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.