Conformational and Coordination Equilibria on DOTA Complexes of Lanthanide Metal Ions in Aqueous Solution Studied by ¹H-NMR Spectroscopy

A variable-temperature, -pressure, and -ionic strength ¹H NMR study of the DOTA complexes of different trivalent cations (Se, Y, La, Ce → Lu) (DOTA = 1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)cyclododecane) yielded data that are in contradiction with the hitherto used model of only two enantiomeric pairs of diastereoisomers that differ in the ligand conformations. A two-isomer equilibrium cannot explain the newly observed apparent reversal of the isomer ratio at the end of the series. As both conformers may lose their inner sphere water molecule, a coordination equilibrium may be superimposed on this conformational equilibrium, as shown by large positive reaction volumes for the isomerization of [Ln(DOTA)(H₂O)_x]^- (Ln = Yb, Lu; x = 1, 0). The isomerization of [Nd(DOTA)(H₂O)]^- and [Eu(DOTA)(H₂O)]^- is purely conformational, as shown by near-zero reaction volumes. The measured isomerization enthalpies and entropies agree with this model. The shift of the isomerization equilibria by a variety of non-coordinative salts depends on the ligand conformation rather than the presence or absence of the inner sphere water molecule. This results from weak ion binding and water solvent stabilization of one ligand conformation, rather than the decrease of the activity of the bulk water in the solution, as shown by UV-vis measurements of the coordination number sensitive transition ⁵F₀ → ⁷D₀ of Eu(III) as a function of ionic strength. Fluoride ions replace a water molecule in the inner coordination sphere, preferentially for one of the conformational isomers, as proven by ¹⁹F-NMR shifts and the appearance of a third set of resonances corresponding to [Eu(DOTA)F]^2- in the ¹H-NMR spectrum of [Eu(DOTA)(H₂O)^-.