Oxidation state delineation via U L III-edge XANES in a series of isostructural uranium coordination complexes

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 tert-butyl (t-Bu) or adamantyl (Ad) ortho substituents. All complexes have approximate C 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 [(( t-BuArO) 3tacnU VI(O)][SbF 6], 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. © 2012 American Chemical Society.