Understanding Reverse-Size-Selective Extraction of Lanthanides Using a Conformationally Adaptive Ligand

Lanthanides (Ln³⁺) are essential elements in modern technology due to their unique electronic, magnetic, and optical properties. However, due to their similar chemical properties, their separation is extremely difficult. In this work, we studied a conformationally adaptive pseudocyclic ligand called octadecyl acyclopa (ODA) that demonstrates remarkable reverse-size-selectivity of larger over smaller Ln³⁺ ions in a biphasic system. ODA forms pseudocyclic molecular complexes at organic/aqueous interfaces with aqueous phase lanthanide ions. These complexes are extracted into the oil phase, where femtosecond structural dynamics are found to correlate with macroscopic selectivity trends. Ultrafast vibrational spectroscopic analysis and ab initio molecular dynamics simulations were used to probe the structural dynamics of metal-ODA complexes. This showed that despite being conformationally flexible, ODA is capable of coiling around and better coordinating lighter Ln³⁺ ions vs the heavier species, thus leading to the selective binding. Overall, these results shed light on understanding how modulation of molecular rigidity and couplings can be used to design a new generation of flexible receptors with enhanced selectivity and efficiency.