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 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.
†Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Martín
M
Regueiro-Figueroa
‡Departamento de Química Física e Enxeñaría Química I, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
José Luis
JL
Barriada
§Centre de Biophysique Moléculaire, CNRS, Rue Charles Sadron, 45071 Orléans, Cedex 2, France.
Agnès
A
Pallier
†Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
David
D
Esteban-Gómez
†Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Andrés de
Ad
Blas
†Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Teresa
T
Rodríguez-Blas
§Centre de Biophysique Moléculaire, CNRS, Rue Charles Sadron, 45071 Orléans, Cedex 2, France.
Éva
É
Tóth
†Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Carlos
C
Platas-Iglesias
D000577:Amides / Q000737:chemistry; D001372:Aza Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D002413:Cations, Divalent; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D043844:Crown Ethers / Q000138:chemical synthesis / Q000737:chemistry*; D055664:Electrochemical Techniques; D005063:Europium / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010848:Picolinic Acids / Q000737:chemistry; D011789:Quantum Theory; D055672:Static Electricity; D014867:Water / Q000737:chemistry*
D000577:Amides; D001372:Aza Compounds; D002413:Cations, Divalent; D056831:Coordination Complexes; D043844:Crown Ethers; D008024:Ligands; D010848:Picolinic Acids; D014867:Water; D005063:Europium; C003142:picolinamide; C030614:picolinic acid