Atomic-Level Fine-Tuning of Phenanthroline-Based Tetradentate Ligands Unlocks Unexpected Lanthanide Affinity

The growing need for high-purity lanthanides demands more selective separation strategies. Existing approaches mainly follow lock-and-key or induced-fit principles: the former uses fixed binding cavities for size-based recognition and the latter relies on adaptive, energy-favored architectures. Although impressive trends such as reversed, V-shaped, and S-shaped selectivity have been achieved, cation-specific selectivity remains rare. Here, we show that an asymmetric ligand design at the atomic level can be highly effective. Introducing sulfur into a phenanthroline-diimine scaffold produced an asymmetrical ligand with unexpected Nd selectivity among light lanthanides. Spectroscopic titrations confirmed this behavior: despite lowering overall binding affinity, sulfur incorporation enhanced Nd selectivity by two orders over La and Eu. This validates the asymmetric ligand design as a promising route to f-element discrimination.