Chemistry Reference
In-Depth Information
4.1 Centered Polyhedral Clusters
4.1.1 N
13
Crystal structures of PGCs with 8
13 were extensively studied before 1990,
and the general preference centered polyhedral geometries have been almost
established. The majority of these centered clusters have skeletal structures based
on centered icosahedron, which can also be described as capped centered hexagonal
chairs. Such icosahedron-based structures are found for a series Au
8
L
7
,Au
8
L
7
,
Au
9
L
8
,Au
10
L
9
, and Au
11
L
10
clusters, which finally complete with the icosahedral
Au
13
L
12
-type clusters (Table
1
). Representative examples are shown in the struc-
tures of 7 (Au
8
L
7
), 8 (Au
8
L
7
), 18 (Au
10
L
9
), 23 (Au
11
L
10
), 30 (Au
11
L
10
), and 34
(Au
13
L
12
) (Fig.
1
). The exceptional structures that cannot be described based on
icosahedron are also found in several cases. For example, Mingos et al. reported
green and golden-brown crystals of [Au
9
(P(4-MeOC
6
H
4
)
3
)
8
](NO
3
)
3
(16·NO
3
) and
showed that the former has a bicapped chair
D
2
h
geometry derived from icosa-
hedron whereas the latter has a centered crown geometry [
38
]. The interconversion
between the two forms seems easy since they share a common structure in solution.
The crown geometries have also been found in the several polyoxometalate salts of
Au
9
P
8
clusters. Jansen et al. recently reported that the appropriate choice of
recrystallization solvents allows the selective formation of either of the two skeletal
geometries of 14·PW
12
O
40
(Fig.
2
)[
61
].
As other examples of deviation from icosahedron-derived structures, capped
centered square antiprism (approximately
D
4
d
symmetry) geometries have been
reported for [Au
11
(PMePh
2
)
10
]
3+
(31) (Fig.
1
)[
42
], which is obviously different
from the icosahedron-based structures with approximately
C
3
v
symmetry found in
the conventional Au
11
(PAr
3
)
10
clusters for various combinations of phosphine,
sub-ligands, and counter anion [
28
,
29
,
33
,
47
,
65
,
84
,
88
,
99
] (Table
1
and 23
and 30 in Fig.
1
). This geometry can also be viewed as a capped crown, which is
also found for Au
10
L
9
clusters (22 in Fig.
1
)[
98
].
As mentioned in previous papers and reviews [
35
,
42
], the center-to-peripheral
radial bond distances are shorter than the peripheral bond distances, indicating the
crucial contribution of radial bonding to the stability of cluster skeleton. The overall
shape of these clusters can be categorized into toroidal (ellipsoidal, 2D) and
hemispherical/spherical (3D), which have been accounted for in terms of poly-
hedral electron counting and molecular orbital calculations [
34
,
51
].
N
4.1.2 Higher-Nuclearity Clusters
Several examples of higher-nuclearity centered clusters have appeared recently and
their structures are shown in Fig.
3
. Simon et al. showed that the crystal structure of
[Au
14
(PPh
3
)
8
(NO
3
)
4
](35) exhibits a unique geometry, in which two face-sharing
tetrahedron dimers are connected by four AuNO
3
units [
90
]. The distance between
