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Figure 5.15 Molecular structure of the anion [Ti
2
(12)
2
(m-OCH
3
)
2
]
2
.
The double-stranded complex anion [Ti
2
(
12
)
2
(m-OCH
3
)
2
]
2
was obtained from [TiO
(acac)
2
] and the
o
-phenylenediamine bridged benzene-
o
-dithiol/catechol ligand H
4
-
12
in
methanol at room temperature [49]. Contrary to the observation for the anion [Ti
2
(
11
)
2
(m-
OCH
3
)
2
]
2
,
1
H NMR spectroscopy indicated the formation of only one pair of enantiom-
ers in solution. Only the amide protons linked to the catechoylamido groups form strong
hydrogen bonds with the catecholato oxygen atoms, which leads to a downfield shift of
the resonance for these protons in the complex anion relative to the resonance of the free
ligand H
4
-
12
. The formation of N
S hydrogen bonds was not observed for the com-
plex [Ti
2
(
12
)
2
(m-OCH
3
)
2
]
2
in solution. An X-ray diffraction analysis with single crystals
of (PNP)
2
[Ti
2
(
12
)
2
(m-OCH
3
)
2
] revealed a parallel orientation of the ligand strands (Fig-
ure 5.15). The reasons for the parallel orientation of the ligand strands is still subject to
speculation. It is reasonable to assume that the formation of strong N
H
O hydrogen
bonds, as observed in the solid state, has an influence on the orientation of the ligand
strands.
Preliminary experiments with an excess of the ligand H
4
-
12
and [TiO(acac)
2
]/Na
2
CO
3
at elevated temperature indicated, via
1
H NMR spectroscopy, the formation of a dinuclear
triple-stranded complex. We assigned this behavior to the elevated reaction temperature.
1
H NMR investigations showed that the isomers with a parallel orientation and with an
antiparallel orientation of ligand strands coexist in solution [49].
H
5.3.2 Dinuclear Triple-Stranded Complexes
Ligands H
4
-
10
and H
4
-
13
react with [TiO(acac)
2
] and Na
2
CO
3
in methanol to give exclu-
sively the triple-stranded helicates Na
4
[Ti
2
(
10
)
3
] [29,49] and Na
4
[Ti
2
(
13
)
3
][49].
Attempts to prepare a dinuclear double-stranded complex Na
2
[Ti
2
(
10
)
2
(m-OCH
3
)
2
]
related to Na
2
[Ti
2
(
11
)
2
(m-OCH
3
)
2
] failed, although H
4
-
10
and H
4
-
11
have the same lig-
and backbone, except for the methyl groups of the bridging unit in H
4
-
10
.Attemptsto
exchange all four sodium cations by PNP
รพ
cations failed in both cases, even after addition
of up to 10 equivalents of (PNP)Cl to methanolic solutions of Na
4
[Ti
2
(
10
)
3
]and
Na
4
[Ti
2
(
13
)
3
]. Only three sodium cations could be exchanged, resulting in the formation
of the complexes Na(PNP)
3
[Ti
2
(
10
)
3
] [29,49] and Na(PNP)
3
[Ti
2
(
13
)
3
] [49].
1
H NMR spectra of Na(PNP)
3
[Ti
2
(
10
)
3
] [29,49] showed only one set of signals for the
ligand strands, which demonstrated the exclusive formation of the geometrical isomer
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