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

Abstract

Oxidation of our previously reported uranium(V) oxo complexes, supported by the chelating ((R)ArO)(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 [(((R)ArO)(3)tacn)U(VI)(O)]SbF(6) (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(s) 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 trans to the oxo (vis-à-vis the ubiquity of the linear UO(2)(2+) moiety). The distortion of the ligand--which stands in contrast to all other complexes of uranium supported by the ((R)ArO)(3)tacn(3-) ligand, including 2-Ad--is most clearly seen in the structures of 2-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)]SbF(6), and 3-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF(3))(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O(ArO) bond length is shortened by 0.1 Å in comparison to the cis U-O(ArO) bonds and the trans U-O-C(ipso) angle is linearized (157.67° versus 147.85° and 130.03°). 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 ∼6 kcal mol(-1).

Covalent Uranium Carbene Chemistry

“…[73] Structural and computational investigations into 45 reveal the oxo to be trans to the carbene carbon which bears all of the hallmarks of the inverse trans influence (ITI). [74][75][76][77][78][79][80][81] The ITI is due to the semi-core 6p z orbital of uranium mixing with the 5f orbitals, leading to a hole in the 6p z orbital directed to the trans position of the strong oxo ligand. In the case of 45, the carbene center is a stronger donor than the chlorides and can compensate for this hole more, and so occupies the trans position to the oxo group.…”

The Nature of the UC Double Bond: Pushing the Stability of High‐Oxidation‐State Uranium Carbenes to the Limit

“…The U À O bond lengths average 2.055 (5) , which compares very well to unique UÀO bond lengths of 2.05(1) and 2.027(8) observed in hexavalent [UA C H T U N G T R E N N U N G (OtBu) 6 ]. [19b] Looking at the three UÀO bonds in detail, it is tantalising to consider that the short U1 À O1 distance of 2.044(5) might constitute an example of the ITI effect [18] as observed in certain O À U=O linkages, [46] but this bond length is statistically indistinguishable to the U1 À O2 and U1 À O3 bond lengths of 2.051(5) and 2.070(5) , respectively. Although the BIPM Dipp ligand is coordinated asymmetrically in 9, the PÀC and PÀN bonds are virtually indistinguishable by the 3s criterion.…”

Oxo–Group‐14‐Element Bond Formation in Binuclear Uranium(V) Pacman Complexes

“…[10] Also, of synthetic interest is uranyl oxo-group functionalization, in which the single-electron reduction of the uranyl dication occurs alongside covalent-bond formation to the oxo group (Figure 1 iv); to date, only bonds to protons [11] and silyl groups [6,12] have been observed. A final class of compounds in oxo-uranium chemistry feature terminal mono-oxo ligands (Figure 1 v) and have been isolated for both uranium(V) [13,14] and uranium(VI), [14,15] and commonly incorporate bulky ligands to prevent further reaction of the highly nucleophilic oxo group. [16] Since 2004, we have been studying the reactivity of mononuclear uranyl complexes of the binucleating Schiff base polypyrrolic macrocycle L, in which the ligand adopts a unique wedge-shaped "Pacman" geometry upon uranyl complexation.…”