Dynamic aggregation of the mid-sized gadolinium complex {Ph4[Gd(DTTA)(H2O)2]−3}

A compound binding three Gd3+ ions, {Ph4[Gd(DTTA)(H2O)2]−3} (where H5DTTA is diethylenetriaminetetraacetic acid), has been synthesized around a hydrophobic center made up of four phenyl rings. In aqueous solution the molecules start to self-aggregate at concentrations well below 1 mM as shown by the increase of rotational correlation times and by the decrease of the translational self-diffusion constant. NMR spectra recorded in aqueous solution of the diamagnetic analogue {Ph4[Y(DTTA)(H2O)2]−3} show that the aggregation is dynamic and due to intermolecular π-stacking interactions between the hydrophobic aromatic centers. From estimations of effective radii, it can be concluded that the aggregates are composed of two to three monomers. The paramagnetic {Ph4[Gd(DTTA)(H2O)2]−3} exhibits concentration-dependent 1H NMR relaxivities with high values of approximately 50 mM−1 s−1 (30 MHz, 25 °C) at gadolinium concentrations above 20 mM. A combined analysis of 1H NMR dispersion profiles measured at different concentrations of the compound and 17O NMR data measured at various temperatures was performed using different theoretical approaches. The fitted parameters showed that the increase in relaxivity with increasing concentration of the compound is due to slower global rotational motion and an increase of the Lipari–Szabo order parameter S2.