Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Abstract

The water exchange rates of water molecules coordinated to the metal ion in lanthanide complexes have been profusely investigated during the last 25 years, especially in the case of Gd³⁺ and Eu³⁺ complexes. This is mainly related to the important application of some Gd³⁺ complexes as contrast agents in magnetic resonance imaging (MRI), and the intensive investigation of Eu³⁺ complexes as contrast agent candidates providing contrast through the chemical exchange saturation transfer mechanism (CEST). Both applications require a fine tunning of the exchange rate of the coordinated water molecule to yield optimal response. Herein we review the progress made in this field to control water exchange in a rational way through ligand design, providing relationships between the observed trends, the structures of the complexes and the mechanisms responsible for the water exchange reaction.