Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA^–); Synthesis and Structure of M^IVTTA₄ (M^IV = Zr, Hf, Ce, Th, U, Np, Pu) and M^III(TTA)₄^– (M^III = Ce, Nd, Sm, Yb)

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating <i>micro</i>scopic amounts of transuranic actinides (i.e., Np and Pu) from <i>macro</i>scopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extractionor find alternativesbecause little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (M^IV = Zr, Hf, Ce, Th, U, Np, and Pu vs M^III = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula M^IV(TTA)₄. Meanwhile, +3 metals formed anionic M^III(TTA)₄^– species. Characterization of these M­(TTA)₄^<i>x</i>– (<i>x</i> = 0, 1) compounds by UV–vis–NIR, IR, ¹H and ¹⁹F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that Np^IV(TTA)₄ and Pu^IV(TTA)₄ were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between M^IV(TTA)₄ and M^III(TTA)₄^– are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np ^IV to Pu^IV, Hf^IV, and Zr^IV

Covalency in Americium(III) Hexachloride

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficultly in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers (Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954, 76, 1461) when it was proposed that covalency from 5<i>f</i>-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding inarguablyone of the most difficult systems to study, the Am–Cl interaction within AmCl₆^3–. We observed both 5<i>f</i>- and 6<i>d</i>-orbital mixing with the Cl-3<i>p</i> orbitals; however, contributions from the 6<i>d</i>-orbitals were more substantial. Comparisons with the isoelectronic EuCl₆^3– indicated that the amount of Cl 3<i>p</i>-mixing with Eu^III 5d-orbitals was similar to that observed with the Am^III 6<i>d</i>-orbitals. Meanwhile, the results confirmed Seaborg’s 1954 hypothesis that Am^III 5<i>f-</i>orbital covalency was more substantial than 4<i>f</i>-orbital mixing for Eu^III

A Pseudotetrahedral Uranium(V) Complex

A series of uranium amides were synthesized from <i>N</i>,<i>N</i>,<i>N</i>-cyclohexyl­(trimethylsilyl)lithium amide [Li]­[N­(TMS)­Cy] and uranium tetrachloride to give U­(NCySiMe₃)_<i>x</i>(Cl)_4–<i>x</i>, where <i>x</i> = 2, 3, or 4. The diamide was isolated as a bimetallic, bridging lithium chloride adduct ((UCl₂(NCyTMS)₂)₂-LiCl­(THF)₂), and the tris­(amide) was isolated as the lithium chloride adduct of the monometallic species (UCl­(NCyTMS)₃-LiCl­(THF)₂). The tetraamide complex was isolated as the four-coordinate pseudotetrahedron. Cyclic voltammetry revealed an easily accessible reversible oxidation wave, and upon chemical oxidation, the U^V amido cation was isolated in near-quantitative yields. The synthesis of this family of compounds allows a direct comparison of the electronic structure and properties of isostructural U^IV and U^V tetraamide complexes. Spectroscopic investigations consisting of UV–vis, NIR, MCD, EPR, and U L₃-edge XANES, along with density functional and wave function calculations, of the four-coordinate U^IV and U^V complexes have been used to understand the electronic structure of these pseudotetrahedral complexes