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 (E 1/2 = −1.34 V in comparison to E 1/2 = −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 (k ex 298 = 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.

Structural Investigation of the Aqueous Eu2+ Ion: Comparison with Sr2+ Using the XAFS Technique

“…The Eu 2+ solutions were prepared electrochemically from an europium(III) triflate stock solution (0.15 M) in 0.1 M triflic acid using controlled potential coulometry in a homemade electrolysis cell 14 at a potential of -0.9 V vs Ag/AgCl. After complete reduction, the samples were taken out with a syringe and filled into a round-bottom flask, sealed with a septum to avoid oxygen contamination.…”

“…Eu 2+ concentrations were checked using the procedure described elsewhere. 14 XAFS Measurements. XAFS measurements of europium and strontium solutions were performed on the beam line BM29 of the 3rd generation synchrotron radiation facility ESRF (Grenoble, France).…”

Metal Complexes as MRI Contrast Enhancement Agents

“…From this perspective, several Eu 2+ complexes have been investigated. [270][271][272][273] Cryptate complexes seem to be an appropriate choice, and the redox stability of these systems has been recently improved by rationally modifying the (2.2.2) cryptate in terms of Lewis basicity, cavity size, and hardsoft acid-base properties (Scheme 18). 274 The best compound has a phenyl ring and four sulfur atoms.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…The activation mode, as suggested principally by the sign and magnitude of the volume of activation, for water exchange on Eu 2 þ chelates varies widely with the nature of the coordinated macrocyclic ligand. [234][235][236] However, in many cases, the water exchange rate is not markedly reduced from that of the fully aquated ion. The properties of coordinating ligands and the effect of overall charge that give rise to these fascinating variations in mechanism may be understood fully by consulting the original literature.…”