Kinetic and equilibrium studies of the complexations of polymer-bound imidazole ligands with a macrocylic cobalt(III) complex

Abstract

The rate and equilibrium constants of the first- and second-step complexations of the axial sites of a macrocyclic Co^III complex, Co(N₄)(OH₂)³⁺₂(N₄= Me₆[14]4,11-dieneN₄), with polymer-bound imidazole ligands such as N-vinylimidazole-co-N-vinylpyrrolidone (Plm) and partially quarternized poly(N-vinylimidazole)(QPlm) in aqueous solutions have been determined by means of spectrophotometry at pH 6.5. A unique aspect of the behaviour of the complex formation of the polymer ligands is that the formation of Co(N₄)(Plm)³⁺₂ from Co(N₄)(B)(Plm)^m+(m= 3 for B = H₂O or m= 2 for B = OH^–) proceeds via an intramolecular process, whereas that of the corresponding ligation for the non-polymer system follows second-order kinetics. For the polymer system the concentration ratio, [Co(N₄)(Plm)³⁺₂]/[Co(N₄)(B)(Plm)^m+], at the equilibrium state is independent of the concentration of the polymer ligand. The effect of polymer ligand on the rate and equilibrium of the complex formations are discussed in the light of the results obtained.