Complexation of Th(IV) and various lanthanides(III) by glycolic acid; potentiometric, C-13-NMR and EXAFS studies
2002 (English)In: Journal of the Chemical Society. Dalton Transactions, ISSN 1472-7773, E-ISSN 1470-479X, no 20, 3805-3812 p.Article in journal (Refereed) Published
The complex formation of tetravalent thorium and various trivalent lanthanides by glycolate HOCH2CO2- = A(-), has been investigated by potentiometry, C-13-NMR spectroscopy and EXAFS. The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pM(n+) (aq) + rA(-) reversible arrow M(p)H(-q)A(r)(np - q - r) + qH(+) at 25 degreesC, 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 Th(2)H(-2)A(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 M(4)H(-q)A(8) are formed, where q varies from 6 to 13 and 6 to 8 for the Th(IV) and Ln(III) systems, respectively. C-13 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 M-4 (OCH2CO2)(4) to which additional glycolate, oxyacetate and hydroxide ligands are coordinated. The identification of new structure and bonding characteristics of alpha-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.
Place, publisher, year, edition, pages
2002. no 20, 3805-3812 p.
metal carbonate equilibria, system, coordination, cluster, binary
IdentifiersURN: urn:nbn:se:kth:diva-21952DOI: 10.1039/b204717aISI: 000178460600007OAI: oai:DiVA.org:kth-21952DiVA: diva2:340650
QC 201005252010-08-102010-08-10Bibliographically approved