Tuning Lanthanide Binding with Phenanthroline-Based Diamides via Electron-Donating and Electron-Withdrawing Groups

Efficient separation of lanthanides is challenging due to their similar sizes and properties but continues to attract intensive theoretical and experimental interest. 1,10-phenanthroline-2,9-diamide (DAPhen) ligands are promising in solvent extraction separations of f-elements due to their combinations of soft N and hard O donors, but how the conjugated electrons and charge densities on the N and O donors can be further manipulated to tune the binding with lanthanides has not been fully explored. By substituting hydrogen atoms on the phenanthroline (Phen) skeleton with various electron-donating and electron-withdrawing groups, here we assess their impact on the complexation properties crucial for lanthanide separation. Employing advanced quantum chemical techniques, we have found that these substitutions significantly affect binding energies, although they have a relatively weak influence on selectivity for the whole lanthanide series. By elucidating the bonding nature in the studied lanthanide complexes, we provide a unified interpretation of these effects, aiming to comprehend the potential role of different electron-donating and electron-withdrawing substituents in lanthanide extraction and separation. These insights provide valuable guidelines for the rational design of DAPhen-based ligands for improved rare-earth element separations.