Unexpected Differences in the Dynamics and in the Nuclear and Electronic Relaxation Properties of the Isoelectronic [Eu^II(DTPA)(H₂O)]³^-and [Gd^III(DTPA)(H₂O)]²^-Complexes (DTPA = Diethylenetriamine Pentaacetate)¹

Abstract

The [EuII(DTPA)(H2O)]3- complex (DTPA = diethylenetriamine pentaacetate) has been prepared by controlled potential coulometry from [EuIII(DTPA)(H2O)]2-. [EuII(DTPA)(H2O)]3- is less stable toward oxidation than Eu2+(aq), as shown by its more negative redox potential (E1/2 = −1.34 V in comparison to E1/2 = −0.61 V vs calomel electrode, respectively). Nevertheless, the rate of oxidation was found to be reasonably slow in highly concentrated solutions. Variable-temperature and -pressure, multiple-field 17O NMR and nuclear magnetic relaxation dispersion (NMRD) measurements have been performed on [EuII(DTPA)(H2O)]3- in aqueous solution. The water-exchange rate (kex298 = 1.3 × 109 s-1) is 3 orders of magnitude higher than that on the corresponding Gd(III) complex, and it is only slightly smaller than that on the Eu(II) aqua ion. The positive activation volume (ΔV⧧ = +4.5 cm3 mol-1) indicates a dissociatively activated water-exchange process. The rotational correlation time is slightly longer for [EuII(DTPA)(H2O)]3- as compared to that for [GdIII(DTPA)(H2O)]2-, which is explained by the higher number of water molecules hydrogen-bonded to the carboxylates of the ligand in the highly charged Eu(II) chelate. The electronic relaxation parameters obtained from NMRD and low-field transverse 17O relaxation rates indicate that electron spin relaxation is considerably faster on [EuII(DTPA)(H2O)]3- than on Eu2+(aq) or on the isoelectronic [GdIII(DTPA)(H2O)]2-. Possibilities to use EuII complexes as MRI contrast agents are discussed.