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Figure 5.9 Molecular structure of the dinuclear complex anion [Ti
2
(5)
2
(m-OCH3)
2
]
2
.
they are connected to, which excludes the presence of intramolecular N
S hydro-
gen bonds. A similar bonding situation as in [Ti
2
(
5
)
2
(m-OCH3)
2
]
2
has been found in the
Ti
IV
complex bearing the analogous dicatecholato ligand where, however, strong intra-
molecular N
H
H
O hydrogen bonds have been observed [39].
5.2.3 Dinuclear Triple-Stranded Complexes
Metallohelicates with N- and O-donors belong to the most thoroughly studied compounds
in metallosupramolecular chemistry [5,40]. While the first metallohelicates derived from
dicatechol ligands were reported almost 20 years ago [5], the first dinuclear triple-
stranded complexes bearing bis(benzene-
o
-dithiolato) ligands were only described in
2006. Reaction of compounds H
4
-
3
[39,41] or H
4
-
4
[39] with [Ti(OC
2
H
5
)
4
] in the pres-
ence of Li
2
CO
3
yielded the salts Li
4
[Ti
2
(
3
)
3
]andLi
4
[Ti
2
(
4
)
3
]. Cation exchange with
(PNP)Cl gave the salts Li(PNP)
3
[Ti
2
(
3
)
3
] and (PNP)
4
[Ti
2
(
4
)
3
] which could be crystallized
from DMF as DMF solvates. In the case of Li
4
[Ti
2
(
3
)
3
] only three of the four lithium
cations could be exchanged, even if a large excess of (PNP)Cl was used in the salt metath-
esis reaction [39,41].
The
1
H NMR spectra for both compounds show only one set of signals for the protons
of the three ligand strands, indicating C3-symmetry for the complex anions in solution.
The resonance signal for the NH protons in the complex anions is shifted downfield rela-
tive to the equivalent signal for the free ligands, indicating the presence of N
S
hydrogen bonds in a six-membered ring formed between the amide protons and the
o
-thiolato sulfur atoms. Both salts crystallize in centrosymmetric space groups. Figure
5.10 shows the molecular structures of the complex anions [Ti
2
(
3
)
3
]
4
(D,D isomer) and
[Ti
2
(
4
)
3
]
4
(L,L isomer). The molecular structure of the anion [Ti
2
(
4
)
3
]
4
resembles that
reported for the dinuclear triple-stranded helicate with the corresponding dicatecholato
ligand [5h]. In both complex anions [Ti
2
(
3
)
3
]
4
and [Ti
2
(
4
)
3
]
4
short nonbonding intra-
ligand N
H
S distances are found between the amide nitrogen atoms and the
ortho
-sulfur
atoms of the adjacent benzene-
o
-dithiolato group. Adjacent benzene-
o
-dithiolato and
amide subunits are arranged in a nearly coplanar fashion. The presence of intrastrand
N
S hydrogen bonds was already indicated by
1
H NMR spectroscopy, and
the observation of a small dihedral angle between the above-mentioned groups
supports their existence. As a consequence, the Ti
H
S bond length to the sulfur atoms in
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