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Formation of Double-Strand Dimetallic Helicates with a Terpyridine-Based Macrocycle
The
macrocyclic ligand (L), containing two terpyridine (terpy) and two
ethylenediamine (en) groups arranged in a cyclic terpy-en-terpy-en
sequence, forms a double-strand helicate Cu<sub>2</sub>L<sup>4+</sup> complex made especially stable by the formation of interstrand π–π
stacking interactions involving opposite pyridine rings. The crystal
structure of this complex shows the Cu<sup>2+</sup> cations in square
pyramidal coordination environments defined by the donor atoms of
half ligand chain composed, in sequence, by one pyridine ring, the
connected ethylenediamine moiety and the two adjacent pyridine rings
of the successive terpyridine. In aqueous solution, L forms both mono-
and binuclear complexes with Cu<sup>2+</sup>. The stability constants
determined for these complexes evidence the combined action of the
two metal ions in the assembly of the very stable helicate species,
the binding of the first metal ion favoring the entrance of the second
one. UV adsorption and emission spectra corroborate these equilibrium
results. Furthermore, the Cu<sub>2</sub>L<sup>4+</sup> complex shows
a significant inertness toward dissociation in acidic solutions. Also
Zn<sup>2+</sup> forms mono- and binuclear complexes with L, although
the Zn<sub>2</sub>L<sup>4+</sup> complex is much weaker than the Cu<sub>2</sub>L<sup>4+</sup> helicate and gives rise to fast dissociation
reactions in acidic media. Experimental evidence allows neither to
say that also the Zn<sup>2+</sup> complex has a helicate structure
nor to exclude it