Coordination features of a terpyridine-containing polyamine receptor. Effect of protonation on the photophysical properties of the complexes

Abstract

The synthesis of the new terpyridine-containing macrocycle 2,6,10,14-tetraaza[15](6,6″)cyclo(2,2′:6′,2″)terpyridinophane (L) is reported. The ligand contains a tetraamine chain linking the 6,6″ positions of a terpyridine unit. A potentiometric, ¹H NMR, UV-vis spectrophotometric and fluorescence emission study on the basicity properties of L in aqueous solutions shows that the first four protonation steps occur on the polyamine chain, while the terpyridine nitrogens are involved in proton binding only in the last protonation step at strongly acidic pH values. Cu(II), Zn(II), Cd(II) and Pb(II) complexation was studied in aqueous solution by means of potentiometric, spectrophotometric and spectrofluorimetric measurements. Cu(II) and Zn(II) can form both mono- and dinuclear complexes in solution, while the larger Cd(II) and Pb(II) give only mononuclear complexes. In the [ML]²⁺ complexes (M = Zn(II) or Cd(II)) the metal is unequivocally bound to the terpyridine unit. Some amine groups are not coordinated and can quench the fluorescence emission of the terpyridine unit thanks to an electron transfer process. Protonation of the unbound amine groups inhibits the eT process, affording fluorescent [MLH_x]^(2+x)+ complexes.