Paramagnetic Relaxation Enhancements in Acetate and Its Fluorine Derivatives Interacting with Gd3+: Complex Formation,Structure, and Transmetallation

The relative spatial distribution and motion with respect to Gd³⁺ of the ¹H and ¹⁹F nuclei in the acetate ion and its fluorine derivatives are studied in D₂O solutions through the paramagnetic relaxation rate enhancements (PREs) of these nuclei. We derive general theoretical expressions of the longitudinal PRE in terms of the analytical concentrations of metal and ligands, formation constants of the complexes, metal–nucleus distances, and coordination lifetimes of the ligands. The observed formation constants of the 1 metal: 1 ligand complexes markedly decrease with increasing number of fluorine atoms, the electronegativity of which reduces the negative partial charge of the coordinating COO^? group. The coordination lifetimes are very short at the scale of the relaxation times of the protons of metal bound acetate, that is, shorter than about 10 μs. The average distance of the acetate protons from Gd³⁺ is in fair agreement with independent crystallographic determination. The release of free Gd³⁺ from the very stable Gddtpa (dtpa=diethylene‐triaminepentaacetate) complex caused by the competition of Zn²⁺ for dtpa, is evidenced by an increase of the PREs with Zn²⁺ concentration. The observed PRE increase is consistent with the known equilibrium constants governing the speciation involving Gd³⁺, Zn²⁺, and dtpa. The present case study illustrates a method which easily yields experimental tunable properties suitable to test the ongoing theories of lanthanide Ln³⁺ complexation in solution.