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 [((t·BuArO)3tacn)UIII(Mecy-C6)]·(Mecy-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–OArO and U–Ntacn 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 [((t·BuArO)3tacn)UIII] moiety is predominantly ionic. The bonding interaction within the [((t·BuArO)3tacn)UIII] moiety is shown to be dispersive in nature and essentially supported by the upper-rim tBu groups of the (t·BuArO)3tacn3– 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.