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resulting metallocycles and two nitrogens capable of coordinating
to metal centers. Therefore, this approach allows us to combine the
coordination bond (thermodynamic control and high yields) with
the presence of π-acceptor units in molecular receptors, suitable
for recognition of aromatic substrates with π-donor properties by
means of establishment of π
interactions. Besides these electronic
characteristics, molecular recognition events demand, shape and
size complementarity to achieve a perfect fitting when the host binds
the guest. Therefore, the π-acceptor units must be about 7.0 Å apart,
the ideal distance to maximize π-stacking interactions, to induce
the insertion of an aromatic system between the
-
π
-
bipyridinium moieties. The insertion of two aromatic units becomes
possible if the distance is increased to ca. 10.4 Å.
N
-monoalkyl-4,4
′
N
N
N
N
X
N
N
N
L1
L3
N
L5
X = N
+
L6
X = C
N
N
N
N
N
N
N
L2
N
N
L4
N
N
L7
N
Figure 11.2
Examples
of
ligands
based
on
N
-monoalkyl-4,4
′
-
bipyridinium.
Self-Assembly of Pd
II
and Pt
II
Metallocycles
11.3
The metallocycles can be obtained by self-assembly of the ligands
with square-planar palladium (II) and platinum (II) complexes
with two labile ligands at
positions, easily displaced by the
corresponding ligands, while the other two
cis
positions are
protected by an ethylenediamine (en) group. Since the pyridine-Pd
cis
II
coordinative bond forms reversibly due to the lability of this bond at
room temperature, a rapid equilibrium between these two species
is reached instantaneously. The equilibrium is shifted completely
towards metallocycle formation as the most thermodynamically
favored species. The solubility of ligands can be controlled by
anion exchange. Although nitrates and halides are soluble in water,
in most cases salts containing relatively large anions such as
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