Formation double-strand dimetallic helicates with a terpyridine-based macrocycle

The macrocyclic ligand (L), containing twoterpyridine (terpy) and two ethylenediamine (en) groups arrangedin a cyclic terpy-en-terpy-en sequence, forms a double-strandhelicate Cu2L4+ complex made especially stable by the formation ofinterstrand π−π stacking interactions involving opposite pyridinerings. The crystal structure of this complex shows the Cu2+ cationsin square pyramidal coordination environments defined by thedonor atoms of half ligand chain composed, in sequence, by onepyridine ring, the connected ethylenediamine moiety and the twoadjacent pyridine rings of the successive terpyridine. In aqueoussolution, L forms both mono- and binuclear complexes with Cu2+.The stability constants determined for these complexes evidencethe combined action of the two metal ions in the assembly of the very stable helicate species, the binding of the first metal ionfavoring the entrance of the second one. UV adsorption and emission spectra corroborate these equilibrium results. Furthermore,the Cu2L4+ complex shows a significant inertness toward dissociation in acidic solutions. Also Zn2+ forms mono- and binuclearcomplexes with L, although the Zn2L4+ complex is much weaker than the Cu2L4+ helicate and gives rise to fast dissociationreactions in acidic media. Experimental evidence allows neither to say that also the Zn2+ complex has a helicate structure nor toexclude it.