Bisphosphonate chelating agents: complexation of Fe(III) and Al(III) by 1-phenyl-1-hydroxymethylene bisphosphonate and its analogues

Bisphosphonate ligands were found to be very efficient chelating agents for both Al(III) and Fe(III) ions. Potentiometric and spectroscopic data allow evaluation of the coordination equilibria in the solutions containing 1-phenyl-1-hydroxymethylene bisphosphonate and its three analogues with Al(III) and Fe(III) ions. At pH below 4 the bis-complexes are formed, while above pH 4 the major species are equimolar, monomeric for Al(III) or dimeric for Fe(III) complexes. The large steric hindrance and high electric charge are the major factors influencing the complex stoichiometry. The formation of the dimeric Fe(III) with μ-oxo or μ-hydroxo bridges were supported by measurements of the magnetic moments using Evans’ method. The stabilities of the complexes formed are higher than those found for deferiprone, L1, used in clinics as a chelating agent.