F-Element complexation by diphosphonate ligands

Abstract

To develop a fundamental understanding of the nature of lanthanide and actinide coordination complexes with ligands bearing phosphonate groups, we have examined the kinetics, thermodynamics and spectroscopy of lanthanide and actinide complexes with phosphonate ligands in aqueous solutions and resolved crystal structures for selected lanthanide complexes. The combined results of two-phase distribution experiments, titration calorimetry, laser-induced fluorescence decay, and molecular mechanics modeling have been used to infer the. structure of 1:2 complexes of Eu(III) with gem-diphosphonic acids in aqueous solution. A complementary crystallographic and FTIR investigation of the structure of lanthanide complexes in the solid state indicates a general similarity between amorphous and crystalline complexes, differing in the relative hydration of the solid. There are no inner sphere water molecules in the solid state. Parallel studies of the kinetics of formation and dissociation of f-element diphosphonate complexes demonstrate that the relative stability of these complexes is governed primarily by differences in the rate of complex formation. A related investigation has established the existence of a correlation between complex formation rate and cationic radius which is independent of the charge on the cation.