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– < ppa18c6– < bpa18c6 with half-wave potentials
of −753 mV (bp18c62–), −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.