Tuned 4,7-Methoxy-Derivatized DAPhen Ligands with Enhanced Selectivity for Am/Eu and An/Np Separations

We report the synthesis and evaluation of two CHON-compliant 4,7-dimethoxy-1,10-phenanthroline-2,9-dicarboxamide (DADOMePhen) ligands: an aromatic variant, Et-Tol-DADOMePhen (1), and an aliphatic analogue, TCH-DADOMePhen (2). Solvent extraction under SANEX conditions shows that ligand 1 has good Am/Eu selectivity in an optimal window of 0.1–0.5 M HNO3, achieving a maximum SFAm/Eu of 108.2 at 0.5 M HNO3, the highest reported for a lipophilic ligand of pure DAPhen-type molecular architecture to date. This performance inverts conventional acidity dependence: electron-donating methoxy substituents enhance metal–ligand covalency at low acidity but make the ligand susceptible to protonation at higher concentrations. Coordination and fluorescence studies indicate a 1:2 metal-to-ligand stoichiometry; single-crystal diffraction reveals the structure of ligand 1. Quantum-chemical calculations establish that methoxy groups tune electrostatic potential and frontier orbitals, strengthening bonding relative to parent Et-Tol-DAPhen. Beyond Am/Eu partitioning, ligand 1 may enable actinide/neptunium separations: at 0.1 M HNO3, calculated SFAm/Np, SFPu/Np, SFU/Np, and SFTh/Np reach 436, 1690, 9766, and 1953, respectively, reflecting mismatch between the dioxo Np(V) monocation and the ligand. This work establishes 4,7-methoxy derivatization as a design strategy to tune DAPhen-based extractant selectivity and introduces a ligand that could be useful for specialized low-acidity actinide/neptunium or lanthanide separations.