling, although a minor contribution from the Ostwald ripening is not completely excluded from the present data alone. In summary, we report herein a simple preparation of sub-2-nm Au:PVP NPs and their first successful application toward the homocoupling of phenylboronic acid in water under aerobic conditions. Although details of the reaction mechanism are not clear at present, the catalytic process is mediated by the interaction of O2 with the smallsized Au NPs. The reaction scheme proposed here opens up the possibility for future development of size-specific and reusable Au NP catalysts through a proper choice of stabilizing agent, which is now under way in our laboratory. Acknowledgment. We thank Professor M. Haruta (AIST) and Drs. O. Oishi and C. Okabe (IMS) for valuable comments on size-dependent catalytic activities of Au NPs and the TEM measurement of Au:PVP NPs, respectively. The present work was financially supported by a CREST program sponsored by JST, the “Nanotechnology Support Project” of MEXT, and the “2002th-year Joint Research Project” of Sokendai (Soken/K02-1). Supporting Information Available: Detailed procedures for structural characterization of the Au:PVP NPs. This material is available free of charge via the Internet at http://pubs.acs.org. LA0478189 (17) (a) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (b) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340.
