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 ß-diketonate adducts that resist dissociation in solution, a prerequisite for photophysical applications using these highly luminescent systems at the molecular level.