Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Spain.
J Am Chem Soc. 2009 Mar 11;131(9):3331-41. doi: 10.1021/ja808534w.
We report a new macrocyclic ligand, N,N'-bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H(2)bp18c6), 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 (1)H and (13)C 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 Delta(deltalambdadelta)(deltalambdadelta), while for the smallest Ln(III) ions (Gd-Lu) our calculations predict the Delta(lambdadeltalambda)(lambdadeltalambda) 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 (1)H 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.
我们报告了一种新的大环配体，N,N'-双[(6-羧基-2-吡啶基)甲基]-4,13-二氮-18-冠-6（H(2)bp18c6），设计用于在水溶液中络合镧系元素离子。电位测定结果表明，bp18c6 对大的 Ln(III)离子具有前所未有的选择性。在不同的 Ln(III)离子中，La(III)和 Ce(III)表现出最高的 log K(ML)值，随着 Ln(III)离子半径的减小，稳定性从 Ce(III)到 Lu(III)急剧下降（log K(CeL) - log K(LuL) = 6.9）。Gd(III)和 Yb(III)配合物的 X 射线晶体结构表明，金属离子直接与 bp18c6 配体的 10 个供体原子配位。通过（1）H 和（13）C NMR 光谱以及在 DFT（B3LYP）水平上进行的理论计算研究了配合物在溶液中的结构。我们的结果表明，构象变化发生在镧系元素系列的中间：对于较大的 Ln(III)离子，最稳定的构象是 Delta(deltalambdadelta)(deltalambdadelta)，而对于最小的 Ln(III)离子（Gd-Lu），我们的计算预测 Delta(lambdadeltalambda)(lambdadeltalambda)形式是最稳定的。这种结构变化通过分析 Ce(III)、Pr(III)和 Yb(III)诱导的顺磁（1）H 位移得到了证实。bp18c6 对大的 Ln(III)离子的选择性可归因于轻镧系元素离子与配体相对较大的冠片段之间更好的匹配。事实上，我们的 DFT 计算表明，随着金属离子半径的减小，Ln(III)离子与冠部分的几个供体原子之间的相互作用减弱。
