Complexation of Th(iv) and various lanthanides(iii) by glycolic acid; potentiometric, 13C-NMR and EXAFS studies

Abstract

The complex formation of tetravalent thorium and various trivalent lanthanides by glycolate HOCH₂CO₂⁻ A⁻, has been investigated by potentiometry, ¹³C-NMR spectroscopy and EXAFS. The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pMⁿ⁺(aq) + rA⁻ ⇄ M_pH_−qA_r^{np − q − r} + qH⁺ at 25 °C, in an ionic medium with a constant concentration of Na⁺ equal to 3.00 M. Mononuclear complexes Th(HOCH₂CO₂⁻)_n; n = 1–4, were identified in the −log[H⁺] range 2.5–4.5. The equilibrium constants of these complexes obtained using a least-squares analysis of the experimental data agree well with previously published information; these test solutions also contain dinuclear ternary complexes Th₂H₋₂A_r, r = 2, 4 and 6. The complex formation in the pH range 5–10 was studied at high and constant concentrations of glycolate, 0.50, 0.75 and 1.0 M, respectively. Under these conditions, in addition to the dinuclear species, also tetranuclear complexes M₄H_−qA₈ are formed, where q varies from 6 to 13 and 6 to 8 for the Th(IV) and Ln(III) systems, respectively. ¹³C NMR spectra show that coordinated and free glycolate are in fast exchange at pH 4.5, while at higher pH there are two separate narrow peaks both in the CH₂ and CO₂⁻ regions for the coordinated ligand, indicating slow exchange between two equally populated sites. The peak integrals correspond to two bonded ligands per metal for both Th(IV) and Ln(III). EXAFS data were used to deduce bond distances within the tetranuclear Th complexes. These data together with the NMR-data indicate that the tetranuclear complexes have a cubane-like core “M₄(OCH₂CO₂)₄” to which additional glycolate, oxyacetate and hydroxide ligands are coordinated. The identification of new structure and bonding characteristics of α-hydroxycarboxylates, in particular at higher pH, may be used to explore new separation schemes between actinides in different oxidation states, but also for group separations between lanthanide(III) and actinide(III) ions.