Chemistry Reference
In-Depth Information
3
þ
Au
13
PR
3
5
þ
Au
13
S
ð
6
Au
13
Cl
2
PR
3
ð
Þ
10
ð
Þ
12
:
Since SR
is capable of donating 4 electrons to gold atoms using 2 dative bonds
(e.g. the bridging mode illustrated in Fig.
24
), the 6(SR) ligands are electronically
equivalent to 12 two-electron phosphine ligands in the original cluster. The charge
difference between PR
3
and SR
is reflected in the overall cluster which changes
from 5+ to
1. The surface staple gold organothiolato-ligands in the resulting
cluster may be formally created by adding neutral AuSR moieties to SR ligands
according to Eq. (
1
) above.
This addition of Au(SR) molecules has no effect on the total charge on the
cluster and does not alter the skeletal electron count. Therefore, the SR ligands may
be replaced completely or partially by up to 6[Au(SR)
2
] or [Au
2
(SR)
3
] staple
ligands, i.e. leading to [Au
13
{Au(SR)
2
}
6
]
or [Au
13
{Au
2
(SR)
3
}
6
]
. However,
geometrically the ligands have the preferred bridging modes shown in Fig.
26
.
Specifically SR favours bonding to an adjacent pair of gold atoms, whereas the
metallothiolates prefer non-adjacent pairs of metal atoms. The formulation
[Au
13
{Au
2
(SR)
3
}
6
]
is preferred to [Au
13
{Au(SR)
2
}
6
]
because Au
2
(SR)
3
spans
two non-adjacent gold atoms of the icosahedron more effectively. This simple
analysis therefore not only reproduces the observed centred icosahedral structure
for [Au
25
(SR)
18
]
illustrated schematically in Fig.
27
and clearly underlines the
relationship with the parent phosphine cluster.
A pair of centred icosahedra sharing a triangular face
is exemplified by
[Au
23
L
18
]
9+
(see Fig.
17
), with 18 of the 21 metal vertices available for bonding
to phosphine-like ligands. The three vertices at the intersection are unavailable
because of the concavity of the surface at this plane. This leads to the following
isoelectronic relationships:
Au
23
SR
9
Au
23
Au
SR
2
9
Au
32
SR
18
9
þ
½
Au
23
L
19
Au
23
Au
2
SR
3
9
Au
41
S
ð
27
:
The homoleptic possibilities [Au
23
{Au(SR)
2
}
9
]
[Au
32
(SR)
18
] and
[Au
23
{Au
2
(SR)
3
}
9
]
[Au
41
(SR)
27
] both satisfy the closed shell electronic require-
ments, but the observed structure has a 3:6 combination of the two staples probably
because the concave surface noted above is more effectively bridged by 3[Au(SR)
2
]
ligands for the geometric reasons illustrated in Fig.
26
. Single crystal X-ray studies
have confirmed that
the [Au
38
(SR)
24
] cluster
indeed has
the [Au
23
{Au
(SR)
2
}
3
{Au
2
(SR)
3
}
6
] structure.
A pair of fused icosahedra
have not been structurally characterised for simple
phosphine gold clusters, but the related platinum carbonyl cluster has been shown
to have the structure shown in Fig.
17
, e.g. [Pt
19
(CO)
17
]
8
has two interstitial atoms
in a structure with fivefold symmetry and successive layers containing 1:5:5:5:1
surface platinum atoms. This leads to the following isoelectronic relationships:
