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

Abstract

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)(H(2)O)]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a comparison with some Ln(III) complexes of a previously reported analogue L(1) in which two opposite amide arms have been replaced by coordinating pyridyl units.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 H(2)O and D(2)O solutions of the [Ln(L(2))(H(2)O)](3+) complexes (Ln = Eu or Tb) were used to investigate the number of H(2)O molecules coordinated to the metal ion, pointing to the presence of an inner-sphere H(2)O 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 [(EuL(1))(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 π-π stacking between the pyridyl rings of two complexes and C-H···F hydrogen-bonding interactions involving the anion and the pyridyl units.