Investigations of the electronic structure of arene-bridged diuranium complexes

The electronic structure of the arene-bridged complex (μ-toluene)U ₂(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.