Self-assembly of homo- and heteronuclear complexes between a ditopic tripodal ligand and selected lanthanides was investigated with spectrophotometry and ESMS. Strong positive cooperativity in favor of dinuclear C3-symmetrical podands and a size-discrimination along the series are revealed using the site-binding thermodynamic model.
We report here on the coordination properties of a series of lanthanide cations (Ln=La, Nd, Eu, Tb, Er) with a tripodal homoditopic ligand L, which is bearing three diamido-dihydroxamate arms anchored on a tertiary amine. The complexes of L with lanthanides were studied in a MeOH/H2O (80/20 by weight) solvent and a marked size-discriminating effect depending on the intramolecular interactions was observed in the tripodal dinuclear lanthanide edifice. Positive cooperative effects favour the formation of the bimetallic LLn2 complexes compared with the monometallic analogues for the heavier lanthanide cations (Eu, Tb, Er), while the lighter ones (La, Nd) show a decrease of cooperative interactions in the corresponding bimetallic species. Moreover, increased levels of heterobimetallic complexes are detected for Ln(III) pairs containing a lighter Ln(III) ion and a heavier one. In contrast with the adjunctive ligand exchange reported for LFe2 and CDTA, the corresponding substitution reaction for LLn2 occurs according to a disjunctive mechanism. Our work thus highlights the importance of the balance between the rigidity of a tripodal structure and the flexibility of the side arms to accommodate two trivalent cations in a supramolecular ensemble.Zum Volltext