Enantioselective recognition between chiral α-hydroxy-carboxylates and macrocyclic heptadentate lanthanide(III) chelates

Three novel heptacoordinated Ln(III) complexes (Ln = Gd and Yb) have been synthesized and investigated by ¹H NMR spectroscopy. These complexes contain two stereogenic centers, one associated with a δδδδ or λλλλ conformation of the ethylenediamine moieties in the tetraazamacrocycle and the latter arises from the orientation (Δ or Δ) of the coordinating arms. Evidence has been gained for the occurrence of a fast exchange between all the possible conformers. Upon addition of several (S)-α-hydroxy-carboxylate substrates, the formation of stable ternary adducts has been obtained. Their ¹H NMR spectra are consistent with the presence of two diastereoisomers differing in the conformation adopted by the macrocyclic ligand wrapping the lanthanide(III) ion. The interaction leading to the formation of the ternary complexes is enantioselective depending on the hydrophilicity of the α-hydroxycarboxylate.