New Chelating Agents based on 8-Hydroxyquinoline

Abstract

Several new potentially terdentate chelating agents based on the 8-hydroxyquinoline structure have been synthesized and characterized. Protonation constants of these ligands and formation constants of their chelates with selected metal-ions have been determined. Where possible, the chelates have been characterized by elemental analysis and other means. The results show that the ligand 2-(2'-thienyl)-8-hydroxyquinoline acts as a sterically hindering bidentate donor; the unusual relationship found between the formation constants of its bis-chelates (K1 < K2) has been explained on the basis of steric effects. The results of studies involving 4-amino-5-hydroxy-acridine and 4,5-dihydroxyacridine indicate that these ligands act as terdentate and bidentate donors, respectively. The failure of 4,5-dihydroxyacridine to act as a terdentate donor is explained in terms of chelate-ring strain.