Coordination equilibria and water exchange kinetics of lanthanide(III) propylenediaminetetraacetates and other magnetic resonance imaging related complexes

Abstract

The UV-Vis absorption spectra of aqueous solutions of Eu 3+ complexes with the hexadentate polyaminocarboxylate ligands EDTA 4−, CDTA 4−, HDTA 3− and PDTA 4− were measured as a function of temperature and pressure in the frequency region corresponding to the 7F 0→ 5D 0 transition of Eu 3+. The results can be explained in terms of equilibria between nine-coordinate and eight-coordinate species where the eight-coordinate species contain one less inner sphere water molecule than the nine-coordinate species. The thermodynamic parameters, including the reaction volume, for these equilibria were determined. 17O NMR transverse relaxation rates and chemical shifts were measured for aqueous solutions of the eight-coordinate complexes [Ln(PDTA)(H 2O) 2] − (LnTb, Dy, Er, Tm, Yb) and [Er(EDTA)(H 2O) 2] − as a function of temperature, pressure and magnetic field. The results were analysed in terms of the water exchange kinetics on the complexes. The water exchange rate on [Ln(PDTA)(H 2O) 2] − decreases dramatically with decreasing ionic radius across the lanthanide series from k ex 298 = (2.4±0.1)×10 7 s −1 for LnTb to k ex 298 = (2.8±0.3)×10 5 s −1 for LnYb. The activation volumes show that this is accompanied by a change of exchange mechanism from associatively activated for LnTb ( ΔV # = −7.6±0.3 cm 3 mol −1) to dissociatively activated for LnYb ( ΔV # = +7.4±0.8 cm 3 mol −1). Water exchange on [Er(EDTA)(H 2O) 2] − ( k ex 298 = (9.8±1.9)×10 6 s −1) is more than an order of magnitude faster than on [Er(PDTA)(H 2O) 2] − ( k ex 298 = (5.6±0.5)×10 5 s −1). These kinetic results can be interpreted in terms of the equilibria measured by UV-Vis spectrophotometry. The implications of these observations for the design of new MRI contrast agents are discussed.