Synthesis, characterisation and complexation behaviour of a series of pyridyl- and bipyridyl-based hydrogel membranes

Abstract

The syntheses of hydrophilic membranes composed of copolymers of 4-vinylpyridine or 4-vinyl-4′-methyl-2,2′-bipyridine with 2-hydroxyethyl methacrylate are described. These coordinating membranes readily take up and complex divalent metal ions (Cu, Co and Fe) from aqueous solution. The types of complexes formed are identified by comparison of analytical and spectroscopic data for the immobilised species with those for the complexes of the monomeric ligands in aqueous solution. Changes in the number or stereochemical arrangements of coordinated ligands occur when the hydrogel membranes are dehydrated. Additionally, it is demonstrated that the redox chemistry of the metal centre can be exploited as if in solution. In the case of 4-vinyl-4′-methyl-2,2′-bipyridine, pre-complexed copolymer membranes were prepared from the tris complexes of Co^II, Fe^II and Ru^II Data from these specimens can be used in the calculation of the total ion-binding capacity of other membranes. For Fe^II ion-binding, up to 80% of the ligands present readily become involved in tris complex formation. This reflects the high degree of polymer-chain mobility possessed by the hydrogel. However, the extent of Fe^II coordination is shown to be anion dependent, with the concentration of Fe^II partitioned within the gel matrix influencing the position of an equilibrium that is established between the mono and tris complexes.