A tripod gadolinium (III) poly(amino carboxylate) relevant to magnetic resonance imaging: structural and dynamical 17O NMR and 1H NMRD studies

Abstract

We present the results of multiple temperature 1H NMRD and variable temperature and pressure, multiple field 17O NMR studies on the gadolinium(III) complex of the tripod ligand H 6ttaha N-tris(2-aminoethyl)amine- N′-hexaacetate in aqueous solution. Of the ten donor atoms, seven are coordinated to the center metal, leaving one of the tripod ‘legs’ free. The coordination number is completed to nine by two water molecules ( q = 2), as confirmed by the value of the scalar coupling constant A/ h = (− 3.5 ± 0.1) × 10 6 rad s −1. The higher number of inner-sphere water molecules causes a higher 1H relaxivity as compared to commercial gadolinium(III) based MRI contrast agents. The water exchange rate was determined as K 298=(8.6±0.6) × 10 6s −1. The enthalpy and volume of activation are ΔH = (40.4±1.4) kJ mol −1 and ΔV = (2.9±1.0) cm 3mol −1, respectively, the latter indicating a dissociatively activated interchanged I d mechanism. The water exchange is unexpectedly fast for an exchange reaction that is not limiting dissociative D; this lability is interpreted as being due to the higher number of inner-sphere water molecules.