A new dioxotetraamine ligand derived from binicotinic acid: synthesis, coordination, and fluorescence behaviour towards divalent transition metal ions

Abstract

A novel bipyridyl-based fluorescent system, N,N′-bis(2-aminoethyl)-2,2′-bipyridine-3,3′-dicarboxamide (BABD), with two different coordination sites, bidentate bipyridyl and tetradentate dioxotetraamine, has been synthesized, and characterized by elemental analysis and spectroscopic (UV–Vis, IR, 1H NMR, and 13C NMR) methods. The lowest energy molecular geometry of BABD was obtained by empirical then quantum mechanical treatment. The binding ability of BABD with H+, Co(II), Ni(II), Cu(II), and Zn(II) ions was investigated in aqueous 0.1 M KCl at 25 ± 1 °C by potentiometric methods. Four protonation constants were determined for BABD, and were used as input data to evaluate the formation constants of the metal complexes. The coordination behaviour of BABD in solution indicated that at low pH the bipyridyl unit coordinates to all metal ions, but at higher pH (>7.0) coordination of the dioxotetraamine unit occurs with the Cu(II) and Ni(II) ions only. The 3D-model structure of the metal complex was predicted by semi-empirical calculation using the AM1/d Hamiltonian. Fluorimetric titrations indicate that at pH 9 BABD exhibits fluorescence enhancement with increasing concentration of Cu(II) and Ni(II) ions, but at pH 7.2 fluorescence enhancement is observed only in the presence of Cu(II) ions. No remarkable effect on the fluorescence of BABD was observed in the presence of other biologically relevant metal ions, for example Ni(II), Fe(II), Mn(II), Co(II), Zn(II), Mg(II), Ca(II), and Hg(II).