Macrocyclic Receptor Exhibiting Unprecedented Selectivity for Light Lanthanides

Abstract

We report a new macrocyclic ligand, N,N′-bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H₂bp18c6), designed for complexation of lanthanide ions in aqueous solution. Potentiometric measurements evidence an unprecedented selectivity of bp18c6 for the large Ln^III ions. Among the different Ln^III ions, La^III and Ce^III show the highest log K_ML values, with a dramatic drop of the stability observed from Ce^III to Lu^III as the ionic radius of the Ln^III ions decreases (log K_CeL − log K_LuL = 6.9). The X-ray crystal structures of the Gd^III and Yb^III complexes show that the metal ion is directly bound to the 10 donor atoms of the bp18c6 ligand. The structure of the complexes in solution has been investigated by ¹H and ¹³C NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. Our results indicate that a conformational change occurs around the middle of the lanthanide series: for the larger Ln^III ions the most stable conformation is Δ(δλδ)(δλδ), while for the smallest Ln^III ions (Gd−Lu) our calculations predict the Δ(λδλ)(λδλ) form being the most stable one. This structural change was confirmed by the analysis of the Ce^III-, Pr^III-, and Yb^III-induced paramagnetic ¹H shifts. The selectivity that bp18c6 shows for the large Ln^III ions can be attributed to a better fit between the light Ln^III ions and the relatively large crown fragment of the ligand. Indeed, our DFT calculations indicate that the interaction between the Ln^III ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases.