Terdentate ligands L10–L14 and
their heteroleptic [L
k
Ln(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.