Zn2+ and Cu2+ complexes of a Fluorescent Scorpiand-type Oxadiazole Azamacrocycle Ligand: Crystal Structures, Solution Study and Optical Properties
A ligand constituted by a macrocyclic pyridinophane core having a pendant arm containing a secondary amine group linked through a methylene spacer to a pyridyl-oxadiazole-phenyl (PyPD) fluorescent system has been prepared (L). The crystal structures of [ZnL](ClO4)2 and [CuL](ClO4)2 show that the M2+ is coordinated by all the nitrogen atoms of the macrocyclic core, the secondary amine of the pendant arm and the nitrogen atom of the pyridine group of the fluorescent moiety, being this latter bond clearly weaker than the one with the pyridine of the macrocycle. Solution studies showed the formation of a highly stable Cu2+ complex of 1:1 stoichiometry, whereas with Zn2+ least stable complexes were formed and, given the right conditions, also a [Zn3L2]6+ specie was detected that was not possible to isolate in the solid state. Following Zn2+ coordination, a strong chelation enhancement of the fluorescence was observed, a behaviour that was not registered with any of the other metal cations tested.