Chemistry Reference
In-Depth Information
3.14 Au
40
Nanoclusters
During the etching synthesis of Au
38
(SC
2
H
4
Ph)
24
nanoclusters, the
Au
40
(SC
2
H
4
Ph)
24
clusters are always produced simultaneously in good yield
[
94
,
97
]. Recently, Qian et al. [
46
] isolated and identified the Au
40
(SC
2
H
4
Ph)
24
species by size-exclusion chromatography. Overall, three steps are needed to
synthesize and isolate the Au
40
(SC
2
H
4
Ph)
24
nanoclusters. In the first step, size-
mixed Au
n
(SG)
m
clusters (38
102) were prepared by reducing an Au(I)-SG
polymer with NaBH
4
in an acetone solution. Subsequently, a thermal thiol etching
process with excess PhC
2
H
4
SH was applied to obtain Au
40
(SC
2
H
4
Ph)
24
and
Au
38
(SC
2
H
4
Ph)
24
size-mixed nanoclusters. During the etching process, the original
polydispersed Au
n
(SG)
m
clusters (38
n
n
102) could be gradually converted to
Au
40
(SC
2
H
4
Ph)
24
and Au
38
(SC
2
H
4
Ph)
24
nanoclusters over 18 h due to the super
stability of Au
40
and Au
38
. Finally, Au
40
(SC
2
H
4
Ph)
24
was isolated by size-
exclusion chromatography (SEC). Figure
12
shows a typical size-exclusion chro-
matogram of the product after 18-h etching, and two peaks (14.0 and 14.4 min)
were obtained. The online-recorded UV-Vis spectra ranging from 12.5 to 14.0 min
and 14.4 to 15.6 min are almost superimposable, respectively, indicating the high
purity of the eluted clusters. MALDI-MS analyses confirmed that cluster eluted
from 12.5 to 14.0 min is Au
40
(SC
2
H
4
Ph)
24
, while the cluster eluted from 14.4 to
15.6 min is Au
38
(SC
2
H
4
Ph)
24
. By replacing PHC
2
H
4
SH with other types of thiols
(e.g., C
6
H
13
SH, C
5
H
11
SH), Au
40
(SC
6
H
13
)
24
and Au
40
(SC
5
H
13
)
24
nanoclusters were
also obtained after different thermal thiol-etching time. These results further con-
firmed the molecular formula of Au
40
(SR)
24
and demonstrated the high stability
of Au
40
.
Recently, the total structure of the isolated Au
40
nanoclusters was predicted and
analyzed by Hakkinen and coworkers [
99
]. By combing structural information
extracted from ligand exchange reactions, circular dichroism and TEM
measurements, the structure of Au
40
(SR)
24
was analyzed in detail via DFT
computations. A novel geometrical motif was proposed where a dimer of two
icosahedral Au
13
cores is protected by six RS-Au-SR and four RS-Au-SR-Au-SR
oligomeric units, analogously to the “Divide and Protect” motif of known clusters
of Au
25
(SR)
18
,Au
38
(SR)
24
, and Au
102
(SR)
44
.
3.15 Au
55
Nanoclusters
From above, Au:SR clusters with different compositions could be isolated by
various techniques, such as fractional crystallization, chromatography, and electro-
phoresis. However, the Au
55
:SC
x
clusters were not easily obtained by the typical
Brust-Schiffrin method. By the ligand exchange reaction of Au
55
(PPh
3
)
12
Cl
6
clusters with hexanethiol, Murray and coworkers [
100
] observed a small amount
of 10 kDa clusters (most likely, Au
55
) in Au:SC
6
clusters. In 2006, Tsunoyama
