Positively Charged Lanthanide Complexes with Cyclen-Based Ligands: Synthesis, Solid-State and Solution Structure, and Fluoride Interaction

The syntheses of a new cyclen-based ligand L-2 containing four N-[2-(2-hydroxyethoxy)ethyl]acetamide pendant arms and of its lanthanide(III) complexes [LnL(2)(H2O)]Cl-3 (Ln = La, Eu, Tb, Yb, or Lu) are reported, together with a comparison with some Ln(III) complexes of a previously reported analogue L-1 in which two opposite amide arms have been replaced by coordinating pyridyl units. The structure and dynamics of the La-III, Lu-III, and Yb-III 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(L-2)(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 L-2 (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 L-1, the X-ray structure of the [(EuL1)(2)F] complex features a bridging fluoride donor with an uncommon linear Eu-F-Eu entity connecting two almost identical [Eu(L-1)](3+) units. Encapsulation of the F- anion within the two complexes is assisted by pi-pi stacking between the pyridyl rings of two complexes and C-H center dot center dot center dot F hydrogen-bonding interactions involving the anion and the pyridyl units.