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 KML values, with a dramatic drop of the stability observed from CeIII to LuIII as the ionic radius of the LnIII ions decreases (log KCeL − log KLuL = 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.

本研究报道了一种新型大环配体（macrocyclic ligand）——N,N'-双[(6-羧基-2-吡啶基)甲基]-4,13-二氮杂-18-冠-6（H2bp18c6），该配体的设计初衷为在水溶液中与镧系离子形成配位配合物。电位滴定测试结果表明，bp18c6对大半径三价镧系离子（LnIII）展现出前所未有的选择性。在各类三价镧系离子中，镧（LaIII）与铈（CeIII）离子的log KML值最高；随着三价镧系离子半径逐渐减小，从CeIII到镥（LuIII）离子，配合物稳定性呈现断崖式下降（log KCeL − log KLuL = 6.9）。钆（GdIII）与镱（YbIII）配合物的X射线单晶衍射结构显示，金属离子直接与配体bp18c6的10个给体原子配位结合。本研究借助1H核磁共振（1H NMR）与13C核磁共振（13C NMR）光谱法，以及在密度泛函理论（DFT，B3LYP）水平上开展的理论计算，对溶液中配合物的结构进行了探究。研究结果表明，在镧系序列中部附近发生了构象转变：对于半径较大的三价镧系离子，最稳定的构象为Δ(δλδ)(δλδ)；而对于半径最小的三价镧系离子（Gd~Lu），理论计算预测最稳定的构象为Δ(λδλ)(λδλ)。这一结构转变通过对CeIII、镨（PrIII）以及YbIII诱导的顺磁性1H化学位移的分析得到了验证。bp18c6对大半径三价镧系离子的选择性，可归因于轻镧系离子与配体中较大的冠醚片段之间更优异的尺寸匹配性。事实上，本研究的DFT计算结果表明，随着金属离子半径减小，三价镧系离子与冠醚部分的多个给体原子之间的相互作用会被削弱。