On the Nature of the Carbonyl versus Phosphoryl Binding in Uranyl Nitrate Complexes

Abstract

The electronic structure of ligands with phosphoryl and carbonyl binding sites and their complexation behavior with uranyl nitrate were investigated using density functional theory (DFT). The quantum chemical calculations indicate that the electronic charges on both phosphoryl and carbonyl groups are more polarized toward oxygen atoms in isolated ligands. This effect is predominant in the case of complexes of the former. Both PO and CO groups are positively charged with the exception in methylisobutylketone (MIBK), where the C=O group is virtually neutral. The fragment molecular orbital analysis suggests that during complexation, a certain amount of charge transfer occurs from the filled pπ-orbitals [π x (CO/PO) and π y (CO/PO) ] of the ligand to 5f, 6d, and 7s orbitals of the uranium atom (fσ* and dsσ*). The NBO analysis reaffirms the charge transfer mechanism. The observed red shift in ν­(CO) and ν­(PO) identified in the simulated infrared spectrum of the corresponding complexes implies a moderate weakening of both carbonyl and phosphoryl bonds upon complexation. The atoms in molecules (AIM) analysis suggests a stronger phosphoryl binding compared to carbonyl interactions and an ionic U–O bond. The estimated complexation energies are considerable for phosphoryl ligands compared to those of the carbonyl analogue, with a reasonably large value derived for tri-n-butyl phosphate (TBP). The energy decomposition analysis marked significant stabilizing orbital interactions for phosphoryl ligands. The contributions of estimated dispersion energies are considerable in all complexes and extensively depend on the alkyl unit.

Insights into the Coordination Behavior of Methyl-Substituted Phosphinic Acids with Actinides

“…Various possible starting geometries were generated by distributing ligands and nitrate anions around UO 2 2+ . During this step, the metal–ligand stoichiometry was considered to be 1:2 because the same was experimentally estimated for various uranyl nitrate complexes with phosphorus-based ligands. ,,− A straightforward geometry optimization and subsequent characterization of harmonic vibrational frequencies resulted in several local minima on the potential energy hypersurface of each complex. The lowest-energy structures of uranyl nitrate complexes with various substituted phosphinic acid ligands are represented in Figure .…”

“…The resulting organic-soluble complexes formed are eventually extracted into the organic medium to complete the extraction process. Plentiful experimental evidence is available to support this mechanism, and most of them, by and large, are based on the equilibrium constant, solubility, and structural characterization of the complexes formed during extraction. − In addition to this, theoretical studies provided further insights into the metal–ligand interactions, − nature of bonding, , solvation, − and so on and furnished ample evidence to support the experimentally proposed extraction mechanism.…”

Selective electrochemical capture and release of uranyl from aqueous alkali, lanthanide, and actinide mixtures using redox-switchable carboranes

“…Together, these results indicate that electrochemically generated PO Cb 2− selectively captures UO 2 2+ from a mixed alkali, lanthanide, and actinide aqueous phase. While the nature of this selectivity remains under investigation, we suspect that optimal covalent bonding interactions between the PO units and the U center – very recently investigated in the PUREX context 29,30 – are likely at play.…”

“…Together, these results indicate that electrochemically generated PO Cb 2À selectively captures UO 2 2+ from a mixed alkali, lanthanide, and actinide aqueous phase. While the nature of this selectivity remains under investigation, we suspect that optimal covalent bonding interactions between the P]O units and the U centervery recently investigated in the PUREX context 29,30 are likely at play. In addition to this selective capture, these biphasic experiments revealed the formation of minor byproduct resonances in the 31 P{ 1 H} NMR spectrum of the DCE phase at 28.6 and 25.8 ppm (Fig.…”

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

“…In the TRPO process, the distribution ratio of UO 2 2+ far exceeds that for other metal ions . The interactions of the phosphoryl group (OP) in these ligands with UO 2 2+ are considered to be responsible to the extraordinary extractability and selectivity and have been investigated by crystallography and theoretical calculations over the past several decades. − The crystal structures suggest the coordination of the oxygen atoms in the phosphoryl group with UO 2 2+ . The theoretical calculations provide the geometry of the complexes and the binding energies between TRPO and UO 2 2+ , and the bonding order analysis indicates the ionic bonding properties between UO 2 2+ and the phosphoryl group. − These studies are, however, insufficient to account for the high extractability and selectivity of the monodentate organophosphorus extractants for UO 2 2+ , and its underlying mechanism requires further investigation.…”