Competitive bulk membrane transport and solvent extraction of transition and post transition metal ions using mixed-donor acyclic ligands as ionophores

Abstract

Competitive transport experiments involving metal ion transport from an aqueous source phase across a chloroform membrane phase into an aqueous receiving phase have been carried out using a series of open-chain mixed-donor ligands as the ionophore in the organic phase. The source phase contained equimolar concentrations of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II), with the source and receiving phases being buffered at pH 4.9 and 3.0, respectively. Clear transport selectivity for silver(I) was observed in all cases. This was also observed in a series of parallel solvent extraction experiments carried out under similar source and membrane phase conditions. Stability constant data confirm that all these ligands show significant affinity for silver(I) in methanol. An X-ray diffraction study of the silver(I) complex of a tripodal NS₃-donor ligand from the present series is reported. This complex occurs as two crystallographically independent molecules with both forms having a 1 ∶ 1 silver ∶ ligand ratio. The metal coordination geometry of the first form is distorted tetrahedral, with three coordination sites occupied by the three sulfur atoms of the ligand and the fourth site filled by an oxygen atom of the nitrate anion. The silver ion in the second form is five-coordinate being bound to the ligand's apical nitrogen as well as to its three sulfur donors. The remaining position is again occupied by a nitrate oxygen, with the metal coordination geometry in this case best described as distorted trigonal bipyramidal.