Lanthanide Complexes with 1H paraCEST and 19F Response for Magnetic Resonance Imaging Applications.

We present a detailed study of the lanthanide(III) complexes with cyclen-based ligands containing phenylacetamide pendants that incorporate CF3 group(s) at different distances from the metal ion. The complexes exhibit square-antiprismatic coordination in solution, as demonstrated by analysis of the Yb3+-induced paramagnetic shifts and the X-ray structure of the [YbL3] complex. Luminescence lifetime measurements and a detailed 1H and 17O relaxometric characterization confirmed the presence of an inner-sphere H2O molecule. The Tm3+ complexes provide chemical-exchange saturation-transfer response upon saturation at the frequency of the amide protons. A 19F relaxation study provided accurate estimates of the Ln···F distances that were used to rationalize the efficiency of the complexes as 19F magnetic resonance imaging (MRI) probes, which was tested in vitro using MRI phantom studies.