Surprising Complexity of the [Gd(AAZTA)(H2O)2]- Chelate Revealed by NMR in the Frequency and Time Domains

Typically, Ln(III) complexes are isostructural along the series, which enables studying one particular metal chelate to derive the structural features of the others. This is not the case for [Ln(AAZTA)(H2O)(x)](-) (x = 1, 2) systems, where structural variations along the series cause changes in the hydration number of the different metal complexes, and in particular the loss of one of the two metal-coordinated water molecules between Ho and Er. Herein, we present a H-1 field-cycling relaxometry and O-17 NMR study that enables accessing the different exchange dynamics processes involving the two water molecules bound to the metal center in the [Gd(AAZTA)(H2O)(2)](-) complex. The resulting picture shows one Gd-bound water molecule with an exchange rate similar to 6 times faster than that of the other, due to a longer metal-water distance, in accordance with density functional theory (DFT) calculations. The substitution of the more labile water molecule with a fluoride anion in a diamagnetic-isostructural analogue of the Gd-complex, [Y(AAZTA)(H2O)(2)](-), allows us to follow the chemical exchange process by high-resolution NMR and to describe its thermodynamic behavior. Taken together, the variety of tools offered by NMR (including high-resolution H-1, NMR as a function of temperature, H-1 longitudinal relaxation rates vs B-0, and O-17 transverse relaxation rates vs T) provides a complete description of the structure and exchange dynamics of these Ln-complexes along the series.

Automated summary

Ln(III) complexes are usually isostructural, but the [Ln(AAZTA)(H2O)(x)](-) systems show structural variations causing changes in hydration number. NMR studies reveal different exchange dynamics processes of water molecules, with one Gd-bound water molecule exchanging 6 times faster than the other due to longer metal-water distance. Substitution of water molecule with fluoride in a diamagnetic-isostructural analogue allows for tracking chemical exchange process and describing thermodynamic behavior. NMR techniques provide a comprehensive understanding of structure and exchange dynamics of Ln-complexes.