Effect of heteroatoms on the binding properties of preorganized claw-type phenanthroline-based ligands towards lanthanide cations

Rational design of highly selective ligands is important for recovering of rare earth elements (REEs) from alternative sources which could be beneficial for the current REEs production in many aspects such as reducing the production cost, relieving the environmental influence and also serving as guidelines for specific REEs binding ligand design. While the complicated coordination properties and the uncertain coordination number of REEs hinder the search for high-selective binding ligands. Current ligand designs are mostly based on trial-and-error method. Structure-function relationship is thus highly desired. In this work, we designed three claw-type phenanthroline ligands with structural differences only on the non-coordination sites. By changing the substituents from benzo[d]imidazole to benzo[d]thiazole and benzo[d]oxazole, the effects of heteroatoms on ligand solid-state packing, photophysics and binding properties were demonstrated. The benzo[d]imidazole flanked phenanthroline ligand showed the most promising sensing ability towards La (III) and Eu (III) with high limit-of-detections of 0.25 and 0.23 μM in methanol. On the other side, benzo[d]thiazole and benzo[d]oxazole decorated ligands showed no and little discrimination ability towards REEs. This study revealed the important role of heteroatoms on the selectivity of the ligands and could contribute to the rational design of high-selective ligands for lanthanides, especially light lanthanides.