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

Abstract

Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO 2 2+) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (OP). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO 2 2+ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO 2 2+ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σand π-type interactions with U 5f and 6d orbitals in the UO 2 Cl 2 (TRPO) 2 complexes. Although both the axial (O yl ) and equatorial (O eq ) oxygen atoms in the UO 2 Cl 2 (TRPO) 2 complexes contribute to the Xray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O eq and is assigned to the transition from O eq 1s orbitals to the unoccupied molecular orbitals of 1b 1u + 1b 2u + 1b 3u symmetries resulting from the σand π-type mixing between U 5f and O eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O eq 2p character in these orbitals for the four UO 2 Cl 2 (TRPO) 2 complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U−TRPO bonding interactions in the UO 2 Cl 2 (TRPO) 2 complexes. The covalent bonding interactions between TRPO and UO 2 2+ , especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

Evaluating f-Orbital Participation in the UV═E Multiple Bonds of [U(E)(NR2)3] (E = O, NSiMe3, NAd; R = SiMe3)

“…Actinide-ligand covalency has been studied using a variety of experimental techniques, including X-ray absorption spectroscopy, − X-ray crystallography, − NMR spectroscopy, − EPR spectroscopy, − and optical spectroscopy. − With respect to optical spectroscopy, complexes with the 5f 1 electronic configuration have proven to be especially useful. ,− The lack of electron–electron repulsion in this configuration allows the spin–orbit coupling to be treated in a relatively straightforward manner. , For example, in 2013, we reported a comprehensive bonding analysis of a series of O h -symmetric 5f 1 complexes, including [U V (CH 2 SiMe 3 ) 6 ] − , [U V (O t Bu) 6 ] − , and [U V X 6 ] − (X = F, Cl, Br), which permitted an estimate of the strengths of the ligand–f-orbital interactions. More recently, Liddle and co-workers analyzed the optical and EPR spectra of the C 3v -symmetric U­(V) nitride, [U V (Tren TIPS )­(N)] − (Tren TIPS = N (CH 2 CH 2 N Si i Pr 3 ) 3 ), and extracted the corresponding crystal field splitting parameters .…”

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.…”

Tuning Phosphine Oxide-Substituted ortho-Carboranes for Improved Biphasic Electrochemical UO22+ Capture and Release

“…4 For instance, only very recently have TRPO derivatives been identified to have excellent binding to uranyl (UO 2 2+ ) due to optimized covalency and the involvement of the U 5f orbitals. 5 We have previously shown that redox-switchable ortho-closocarboranes bearing coordinating phosphine oxide donors, 1,2-(Ph 2 PO) 2 -1,2-C 2 B 10 H 10 ( PO Cb), could be electrochemically reduced to the nido-carborane, PO Cb 2− , an efficient extractant for biphasic UO 2 2+ capture (Figure 1a). Release of UO product back to its closo ( PO Cb) form.…”

“…In most cases, however, phosphine oxide-based extractants are most effective at capturing actinides, but there remains a surprising knowledge gap in the literature between structure–activity relationships of the ligands and corresponding UO 2 2+ extraction properties . For instance, only very recently have TRPO derivatives been identified to have excellent binding to uranyl (UO 2 2+ ) due to optimized covalency and the involvement of the U 5f orbitals …”