Selectivity in transport

Abstract

A series of bis benzimidazole ligands, bridged by additional donor groups (ether, thioether) and with pendant carboxylic amide groups, was prepared. In three-phase transport experiments using one quadridentate ligand and metal nitrates, the sequence Ag⁺≈ Cu²⁺ > Cd²⁺ > Zn²⁺ > Pb²⁺ was observed for rates of single-ion transport. In pairwise comparisons of mixtures of two salts, potentially useful discriminations between Ag⁺ and Pb²⁺, Ag⁺/Cu²⁺, Ag⁺/Zn²⁺, Ag⁺/Cd²⁺, Cu²⁺/Zn²⁺, and Cd²⁺/Zn²⁺ were also observed. The complexation behaviour of several ligands with Pb²⁺ and Ag⁺ in a single phase was further screened in an attempt to explain the observed discrimination between these cations, either in solid or solution, with ¹H and ¹³C n.m.r., i.r., and fast atom bombardment (f.a.b.) mass spectrometric techniques. Evidence for complexes of Ag⁺ was found for each ligand, with each technique. However for Pb²⁺ only the ligands with benzimidazole plus ether donors showed any tendency to complex; although solids with a 1:1 ligand: metal salt composition could be isolated, only one could be verified as a complex in the f.a.b. experiment. Solid complexes with a 1:1 composition were isolated for the nitrate salts of Cd²⁺, Cu²⁺, and Zn²⁺.