The thermodynamics of the formation of Sm(III) mixed-ligand complexes carrying α-amino acids

Taking into account the low stability of the Sm(III)–amino acids compounds and searching for thermodynamically stable compounds which contain biologically relevant ligands in the coordination sphere, mixed-ligand compounds have been studied by potentiometric measurements. The strategy was to use a stable core with Sm(III) and polycarboxylates (edta, nta, ida, etc.) and to substitute the labile positions (occupied by water molecules) by amino acids. Experiments were performed in aqueous solution at 37.0°C and 0.15 M NaClO4, resembling physiological medium. In all cases, mixed-ligand species were detected in solution. If cores like nta or ida were used, it was possible to get amino acid containing complexes stable enough to be used at physiological pH, avoiding precipitation of Sm(OH)3. Single-crystal X-ray structure of K2.5(H3O)0.5[Sm(nta)2(H2O)]·5H2O is also reported. A nine coordinate Sm atom is found, surrounded by two N atoms and six O atoms from two nta ligands and one O from a water molecule. Geometry around Sm can be described as a tricapped trigonal prism.