Unfolding the Extraction and Complexation Behaviors of Trivalent f-Block Elements by a Tetradentate N,O-Hybrid Phenanthroline Derived Phosphine Oxide Ligand

In this work, a tetradentate N,O-hybrid 2,9-bis(diphenylphosphine oxide)-1,10-phenanthroline (Ph-2-BPPhen) ligand was studied for the coextraction of trivalent f-block elements from nitric acid media. The extraction as well as the complexation behaviors of Ph-2-BPPhen with f-block elements were thoroughly investigated using P-31 and H-1 NMR spectrometry, UV-vis spectrophotometry, single crystal X-ray diffraction, and density functional theoretical (DFT) calculation. Ph-2-BPPhen exhibits remarkably extraction ability for both Am(III) and Eu(III) and more than 99.5% of Am(III) and Eu(III) were extracted from 1.0 M HNO3 solution. Slope analysis suggests that both 2:1 and 1:1 ligand/metal complexes were probably formed during the extraction. The 1:1 and 2:1 Ln(III) complexes with Ph-2-BPPhen were also identified in CH3OH solution by NMR spectrometry, and the stability constants were determined via UV-vis spectrophotometry. Structures of the 1:1 Eu(Ph-2-BPPhen)(NO3)(3) and Am(Ph-2-BPPhen)(NO3)(3) complexes were further elucidated by single X-ray crystallography and DFT calculations. The higher extractability of Ph-2-BPPhen toward trivalent Am(III) and Eu(III) compared with the previously reported phenanthroline-derived amide and phosphonate ligands was attributed to the stronger affinity of the -P=O(R)(2) group to metal ions. The results from this work indicate that the N,O-hybrid 1,10-phenanthroline derived phosphine oxide ligand can serve as a new and promising candidate for coextraction of trivalent f-block elements in the treatment of nuclear waste.

Automated summary

The tetradentate N,O-hybrid 2,9-bis(diphenylphosphine oxide)-1,10-phenanthroline (Ph-2-BPPhen) ligand showed high extraction ability for trivalent f-block elements, particularly Am(III) and Eu(III), making it a promising candidate for nuclear waste treatment.