Chemistry Reference
In-Depth Information
with neighboring tetrahedrons. Six five-fold (
C
5
) rotation axes can be readily
identified on each opposite pair of
vertices
, and fifteen two-fold (
C
2
) axes on
each opposite pair of
edges
, and ten three-fold (
C
3
) axes on each opposite pair of
faces
. Accordingly, one can have three views of an icosahedron based on its
C
2
,
C
3
,
or
C
5
axis (Fig.
11
). If viewed with respect to a
C
2
axis, a 2-2-4-2-2 five layers of
atoms can be identified (the central atom not considered). Similarly, when viewing
from a
C
3
axis, there are four layers of atoms 3-3-3-3, and from
C
5
, there are also
four layers of atoms but with a 1-5-5-1 configuration.
The icosahedral Au
13
kernel is protected by six “V-shaped” -SR-Au-SR-Au-
SR-chains or oligomers (Fig.
10
). This oligomeric structure is often called a staple
motif, because it has two sulfur ends binding two surface gold atoms of the kernel in
a way resembling a staple. Based on the number of gold atoms in the chain, there
are monomeric staples, dimeric staples, and so on. For example, the six V-shaped
staples in Au
25
(SR)
18
are called dimeric staples since there are two gold atoms
incorporated in the chain. In regard to the distribution of the six dimeric staples on
the icosahedron, if one views from the three mutually perpendicular C
2
axes of the
icosahedron (Fig.
10
), two dimeric staples are seen on the top and the bottom,
another two on the front and the back, and two more on the left and the right. Since
six staple motifs have 12 sulfur ends, all the 12 surface gold atoms of the Au
13
icosahedron are protected by the -SR groups. In terms of symmetry, the Au
13
kernel
has the highest
I
h
symmetry, but the Au
25
S
18
skeleton reduces its symmetry to
D
2h
.
Following the above anatomy, the Au
25
(SC
2
H
4
Ph)
18
structure may be represented
as Au
13
@[Au
2
(SR)
3
]
6
to illustrate the kernel and the exterior shell.
It should be stressed that the staple gold atoms also interact with the gold atoms
of the Au
13
kernel via face capping, with one shorter Au-Au contact (3.02-3.12 Å)
and two longer ones (3.18-3.27 Å) to the other two Au atoms on the face it is
capping.
The Au
25
(SR)
18
nanocluster can have different, stable charge states (e.g.,
q
1, 0, +1, etc.) [
13
,
83
-
88
]. The initially reported Au
25
(SR)
18
nanocluster
was negatively charged, with a positive counterion [N(
n
-C
8
H
17
)
4
]
+
in the unit cell.
The negatively charged Au
25
can be oxidized to neutral [Au
25
(SR)
18
]
0
, and the
Au
25
S
18
framework is preserved in the neutral [Au
25
(SR)
18
]
0
, with the only differ-
ence in that the [Au
25
(SR)
18
]
0
framework is less distorted than the negatively
charged [Au
25
(SR)
18
]
[
13
]. The [Au
25
(SR)
18
]
0
is paramagnetic [
89
], while the
[Au
25
(SR)
18
]
and [Au
25
(SR)
18
]
+
are diamagnetic [
84
].
¼
5.1.2 The Case of Au
38
(SC
2
H
4
Ph)
24
The Au
38
(SC
2
H
4
Ph)
24
nanocluster possesses an Au
23
kernel which is composed of
two icosahedrons (building blocks) via face-sharing (Fig.
12a
)[
10
]. If we start with
one icosahedron (with two opposite triangular faces on the top and the bottom,
i.e. the
C
3
orientation, see Fig.
11
above), next, place the second icosahedron on the
