Sulfur K-edge X-ray Absorption Spectroscopy and Time-Dependent Density Functional Theory of Dithiophosphinate Extractants: Minor Actinide Selectivity and Electronic Structure Correlations

The dithiophosphinic acid HS2P(o-CF3C6H4)(2) is known to exhibit exceptionally high extraction selectivities for trivalent minor actinides (Am and Cm) in the presence of trivalent lanthanides. To generate insight that may account for this observation, a series of [PPh4][S2PR2] complexes, where R = Me (1), Ph (2), p-CF3C6H4 (3), m-CF3C6H4 (4), o-CF3C6H4 (5), o-MeC6H4 (6), and o-MeOC6H4 (7), have been investigated using sulfur K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT). The experimental analyses show distinct features in the spectrum of S2P(o-CF3C6H4)(2)(-) (5) that are not present in the spectrum of 4, whose conjugate acid exhibits reduced selectivity, or in the spectra of 2 and 3, which are anticipated to have even lower separation factors based on previous studies. In contrast, the spectrum of 5 is similar to those of 6 and 7, despite the significantly different electron-donating properties associated with the o-CF3, o-Me, and o-OMe substituents. The TDDFT calculations suggest that the distinct spectral features of 5-7 result from steric interactions due to the presence of the ortho substituents, which force the aryl groups to rotate around the P-C bonds and reduce the molecular symmetry from approximately C-2v in 2-4 to C-2 in 5-7. As a consequence, the change in aryl group orientation appears to make the ortho-substituted S2PR2- anions softer extractants compared with analogous Ph-, p-CF3C6H4-, and m-CF3C6H4-containing ligands (2-4) by raising the energies of the sulfur valence orbitals and enhancing orbital mixing between the S2P molecular orbitals and the aryl groups bound to phosphorus. Overall, we report that sulfur K-edge XAS experiments and TDDFT calculations reveal unique electronic properties of the S2P(o-CF3C6H4)(2)(-) anion in S. These results correlate with the special extraction properties associated with HS2P(o-CF3C6H4)(2), and suggest that ligand K-edge XAS and TDDFT can be used to guide separation efforts relevant to advanced fuel cycle development.