Positively Charged Lanthanide Complexes with Cyclen-Based Ligands : Synthesis, Solid-State and Solution Structure, and Fluoride Interaction
Résumé
The syntheses of a new cyclen-based ligand L2 containing four N-[2-(2-hydroxyethoxy)ethyl]acetamide pendant arms and of its lanthanide(III) complexes [LnL2(H2O)]Cl3 (Ln = La, Eu, Tb, Yb, or Lu) are reported, together with a comparison with some LnIII complexes of a previously reported analogue L1 in which two opposite amide arms have been replaced by coordinating pyridyl units. The structure and dynamics of the LaIII, LuIII, and YbIII complexes in solution were studied by using multinuclear NMR investigations and density functional theory calculations. Luminescence lifetime measurements in H2O and D2O solutions of the [Ln(L2)(H2O)]3+ complexes (Ln = Eu or Tb) were used to investigate the number of H2O molecules coordinated to the metal ion, pointing to the presence of an inner-sphere H2O molecule in a buffered aqueous solution. Fluoride binding to the latter complexes was investigated using a combination of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy, pointing to a surprisingly weak interaction in the case of L2 (log K = 1.4 ± 0.1). In contrast to the results in solution, the X-ray crystal structure of the lanthanide complex showed the ninth coordination position occupied by a chloride anion. In the case of L1, the X-ray structure of the [(EuL1)2F] complex features a bridging fluoride donor with an uncommon linear Eu-F-Eu entity connecting two almost identical [Eu(L1)]3+ units. Encapsulation of the F- anion within the two complexes is assisted by π-π stacking between the pyridyl rings of two complexes and C-H***F hydrogen-bonding interactions involving the anion and the pyridyl units.