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

Daniela Lalli,Lorenzo Tei,Mauro Botta,Carlos Platas-Iglesias,Fabio Carniato

doi:10.1021/acs.inorgchem.1c03194

Daniela Lalli, Lorenzo Tei + Show 3 more

Open Access

https://doi.org/10.1021/acs.inorgchem.1c03194

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Abstract

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 1H field-cycling relaxometry and 17O 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 ∼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 1H, 19F NMR as a function of temperature, 1H longitudinal relaxation rates vs B0, and 17O transverse relaxation rates vs T) provides a complete description of the structure and exchange dynamics of these Ln-complexes along the series.

Highlights

Magnetic resonance imaging (MRI) has rapidly emerged as one of the most important and widespread tools in diagnostic clinical medicine and biomedical preclinical research
The topic that we address in this work is of interest both for the area of MRI biomedical imaging and, above all, for basic coordination chemistry as it considers fundamental aspects of the properties of lanthanide complexes
The complex [Gd(AAZTA)(H2O)2]− represents one of the few bis-aquated Gd3+ derivatives that displays favorable properties for in vivo applications as an MRI contrast agent

Summary

■ INTRODUCTION

Magnetic resonance imaging (MRI) has rapidly emerged as one of the most important and widespread tools in diagnostic clinical medicine and biomedical preclinical research. By reasonably assuming that F− more substitutes the more labile water molecule, characterized by a shorter residence lifetime (τMA = 29 ns), the longer bound distance from the metal center (r = 2.505 Å),[20,25] and lower energy cost associated with breaking of the Gd−Ow bond (ΔHMA = 20 kJ mol−1) and that its binding reduces the charge density at the metal center and labilizes the coordinated water molecule, we expect an acceleration of the water exchange rate This is in agreement with the experimental data showing a remarkable (∼50-fold) decrease in the residence lifetime (τM = 3.4 ns), followed by a slight reduction of the enthalpy associated with the exchange process (ΔHM = 23.4 kJ mol−1) with respect to the more strongly bound water molecule of the binary complex.

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES