Regueiro-Figueroa, M., Barriada, J. L., Pallier, A., Esteban-Gómez, D., de Blas, A., Rodríguez-Blas, T., Tóth, É., Platas-Iglesias, C.
Inorganic Chemistry (2015) 54 (10) 4940-4952 - doi : 10.1021/acs.inorgchem.5b00548
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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 Sr(2+) and Eu(2+). 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 La(3+) and Eu(3+). Both ligands present a very high selectivity for the large La(3+) ion over the smaller Eu(3+), with a size-discrimination ability that exceeds that of the analogous ligand containing two picolinate pendant arms reported previously (bp18c6(2-)). 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 Ln(3+) ions. Cyclic voltammetry measurements show that the stabilization of Eu(2+) by these ligands follows the sequence bp18c6(2-) < ppa18c6(-) < bpa18c6 with half-wave potentials of -753 mV (bp18c6(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.