Positively Charged GdIII Cryptates: Slow, Associative Water Exchange
A combined variable-temp. and multiple field 17O NMR, EPR and NMRD study was performed for the 1st time on Gd(III) complexes of cryptand ligands, L1 and L2, where L1 contains three 2,2'-bipyridine units ([bpy.bpy.bpy]) and L2 is the disubstituted Me ester deriv. of L1. The exptl. data were analyzed in a simultaneous fit to det. parameters for H2O exchange, rotational dynamics and electronic relaxation for both complexes. The cryptates have three H2O mols. in the inner sphere which exchange with a rate of kex298 = 1.8 ´ 106 s-1 and 0.97 ´ 106 s-1 for [GdL1(H2O)3]3+ and [GdL2(H2O)3]3+, resp. The kex298 values obtained for these pos. charged cryptates are smaller than those of the neg. charged Gd-poly(amino carboxylate) complexes. The H2O exchange mechanism was assessed for [GdL2(H2O)3]3+ by variable-pressure 17O NMR relaxation measurements. Based on the activation vol., DV.dbldag. = -2.5 cm3 mol-1, the H2O exchange is an associative interchange process. The proton relaxivities, r1, 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 H2O mols., represent much higher values than those obtained for Gd3+ poly(amino carboxylate) complexes of similar mol. wt.