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Speciation 98: Abstracts
Kaspar Hegetschweiler,1 Roman Hedinger,2 Eva Toth,3 Andre E. Merbach3
1 Universität des Saarlandes, Fachrichtung
Anorganische Chemie, Saarbrücken, Germany;
2 Laboratorium für Anorganische Chemie,
ETH-Zürich, CH-8092 Zürich, Switzerland;
3 Institute of Inorganic and Analytical Chemistry, University
of Lausanne, Switzerland
Due to the four different metal binding sites, 1,3,5-triamino-1,3,5-trideoxy-cis-inositol proved to be a very versatile ligand for metal ions.1 With trivalent Lanthanide cations, a series of trinuclear complexes of the composition [Ln3(H-3taci)2(H2O)6]3+ is formed in aqueous solution. The species distribution of the Ln3+-taci system was analyzed by potentiometric titrations and NMR spectroscopy. Solid state structures were investigated by crystal structure analysis. The trinuclear Gd complex was investigated with respect to a possible application as paramagnetic contrast agent for MRI. [Gd3(H-3taci)2(H2O)6]3+ undergoes a relatively slow water exchange and shows a high volume of activation which can be interpreted in terms of an associative exchange mechanism. [Gd3(H-3taci)2(H2O)6]3+ is the sole species formed in the range 7 < pH < 9. At higher pH, deprotonation of a coordinated water molecule, followed by further aggregation, is observed. The complex decays rapidly in slightly acidic media which limits the potential of a medical application. The N-methylated ligand 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol forms a more stable complex. A strategy for further increase of the complex stability by some additional structural modifications of taci will be discussed.
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