Synthesis and Evaluation of Hydrophilic 1,10-Phenanthroline Derivative for the Separation of Scandium(III) and Yttrium(III)

This work addresses a critical scientific challenge for the field of lanthanide solvent extraction by developing a novel hydrophilic ligand system based on a phenanthroline skeleton. Through the design and synthesis of 2,2'-((1,10-phenanthroline-2,9-dicarbonyl)bis(azanediyl))bis(ethane-1-sulfonic acid) (DS-DAPhen) featuring hard-soft hybrid donors, and its synergistic use with the conventional hydrophobic extractant <i>N,N,N</i>'<i>,N</i>'-tetraoctyl diglycolamide (TODGA), we successfully established a novel competitive extraction system for Sc(III)/Y(III) separation. Experimental results demonstrate that this system exhibits exceptional selectivity toward Sc(III) within a 0.5-2.0 M HNO₃ concentration range, achieving a remarkably high maximum separation factor of 1031.6 (Y/Sc). Through comprehensive characterization techniques including ESI-HRMS and single-crystal X-ray diffraction, we systematically investigated the concentration-dependent speciation of complexes: under ligand-deficient conditions, both 1:2 and 1:1 complex species predominated, while at equimolar concentrations, the 1:1 complex became the dominant species. DFT calculations provided mechanistic insights at the electronic structure level, confirming significantly stronger coordination bonds between DS-DAPhen and Sc(III) compared to Y(III), along with greater thermodynamic stability of Sc(III) complexes.