Investigations of the Electronic Structure of Arene-Bridged Diuranium Complexes

The
electronic structure of the arene-bridged complex (μ-toluene)­U2(N­[tBu]­Ar)4 (1a2-μ-toluene, Ar = 3,5-C6H3Me2) has been studied in relation to a variety of mononuclear
uranium amide complexes, and
their properties have been discussed comparatively. The syntheses,
molecular structures (X-ray crystal structures and solution behavior
based on variable-temperature NMR spectroscopic data), and corresponding
spectroscopic (X-ray absorption near-edge structure and UV–vis–near-IR
absorption) and magnetic properties are presented and interpreted
with reference to results of density functional theory (DFT) and complete
active space self-consistent field with corrections from second-order
perturbation theory (CASSCF/CASPT2) calculations performed on model
compounds. While the mononuclear compounds display expected electronic
and magnetic properties for uranium complexes, 1a2-μ-toluene shows complicated properties
in contrast. XANES spectroscopy, X-ray crystallography, and both density
functional and CASSCF/CASPT2 results are consistent with the following
electronic structure interpretation: f orbitals host the unpaired
electrons, followed energetically by two δ bonds formed by filled
uranium f orbitals and LUMOs of toluene.