A computational study about the lability of H₂O molecules in complexes with lanthanide ions and EDTA ligand

Full text: Complexes with trivalent lanthanide ions Ln³⁺ have been extensively studied because of the diversity of their properties and applications, such as contrast agents for diagnosis by NMR imaging, fluorescent lighting materials, luminescent chemosensors and (electro)luminescent devices. These and other applications may be influenced by the structural diversity and electron configuration presented by complexes with Ln³⁺. Furthermore, the unpaired 4f-electrons and the lability of water ligands in these complexes may have implications in their magnetic properties. Coordination compounds containing Ln³⁺ generally have coordination numbers of 8 or 9 in aqueous solution, such as aqua-complexes [Ln(H₂O)_n]³⁺. The addition of the EDTA chelating agent, which has six coordination sites, leads to the formation of complexes such as [Ln(EDTA)(H₂O)_m]^- (with m = 2-3), which are highly stable; however, the water ligands are very labile. Thus, in this study we have studied the lability of water molecules in complexes containing Ln³⁺ and EDTA ligand using electronic structure methods (DFT), implemented in the Gaussian09 program. Our results suggest that the water molecules coordinated to the complex [Ln(EDTA)(H₂O)_m]^- have high lability, presenting low values of activation energy barriers for de-coordination the water molecules into the solution. In the complex [Eu(EDTA)(H₂O)₂]^-, this value was ca. 20 kJ/mol. The barriers obtained with the La³⁺ and Lu³⁺ complexes correlate quite well with their ionic radius. This lability of water ligands is important to explain, predict and design new stable complexes with magnetization transfer properties, as well as to understand the structural fluxionality mechanisms, which may alter several properties of the complex in solution. (author)