Chemistry Reference
In-Depth Information
Each titanium atom in [Ti
2
(
9
)
2
]
4
is coordinated by six sulfur atoms in a strongly dis-
torted octahedral fashion. Both titanium atoms exhibit the same absolute configuration,
namely D in the depicted complex anion. Each {TiS
6
}moietyincomplexanion
[Ti
2
(
9
)
2
]
4
shows two long N
3.5 A
S separations (
) and one short N
S separation
2.9 A
(
), indicative for the formation of just one N
H
S hydrogen bond per {TiS
6
}
unit.
A side view of the complex anion (Figure 5.13, right) shows the uncommon banana-
like shape of the anion. Remarkable is the very long Ti
Ti distance of about 19 A
. Since
the intramolecular separation of the triazine moieties measures about 9 A
, the two ligands
span a large groove and the complex anion can serve as a molecular host. The molecular
structure determination shows one Me
3
PhN
þ
cation located inside this cavity (Figure
5.13, left), while the other three cations remain outside maintaining no remarkable inter-
actions with the complex anion.
Encapsulation and release from the cavity in solution is too fast to be detected by NMR
spectroscopy. Even at low temperatures neither a broadening of the proton resonances nor
a chemical shift to a higher field was observed for the Me
3
PhN
þ
cation, as would be
expected for an encapsulated cation.
5.3 Coordination Chemistry of Mixed Bis(Benzene-
o
-Dithiol)/Catechol
Ligands
Tetradentate ligands with different donor groups (directional ligands) [47,48] like
benzene-
o
-dithiol/catechol derivatives are of special interest, since they offer the
opportunity to prepare heterodinuclear complexes (different preferences of the donor
groups for different metal ions) as well as complexes with a different orientation of
the ligand strands (parallel or antiparallel). The versatile coordination chemistry of
bis(benzene-
o
-dithiol) ligands described in Section 5.2 prompted attempts to substi-
tute one of the benzene-
o
-dithiol groups for a catechol group, leading to the ligands
H
4
-
10
-H
4
-
13
(Figure 5.4).
5.3.1 Dinuclear Double-Stranded Complexes
Initial experiments regarding the coordination chemisty of directional benzene-
o
-dithiol/
catechol ligands were undertaken with the ligand H
4
-
11
which was reacted with
[TiO(acac)
2
]andNa
2
CO
3
. Since the analogous bis(benzene-
o
-dithiol) ligand H
4
-
3
[41]
(Figure 5.3) and its catechol anologue [40a] with the same topology as H
4
-
11
both form
dinuclear triple-stranded helicates [Ti
2
L
3
]
4
, the formation of a triple-stranded helicate
Na
4
[Ti
2
(
11
)
3
] was expected. Surprisingly, all attempts to prepare the triple-standed com-
plex anion [Ti
2
(
11
)
3
]
4
failed and the double-stranded complex Na
2
[Ti
2
(
11
)
2
(m-OCH
3
)
2
]
was exclusively obtained [49].
The directionality of the ligand
11
4
enables the formation of two regioisomeric com-
plex anions [Ti
2
(
11
)
2
(m-OCH
3
)
2
]
2
, one with a parallel orientation of the ligand strands
and one with an antiparallel orientation. In addition, the chirality at the metal centers
allows the generation of up to seven stereoisomers (Figure 5.14). Since the ligand H
4
-
11
is achiral, six of the seven stereoisomers form three pairs of enantiomers.
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