Stabilizing Divalent Europium in Aqueous Solution Using Size-Discrimination and Electrostatic Effects

We report two macrocyclic ligands containing a 1,10-diaza-18-crown-6 fragment functionalized with either two picolinamide pendant arms (bpa18c6) or one picolinamide and one picolinate arm (ppa18c6–). The X-ray structure of [La­(ppa18c6)­(H2O)]2+ shows that the ligand binds to the metal ion using the six donor atoms of the crown moiety and the four donor atoms of the pendant arms, 11-coordination being completed by the presence of a coordinated water molecule. The X-ray structure of the [Sr­(bpa18c6)­(H2O)]2+ was also investigated due to the very similar ionic radii of Sr2+ and Eu2+. The structure of this complex is very similar to that of [La­(ppa18c6)­(H2O)]2+, with the metal ion being 11-coordinated. Potentiometric measurements were used to determine the stability constants of the complexes formed with La3+ and Eu3+. Both ligands present a very high selectivity for the large La3+ ion over the smaller Eu3+, with a size-discrimination ability that exceeds that of the analogous ligand containing two picolinate pendant arms reported previously (bp18c62–). DFT calculations using the TPSSh functional and the large-core pseudopotential approximation provided stability trends in good agreement with the experimental values, indicating that charge neutral ligands derived from 1,10-diaza-18-crown-6 enhance the selectivity of the ligand for the large Ln3+ ions. Cyclic voltammetry measurements show that the stabilization of Eu2+ by these ligands follows the sequence bp18c62– – 2–), −610 mV (ppa18c6–), and −453 mV (bpa18c6) versus Ag/AgCl. These values reveal that the complex of bpa18c6 possesses higher stability against oxidation than the aquated ion, for which an E1/2 value of −585 mV has been measured.