Chemistry Reference
In-Depth Information
Although phosphine-stabilized gold nanoclusters have been studied extensively,
the lower stability compared to thiol-capped clusters further inhibited their practical
applications. Zhu et al. [
80
] reported a kinetically controlled approach for
synthesizing thiolate-capped Au
20
nanoclusters.
In a
typical
experiment,
HAuCl
4
∙
3H
2
O was phase transformed from an aqueous solution into toluene
phase in the presence of TOABr. After the toluene phase was cooled to 0
C,
phenylethylthiol was added to the solution at a low stirring rate (~50 rpm). After
the formation of the [Au
(I)
SR]
x
aggregates, NaBH
4
(1 equiv. of gold) solution was
slowly added over a 30-min period to reduce the [Au
(I)
SR]
x
aggregates. Here, the
amount of NaBH
4
(~1 equiv.) and slow reduction of [Au
(I)
SR]
x
aggregates were
found to be critical for the formation of Au
20
(SR)
16
clusters. Only one peak eluting
at ~16.05 min in SEC analysis indicated the high purity of the as-synthesized Au
nanoclusters. The composition of Au
20
(SCH
2
CH
2
Ph)
16
can be determined from the
ESI-MS-TOF analysis, which is in agreement with the simulated isotopic patterns.
More interestingly, the Au
20
nanoclusters were found to be particularly stable
against excess thiol etching. Lately, Zeng and coworkers [
81
] performed a calcula-
tion on this Au
20
nanoclusters and predicted that it has a prolate Au
8
core and four
level-3 extended staple motifs -RS-Au-RS-Au-RS-Au-RS-.
3.10 Au
24
Nanoclusters
Das et al. [
82
] reported the synthesis and total structural determination of Au
24
nanocluster protected by mixed ligands of phosphines and thiolates. The precursor
used in the synthesis is HAuCl
4
∙
3H
2
O rather than the commonly used Au(PPh
3
)Cl
salt. In a typical synthesis, HAuCl
4
3H
2
O was first phase transferred from water to
toluene in the presence of TOABr. Upon the addition of PPh
3
, Au(III) was reduced
to Au(I), followed by further reduction by NaBH
4
. After 16 h, the reddish-brown
product was extracted by dichloromethane. Phenylethylthiol was then added to this
solution and then reacted at 313 K for ~4 h. [Au
24
(PPh
3
)
10
(SCH
2
CH
2
Ph)
5
X
2
]
+
(counterion (X)
halide ions) was obtained after the reaction continued for
another 24 h with adding excess PPh
3
into the solution. In the process, the addition
of excess PPh
3
is critical for producing Au
24
nanoclusters. By comparing the
UV-Vis absorption spectra displayed in Fig.
7
, the Au
24
clusters exhibited much
different optical properties from that of Au
25
clusters although there is only
one-atom difference between the Au
24
and Au
25
clusters. The computed absorption
spectrum of [Au
24
(PPh
3
)
10
(SCH
2
CH
2
Ph)
5
X
2
]
+
from DFT calculation (Fig.
7c
)
agrees well with the experimental one. The absorption peaks of Au
24
at 560 nm
and Au
25
clusters at 670 nm are from the HOMO-LUMO transition (LUMO + 2
molecular orbital is shown in Fig.
7d
). From the single-crystal X-ray crystallo-
graphic analysis, the Au
24
core consists of two incomplete icosahedral Au
12
units
joined together in an eclipsed fashion through five thiolate linkages.
¼
