Dispersion Forces Drive the Formation of Uranium-Alkane Adducts

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η²-H, C metal-alkane interactions in the [((ArO)₃tacn)U(cy-C6)]·(cy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-O and U-N bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap. For that reason, it is deduced that metal-ligand bonding in the [((ArO)₃tacn)U] moiety is predominantly ionic. The bonding interaction within the [((ArO)₃tacn)U] moiety is shown to be dispersive in nature and essentially supported by the upper-rim Bu groups of the (ArO)₃tacn^3- ligand. Our findings indicate that the axial alkane molecule is held in place by the guest-host effect rather than direct metal-alkane ionic or covalent interactions.