We report the first solid state X-ray crystal structure for a Eu(II) chelate, [C(NH2)3]3[Eu(II)(DTPA)(H2O)].8H2O, in comparison with those for the corresponding Sr analogue, [C(NH2)3]3[Sr(DTPA)(H2O).8H2O and for [Sr(ODDA)].8H2O (DTPA5 = diethylenetriamine-N,N,N',N",N"-pentaacetate, ODDA2- =1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate ). The two DTPA complexes are isostructural due to the similar ionic size and charge of Sr(2+) and Eu(2+). The redox stability of [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- complexes has been investigated by cyclovoltammetry and UV/Vis spectrophotometry (ODDM4- =1,4,10,13-tetraoxa-7,16-diaza-cyclooctadecane-7,16-++ +dimalonate). The macrocyclic complexes are much more stable against oxidation than [Eu(II)(DTPA)(H2O)]3- (the redox potentials are E1/2 =-0.82 V, -0.92 V, and -1.35 V versus Ag/AgCl electrode for [Eu(III/II)(ODDA)(H2O)],[Eu(III/II)(ODDM)], and [Eu(III/II)(DTPA)(H2O)], respectively, compared with -0.63 V for Eu(III/II) aqua). The thermodynamic stability constants of [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2- were also determined by pH potentiometry. They are slightly higher for the EuII complexes than those for the corresponding Sr analogues (logK(ML)=9.85, 13.07, 8.66, and 11.34 for [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2-, respectively, 0.1M (CH3)4NCl). The increased thermodynamic and redox stability of the Eu(II) complex formed with ODDA as compared with the traditional ligand DTPA can be of importance when biomedical application is concerned. A variable-temperature 17O-NMR and 1H-nuclear magnetic relaxation dispersion (NMRD) study has been performed on [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- in aqueous solution. [Eu(II)(ODDM)]2- has no inner-sphere water molecule which allowed us to use it as an outer-sphere model for [Eu(II)(ODDA)(H2O)]. The water exchange rate (k298(ex)= 0.43 x 10(9)s(-1)) is one third of that obtained for [Eu(II)(DTPA)(H2O)]3-. The variable pressure 17O-NMR study yielded a negative activation volume, deltaV (not=) = -3.9cm3mol(-1); this indicates associatively activated water exchange. This water exchange rate is in the optimal range to attain maximum proton relaxivities, which are, however, strongly limited by the fast rotation of the small molecular weight complex.

The effect of temperature and pressure on the water-exchange reactions of complexes of the type [FeIII(L)(H2O)x]n-, where L = edta4- (ethylenediaminetetraacetate), Hedta3- (monoprotonated form of edta), cdta4- (trans-1,2-diaminocyclohexanetetraacetate), edds4- (s,s-ethylenediaminedisuccinate), 1,3-pdta4- (1,3-propylenediaminetetraacetate), and alpha,beta-eddadp4-(alpha,beta-ethylenediaminediaceatedipropionate), was studied by employing 17O NMR techniques. The effect of potentially hexadentate ligands, covering a systematic variation of the size, substituents, and overall coordination geometry, on iron(III) complexes was investigated in terms of the lability of the coordinated water and the underlying exchange mechanism. For most of the systems studied, the results are in agreement with a dissociatively activated water-exchange mechanism for the seven-coordinate complexes. The absolute magnitudes of the volumes of activation are small and fit an I(d) mechanism. The results contribute to a better understanding of the nature, reactivity, and substitution mechanism of the selected complexes in solution.

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.

No abstract

The complicated oxidation kinetics of the reaction of reacts with O 2 by a substitution process to form [Fe II (cdta)O 2 ] 2-; (2) electron-transfer to form an Fe III -[Fe II (cdta)] [cdta = 1,2-(N,NЈ-cyclohexanediamine)tetraacetate] with molecular oxygen was investigated as a superoxo species; (3) subsequent bridge formation followed by electron-transfer to give [(cdta)Fe III -O 2 2--Fe III (cdta)] 4-; function of [Fe II ], [O 2 ], pH, temperature and pressure. In the presence of an excess of [Fe II (cdta)] three steps could be and (4) a fast decomposition of the peroxide intermediate yielding the monomeric [Fe III (cdta)] and H 2 O 2 .Rate and observed, for which the following rate constants were found at 25°C; k 1 = 1080 ± 16 M -1 s -1 , k 2 = 103 ± 4 M -1 s -1 and k 3 = activation parameters for these steps are reported and discussed in terms of the postulated mechanism and in 59 ± 5 M -1 s -1 . These reaction steps can be accounted for in terms of the following mechanism: (1) [Fe II (cdta)H 2 O] 2reference to available literature data.The present work describes how the selected chelate can Kinetic Measurements [a] Kinetic traces measured in the presence of an excess of Nürnberg,[Fe II (cdta)] showed a complicated multistep process. In con-Egerlandstraße 1, 91058 Erlangen, Germany trast to the [Fe II (edta)] system, we could now fit the kinetic Fax: (internat.) ϩ49 (0)9131/ 85-27387