Coordination Behavior of Th(IV) with Tetradentate N,O-Ligands: Solvent Extraction, Complexation, and XRD Study

The development of the thorium fuel cycle has made the efficient extraction and separation of Th(IV) an important task. In this work, the extraction, complexation, and coordination behavior of Th(IV) with a series of 1,10-phenanthroline-2,9-diamide ligands were investigated by solvent extraction, spectrophotometric titration, single-crystal X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). The results showed that Th(IV) forms neutral complexes of the type ThL(NO3)4, in which the metal ion is coordinated by a tetradentate N,O-donor ligand and four bidentate nitrate anions. XRD, EXAFS, and XANES data demonstrated that the same coordination motif was retained in the solid state and in the organic phase. The extraction of Th(IV) from nitric acid solutions was found to be strongly dependent on the ligand structure, with ligands bearing aryl substituents at the amide nitrogen atoms exhibiting exceptionally high efficiency and distribution ratios reaching nearly 104. Spectrophotometric titration revealed the formation of stable 1:1 and 2:1 metal-to-ligand complexes and confirmed the pronounced influence of ligand substituents on complex stability and extraction performance. These results demonstrate the high potential of phenanthroline diamides for the efficient binding and extraction of tetravalent actinides.