Inverse Trans Influence and Uranium-Arene σ‑Bonding Drive Molecular Geometry: Ligand Modification from Hard to Soft Flips the Oxide from Axial to Equatorial

A rare example of an equatorially bound terminal uranium(V)
oxo
complex in a chelating sulfur-based ligand environment, namely [(mes(Me,AdArS)3)UV(Oeq)(THF)] (2), is presented. Octahedrally coordinated 2 is
obtained by reaction of the mesitylene-anchored tris-thiophenolate-coordinated
uranium(III) complex [UIII((SArAd,Me)3mes)] (1) with the oxygen-atom transfer reagent N2O. The observed, equatorially bound oxo ligand in 2 is in stark contrast to its known tris-aryloxide analog, [(mes(Me,AdArO)3)UV(Oax)(THF)] (A), where the oxo ligand occupies the typically observed axial
coordination site. Complexes 1 and 2 are
characterized by single-crystal X-ray diffraction analyses and spectroscopic
and magnetochemical methods, including 1H NMR, UV/vis/NIR
electronic absorption, as well as EPR spectroscopy and SQUID magnetometry,
thus confirming the CS symmetry and the
pentavalent oxidation state of 2. Encompassing quantum
chemical calculations (DFT and CASPT2) on 2 and its tris-phenolate
analog A, support and rationalize the structural and
electronic differences. The molecular orbital pictures show that a
stabilizing σ-bonding interaction arising from the U–Oeq inverse trans influence (ITI) is present in 2 but missing in A. In 2, the sulfur 3p
orbitals are closer in energy to the uranium 5f manifold than the
arene π-system, leading to an ITI, while U–arene σ-
or δ-bonding is not observed. Although the arene orbitals remain
separated from the uranium 5f orbitals in A, the absence
of an ITI allows the arene a2u orbital
to engage in a σ-type interaction with the metal. Thus, incorporating
a tris-thiophenolate to an arene anchor introduces a new design concept
in molecular f-element chemistry. This approach stabilizes an equatorially
bound U(V) oxo center, contrasting with its tris-phenolate counterpart,
where oxo coordination is axial. The observed geometric divergence,
driven by competing ITI and U–arene interactions, not only
tunes electronic structure but also leads to differentiated reactivity:
only the phenolate analogs activate H2O, while the thiolates
do not.