Spectroscopic Characterization of the Europium(III) Complexes of a Series of N,N'-bis(carboxymethyl) Macrocyclic Ether Bis(lactones)
The Eu3+ and Y3+ complexes of a series of N,N'-bis(carboxymethyl) macrocyclic ether bis(lactone) ligands derived from ethyl-enediaminetetraacetic acid (EDTA) were characterized in solution by using Eu3+ laser-induced luminescence and 1H and 13CNMR spectroscopy. The complexation of EDTA was also studied for comparison purposes. These ligands form 1:1 complexes with Eu3+ at the concentrations studied (10 µM), with the luminescence lifetimes in H2O and D2O providing the number of coordinated water molecules. The 7F0 -> 5D0 excitation spectra indicate that for each of the complexes two isomers are present in fast exchange on the
5D0 time scale. The spectra obtained for the macrocyclic complexes are identical with that of [EuEDTA]-, suggesting that the ether moieties of the macrocyclic ligands are not involved in direct coordination to the metal ion. These results are corroborated by the 1H and 13CNMR spectra of the Y3+ complexes. Changes in the coordination environment of the Eu3+ ion as a function of pH and temperature were observed for each complex. These results suggest that two species, which involve different numbers of coordinated carboxylate moieties, are present in rapid interconversion in solution. The interchange rates between the two species observed in aqueous solution decreases to the slow exchange limit in methanol, allowing each species to be probed independently.