High-pressure NMR kinetics, part 95 - 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)]. 8H(2)O, in comparison with those for th e corresponding Sr analogue, [C(NH2)(3)](3)[Sr(DTPA)(H2O)]. 8H(2)O and for [Sr(ODDA)]. 8H(2)O (DTPA(5-) = diethylenetriamine-N,N,N',N " ,N " -pentaace tate, ODDA(2-) = 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetat e). The two DTPA complexes are isostructural due to the similar ionic size and charge of Sr2+ and Eu2+. The redox stability of [Eu-II(ODDA)(H2O)] and [Eu-II(ODDM)](2-) complexes has been investigated by cyclovoltammetry and U V/Vis spectrophotometry (ODDM4- = 1,4,10,13-tetraoxa-7,16-diaza-cyclooctade cane-7,16-dimalonate). The macrocyclic complexes are much more stable again st oxidation than [Eu-II(DTPA)(H2O)](3-) (the redox potentials are E-1/2 = -0.82 V, -0,92 V, and -1,35 V versus Ag/AgCl electrode for [Eu-III/II(ODDA) (H2O)1. [Eu-III/II(ODDM)], and [Eu-III/II(DTPA)(H2O)], respectively, compar ed 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 t he Eu-II 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)], [Sr(ODDM)](2-), respectively, 0.1 M (CH,),NCI). The increas ed thermodynamic and redox stability of the Eu-II complex formed with ODDA as compared with the traditional ligand DTPA can be of importance when biom edical application is concerned. A variable-temperature O-17-NMR and H-1-nu clear 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 (k(ex)(298) = 0.43 x 10(9) s(-1)) is one third of that obtained for [Eu-II(DTPA)(H2O)](3 -). The variable pressure O-17-NMR study yielded a negative activation volu me, DeltaV(double dagger) = -3.9 cm(3)mol(-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 limite d by the fast rotation of the small molecular weight complex.