Lanthanide complexes with 1-hydroxyethane-1,1-diphosphonic acid: solvent organization and coordination geometry in crystalline and amorphous solids

Abstract

In has been suggested in an earlier report that a 1:2 complex between europium(III) and 1-hydroxyethane-1,1-diphosphonic acid, Eu(H3HEDP)2, adopts an unexpected coordination geometry in aqueous solution. Thermodynamic and spectroscopic results were interpreted to indicate extensive hydrogen bonding within the complex, and between the complex and the water molecules in its second coordination sphere. In an attempt to confirm some of these features, samples of six amorphous and ten crystalline 1:2 complexes of selected lanthanides with HEDP have been prepared and characterized by single crystal X-ray diffraction, FT-IR spectroscopy and thermogravimetric analysis. For the heavy lanthanides (TbLu plus Y) the crystals are polymeric chains in which the metal ions are seven-coordinate, five water molecules are dispersed in interlamellar spaces, and the fifth hydrogen ion is bound to a free phosphonate oxygen on HEDP. The structure parameters for Tb(H3HEDP)(H2HEDP)·5H2O (representative of the heavy lanthanides) are a = 10.465, b = 11.415, c = 17.252Å, α = 17.90, β = 94.22, γ = 90° and the crystal system is monoclinic. In the corresponding crystalline complexes of the light lanthanides (Eu and Nd), the metal ion is eight-coordinate and the hydration numbers are higher. The fifth hydrogen ion is not closely associated with the phosphonate group in the Eu complex but is bound in the Nd crystal. The crystal indices for [H3O] [Eu(H2HEDP)2]·12H2O are a = 10.0988, b = 11.4846, c = 11.9303Å, α = 99.637, β = 91.277, γ = 95.828°. The corresponding values for Nd(H3HEDP)(H2HEDP)·TH2O are a = 9.318, b = 10.272, c = 11.766Å, α = 91.09, β = 111.24, γ = 98.50°. Both crystallize in the triclinic crystal system. The amorphous analogs for both light and heavy lanthanides are more strongly hydrate (Ln(H3HEDP)(H2HEDP)·18H2O). FT-IR spectra confirm that there are no inner sphere water molecules in either the crystalline or amorphous solids, but the water stretching and bending frequencies are proportional to the cation radius in the amorphous species (though not in the crystalline analogs). The crystal position parameters indicate extensive hydrogen bonding between the interstitial water molecules and the bound ligands, and a hydrogen bonding interaction between the bound ligands. Though the polymeric nature of the solids is different from that in solution, the solvent organization features are consistent with the previously postulated structure of the 1:2 complex between Eu(III) and HEDP in aqueous solution.