Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory

Abstract

The dithiophosphinic acids (HS2PR 2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR 2 − anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR 2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character in these orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U—S bonds for the two UO2(S2PR 2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

Covalency in AnCl2 (An = Th–No)

“…5 This combination of DFT and spectroscopy has also been employed to study covalency in actinide ligand bonds by several other groups. 6–11 Furthermore, experiment has also validated the use of QTAIM parameters to measure covalency; for example, changes in chemical shifts in both 13 C and 15 N nuclear magnetic resonance (NMR) spectra show good correlation with bond order metrics, for a range of actinide ligand bonds. 12,13…”

Semiconductive Behavior and Photoconductivity of Uranyl Dithiophosphinate Single Crystal

“…To ascertain that O1 connected to U originates from the methanol molecule, we measured the C–O bond length of O1, which is 1.431(5) Å (Table S2). This discrepancy may be attributed to charge attraction subsequent to coordination with the metal . The stretching vibration in the IR spectrum at 3500 cm –1 in Figure S2 can also be attributed to the −OH group originating from the coordinating methanol .…”

“…23 This discrepancy may be attributed to charge attraction subsequent to coordination with the metal. 24 The stretching vibration in the IR spectrum at 3500 cm −1 in Figure S2 can also be attributed to the −OH group originating from the coordinating methanol. 25 In the axial position, occupied by two acyl oxygen atoms, the uranium atomic center forms a seven-coordinated pentagon−bipyramidal coordination geometry between O and S atoms, as shown in Figure 1b.…”

Foreword to the special virtual issue on Actinide physics and chemistry with synchrotron radiation

“…(2) Alexander Scott Ditter and co-authors showed the power of soft X-ray spectromicroscopy at the O K-, U N 4,5 -and Ce M 4,5 -edges for spent nuclear fuel investigations, yielding chemical information on the sub-micrometre scale (Ditter et al, 2022). Yusheng Zhang and co-workers investigated the covalency effects in the bonding between the uranyl ion and dithiophosphinate by combining sulfur K-edge XANES and density functional theory (Zhang et al, 2022).…”