Characterization of Lanthanide(III) DOTP Complexes: Thermodynamics, Protonation, and Coordination to Alkali Metal Ions
Résumé
Lanthanide complexes of the tetraazatetrakis(methylenephosphonate) ligand DOTP8- have been examined by spectrophotometry, potentiometry, osmometry, and 1H, 31P, and 23Na NMR spectroscopy. The LnDOTP5- complexes undergo four protonations between pH 9 and 3, and their stability constants (log KML) range from 27.6 to 29.6 across the lanthanide series. TmDOTP5- acts as a particularly good aqueous shift reagent, inducing paramagnetic shifts in 23Na nuclei of over 400 ppm. 23Na NMR titrations and osmometry measurements indicated that a single Na+ was bound to each TmDOTP5- at low Na+/TmDOTP5- ratios, while three Na+ ions were bound at high Na+/TmDOTP5- ratios.