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opposite chiralities. This leads again to the formation of a
meso
-complex. Additional
bond parameters in the [Et
4
N
Ti
2
(
6
)
3
]
3
anion are similar to those found for the
[Ti
2
(
6
)
3
]
4
anion. Long and variable N
S separations (range 2.981-4.132 A
)exclude
the presence of N
S hydrogen bonds. The encapsulated tetraethylammonium cation
does not lead to a second set of resonances for the NEt
4
þ
cations, which would indicate
that it is rapidly exchanged in solution. This assumption is corroborated by the observa-
tion of only one set of resonances for all four tetraethylammonium cations and one set of
resonances for the three ligand strands in the NMR spectra.
Finally, the complex anion [Ti
2
(
6
)
3
]
4
was prepared in the presence of Bu
4
N
þ
cations,
leading to the compound (Bu
4
N)
4
[Ti
2
(
6
)
3
] [43b]. The molecular structure of the complex
anion is shown in Figure 5.11 (right). Again, each titanium atom is surrounded by six
sulfur atoms in a strongly distorted octahedral fashion. Surprisingly in this case, both tita-
nium atoms possess the same absolute configuration L, and therefore this tetraanion
[Ti
2
(
6
)
3
]
4
is a dinuclear, triple-stranded helicate with a calculated helix angle of 58.2
.
Since (Bu
4
N)
4
[Ti
2
(
6
)
3
] crystallized in the centrosymmetric space group
C
2/
c
,boththe
L,L and D,D enantiomers are present in the same crystal. A comparison of the molecular
structure of the helical (Bu
4
N)
4
[Ti
2
(
6
)
3
] to both
meso
-complexes (Ph
4
As)
4
[Ti
2
(
6
)
3
]and
(Et
4
N)
3
[Et
4
NTi
2
(
6
)
3
] revealed similar Ti-S distances and STiS bite angles. Since
only the size of the counter ions varies in the three compounds, they are most likely
responsible for the structural differences with the encapsulated tetraethyl ammonium cat-
ion, accounting for the structural differences between the two
meso
-complexes
(Ph
4
As)
4
[Ti
2
(
6
)
3
] and (Et
4
N)
3
[Et
4
N
H
Ti
2
(
6
)
3
].
Ligand H
4
-
6
is the first bis(benzene-
o
-dithiolato) derivative which has been shown to
be capable of forming isomeric complexes (
meso
-complex and helicate) in metal-directed
self-assembly reactions with Ti
4þ
. The formation of isomeric complex anions [Ti
2
(
6
)
3
]
4
is most likely caused by the different cations present during their synthesis. The influence
of the counterions on the coordination geometry of mononuclear tungsten and molybde-
num complexes of type [M(bdt)
3
]
n
has been previously studied [20].
A detailed analysis using density functional theory showed different mechanisms for
the L!D interconversion of tris(catecholato) and tris(benzene-
o
-dithiolato) titanium
complexes [43b]. The L!D interconversion of the mononuclear [Ti(bdt)
3
]
2
complex
proceeds via a C
3h
-symmetric transition structure while a D
3h
transition state was previ-
ously found for the [Ti(cat)
3
]
4
complex anion. The DFT calculations also reveal a small
activation energy for the L!D interconversion which transforms a
meso
-complex anion
of type [Ti
2
(
3
)]
4
into the helicate.
Initially it was surprising that the dicatechol ligand H
4
-
F
reacts with metal ions under
the formation of the tetranuclear cluster anion [Et
4
N
Ga
III
,Fe
III
) con-
taining an encapsulated tetraethylammonium cation [23] while the analogous bis(ben-
zene-
o
-dithiol) ligand H
4
-
6
, both in the absence and in the presence of
tetraalkylammonium cations, gave only the dinuclear complexes [Ti
2
(
6
)
3
]
4
and
[Et
4
N
M
4
(
F
)
6
]
11
(M
¼
Ti
2
(
6
)
3
]
3
, respectively. A close inspection of the coordination mode of dicate-
cholato and bis(benzene-
o
-dithiolato) ligands reveals significant differences which might
well explain the different coordination chemistry. The MO
2
C
6
H
4
fragment in dinuclear or
tetranuclear tetrahedral complexes with dicatecholato ligands is almost planar, while the
TiS
2
C
6
H
4
fragments in [Ti
2
(
6
)
3
]
4
,[Et
4
N
Ti
2
(
6
)
3
]
3
and other [TiL(bdt)
x
] complexes
[33] is severely bent about the S
S vector (see Figures 5.2 and 5.5). This bend changes
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