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hexafluorophosphate are only soluble in organic solvents. Thus, the
self-assembly process can be carried out either in aqueous or organic
media. Simply mixing at room temperature the stoichiometric
amounts of ligand and (en)Pd(NO
)
in water or (en)Pd(OTf )
in
3
2
2
CH
, leads to self-assembled metallocycles (Fig. 11.3).
The formation of these metallocycles is clearly supported by NMR
spectroscopy. Typically, the protons and carbons at the
CN or CH
NO
3
3
2
α
positions
of the pyridine rings that coordinate to the metal center show the
characteristic downfield shiftings expected for coordination of
the pyridyl nitrogen atom to the Pd center. Moreover, diffusion
ordered spectroscopy (DOSY) experiments show that the diffusion
coefficients of the new species are significantly smaller than those of
the free ligand or palladium complex and that both components are
part of the same species, which diffuses as a whole, as they show the
same diffusion coefficients.
Figure 11.3
Examples of metallocycles.
As described earlier, the lability of the coordination bonds
between pyridine units and Pd
centers may be useful to obtain
the desired structures under thermodynamic control in very
mild conditions. Nevertheless, in some cases, this lability may be
disadvantageous owing to the low stability of the structures obtained
whose identity can only be asserted in solution. The use of more
inert metal-ligand bonds should stabilize the metallomacrocyclic
structures
II
and
allow
their
isolation
and
a
more
accurate
II
characterization. Pt
square-planar metal centers, geometrically
analogous to those of Pd
, afford much more inert coordination bonds
with pyridine ligands. Furthermore, this coordination bond exhibits
II
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