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The situation becomes further complicated by the non-innocent nature of the bdt
2
lig-
and. Wieghardt
et al.
showed that an intramolecular redox reaction can convert the coor-
dinated bdt
2
ligand into a coordinated
o
-dithio-benzosemiquinonate(1
) radical anion.
Under such conditions the observable
spectroscopic oxidation state
of a metal center
may differ from the
formal oxidation state
. The geometrical changes within the bdt
n
ligand associated with intramolecular redox reactions have been outlined for some hexa-
coordinated [17c] and tetracoordinated square-planar complexes [21]. Likewise, manga-
nese complexes containing two toluene-3,4-dithiolato ligands
B
2
(Figure 5.1) show a
change in their coordination geometry during the course of a redox reaction. The tetrahe-
dral complex [Mn
II
(
B
)
2
]
2
converts into the square-planar complex [Mn
III
(
B
)
2
]
by a for-
mal one-electron oxidation [22]. In summary and regardless of the ultimate reasons,
changes in coordination geometry have been observed in the series of homologous molyb-
denum and tungsten tris(benzene-
o
-dithiolate) complexes, depending on the
formal
oxi-
dation state of the metal center.
We intended to study this type of geometry change in dinuclear (helical) complexes
generated from bis(benzene-
o
-dithiolato) ligands. Related dicatecholato ligands are
known and have been used for the synthesis of several structural motifs. Particularly
important among these are the dinuclear triple-stranded helicates [5] of type [M
2
L
3
]
n
and tetranuclear tetrahedral clusters [23] [M
4
L
6
]
m
(L
dicatecholate ligand).
The first poly(benzene-
o
-dithiol) ligands were described in 1995 [24]. The synthetic
methodology was subsequently improved [25] and a general synthetic strategy for the
preparation of bis(benzene-
o
-dithiols ligands with different spacers is presented in
Scheme 5.1.
A number of bis(benzene-
o
-dithiol) ligands featuring different aromatic or aliphatic
spacers (Figure 5.3) have been prepared using the methodology depicted in Scheme 5.1.
The majority of these have been obtained from 2,3-dimercaptobenzoic acid and a suitable
diamine via the formation of two amide bonds [25]. The introduction of either an aro-
matic or a purely aliphatic spacer like in H
2
-
1
(Figure 5.3) is equally possible [25]. The
use of 2,3-dimercaptobenzoic acid and different tripodal amines leads, after removal of
the
S
-protection groups, to the tripodal hexadentate ligands H
6
-
7
[26], H
6
-
8
[27] and H
6
-
9
[28] (Figure 5.3) which, in spite of their topological similarity, form different complexes
with transition metals.
ΒΌ
S
i
Pr
SH
S
i
Pr
SH
HO
O
O
O
HN
HN
1. SOCl
2
S
i
Pr
S
i
Pr
1.
n
BuLi / TMEDA
1. Na / naphthalene
2. CO
2
3. HCl / H
2
O
NH
2
2. HCl / H
2
O
S
i
Pr
S
i
Pr
HN
O
HN
2.
O
S
i
Pr
SH
NH
2
S
i
Pr
SH
Scheme 5.1 General procedure for the synthesis of bis(benzene-o-dithiol) ligands.
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