Pyridine and phosphonate containing acyclic ligands for stable lanthanide complexation. Extremely fast water exchange on the GdIII chelates.
Résumé
Two novel ligands containing pyridine units and phosphonate pendant arms, with ethane-1,2-diamine (L2) or cyclohexane-1,2-diamine (L3) backbones, have been synthesized for Ln complexation. The hydration numbers obtained from luminescence lifetime measurements in aqueous solutions of the EuIII and TbIII complexes are q = 0.6 (EuL2), 0.7 (TbL2), 0.8 (EuL3), and 0.4 (TbL3). To further assess the hydration equilibrium, we have performed a variable-temperature and -pressure UV-vis spectrophotometric study on the EuIII complexes. The reaction enthalpy, entropy, and volume for the hydration equilibrium EuL EuL(H2O) were calculated to be H = -(11.6 ± 2) kJ mol-1, S = -(34.2 ± 5) J mol-1 K-1, and = 1.8 ± 0.3 for EuL2 and H = -(13.5 ± 1) kJ mol-1, S = -(41 ± 4) J mol-1 K-1, and = 1.7 ± 0.3 for EuL3, respectively. We have carried out variable-temperature 17O NMR and nuclear magnetic relaxation dispersion (NMRD) measurements on the GdL2(H2O)q and GdL3(H2O)q systems. Given the presence of phosphonate groups in the ligand backbone, a second-sphere relaxation mechanism has been included for the analysis of the longitudinal 17O and 1H NMR relaxation rates. The water exchange rate on GdL2(H2O)q, = (7.0 ± 0.8) × 108 s-1, is extremely high and comparable to that on the GdIII aqua ion, while it is slightly reduced for GdL3(H2O)q, = (1.5 ± 0.1) × 108 s-1. This fast exchange can be rationalized in terms of a very flexible inner coordination sphere, which is slightly rigidified for L3 by the introduction of the cyclohexyl group on the amine backbone. The water exchange proceeds via a dissociative interchange mechanism, evidenced by the positive activation volumes obtained from variable-pressure 17O NMR for both GdL2(H2O)q and GdL3(H2O)q (V = +8.3 ± 1.0 and 8.7 ± 1.0 cm3 mol-1, respectively).