Chemistry Reference
In-Depth Information
S separations in the range 3.309-4.127 A
metal centers. Long and variable N
confirm
the absence of intramolecular N
S hydrogen bonds in the solid state, as was already
concluded from the
1
H NMR spectrum for the situation in solution. The C
2
S
2
Ti hetero-
cycles are bent along the S
H
S vector (range of dihedral angles between the C
6
H
3
S
2
and
S
2
Ti planes 2.7-15.2
), as was observed earlier for related complexes of bdt
2
[39,41,43,46].
The anion [Ti
4
(
7
)
4
]
8
encapsulates not only a potassium cation but also four additional
Me
4
N
þ
cations
(Figure 5.12,
top)
and is
therefore best described as
[K
Ti
4
(
7
)
4
]
3
. This behavior is in remarkable contrast to the situation described
for a complex anion with the analogous tricatecholato ligand [42a] where no encapsula-
tion of any cations has been observed due to the limited space within the cluster anion.
The tricatecholato ligand in [Ti
4
(tricatecholato ligand)
4
]
8
is essentially planar with
strong intramolecular N
(Me
4
N)
4
O hydrogen bonds which cause coplanar orientation of the
phenylene backbone with the catecholato donor groups (Figure 5.12, bottom right) [42a].
Ligand [
7
]
6
in [Ti
4
(
7
)
4
]
8
is not planar and no N
H
S hydrogen bonds have been
observed. The benzene-
o
-dithiolato donor groups are oriented essentially perpendicular
to the central phenylene group (range of dihedral angles between the C
6
H
3
and C
6
H
3
S
2
planes 73.5-89.9
; see ligand bridging Ti1, Ti1
and Ti2
in Figure 5.12, top). This ligand
conformation (Figure 5.12, bottom left) together with the nonplanar C
2
S
2
Ti heterocycles
generates a much larger and much more open cavity in [Ti
4
(
7
)
4
]
8
than was found for
[Ti
4
(tricatecholato ligand)
4
]
8
, which in turn allows the encapsulation of the five cations
(one K
þ
and four NMe
4
þ
).
Encouraged by the encapsulation of tetramethylammonium cations observed with
[Ti
4
(
7
)
4
]
8
, encapsulation of the larger benzimidazolium cation was attempted. Reaction
of ligand H
6
-
7
with [Ti(OPr)
4
] and Li
2
CO
3
/K
2
CO
3
in methanol followed by the addition
of four equivalents of
N
,
N
0
-dimethylbenzimidazolium bromide gave a red precipitate. The
1
H NMR spectrum of this solid in DMF-
d
7
/CD
2
Cl
2
showed the signals for the [Ti
4
(
7
)
4
]
8
complex anion and four broad resonances for the benzimidazolium cations. Compared to
the
1
H NMR spectrum of benzimidazolium bromide, measured in the absence of complex
anion [Ti
4
(
7
)
4
]
8
, all resonances for the benzimidazolium cations are shifted highfield in
the presence of [Ti
4
(
7
)
4
]
8
. Both the highfield shift of the resonances of the benzimidazo-
lium cation and the observed line broadening were taken as indications for a fast
exchange of benzimidazolium cations between the inside and the outside of the octanu-
clear [Ti
4
(
7
)
4
]
8
octaanion [26].
Surprising results were obtained with the tripodal ligand H
6
-
9
(Figure 5.3) [28]. Equi-
molar amounts of H
6
-
9
and Ti(OPr)
4
react in the in the presence of Na
2
CO
3
in methanol
to give a deep red solution (l
max
¼
H
540 nm), typical for the [Ti(bdt)
3
]
2
chromophore
[46]. The addition of (Me
3
PhN)Cl to the methanolic solution yielded a deep red pre-
cipitate which was shown to contain a dinuclear complex anion in compound
(Me
3
PhN)
4
[Ti
2
(
9
)
2
] (Scheme 5.5) instead of the expected M
4
L
4
complex [28].
Figure 5.13 (left) shows the molecular structure of the trianion [Me
3
PhN
Ti
2
(
9
)
2
]
3
with one Me
3
PhN
þ
cation encapsulated within the interior of the cavity. To the best of
our knowledge, the complex anion [Ti
2
(
9
)
2
]
4
constitutes the first example of two C
3
-
symmetric ligands forming a dinuclear complex, with two ligand arms of each ligand
coordinating to one metal center and the remaining one coordinating to the second metal
center (and vice versa).
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