Tuning Selectivity and Stability in Heteroleptic Lanthanide Adducts by Ligand Design

Terdentate ligands L10−L14 and their heteroleptic [LkLn(hfac)3] complexes (Ln
= La, Eu, Gd, Er, or Y; H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) exhibit multifactorial
correlations between the ligand’s structural frameworks, including their level of preorganization
and steric congestion and their affinities and selectivities for catching the trivalent lanthanide
containers [Ln(hfac)3]. The polyaromatic ligand scaffolds could be stepwise modulated via
lanthanide−template synthetic strategies, using intermolecular rhodium-catalyzed insertion
reactions. The increasing level of preorganization along the L10 → L11 → L12 series leads to
a duality in which larger thermodynamic formation constants with lanthanides in CD2Cl2 are
accompanied by an unexpected decrease in the Ln−N affinities in the solid state, which could be
assigned to a limited match between the lanthanide size and the enlarged preorganized cavities.
On the contrary, a reduced stability is induced by the connection of additional methyl groups at
position 1 of the benzimidazole moieties in L13 and L14, which is accompanied by an
optimization of metal−nitrogen bond lengths. This study contributes to the rational design of
highly stable neutral heteroleptic lanthanide β-diketonate adducts that resist dissociation in solution, a prerequisite for photophysical applications using these highly luminescent systems at the molecular level.