Polyamine Receptors Containing Dipyridine or Phenanthroline Units:Clues for the Design of Fluorescent Chemosensors for Metal Ions

The synthesis of the macrocyclic receptor L1, which contains a tetraamine chain linking the 6,6′-positions of a 2,2′-dipyridine moiety, is reported. Its basicity properties and complexation features toward CuII, ZnII, CdII and PbII have been studied in aqueous solutions by means of potentiometric, UV/Vis spectroscopy and fluorescence emission measurements and compared with those of ligand L2, in which a 1,10-phenanthroline moiety replaces the dipyridine unit of L1. In metal coordination, L1 shows a marked selectivity toward CdII over ZnII and PbII. The crystal structures of its metal complexes shows that L1 possesses a preferential tetradentate binding site for metal cations, composed of the dipyridine unit and the two adjacent benzylic amine groups. This binding site has the proper dimension and conformation to selectively coordinate the CdII ion, as confirmed by DFT calculations carried out on the complexes. This coordinative zone is lost in L2. The rigidity of phenanthroline does not allow the simultaneous binding of both the benzylic amine groups to ZnII and CdII and, in fact, one benzylic amine is not coordinated to these metal cations. The fluorescence emission properties of the L1 and L2 complexes are strongly pH dependent. Only the ZnII and CdII complexes with L1 display fluorescence emission at neutral pH. This feature is related to the formation in solution at pH 7 of emissive protonated complexes of the type [M(HxL)](2+x)+ (x=1–3), in which all the nitrogen donors are involved in metal or proton binding. The emissive characteristics of these protonated complexes are confirmed by the fluorescence emission spectra collected on the [Zn(HL1)Br][ClO4]2 and [Cd(HL1)Br][ClO4]2 solid compounds dissolved in CH3CN. Conversely, the ZnII and CdII complexes with L2 are not emissive; in fact, they contain a benzylic amine group not involved in metal or proton binding that can quench the fluorescence emission of the fluorophore, thanks to a photoinduced electron-transfer process.