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The way to prepare gold clusters -Every atom makes a difference Naoaki Shinjo Department of Chemistry, School of Science The University of Tokyo

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The strategy for preparing metal clusters Bottom-up preparation Metal ions – electrostatic repulsion ↓ reduction Neutral metal atoms 　　↓ aggregation Metal clusters Problems: Continuous aggregation 　　 Difficulty in size control 　　

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“Magic numbers” for bare gold clusters

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Protection of metal clusters by ligands I. Protection by phosphine ligands Dative ligands　　

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Protection of metal clusters by ligands I. Protection by phosphine ligands Chiral clusters with BINAP　　

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Protection of metal clusters by ligands II. Protection by thiolate ligands – high affinity between Au and S “Brust-Schiffrin Method” (B) Ligand exchange of phosphine-protected clusters Thiolation of polymer-stabilized clusters

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Protection of metal clusters by ligands II. Protection by thiolate ligands “Brust-Schiffrin Method” (i) Phase transfer of a gold salt (reaction is not balanced) HAuCl4 (aq) + TOABr (tol) → [AuCl4-xBrx]TOA (tol) + HCl (aq) (ii) Reduction of Au (III) to Au(I) [AuCl4-xBrx]TOA + 3HSR → Au:SR + RSSR (ii) Reduction of Au (I) to Au(0) → Aggregation Au:SR + NaBH4 (aq, excess) → Aun(SR)m Competition between thiolate protection and aggregation ⇒ Kinetic control of size distribution

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Protection of metal clusters by ligands II. Protection by thiolate ligands (B) Ligand exchange of phosphine-stabilized clusters e.g. [Au11(PPh3)8X2]+, [Au11(BINAP)4X2]+ Thiolation of polymer-stabilized clusters e.g. Au:PVP Dependent on the stability of parent clusters ⇒ Thermodynamic control of size distribution

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Fractionation of gold clusters Atomic monodispersity is difficult to achieve ↓ Fractionation (A) Polyacrylamide gel electrophoresis (PAGE) (B) Gel permeation chromatography (GPC) (C) Size-selective etching

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Fractionation by PAGE Fractionation by PAGE – suitable for hydrophilic, charged clusters

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Fractionation by PAGE Fractionation by PAGE Au:SG, Au:S(h-G) GSH: glutathione h-GSH: homo-glutathione

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Fractionation by PAGE Fractionation by PAGE

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(B) Fractionation by GPC (B) Fractionation by GPC – suitable for hydrophobic clusters

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(B) Fractionation by GPC (B) Fractionation by GPC

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Fractionation of gold clusters (C) Size-selective etching

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Fractionation of gold clusters Stability of Au25(SG)18

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The latest results Bond stiffness of Aun(SR)m clusters

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The latest results Bond stiffness of Aun(SR)m clusters

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The latest results Bond stiffness of Aun(SR)m clusters

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$Conclusion Bottom-up preparation of gold clusters can be achieved by protection with ligands. Monodisperse clusters are obtained by the fractionation...$

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Conclusion Bottom-up preparation of gold clusters can be achieved by protection with ligands. Monodisperse clusters are obtained by the fractionation (PAGE, GPC, and size-selective etching). Magic-numbered clusters are thermodynamically stable and forms dominantly. The stability is explained by their electronic or geometric structures.