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. Ministerio de Economía y Competitividad; CTQ2016-76756-P Xunta de Galicia; ED431B 2017/59 Xunta de Galicia; ED431D 2017/01 Ministerio de Ciencia, Innovación y Universidades; PRX18/00201 Estados Unidos. U.S. National Institute of Biomedical Imaging and Bioengineering; R01EB009062 Estados Unidos; National Institute of Diabetes and Digestive and Kidney Diseases; U01DK104302