Chemistry Reference
In-Depth Information
[Au
18
Ag
20
(PR
3
)
14
Cl
12
]
2+
[
19
-
22
]. Dahl and coworkers synthesized and character-
ized a series of phosphine-carbonyl Pd
n
nanoclusters [
23
-
26
].
In recent development of gold-phosphine nanoclusters, Shichibu et al. reported
diphosphine-protected Au
13
icosahedral clusters [
27
]. Pettibone et al. carried out
detailed work on the synthesis and growth mechanism of small gold-phosphine
clusters [
28
]. Wan et al. reported the structure of [Au
20
(PPhpy
2
)
10
Cl
4
]
2+
cluster
(where Phpy
2
¼
pyridyl phosphine), in which the core consists of two edge-shared
Au
11
units [
29
]. With phosphine/thiolate ligands, a biicosahedral
[Au
25
(PPh
3
)
10
(SR)
5
Cl
2
]
2+
cluster (SR
¼
thiolate) has been obtained [
30
,
31
]. Recently, Das et al. report a [Au
24
(PPh
3
)
10
(SC
2
H
4
Ph)
5
X
2
]
+
nanocluster
(where X
¼
Cl/Br) [
32
]. Zheng and coworkers recently reported Au
13
Cu
x
(
x
¼
2,
4, 8) nanoclusters protected by mixed phosphine and thiolate ligands [
33
].
Thiol was extensively used in the synthesis of gold(I)-thiolate complexes in
early research, and later thiol was used to prepare gold nanoparticles. In this
chapter, we focus on the thiolate-protected
nanoclusters
, while the research on
conventional gold-thiolate
nanoparticles
is not discussed herein.
3 Thiolate-Protected Gold Nanoclusters
The protecting molecules are very important for the stability of nanoclusters.
Generally speaking, the protecting molecules provide barriers such as electrostatic
and steric repulsions between particles to prevent them from aggregation into
precipitate. Different types of protecting molecules impart different stability to
nanoclusters. For conventional gold nanoparticles, simple ions (e.g., citrate), poly-
mers, surfactants, as well as ligands have been used for stabilization (Fig.
2
).
Among these reagents, ligands - especially thiolate - render highly stable gold
nanoparticles and nanoclusters, and thus are of wide interest. The high stability of
thiolate-protected gold nanoparticles originates from the strong covalent bonding
between thiolate and gold - the ligand is thus hard to dissociate from the nano-
particle surface. The carbon tails of the thiolate ligands provide further steric
repulsion between nanoparticles, hence preventing aggregation.
The study of thiolate-protected gold nanoclusters experienced several stages,
i.e. from polydispersed nanoclusters to monodispersed ones and finally to atomi-
cally precise nanoclusters [
2
,
34
-
37
]. In early years, separation was done on the
polydisperse nanoclusters in order to obtain relatively monodisperse ones [
4
,
35
].
In recent years, the research progress has evolved to large-scale, controlled synthe-
sis [
37
-
43
].
Whetten's group found that the thiolate-protected gold nanoclusters had the
trend to form a series of
discrete
sizes [
35
]. The mixture of clusters was separated
by solvent fractionation, and each fraction was characterized by laser desorption
ionization mass spectrometry (LDI-MS). Distinct species with molecular weight of
5
k
,8
k
,14
k
,22
k
,29
k
, etc., where
k
1,000 Da were identified; of note, these mass
values correspond to the mass of Au
x
S
y
[
36
,
44
-
47
]. Those species were quite
¼
