Electrochemical studies of metal-ligand equilibria involving chelating ligands

Abstract

Metal-ligand models and complex stability constants of newly synthesised chelating ligands, N,N’-bis(2-hydroxycyclopentyl)-ethylenediamine (Cyp2EN) and N,N’-bis(2-hydroxycyclohexyl)-ethylenediamine (Cy2EN), with metal ions Cd2+, Cu2+, Ni2+, Pb2+ and Zn2+ were established in this work. Stability constants were determined by Glass Electrode Potentiometry (GEP) and polarography as electrochemical techniques. A new concept, termed Virtual Potentiometry (VP), was also used for the evaluation of stability constants. In this concept, polarographic or polarographic + potentiometric data were evaluated using potentiometric computer software (ESTA). This concept assisted in obtaining a final model for Cd–Cyp2EN and Pb–Cyp2EN systems. It could refine M(HL) complex inaccessible via polarographic study of Pb–Cyp2EN system and refined the hydroxo–complex ML2(OH) that in turn was inaccessible using GEP for Cd– Cyp2EN system. For all metals studied, the complexes ML formed with the ligand Cy2EN were found more stable than the complexes ML formed with the ligand Cyp2EN. The complex M(HL) was obtained for all systems studied, but it seemed to be a minor species. The complex ML2 was obtained in different systems studied with the ligand Cy2EN while this complex was only found in Cd–Cyp2EN system. In several systems potentiometric (ESTA) and voltammetric (3D–CFC) software could not distinguish which hydroxo–complexes were present as these species were formed in the pH range where the ligand was fully deprotonated. Selectivity trends for Cyp2EN and Cy2EN were compared and related to DHEEN as a function of metal ion radius. It was observed that the large metal ions were favoured by the addition of cyclopentyl bridges in DHEEN while the small metal ions were favoured when cyclohexyl bridges were added.