Solution and solid-state characterization of Eu(II) chelates: a possible route towards redox responsive MRI contrast agents.

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

Bis-Tris Propane as a New Polydentate Linker in the Synthesis of Iron(III) and Manganese(II/III) Complexes

We describe the synthesis, structure, and magnetic properties of two new complexes, one decanuclear iron(III) cluster and one hexanuclear mixed-valence manganese(II/III) cluster, where the previously unexplored polydentate ligand Bis-tris propane {(CH2OH)(3)CNH(CH2)(3)NHC(CH2OH)(3)} is used to link small cluster fragments into high-nuclearity complexes

Generation of ferryl species through dioxygen activation in Iron/EDTA systems : a computational study

The ferryl species (oxidoiron(IV), Fe

O2 activation in a dinuclear Fe(II)/EDTA complex : spin surface crossing as a route to highly reactive Fe(IV)oxo species

We study the cleavage of O2́ in gas phase [(EDTAH)Fe(

Rates of ligand exchange between > Fe-III-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution

Balogh E, Todea AM, Müller A, Casey WH. Rates of ligand exchange between > Fe-III-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution. INORGANIC CHEMISTRY. 2007;46(17):7087-7092.Variable-temperature O-17 NMR experiments were conducted on the nanometer-sized Keplerate MO72Fe30 cluster, with the stoichiometry [MO72Fe30O252(CH3COO)(12)[MO2O7(H2O)](2)[H2MO2O8(H2O)(91)]center dot similar to(H2O)91]center dot similar to 150H(2)O. This molecule contains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the rates of exchange of the isolated >Fe-III-OH2 waters with bulk aqueous solution, Both longitudinal and transverse O-17-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: k(ex)(298) = 6.7(+/- 0.8) x 10(6) s(-1), which is about a factor of similar to 4 x 10(4) times larger than the corresponding rate coefficient for the Fe(OH2)(6)(3+) ion of k(ex)(298) = 1.6 x 10(2) S-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and Delta H double dagger and Delta S double dagger are 26.3 +/- 0.6 kj mol(-1) and -26 +/- 0.9 J mol(-1) K-1, respectively. High-pressure 110 NMR experiments were also conducted, but the cluster decomposed slightly under pressure, which precluded confident quantitative estimation of the AV. However, the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as a D or Id mechanism. The enhanced reactivity of waters on the MO72Fe30 cluster is associated with an increase in the Fe-III-OH2 bond length in the solid state of similar to 0.1 angstrom relative to the Fe(OH2)(6)(3+) ion, suggesting that a correlation exists between the Fe-III-OH2 bond length and k(ex)(298). Although there are only few high-spin Fe(III) complexes where both exchange rates and structural data are available, these few seem to support a general correlation