Development of a Polyfunctional Dipodal Schiff Base: An Efficient Chelator and a Potential Zinc Sensor

Abstract

A novel polyfunctional dipodal ligand, L = N,N'-bis[2-[(2-hydroxy-1-naphthyl)methyleneamino]ethyl]propanediamide (DOTA2HNAP) was developed and characterized through elemental and spectral analyses. The complexation behavior of the ligand was investigated with Co2+, Cu2+, and Zn2+ metal ions by potentiometric and spectrophotometric methods in the H2O-DMSO mixture (99:1) at µ = 0.1M KCl and 25 ± 1 °C. Four protonation constants for –OH of naphtholate groups and –N of imine were determined for the ligand. The ligand forms monomeric complexes of ML type with the metal ions, where coordination occurs through N-imine and O-naphtholate donors (N2O2). In the case of a complex of copper, an additional species, MLH-2, was formed due to ionization of the amide groups in a higher pH. The minimum energy structures of the metal complexes in solution have been obtained through molecular modeling studies by using the semi-empirical/ PM3 method. The photophysical properties of DOTA2HNAP were investigated in the presence of a wide range of biologically relevant metal ions. The fluorescence emission of the ligand at 450 nm (λex = 361 nm) exhibited a remarkable enhancement with Zn2+ ions (1 equivalent) at physiological pH amongst all metal ions. Such behavior enables the ligand to be considered as a suitable model for the detection of Zn2+ towards environmental applications.