Positively charged Gd-III cryptates: slow, associative water exchange

摘要

A combined variable-temperature and multiple field O-17 NMR, EPR and NMRD study has been performed for the first time on gadolinium(III) complexes of cryptand ligands, L-1 and L-2, where L-1 contains three 2,2'-bipyridine units ([bpy.bpy.bpy]) and L-2 is the disubstituted methyl ester derivative of L-1. The experimental data have been analysed in a simultaneous fit in order to determine parameters for water exchange, rotational dynamics and electronic relaxation for both complexes. The cryptates have three water molecules in the inner sphere which exchange with a rate of k(ex)(298) = 1.8 x 10(6) s(-1) and 0.97 x 10(6) s(-1) for [GdL1(H2O)(3)](3+) and [GdL2(H2O)(3)](3+), respectively. The k(ex)(298) values obtained for these positively charged cryptates are smaller than those of the negatively charged Gd-poly(amino carboxylate) complexes. The water exchange mechanism was assessed for [GdL2(H2O)(3)](3+) by variable-pressure O-17 NMR relaxation measurements. Based on the activation volume, Delta V-double dagger = -2.5 cm(3) mol(-1), the water exchange is an associative interchange process. The proton relaxivities, r(1), of the cryptate complexes are 9.79 mM(-1) s(-1) for [GdL1(H2O)(3)](3+) and 11.18 mM(-1) s(-1) for [GdL2(H2O)(3)](3+) (298 K, 20 MHz), which, due to the presence of three inner sphere water molecules, represent much higher values than those obtained for Gd3+ poly(amino carboxylate) complexes of similar molecular weight.