Chemistry Reference
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Figure 5.11 Molecular structures of the dinuclear complex anions
meso
-[Ti
2
(6)
3
]
4
(left),
meso
-
[Et
4
NTi
2
(6)
3
]
3
(middle, the encapsulated NEt
4
þ
cation is drawn in black) and L,L-[Ti
2
(6)
3
]
4
.
sulfur atoms in a strongly distorted octahedral fashion. The titanium atoms exhibit oppo-
site absolute configurations (L and D) due to the inversion center present. Consequently
[Ti
2
(
6
)
3
]
4
is an achiral dinuclear triple-stranded
meso
-complex.
The nonbonding intrastrand N
S separations in the range 3.026-3.043 A
are indica-
tive of the presence of only weak or no N
S hydrogen bonds. This assumption is
confirmed by the observation of Ti-S
ortho
distances which are not longer than the Ti-S
meta
distances, as would be expected in the case of formation of N
H
S
ortho
hydrogen
bonds [39]. The NMR spectra of (Ph
4
As)
4
[Ti
2
(
6
)
3
] are rather simple, exhibiting only reso-
nances for one ligand strand, which was taken as an indication that the complex anion
adopts the C
3
-symmetry in solution.
Raymond and coworkers described the influence of ammonium templates on the coor-
dination chemistry of an anthracene bridged dicatechol ligand. Molecular modeling stud-
ies showed the capability of the ligand to form both helicates of type [M
2
L
3
]
m
and
tetrahedral clusters of type [M
4
L
6
]
n
. Reaction of the ligand with Ti
4þ
or Ga
3þ
gave dinu-
clear triple-stranded helicates, whereas the presence of a tetraalkylammonium salt caused
the formation of a tetrahedral cluster with one template ion in the interior of its cavity
[44]. Based on these results we investigated the influence of the tetraethylammonium tem-
plate on the reaction of H
4
-
6
with Ti
4þ
. It was hoped that the ammonium template might
induce the formation of the tetranuclear [Ti
4
(
6
)
6
]
8
cluster anion instead of the dinuclear
complex anion [Ti
2
(
6
)
3
]
4
. The reaction of two equivalents of [Ti(OPr)
4
]withthree
equivalents of H
4
-
6
in the presence of four equivalents of Et
4
NBr (Scheme 5.3) in metha-
nol yielded a dark red precipitate which was characterized by X-ray diffraction as the
dinuclear complex (Et
4
N)
3
[Et
4
N
H
Ti
2
(
6
)
3
].
Ti
2
(
6
)
3
] crystallized in a centrosymmetric space group.
Figure 5.11 (middle) shows the molecular structure of the anion [Et
4
N
Complex (Et
4
N)
3
[Et
4
N
Ti
2
(
6
)
3
]
3
that
contains one molecule of the tetraethylammonium cation located between the two metal
atoms. The two titanium atoms in the complex anion are each coordinated by three ben-
zene-
o
-dithiolato moieties differing slightly in their coordination geometry and exhibiting
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