Functionalized Triptycene-Derived Tripodal Ligands: Privileged Formation of Tetranuclear Cage Assemblies with Larger Ln(III)

In
this Article, we report the self-assembly of lanthanide complexes
formed with two new tripodal ligands, L2 and L3, where binding strands are connected to a rigid triptycene anchor.
The pyridine moieties are functionalized with methoxy and PEG groups
to enhance ligand solubility and to evaluate the effect of these substituents
on lanthanide coordination. These ligands were successfully synthesized
and characterized, and their coordination properties were examined
along the lanthanide series through speciation studies with NMR and
ESI-MS. Well-defined tetranuclear complexes are formed with both ligands,
but their stabilities with heavier lanthanides are considerably reduced,
especially for complexes with L3. This is attributed
to a destabilizing effect of pending PEG arms in combination with
increased steric hindrance between binding strands upon complexation
with smaller cations. The sensitization of lanthanide luminescence
in tetranuclear complexes occurs despite one water molecule being
coordinated to a metal ion.