Polynuclear complex formation of trivalent lanthanides by 5-sulfosalicylate in an aqueous system—potentiometric, 1H NMR, and TRLIFS studies

Abstract

The complex formation between several trivalent lanthanides (Ln) and 5-sulfosalicylate, (SSA) 3−, was investigated by potentiometry, 1H NMR, and time resolved laser-induced fluorescence spectroscopy (TRLIFS). The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pLn 3+ + rL ⇆ Ln p H − q L r + qH + at 298 K in an ionic medium with a constant concentration of Na + equal to 1.00 M. Note that “L” denotes the SSA ligand where all protons are dissociated. Two mononuclear chelating complexes, LnL(aq) and LnL 2 3 - , were identified. Their stability constants obtained by least-squares refinement of the potentiometric data agree well with previously published information. In addition, two additional dinuclear complexes, Ln 2 H 0 L 5 9 - and Ln 2 H 0 L 6 12 - , which have very different 1H NMR and fluorescence characteristics, were identified by least-squares refinement in the −log[H +] range of 6.0–10.0. 1H NMR spectra from the ligand in the complex Ln 2 H 0 L 6 12 - showed separate peaks having two different rates of exchange with free ligand in the bulk solution besides a signal from the free and carboxylate-coordinated ligands. This indicates that the dinuclear complex, Ln 2 H 0 L 6 12 - , consists of two different types of chelating ligands: μ–{OR}-type chelating ligands between metals to form the {Ln 2L 2}-type core structure and the bidentate ligands outside the {Ln 2L 2}-type core. This core structure is different from the An(IV)–SSA case (An(IV): tetravalent actinide), in which hydroxides play the role of forming the {An 2(OH) 2}-type core structure. TRLIFS measurement gave further information about the dynamics and molecular structures of the complexes.