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 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with a comparison with some LnIII complexes of a previously reported analogue L1 in which two opposite amide arms have been replaced by coordinating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.