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The self-assembly of these palladium oligomeric structures (that
proceeds again under thermodynamic control) is extremely simple,
being accomplished instantly by mixing in aqueous or organic media
the components in the suitable molar ratio, and producing discrete
oligomeric species as a function of the relative proportions of building
blocks available (4:2:1 molar ratio for the dimer
M5a
2
, 6:2:2
for the
trimer
). The formation of
the different oligomers was supported by NMR spectroscopy. Apart
from the downfield shift of the corresponding pyridine protons and
carbons indicating complexation of the pyridine moieties to the metal
centers, new signals attributable to the oligomeric species appear,
exhibiting the adequate relative integration. In this occasion, DOSY
experiments were crucial in order to support the presence of only
one species in the solution. The value of the diffusion coefficients
decreases from monomer to tetramer indicating a rise in the
hydrodynamic radius. Moreover, the calculated diffusion coefficients
from geometrical parameters were in excellent agreement with the
experimental values.
The utilization of the previously described “molecular lock”
strategy, by using the appropriate Pt
M5a
3
, and 8:2:3 for the tetramer
M5a
4
II
complexes, allowed us to
isolate and characterize the dimeric and trimeric Pt
II
analogues by
mass spectrometry, providing additional insights into the formation
of the oligomeric metallocycles.
At thermodynamic equilibrium in a true dynamic system, external
physical or chemical stimuli have to afford new constitutional
species, which are thermodynamically more stable under these new
conditions. Thus, we designed a
1
H NMR experiment to visualize
the dynamic behavior of the three-component system under
stoichiometric control. With the monomer
M5a
·8NO
as starting
3
point, the addition of two equiv. of ligand
L5
·2NO
and one equiv.
3
of Pd(CH
led to the formation of the dimer, a second
addition of two equiv. of
CN)
·2BF
3
4
4
L5
·2NO
and one equiv. of Pd(CH
CN)
·2BF
4
afforded the trimer, and finally after a third addition the system
evolved producing the tetramer in an increasing-size process. From
the solution of the tetramer, a decreasing route to the dimer can be
followed by successive addition of one equiv. of
3
3
4
L5
·2NO
and one
3
equiv. of (en)Pd(NO
(Scheme 11.1). This experiment shows that
the system can be self-assembled into new constitutional species of
increasing (growing route) or decreasing complexity (decreasing
route) by means of stoichiometric control.
)
3
2
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