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.