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Size Effect on the Catalysis of Gold Clusters Dispersed in Water for Aerobic Oxidation of Alcohol
H. Tsunoyama, H. Sakurai, T. Tsukuda*
Chem. Phys. Lett., 429, 528-532 (2006).
We prepared a set of nearly monodisperse gold clusters ranging from 1.3 to 10 nm by a seed-mediated growth in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). The seed clusters with a diameter of 1.3±0.3 nm were prepared by reducing AuCl4− with NaBH4 in a low-temperature aqueous solution of PVP. Subsequent reduction of more AuCl4− by Na2SO3 in the presence of the seed clusters yielded a series of larger Au:PVP clusters. Catalytic activities of the Au:PVP clusters for aerobic oxidation of p-hydroxybenzyl alcohol increased with decrease in the core size. The size dependence is discussed in light of the electronic structures of the cores probed by optical spectroscopy.
X-ray Magnetic Circular Dichroism of Size-Selected, Thiolated Gold Clusters
Y. Negishi, H. Tsunoyama, M. Suzuki, N. Kawamura,
M. M. Matsushita, K. Maruyama, T. Sugawara, T. Yokoyama*, and T.
J. Am. Chem. Soc. (communications), 128, 12034-12035 (2006).
We report herein the X-ray magnetic circular dichroism (XMCD) at the Au L2,3 edges of a series of Au clusters protected by glutathione (GSH). The samples used here included AuN(SG)M with (N, M)=(10, 10), (15, 13), (18, 14), (22, 16), (25, 18), (29, 20), (39, 24) and a sodium gold(I) thiomalate (SGT) as a reference. Magnetic moments per cluster were found to be increased with size, whereas those per Au-S bond were nearly constant. This finding sugests that a localized hole created by Au-S bonding at the gold/ glutathione interface, rather than the quantum size effect, is responsible for the spin polarization of gold clusters.
Kinetic Stabilization of Growing Gold Clusters by Passivation with Thiolates
Y. Negishi, Y. Takasugi, S. Sato, H. Yao, K.
Kimura, and T. Tsukuda*
J. Phys, Chem. B (Letters), 110, 12218-12221 (2006).
Small gold clusters (<1 nm), protected by monolayers of glutathione, N-(2-mercaptopropionyl)glycine or mercaptosuccinic acid, were prepared by reducing the corresponding Au(I)-thiolate polymers, and were fractionated by size using polyacrylamide gel electrophoresis (PAGE). Mass analysis of the fractionated clusters revealed that their core sizes varied with the molecular structures of the thiolates. This finding indicates that the reduction of the Au(I)-thiolate polymers yields small clusters whose growth is kinetically hindered by passivation with thiolates. Optical spectra of the clusters with identical compositions exhibited different profiles depending on the thiolate molecular structures, implying that deformation of the underlying gold cores is induced by inter-ligand interactions.
Chiroptical Activity of BINAP-stabilized Undecagold Clusters
Y. Yanagimoto, Y. Negishi, H. Fujihara, and T.
J. Phys, Chem. B (Letters), 110, 11611-11614 (2006).
Undecagold cluster compounds [Au11(BINAP)4X2]+ (X=Cl and Br) were synthesized by chemical reduction of the corresponding precursor complexes, Au2X2(BINAP), where BINAP represents the bidentate phosphine ligand 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. The circular dichroism spectra of Au11 stabilized by the enantiomers Au11 (R-BINAP)4X3 and Au11(S-BINAP)4X3 exhibited intense and mirror-image Cotton effect, whereas those of Au113+ clusters stabilized by achiral monodentate phosphine ligands did not. The origin of the chiroptical activity of [Au11(BINAP)4X2]+ is discussed in the context of the structural deformation of the Au113+ core.
Aerobic Oxidation Catalyzed by Gold Nanocluster as a quasi-Homogeneous Catalyst: Application to Generation of Hydrogen Peroxide using Ammonium Formate
H. Sakurai,* H. Tsunoyama, and T. Tsukuda*
Trans. MRS-J., 31, 521-524 (2006).
Gold(0) nanocluster with dav=1.3±0.3 nm stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP-1) shows high catalytic activity toward the generation of hydrogen peroxide in water under ambient conditions using ammonium formate as a reducant. Generation of hydrogen peroxide is monitored by the conversion reaction of o-tolylboronic acid to o-cresol. In the presence of excess amount of ammonium formate, o-cresol was obtained from o-tolylboronic acid with ~60% yield within 24 hours at 300 K. In order to investigate the cluster-size dependence on the reactivity, the monodispersed gold nanoclusters (Au:PVP-n; n=2-4) with the average diameters up to ~5 nm are prepared by seed-mediated growth method using Na2SO2 as a reductant. It is found that the smaller cluster shows superior catalytic activity to larger clusters. The results suggest that the molecular oxygen species adsorbed preferentially on small-sized Au cluster should play an essential role in the formation of hydrogen peroxide
Chromatographic Isolation of "Missing" Au55 Clusters Protected by Alkanethiolates
H. Tsunoyama, Y. Negishi, and T. Tsukuda*
J. Am. Chem. Soc. (communications), 128, 6036-6037 (2006).
We report on the first synthesis of alkanethiolate-protected Au55 (11 kDa), which has been a "missing" counterpart of Schmid's Au55(PR)12Cl6. Au:SR clusters were prepared by the reaction of alkanethiol (RSH) with polymer-stabilized Au clusters (~1.3 nm) and subsequently incubated in neat RSH. The resulting clusters were successfully fractionated by recycling gel permeation chromatography into Au~38:SR and Au~55:SR, and identified by laser-desorption ionization mass spectrometry. The Au~55:SR clusters exhibited structured optical spectra, suggesting molecular-like properties. The thiolate monolayers were found to be liquid-like on the basis of the IR spectrum and the monolayer thickness estimated from the hydrodynamic diameter.