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Amplification of Optical Activity of Gold Clusters by the Proximity of BINAP
Shinjiro Takano and Tatsuya Tsukuda*
J. Phys. Chem. Lett. 7, 4509-4513 (2016).
Selected as ACS Editors' Choice
Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au11(R/S-DIOP)4Cl2]+ (DIOP = 1,4-bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol) and [Au8(R/S-BINAP)3(PPh3)2]2+ (BINAP = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl). [Au8(BINAP)3(PPh3)2]2+ showed stronger rotatory strengths than [Au11(DIOP)4Cl2]+ in the visible region while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core.
Tuning the electronic structure of thiolate-protected 25-atom clusters by co-substitution with metals having different preferential site
Trimetallic Au24−xAgxPd and tetrametallic Au24−x−yAgxCuyPd clusters were synthesized by the subsequential metal exchange reactions of dodecanethiolate-protected Au24Pd clusters. EXAFS measurements revealed that Pd, Ag, and Cu dopants preferentially occupy the core, edge and staple sites, respectively. Spectroscopic and theoretical studies demonstrated that the synergistic effects of multiple substitutions on the electronic structures are additive in nature.
Rayleigh Instability and Surfactant-Mediated Stabilization of Ultrathin Gold Nanorods
Ryo Takahata, Seiji Yamazoe, Chompunuch Warakulwit, Jumras Limtrakul, and Tatsuya Tsukuda*
J. Phys. Chem. C 120, 17006-17010 (2016).
Ultrathin gold nanorods (AuUNRs; diameter ~2 nm) stabilized by oleylamine (OA) were spheroidized when dispersed in chloroform containing a small amount of OA. Time-resolved optical spectroscopy and TEM analysis indicated that the AuUNRs were gradually shortened with the release of small Au nanospheres (AuNSs) because of Rayleigh instability, followed by transformation into plasmonic AuNSs (diameter > 2 nm). The OA surfactants play an essential role in stabilizing the morphology of AuUNRs by suppressing the diffusion of Au surface atoms.
Controlled Synthesis of Carbon Supported Gold Clusters for Rational Catalyst Design
Seiji Yamazoe, Tatchamapan Yoskamtorn, Shinjiro Takano, Sudarat Yadnum, Jumras Limtrakul, and Tatsuya Tsukuda*
Chem. Rec. 16, 2338-2348 (2016).
The development of novel catalysts based on metal clusters requires a rational design principle as well as atomically precise synthetic methods. Toward this goal, we have developed a method to precisely and independently control the size, composition, and surface modification of heterogeneous gold clusters by calcination of the ligand-protected Au clusters on carbon supports. We studied the effects of these structural parameters on benzyl alcohol oxidation as a test reaction. Unexpectedly, Au144 and Au~330 on hierarchically porous carbon exhibited significantly larger turnover frequency than Au25 and Au38. This size dependence is ascribed to the difference in the geometric structures of the Au clusters; Au144 and Au~330 have an icosahedral-based structure whereas Au25 and Au38 have a face centered cubic (FCC) structure. Doping of a single Pd atom to Au25 supported on carbon nanotubes remarkably enhanced the catalytic activity. The doping effect is explained in terms of the accelerated formation of the carbocation intermediate due to electron transfer from Pd to Au since the doped Pd is buried within the Au clusters and is located at the interface between the supports. Residual thiolates on Au25 affected both the activity and selectivity; selective oxidation to benzaldehyde was achieved at optimized coverage. Non-formation of benzoic acid is due to the suppression of oxidation activity by electron withdrawal by thiolates and nonformation of benzyl benzoate is due to the site isolation effect by thiolates. These results will provide useful information for the rational design of gold-cluster-based catalysts with desired performance.
Halogen adsorbates on polymer-stabilized gold clusters: Mass spectrometric detection and effects on catalysis
Ryo Ishida, Setsuka Arii, Wataru Kurashige, Seiji Yamazoe, Kiichirou koyasu, Yuichi Negishi, and Tatsuya Tsukuda*
Chin. J. Catal. 37, 1656-1661 (2016).
Mass spectrometry of gold clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) revealed the presence of chlorine (Cl) adsorbates derived from synthetic precursors mainly on the Au34 and Au43 clusters. The amount of Cl adsorbates on the Au clusters did not affect the catalytic properties for the aerobic oxidation of benzyl alcohol, suggesting that the Cl atoms were only weakly bound to the Au clusters. In contrast, the replacement of Cl with Br on the Au34 and Au43 clusters significantly suppressed the activity without any influence on the electronic structure. This result indicates that the Br atoms were strongly bound to the Au clusters and sterically blocked their active sites. Substantial reduction of catalytic activity by Br adsorbates suggests that the Au34 and Au43 clusters made a major contribution to the catalytic activity of Au:PVP.
Size-Specific, Dissociative Activation of Carbon Dioxide by Cobalt Cluster Anions
Akimaro Yanagimachi, Kiichirou Koyasu, David Yubero Valdivielso, Sandy Gewinner, Wieland Schöllkopf, André Fielicke, and Tatsuya Tsukuda*
J. Phys. Chem. C 120, 14209-14215 (2016).
The reaction of cobalt cluster anions Con− (3 ≤ n ≤ 17) with CO2 was studied experimentally and theoretically to explore the size-specific activation mode of CO2 by Con−. Mass spectrometric measurements revealed that the reactivity depends strongly on cluster size: the reactivity emerges abruptly at n = 7, peaks at n = 8−10, and then gradually decreases with increasing n. Infrared multiple photon dissociation spectra of ConCO2− exhibit a single peak at ~1870 cm−1, similarly to the previously reported spectra of ConCO−. Density functional theory calculations for Co7CO2− as an example revealed that the dissociative adsorption of CO2 into CO and O is energetically more favorable than non-dissociative adsorption. The infrared spectra calculated for dissociated isomers Co7(CO)O− reproduced the experimental results, whereas those for non-dissociated isomers Co7CO2− did not. The photoelectron spectra of ConCO2− were shifted dramatically toward higher energies relative to those of Con−, suggesting electron transfer from Con− to the CO and O ligands. These results indicate that the CO2 molecule adsorbs dissociatively on Con−, in sharp contrast to its non-dissociative adsorption onto the Co monomer anion.
Partially Oxidized Iridium Clusters Within Dendrimers: Size-Controlled Synthesis and Selective Hydrogenation of 2-Nitrobenzaldehyde
Tatsuya Higaki, Hirokazu Kitazawa, Seiji Yamazoe, and Tatsuya Tsukuda*
Nanoscale 8, 11371-11374 (2016).
Iridium clusters nominally composed of 15, 30 or 60 atoms were size-selectively synthesized within OH-terminated poly(amidoamine) dendrimers of generation 6. Spectroscopic characterization revealed that the Ir clusters were partially oxidized. All the Ir clusters efficiently converted 2-nitrobenzaldehyde to anthranil and 2-aminobenzaldehyde under atmospheric hydrogen at room temperature in toluene via selective hydrogenation of the NO2 group. Selectivity toward 2-aminobenzaldehyde over anthranil was improved with the reduction of the cluster size. The improved selectivity is ascribed to more efficient reduction than intramolecular heterocyclization of a hydroxylamine intermediate on smaller clusters that have a higher Ir(0)-phase population on the surface.
Selective and High-Yield Synthesis of Oblate Superatom [PdAu8(PPh3)8]2+
We report herein a selective and high-yield synthesis of [PdAu8(PPh3)8]Cl2 via kinetic control of the co-reduction of AuCl(PPh3) and Pd(PPh3)4 and post-synthesis annealing. Single-crystal X-ray diffractometry confirmed that the PdAu8 core has a centered-crown motif with the Pd atom located at the center. The energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was determined to be 1.66 eV via electrochemical measurement. Density functional calculation showed that both the HOMO and LUMO are assigned to 1P superatomic orbitals and that the PdAu82+ core can be viewed as an oblate superatom with an electronic configuration of (1S)2(1P)4. The optical gap (1.95 eV) was larger than the electrochemical HOMO-LUMO gap because electronic transition from the HOMO to the LUMO is optically forbidden.
Oxidative Addition of CH3I to Au− in the Gas Phase
Satoru Muramatsu, Kiichirou Koyasu, and Tatsuya Tsukuda*
J. Phys. Chem. A 120, 957-963 (2016).
Reaction of the atomic gold anion (Au−) with CH3I under high-pressure helium gas affords the adduct AuCH3I
Application of Group V Polyoxometalate as Efficient Base Catalyst: a Case Study of Decaniobate Cluster
Shun Hayashi, Seiji Yamazoe, Kiichirou Koyasu, and Tatsuya Tsukuda*
RSC Advances 6, 16239-16242 (2016).
Base catalytic activity of the decaniobate cluster (TMA)6[Nb10O28]·6H2O (TMA+ = tetramethylammonium cation) was studied theoretically and experimentally. Density functional theory calculations showed that the oxygen atoms in the cluster are highly negatively charged and suggested the possibility that the cluster can act as a base catalyst. We demonstrate for the first time that [Nb10O28]6− actually exhibits base catalytic activity for aldol-type condensation reactions including Knoevenagel and Claisen-Schmidt condensation reactions. The catalytic reactions proceeded under ambient condition, suggesting that [Nb10O28]6− holds promise as a practical strong base catalyst.
Hierarchy of Bond Stiffnesses within Icosahedral-based Gold Clusters Protected by Thiolates
Seiji Yamazoe, Shinjiro Takano, Wataru Kurashige, Toshihiko Yokoyama, Kiyofumi Nitta, Yuichi Negishi, and Tatsuya Tsukuda*
Nature Communications 7, 10414 (2016).
Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au-Au bonds have different stiffnesses depending on their lengths. The long Au-Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au-Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au-SR oligomers.
The Electrooxidation-induced Structural Changes of Gold Di-superatomic Molecules: Au23 vs. Au25
Shota Matsuo, Seiji Yamazoe, Jing-Qiang Goh, Jaakko Akola, and Tatsuya Tsukuda*
Phys. Chem. Chem. Phys. 18, 4822-4827 (2016).
The gold cluster compounds Au38(SC2H4Ph)24 and [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ are known to possess bi-icosahedral Au23 and Au25 cores, respectively, inside their ligand shells. These Au cores can be viewed as quasi-molecules composed of two Au13 superatoms sharing three and one Au+ atoms, respectively. In the present work, we studied the structural changes of these gold di-superatomic molecules upon electrooxidation via spectroelectrochemical techniques, X-ray absorption fine structure analysis, and density functional theory calculations. The Au23 core was electrochemically stable, but the Au25 core underwent irreversible structural change. This marked difference in the stability of the oxidized states is ascribed to differences in the bonding scheme of Au13 units and/or the bonding nature of the protecting ligands.
Repeated Appearance and Disappearance of Localized Surface Plasmon Resonance in 1.2 nm Gold Clusters Induced by Adsorption and Desorption of Hydrogen Atoms
Addition of an aqueous solution of NaBH4 to a dispersion of small (~1.2 nm) gold clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) induced a localized surface plasmon resonance (LSPR) absorption for a certain period while maintining the cluster size. The duration of the LSPR band could be lengthened by increasing the NaBH4 concentration and be shortened by increasing the concentration of dissolved O2, and the LSPR band could be made to appear and reappear repeatedly. The appearance of the LSPR band is explained by the electron donation to the Au core from the adsorbed H atoms that originate from NaBH4, whereas its disappearance is ascribed to the removal of H atoms by their reaction with O2. These results suggest that the transition between the metallic and nonmetallic electronic structures of the Au clusters can be reversibly induced by the adsorption and desorption of H atoms, which are electronically equivalent to Au.