Pyridine- and phosphonate-containing ligands for stable Ln complexation.: Extremely fast water exchange on the GdIII chelates

Two novel ligands containing pyridine units and phosphonate pendant arms, with ethane-1,2-diamine (L-2) or cyclohexane-1,2-diamine (L-3) backbones, have been synthesized for Ln complexation. The hydration numbers obtained from luminescence lifetime measurements in aqueous solutions of the Eu-III and Tb-III 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 Eu-III complexes. The reaction enthalpy, entropy, and volume for the hydration equilibrium EuL <-> EuL(H2O) were calculated to be Delta H degrees-(11.6 +/- 2) kJ mol(-1), Delta S degrees-(34.2 +/- 5) J mol(-1) K-1, and K-Eu(298)=1.8 +/- 0.3 for EuL2 and Delta H degrees-(13.5 +/- 1) kJ mol(-1), Delta S degrees-(41 +/- 4) J mol(-1) K-1, and K-Eu(298)=1.7 +/- 0.3 for EuL3, respectively. We have carried out variable-temperature O-17 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 O-17 and H-1 NMR relaxation rates. The water exchange rate on GdL2(H2O)(q), k(ex)(298)=(7.0 +/- 0.8) x 10(8)s(-1), is extremely high and comparable to that on the Gd-III aqua ion, while it is slightly reduced for GdL3(H2O)(q), k(ex)(298)=(1.5 +/- 0.1) x 10(8)s(-1). This fast exchange can be rationalized in terms of a very flexible inner coordination sphere, which is slightly rigidified for L-3 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 O-17 NMR for both GdL2(H2O)(q) and GdL3(H2O)(q) (Delta V-# =+8.3 +/- 1.0 and 8.7 +/- 1.0cm(3)mol(-1), respectively).