Solution Structures and Dynamics of Diamagnetic Lu– and Y–DOTAM/Fluoride Ternary Complexes: The Role of ¹⁹ F-CEST in the Detection of the Complex Fluoride Speciation

This study deals with the synthesis and ¹H, ¹³C, and ¹⁹F-NMR characterization of ternary complexes formed upon the addition of fluoride ions to Lu- and Y-DOTAM. The application of the ¹⁹F-CEST protocols allowed to get more insight into the complex speciation of the products of this reaction. In fact, the enhanced sensitivity associated with this technique revealed the presence of other fluoride-containing complexes beyond the main product represented by the simple replacement of the coordinated water molecule in the axial position of the square antiprismatic geometry usually adopted when the macrocycle DOTAM ligand wraps around a metal ion. Both Lu(III) and Y(III) yielded low amounts of a derivative containing a fluoride anion bridging two DOTAM complexes. In the case of the larger Y(III) ion, an additional species was detected, likely due to the expansion to ten of its coordination number. The exchange of the coordinated fluoride with the free anion has also been investigated by 2D-¹⁹F-EXSY and selective inversion-recovery experiments, thus yielding additional routes to assess the lifetime of the fluoride anion at its coordination site.