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, (1)H 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-Bu)ArO)(3)tacnU(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.

Uranium(III)-carbon multiple bonding supported by arene δ-bonding in mixed-valence hexauranium nanometre-scale rings

“…Samples were collected on two separate beam lines to ensure consistency. The sample spectra have strong white lines and are similar in shape to the UO 2 and uranium(III) standards and other uranium(III) and uranium(IV) L 3 -edge XANES described in the literature 67 – 69 . Complexes 3 and 4 lack a shoulder feature on the high energy side of the white line indicating the absence of uranium(V) or uranium(VI), since a shoulder feature would be indicative of multiple scattering along the trans-dioxo moiety 67 .…”

X-ray Absorption Spectroscopy Systematics at the Tungsten L-Edge

“…71−73 Departure from linearity is ascribed to variation in the peak profile of the tail of the white line above the ionization threshold due to transitions to quasi-bound states leading to a deviation of the peak from a pseudo-Voigt line shape. 69,72 Also 2p–5d multiplet effects will affect the spectral shape and peak area. 74 A similar increase is noted for the L 2 -edge (Figure S6), though [W 0 (PMe 3 ) 6 ] departs significantly from expectation without an obvious explanation.…”

Exploring Actinide Materials Through Synchrotron Radiation Techniques

“…In the complexes, the α‐diimine was reduced to ene‐diamides and the oxidation state of uranium was +4. Interestingly, Kosog et al synthesized a series of isostructural hexa‐ and hepta‐coordinate uranium complexes that possessed nearly identical first‐coordination spheres and geometries in the full range of oxidation states from U(III) to U(VI). The uranium cations with different oxidation states could be supported by the triazacyclononane anchor bearing three aryloxide side arms with bulky tert‐butyl or adamantyl ortho substituents.…”