Due to the ever‐increasing demand for high‐purity individual rare‐earth elements, novel and highly selective separation processes are increasingly sought after. Herein, we report a separation protocol that employs shape‐persistent 2,9‐bis‐lactam‐1,10‐phenanthroline (BLPhen) ligands exhibiting unparalleled selectivity for light trivalent lanthanides. The highly preorganised binding pockets of the ligands allowed for the separation of lanthanides with high fidelity, even in the presence of competing transition metals, in a biphasic separation system. Notably, the selectivity trends of the BLPhen ligands towards metal ions across the lanthanide series can be chemically modulated by altering the molecular rigidity of the extractant.

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The separation of adjacent lanthanides continues to be a challenge worldwide because of the similar physical and chemical properties of these elements and a necessity to advance the use of clean-energy applications. Herein, a systematic structure−performance relationship approach toward understanding the effect of N-alkyl group characteristics in diglycolamides (DGAs) on the separation of lanthanides(III) from a hydrochloric acid medium is presented. In addition to the three most extensively studied DGA complexants [N,N,N′,N′-tetra(n-octyl)diglycolamide, TODGA; N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide, TEHDGA; N,N′-dimethyl-N,N′-di(n-octyl)diglycolamide, DMDODGA], 12 new extracting agents with varying substitution patterns were designed to study the interplay of steric and electronic effects that control rare-earth element extraction. Subtle changes in the structure around diglycolamide carbonyl oxygen atoms result in dramatic shifts in the lanthanide extraction strength and selectivity. The effects of the chain length and branching position of N-alkyl substituents in DGAs are elaborated on with the use of experimental, computational, and solutionstructure characterization techniques.