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.

本研究报道了一种全新的大环配体（macrocyclic ligand）：N,N'-双[(6-羧基-2-吡啶基)甲基]-4,13-二氮杂-18-冠-6（H2bp18c6），该配体专为水溶液中镧系离子的配位络合而设计。电位滴定测试结果表明，bp18c6对大尺寸三价镧系离子（LnIII）展现出前所未有的选择性。在各类三价镧系离子中，镧离子（LaIII）与铈离子（CeIII）的络合稳定常数对数（log KML）数值最高；随着三价镧系离子离子半径减小，从CeIII到镥离子（LuIII）的络合稳定性出现急剧下降（log KCeL − log KLuL = 6.9）。钆离子（GdIII）与镱离子（YbIII）的络合物X射线晶体结构显示，金属离子直接与bp18c6配体的10个给体原子配位结合。本研究通过氢谱（¹H NMR）、碳谱（¹³C NMR）光谱分析，以及基于密度泛函理论（DFT, B3LYP泛函）的理论计算，对溶液中络合物的结构进行了表征。研究结果表明，在镧系序列的中段发生了构象转变：对于大尺寸三价镧系离子，最稳定的构象为Δ(δλδ)(δλδ)；而对于尺寸最小的三价镧系离子（Gd−Lu），计算结果显示Δ(λδλ)(λδλ)构象为最稳定形式。该结构转变通过铈离子（CeIII）、镨离子（PrIII）与镱离子（YbIII）诱导的顺磁性氢谱位移分析得到了验证。bp18c6对大尺寸三价镧系离子的选择性可归因于轻质量三价镧系离子与配体中较大的冠醚片段具有更优的尺寸匹配性。实际上，密度泛函理论计算结果表明，随着金属离子半径减小，三价镧系离子与冠醚部分多个给体原子之间的相互作用会被削弱。