Synthesis of Uranium(VI) Terminal Oxo Complexes: Molecular Geometry Driven by the Inverse <i>Trans</i>-Influence

Oxidation of our previously reported uranium­(V) oxo complexes, supported by the chelating (^RArO)₃tacn^3– ligand system (R = <i>tert</i>-butyl (<i>t-</i>Bu), <b>1-</b><i><b>t</b></i><b>-Bu</b>; R = 1-adamantyl (Ad), <b>1-Ad</b>), yields terminal uranium­(VI) oxo complexes [((^RArO)₃tacn)­U­(VI)­(O)]­SbF₆ (R = <i>t</i>-Bu, <b>2-</b><i><b>t</b></i><b>-Bu</b>; R = Ad, <b>2-Ad</b>). These complexes differ in their molecular geometry in that <b>2-</b><i><b>t</b></i><b>-Bu</b> possesses pseudo-<i>C</i>_<i>s</i> symmetry in solution and solid state as the terminal oxo ligand lies in the equatorial plane (as defined by the three aryloxide arms of the ligand) in order to accommodate the thermodynamic preference of high-valent uranium oxo complexes to have a σ- and π-donating ligand <i>trans</i> to the oxo (<i>vis-à-vis</i> the ubiquity of the linear UO₂²⁺ moiety). The distortion of the ligand  which stands in contrast to all other complexes of uranium supported by the (^RArO)₃tacn^3– ligand, including <b>2-Ad</b>  is most clearly seen in the structures of <b>2-</b><i><b>t</b></i><b>-Bu</b>, [((^{<i>t</i>‑Bu}ArO)₃tacn)­U­(VI)­(O)_eq]­SbF₆, and <b>3-</b><i><b>t</b></i><b>-Bu</b>, [((^{<i>t</i><b>‑</b>Bu}ArO)₃tacn)­U­(VI)­(O)_eq(OC­(O)­CF₃)_ax]. The solid-state structure of <b>3-</b><i><b>t</b></i><b>-Bu</b> reveals that the <i>trans</i> U–O_ArO bond length is shortened by 0.1 Å in comparison to the <i>cis</i> U–O_ArO bonds and the <i>trans</i> U–O–C_<i>ipso</i> angle is linearized (157.67° versus 147.85° and 130.03°). Remarkably, the minor modification of the ligand to have Ad groups at the <i>ortho</i> positions of the aryloxide arms is sufficient to stabilize a <i>C</i>_3<i>v</i>-symmetric terminal uranium­(VI) oxo complex (<b>2-Ad)</b> without a ligand <i>trans</i> to the oxo. These experimental results were reproduced in DFT calculations and allow the qualitative bracketing of the relative thermodynamic stabilization afforded by the inverse <i>trans</i>-influence as ∼6 kcal mol<sup>–1