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

Oxidation of our previously reported uranium-V) oxo complexes, supported by the chelating ((ArO)-Ar-R)(3)tacn(3-) ligand system (R = tert-butyl (t-Bu), 1-t-Bu; R = 1-adamantyl (Ad), 1-Ad), yields terminal uranium(VI) oxo complexes [(((ArO)-Ar-R)(3)tacn)U(VI)(O)]SbF6 (R = t-Bu, 2-t-Bu; R = Ad, 2-Ad). These complexes differ in their molecular geometry in that 2-t-Bu possesses pseudo-C, 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 sigma- and pi-donating ligand trans to the oxo (vis-a-vis 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 ((ArO)-Ar-R)(3)tacn(3-) ligand, including 2-Ad - is most clearly seen in the structures of 2-t-Bu, [((t-BuArO)(3)tacn)U(VI)(O)(eq)]SbF6, and 3-t-Bu, [((t-BuArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF3)(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O-ArO bond length is shortened by 0.1 angstrom in comparison to the cis U-O-ArO bonds and the trans U-O-C-ipso, angle is linearized (157.67 degrees versus 147.85 degrees and 130.03 degrees). Remarkably, the minor modification of the ligand to have Ad groups at the ortho positions of the aryloxide arms is sufficient to stabilize a C-3v-symmetric terminal uranium(VI) oxo complex (2-Ad) without a ligand trans 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 trans-influence as similar to 6 kcal mol(-1).