Efficient UO22+ Extraction by DAPhens with Asymmetric Terminal Groups: the Molecular Design, Spectral Titration, Liquid-Liquid Extraction and Mechanism Study

Abstract Four asymmetric alkyl-substituted DAPhens (N,N’-Dimethyl-N,N’-Dialkyl-2,9-Diamide-1,10-Phenanthrolines) were synthesized and investigated as selective ligands for UO22+. The results confirm that the ligands have higher affinity for uranyl comparing to the symmetric analogues with the same 1,10-phenanthroline core moiety. The slope analysis indicated the formation of 1:1 (metal/ligand) extracting complex, which was identified in the organic phase as [UO2(DAPhen-4)(NO3)]+ complex ion by electrospray ionization mass spectrometry (ESI-MS). The superior affinity of the ligands for UO22+ over Th4+ and Eu3+ in solvent extraction was consistent with the stability constants of their complexes in acetonitrile obtained by UV-Vis spectroscopic titrations. The observed better affinity of the asymmetric alkyl-substituted DAPhens could originate from the less steric hindrance of methyl in the terminal groups, which was confirmed by the relatively mild distortion of the ligand structure in the uranyl complex according to the crystal and molecular structure of the solid-state complex [UO2(DAPhen-1)(NO3)](NO3)·(CH3OH) as characterized by single crystal X-ray diffraction.