Anion Control Selectivity of Neutral N4-Type Schiff Base Extractants Towards Transition Metal Ions

Two N4-type Schiff base ligands named N,N'-bis(2-pyridylmethylidene)-1,2-diiminoethane (L1) and N,N'-bis(2-pyridylmethylidene)-1,3-diiminopropane (L2) have been synthesized by the reaction of pyridine-2-carbaldehyde with ethylenediamine and propylenediamine, respectively. The binding abilities of L1 and L2 in dichloromethane towards Cu2+, Cd2+, Co2+, Ni2+, Zn2+ and Pb2+ ions were evaluated by performing competitive extraction experiments on the mixtures containing theses ions from aqueous picrate and nitrate media as a function of the aqueous phase pH. The results show a pronounced selectivity of the ligands towards copper and cadmium ions among the tested metal ions. It is shown that the copper/cadmium selectivity of the ligands is profoundly influenced by the nature of anions accompanying the extracted complexes. In fact, the ligands are more efficient complexing agent for cadmium ions in the presence of nitrate and acetate anions, whereas this selectivity changes towards copper ions in the presence of picrate, perchlorate and thiocyanat ions.