Potentiometric and 1H and 19F NMR studies of the Th(iv)–5-sulfosalicylate–OH−–F− system

Abstract

The complex formation between Th(IV) and 5-sulfosalicylate, (SSA)³⁻, has been investigated, in both the presence and absence of fluoride, by potentiometry, ¹H and ¹⁹F NMR spectroscopy. The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pTh⁴⁺(aq) + r(SSA)³⁻ ⇌Th_pH_−q(SSA)_r^4p−q−3r + qH⁺ at 25 °C, in an ionic medium with a constant concentration of Na⁺ equal to 1.00 M. Three mononuclear complexes, Th(HSSA)²⁺, Th(SSA)⁺ and Th(SSA)₂²⁻, were identified in the −log [H⁺] range 1.7–3.5. Their stability constants obtained by least-squares refinement of the experimental data agree well with previously published information; their magnitude, and additional NMR data, indicate coordination at the carboxylate end only in Th(HSSA)²⁺, and the formation of chelates involving deprotonation of the aromatic OH group in the other two species. A maximum of two SSA ligands are coordinated even at high ligand concentration. However, this does not indicate tetrahedral coordination around Th, additional fluoride can coordinate to these complexes even at very low free fluoride concentrations. We expect that additional fluorides may coordinate at higher fluoride concentration, but these systems could not be investigated by potentiometry due to precipitation. We could demonstrate the coordination of one fluoride to these complexes. We suggest that Th is nine- or ten-coordinated in these species as in other mononuclear Th-complexes. The bi- and tetra-nuclear complexes, Th₂(OH)₂(SSA)₄⁶⁻ and Th₄(OH)₄(SSA)₁₀¹⁸⁻, were identified at pH > 3.5 using both potentiometric data and ¹H NMR spectra. The binuclear complex can bind three fluoride ions per Th, indicating nine-coordination with a bridging “Th₂(OH)₂” core. The tetranuclear complex Th₄(OH)₄(SSA)₁₀¹⁸⁻ cannot bind additional fluoride, and from this we can suggest that Th(IV) is eight-coordinated. The stoichiometry and the NMR data show that the complex has a “cubane-like” hydroxide core “Th₄(OH)₄”. This structure is different from the previously investigated tetranuclear glycolate complex with a “Th₄(OR)₄” core, where OR is ⁻OCH₂COO⁻.