Macrocyclic Ligands with an Unprecedented Size-Selectivity Pattern for the Lanthanide Ions

Lanthanides
(Ln3+) are critical materials used for many
important applications, often in the form of coordination compounds.
Tuning the thermodynamic stability of these compounds is a general
concern, which is not readily achieved due to the similar coordination
chemistry of lanthanides. Herein, we report two 18-membered macrocyclic
ligands called macrodipa and macrotripa that show for the first time
a dual selectivity toward both the light, large Ln3+ ions
and the heavy, small Ln3+ ions, as determined by potentiometric
titrations. The lanthanide complexes of these ligands were investigated
by NMR spectroscopy and X-ray crystallography, which revealed the
occurrence of a significant conformational toggle between a 10-coordinate
Conformation A and an 8-coordinate Conformation B that accommodates
Ln3+ ions of different sizes. The origin of this selectivity
pattern was further supported by density functional theory (DFT) calculations,
which show the complementary effects of ligand strain energy and metal–ligand
binding energy that contribute to this conformational switch. This
work demonstrates how novel ligand design strategies can be applied
to tune the selectivity pattern for the Ln3+ ions.

镧系元素（Ln3+）是众多重要应用中的关键材料，通常以配位化合物的形式存在。调节这些化合物的热力学稳定性是一个普遍关注的问题，然而，由于镧系元素的相似配位化学，这一目标并不容易实现。本研究报道了两种称为macrodipa和macrotripa的18元环状配体，它们首次表现出对轻质大离子和重质小离子的双重选择性，这一结论是通过电位滴定法得出的。通过核磁共振波谱学和X射线晶体学对由这些配体形成的镧系元素络合物进行了研究，揭示了在10配位构象A和8配位构象B之间存在显著的构象转换，后者能够适应不同尺寸的镧系离子。这种选择性的起源进一步得到了密度泛函理论（DFT）计算的证实，计算结果表明配体应变能和金属-配体结合能的互补效应是导致这种构象转换的原因。这项工作展示了如何通过新颖的配体设计策略来调节镧系离子选择性的模式。