The way to prepare gold clusters - Every atom makes a difference

1.

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

2.

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
Au3+
Au
Au3+
Au
Au Au Au
Au3+
Au

3.

“Magic numbers” for bare gold clusters
n=9
(8e-)
Au: [Xe] 4f14 5d10 6s1
n = 21 (20e-)
n = 35 (34e-)
n = 58 (57e-)
Katakuse, I. et al. Int. J. Mass Spec. Ion Proc. 1985, 67, 229.
“Superatom”
– Stability is explained by electronic
shell structure.

4.

Protection of metal clusters by ligands
I. Protection by phosphine ligands
Dative ligands
Electronic structure of clusters
is preserved
Au11(PPh3)7(SCN)3
(8e-, magic number)
McPartlin, M. et al.
Chem. Commun. 1969, 1, 334.
[(PPh3)14Au39Cl6]Cl2
(31e-, hcp)
Teo, B. K. et al.
J. Am. Chem. Soc. 1992, 114, 2743.

5.

Protection of metal clusters by ligands
I. Protection by phosphine ligands
Chiral clusters with BINAP
[Au11(BINAP)4Br2]+
(8e-, magic number)
[Au11(R-BINAP)4Br2]+
[Au11(S-BINAP)4Br2]+
[Au11(R-BINAP)4Br2]+
[Au11(S-BINAP)4Br2]+
Au
P
Br
Yanagimoto, Y. et al. J. Phys. Chem. B 2006, 110,
11611.

6.

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

7.

Protection of metal clusters by ligands
II. Protection by thiolate ligands
(A) “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

8.

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]+
(C) Thiolation of polymer-stabilized clusters
e.g. Au:PVP
Dependent on the stability of parent clusters
⇒ Thermodynamic control of size distribution

9.

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

10.

Fractionation of gold clusters
(A)Fractionation by PAGE
– suitable for hydrophilic, charged clusters
Core size
Large
Small
Clusters with larger cores subject to stronger
resistance.

11.

Fractionation of gold clusters
(A)Fractionation by PAGE
Au:SG, Au:S(h-G)
GSH: glutathione
Hydrophilic
h-GSH: homo-glutathione
Negishi, Y. et al. J. Am. Chem. Soc. 2005, 127, 5261.
Au:SG
Au:S(h-G)

12.

Fractionation of gold clusters
(A)Fractionation by PAGE
Aun(SG)m
n-m
10437 Da
10415 Da
High-resolution ESI MS of
the fractions enabled
correct determination of
the compositions.
Negishi, Y. et al. J. Am. Chem. Soc. 2005, 127, 5261.

13.

Fractionation of gold clusters
(B) Fractionation by GPC
– suitable for hydrophobic clusters
dH
Small
dH: Hydrodynamic
diameter
Large
Size exclusion chromatography:
Clusters with larger hydrodynamic diameter
are excluded by pores of the gel.

14.

Fractionation of gold clusters
(B) Fractionation by GPC
Size separation with high reproducibility and
resolution by recycling GPC
Aun:SC18
n = 53 ± 10
⇒ n = 55
n = 35 ± 6
⇒ n = 38
Tsunoyama, H. et al. J. Am. Chem. Soc. 2006, 128,
6036.
2
4

15.

Fractionation of gold clusters
(C) Size-selective etching
“Size focusing”
― Thermodynamically stable cluster
Au25(SG)18 is preferentially produced from
larger precursors.
Aun(SG)m
n < 25
⇒ oxidized to AuI:SG
n ≥ 25
⇒ etched into Au25(SG)18
Shichibu, Y. et al. Small 2007, 3, 835.

16.

Fractionation of gold clusters
Stability of Au25(SG)18
“Magic-numbered cluster”
― Anionic form [Au25(SG)18]- is stabilized by TOA+
8 valence electrons ⇒ closed shell structure
Au(I)-SG polymers are
reduced to [Au25(SG)18]in the Brust-Schiffrin
Method.
Negishi, Y. et al. J. Am. Chem. Soc. 2007, 129, 11322.

17.

The latest results
Bond stiffness of Aun(SR)m clusters
Au-Au bonds were divided into 3
groups.
The contribution from the longest
group was negligible.
⇒ FT-EXAFS data were analyzed
with Au-S, Au-Au(S) and Au-Au(L).
Yamazoe, S. et al. Nat. Commun. 2016, 7, 10414.

18.

The latest results
Bond stiffness of Aun(SR)m clusters
Bond Stiffness:
Au-Au(S) > bulk Au-Au > Au-Au(L)
Bond length:
Au-Au(S) < bulk Au-Au < Au-Au(L)
⇒ stiffness is related to length
Yamazoe, S. et al. Nat. Commun. 2016, 7, 10414.

19.

The latest results
Bond stiffness of Aun(SR)m clusters
Au-S and surface Au-Au(S) bonds constitute
rigid ring structure
⇒ Contribution to the stability
Yamazoe, S. et al. Nat. Commun. 2016, 7, 10414.

20.

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.