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double characteristics owing to its inertness at low temperature,
which is lost when the temperature is increased. This dual feature
has been beautifully exploited by Fujita and co-workers introducing
the “molecular lock” concept [18,19]. Our “molecular lock” strategy
is similar to that originally developed by Fujita but with slight
modifications. In our case, after the lowering of the temperature,
the product can be precipitated as its hexafluorophosphate salt and
isolated from the original reaction media with yields between 50%
and 99% (Fig. 11.3). In general, the size and shape of palladium and
platinum metallocycles are very similar as it can be verified by DFT
calculations and X-ray crystallography. As a consequence of this
resemblance, their behavior as receptors is almost identical.
11.3.1
Dynamic Formation of Self-Organized
Corner-Connected Square Metallocycles by
Stoichiometric Control
The structure of a supramolecular assembly can be controlled by the
chemical and geometrical information encoded within both the metal
and the ligand components, which self-assemble in a predictable
manner. However, the use of more than two-component systems
should open the door for the preparation of supramolecules with
different shape and size under stoichiometric control. This implies
that a careful control of the ratio between the different building
blocks may be used to change the constitution of the resulting self-
assembled supramolecule at will. Therefore, the stoichiometric
control in combination with metal-directed self-assembly should
produce constitutional dynamic systems at the supramolecular
level.
Moving one step forward, the introduction of a third type of
building block on the self-assembly process (namely a Pd
II
II
or Pt
square-planar metal complex with four labile positions) opens the
possibility for the stoichiometry-dependent synthesis of oligomers
of the aforementioned metallocyclic “monomers.” This possibility
was explored by our research group for the dynamic synthesis of
self-organized corner-connected square metallocycles formed by
a 4,4
′
-bipyridinium-based bidentate ligand (
L5
), (en)Pd(NO
)
/
3
2
(en)Pd(OTf )
as the Pd
II
center with two labile
cis
positions and
2
Pd(MeCN)
(BF
)
as the metallic center with four labile ligands
4
4
2
[20].
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