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

摘要

The complex formation between several trivalent lanthanides (Ln) and 5-sulfosalicylate, (SSA)(3-), was investigated by potentiometry, H-1 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 reversible arrow Ln(p)H-L-q(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 H-1 NMR and fluorescence characteristics, were identified by least-squares refinement in the -log[H+] range of 6.0-10.0. H-1 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: mu-(OR)-type chelating ligands between metals to form the {Ln(2)L(2)}-type core structure and the bidentate ligands outside the {Ln(2)L(2)}-type core. This core structure is different from the An(IV)-SSA case (An(IV): tetra valent 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. (c) 2006 Elsevier B.V. All rights reserved.