Macrocyclic Ligands with an Unprecedented Size-Selectivity Pattern for the Lanthanide Ions

Lanthanides
(Ln3+) are critical materials used for many
important applications, often in the form of coordination compounds.
Tuning the thermodynamic stability of these compounds is a general
concern, which is not readily achieved due to the similar coordination
chemistry of lanthanides. Herein, we report two 18-membered macrocyclic
ligands called macrodipa and macrotripa that show for the first time
a dual selectivity toward both the light, large Ln3+ ions
and the heavy, small Ln3+ ions, as determined by potentiometric
titrations. The lanthanide complexes of these ligands were investigated
by NMR spectroscopy and X-ray crystallography, which revealed the
occurrence of a significant conformational toggle between a 10-coordinate
Conformation A and an 8-coordinate Conformation B that accommodates
Ln3+ ions of different sizes. The origin of this selectivity
pattern was further supported by density functional theory (DFT) calculations,
which show the complementary effects of ligand strain energy and metal–ligand
binding energy that contribute to this conformational switch. This
work demonstrates how novel ligand design strategies can be applied
to tune the selectivity pattern for the Ln3+ ions.