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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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