Macrocyclic Receptor Exhibiting Unprecedented Selectivity for Light Lanthanides

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 LnIII ions. Among the different LnIII ions, LaIII and CeIII show the highest log K ML values, with a dramatic drop of the stability observed from CeIII to LuIII as the ionic radius of the LnIII ions decreases (log K CeL − log K LuL = 6.9). The X-ray crystal structures of the GdIII and YbIII 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 LnIII ions the most stable conformation is Δ(δλδ)(δλδ), while for the smallest LnIII 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 CeIII-, PrIII-, and YbIII-induced paramagnetic 1H shifts. The selectivity that bp18c6 shows for the large LnIII ions can be attributed to a better fit between the light LnIII ions and the relatively large crown fragment of the ligand. Indeed, our DFT calculations indicate that the interaction between the LnIII ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases.