Oxidation State Delineation via U L_III-Edge XANES in a Series of Isostructural Uranium Coordination Complexes

Abstract

We present an X-ray absorption near-edge structure (XANES) study of a series of uranium coordination complexes that possess nearly identical first coordination spheres and geometries in a range of oxidation states from U^III to U^VI. These compounds were obtained through the activation of small molecules, such as ketones, azides, and carbon dioxide, and upon oxidation of a high-valent U^VO to [U^VIO]⁺. Most of the compounds have been reported previously. All of them are fully characterized and their oxidation states have been confirmed by various spectroscopic methods (SQUID, ¹H NMR, and UV/vis/near-IR). Each uranium complex consists of a triazacyclononane anchor bearing three aryloxide side arms with bulky <i>tert</i>-butyl (<i>t</i>-Bu) or adamantyl (Ad) ortho substituents. All complexes have approximate <i>C</i>₃ symmetry and possess an axial cavity that is either empty (U^III) or occupied by a seventh ligand, namely, terminal oxygen (U^V and U^VI) or an oxygen-containing ligand (U^IV). The only exception is [((^{<i>t</i>‑Bu}ArO)₃tacnU^VI(O)]­[SbF₆], which is the rare case of a complex that shows a strong inverse trans influence. The determined correlation between the uranium oxidation state and the U L_III-edge XANES absorption in this series includes a single terminal oxo ligand bonded uranium­(V,VI), for which data are essentially nonexistent. The correct assignment of the uranium valence in a U^IV–L^•– compound (L^•– = ketyl radical) is shown to be only possible by a comparison to structurally similar compounds