Fluoride Binding in Unlikely Partners: The Formation of Anion-Anion Complexes with [M(EGTA)]⁻ and [M(OBETA)]⁻ (M = Gd3+, Y3+)

Abstract

Anionic metal complexes (M = Gd³⁺, Y³⁺) with two homologous acyclic aminopolycarboxylate ligands, heptadentate (OBETA) and octadentate (EGTA), were prepared and characterized using both relaxometric NMR (for Gd³⁺) and high-resolution NMR (for Y³⁺). The addition of fluoride to aqueous solutions of these complexes led to the formation of ternary complexes where F⁻ displaces a coordinated water molecule from the metal ion's inner coordination sphere. In the Gd³⁺ complexes, this exchange process was tracked by monitoring changes in the nuclear magnetic relaxation rate of water protons, allowing calculation of the binding affinity. For the diamagnetic Y³⁺ complexes, the exchange was followed through variable-temperature high-resolution ¹⁹F NMR experiments. Calculated enthalpic and entropic contributions to the activation free energy suggest a dissociative exchange mechanism for the monohydrated [M(EGTA)(H₂O)]⁻ and an associative mechanism for the dihydrated [M(OBETA)(H₂O)₂]⁻. Additionally, an unusual dimeric structure was observed for the dihydrated complexes, where two anionic complexes are bridged by fluoride. Detailed DFT calculations confirmed the presence of the dimer, showing a Y–F bond length of 2.33 Å and a ¹J_Y–F NMR coupling constant of 38.0 Hz, in excellent agreement with the experimental value.