Coordination behavior of uranyl with PDAM derivatives in solution: Combined study with ESI-MS and DFT

Abstract

Although the extraction behaviors of the tetradentate 1,10-phenanthroline-2,9-dicarboxamide (PDAM, L) derivative ligands toward actinide and lanthanide ions have been reported, the coordination structures and species determination of the complexes are still involved and are also of great significance. To reveal the coordination structure of U(VI) ion and three PDAM derivative ligands with different alkyl chain, electrospray ionization mass spectrometry (ESI-MS) combined with density functional theory (DFT) calculations were carried out. The [UO2L(NO3)]+ complexes are the dominant species for the three PDAM derivative ligands based on the ESI-MS spectra. Collision-induced dissociation (CID) results further show that the U(VI) ions strongly bind with the phenanthroline fragment, while the amine groups and the alkyl moieties are apt to lost. Furthermore, DFT calculations further confirm that three PDAM derivative ligands can be well combined with uranyl, and [UO2L(NO3)]+ species are more stable than [UO2L(OH)·H2O]+ one. The length of the alkyl chain has little effect on the coordination structure of uranyl complexes, but affects their thermodynamic behaviors due to the steric hindrance effect. Therefore, the studies on the coordination chemistry of uranyl with three PDAM derivative ligands at the molecular level provide basic information for efficient actinides separation in spent fuel reprocessing.