Kinetic and mechanistic aspects of iron(II) coordination to bipyridyl-based hydrogel pofymer membranes

Abstract

A range of hydrophilic membranes composed of copolymers of 4-methyl-4′-vinyl-2, 2′-bipyridyl with 2-hydroxyethyl methacrylate have been synthesized. These membranes readily coordinate iron(II) from aqueous solution to form the tris(2,2′-bipyridyl)iron(II) species, but at a rate very much slower than that of the free ligand in solution. Kinetic studies on the rate of development of these colour centres have shown the process to be anion-dominated and pseudo-first order for iron(II) sulfate and second order for the chloride and perchlorate. The extent of coordination within the membrane is also dependent on the salt used, as the anion influences the concentration of Fe^II partitioned within the gel matrix, and ultimately the position of the equilibrium established between the mono and tris complexes. Mechanisms are proposed incorporating the known water-structuring effects that anions impose on this type of hydrogel environment and accounting for the presence of ion pairs and their effect on the molecular reorganisations that are necessary in order for tris complexation to occur.