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the approach angle as defined by Raymond
et al.
[42] and removes the geometric factors
favoring M
4
L
6
cluster formation over the entropically favored formation of dinuclear
complexes of type M
2
L
3
. In addition, the nature of the donor atoms and particularly the
longer Ti
O distances and the higher flexibility in the
sulfur-sulfur bite angle can aid in the formation of the M
2
L
3
helicates versus M
4
L
6
clusters.
S distances compared to the Ti
5.2.4 Coordination Chemistry of Tripodal Tris(Benzene-o-Dithiolato) Ligands
While 1,5-naphthalenediamido-linked dicatecholato ligands form tetrahedral M
4
L
6
clus-
ters, planar C
3
-symmetric tricatecholato ligands have been shown by Raymond [23b,42]
and Albrecht [45] to yield tetranuclear tetrahedral clustes of type M
4
L
4
with selected
metal ions. This ligand building principle was transferred to C
3
-symmetric tris(benzene-
o
-dithiolato) ligands [26]. While ligand H
6
-
8
(Figure 5.3) with a flexible backbone reacts
with Ti
IV
ions with formation of a mononuclear chelate complex (Figure 5.6) [27], the
more rigid essentially planar tris(benzene-
o
-dithiol) ligand H
6
-
7
(Figure 5.3) is incapable
of forming mononuclear chelate complexes.
Reaction of H
6
-
7
with [Ti(OPr)
4
] in methanol, in the presence of Li
2
CO
3
/K
2
CO
3
, led to
the formation of a dark red solution. The complex Li
x
K
8
x
[Ti
4
(
7
)
4
]wasnotisolated.
Instead the alkali metal cations were exchanged for tetraethylammonium cations. The use
of the organic cations led to a red precipitate which was isolated. Since a ligand:metal
ratio of 1 : 1 was used and ligand H
6
-
7
is not capable of forming mononuclear complexes,
formation of the tetranuclear complex (Et
4
N)
8
[Ti
4
(
7
)
4
] was assumed [26]. The small
number of resonances in the
1
H NMR spectrum of (Et
4
N)
8
[Ti
4
(
7
)
4
] (in DMF-
d
7
/CD
3
CN)
indicated the presence of only one highly symmetric species in solution. In the case of
encapsulation of some tetraethylammonium cations within the cavity of the complex
anion [Ti
4
(
7
)
4
]
8
, two sets of proton resonances would be expected, with a highfield shift
for those of the encapsulated cations [23a]. However, only one set of sharp resonances
was observed for all tetraethylammonium cations at ambient temperature.
In a second attempt, ligand H
6
-
7
was reacted with [Ti(OPr)
4
] in the presence of
Li
2
CO
3
/K
2
CO
3
followed by the addition of Me
4
NCl. This reaction yielded a dark red
solid. The ESI (negative ions) mass spectrum showed peaks for the anions
{(Me
4
N)
3
[Ti
4
(
7
)
4
]}
5
and {(Me
4
N)
5
[Ti
4
(
7
)
4
]}
3
with the correct isotope distribution,
respectively. Recrystallization of the solid from DMF/CH
3
CN yielded red crystals of LiK
(Me
4
N)
6
[Ti
4
(
7
)
4
]
6DMF (Scheme 5.4) [26].
The complex LiK(Me
4
N)
6
[Ti
4
(
7
)
4
]
6DMF crystallized in the monoclinic space group
C
2/
c
with the octaanion residing on a crystallographic twofold axis (Figure 5.12, top).
The asymmetric unit contains half of the octaanion, three Me
4
N
รพ
cations and three DMF
molecules. The potassium cation resides on the twofold axis passing through the center of
the octaanion. One half of a positive charge per asymmetric unit could not be located and
it was assumed either that each asymmetric unit contains half of a disordered lithium cat-
ion or that a lithium cation is also located on the twofold axis.
Each titanium atom in the anion [Ti
4
(
7
)
4
]
8
is coordinated by six sulfur atoms in a
strongly distorted octahedral fashion. Two titanium atoms adopt the D configuration (Ti1,
Ti1
) and the other two the L configuration (Ti2, Ti2
), which is a rare feature for supra-
molecular tetrahedral clusters of type [M
4
(L)
4
]
n
normally possessing four homochiral
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