Probing the Electronic Structure of a Thorium Nitride Complex by Solid-State N NMR Spectroscopy.

The solid-state N NMR powder spectra of the thorium nitride complex, [K(18-crown-6)(THF)][(RN)Th(μ-N)Th(NR)] (, R = SiMe), and the thorium amide complex, [Th(NR)(NH)] (), were recorded. The spectrum for represents the first reported solid-state N NMR data for an actinide complex. The experimentally measured tensor spans are Ω = 847 ppm for and Ω = 237 ppm for . Both shielding tensors exhibit axial symmetry, which for is consistent with a local rotational symmetry of its N-labeled nitride ligand. For , the axial asymmetry can be rationalized by a quasi-free Th-NH bond rotation in the solid-state. Density functional theory calculations overestimate the tensor span somewhat for , but provide isotropic shifts in good agreement with both the solid-state and solution values for both complexes. Natural localized molecular orbital analyses of the nuclear shielding reveal that the larger tensor span in vs is primarily a consequence of more pronounced covalency of the σ(N-Th) bonds and large spin-orbit coupling due to significant Th 5f orbital contribution to those bonds, impacting the principal components of the shielding tensor perpendicular to the Th-N-Th axis. Overall, our analysis confirms the involvement of the 5f orbitals in Th-N multiple bonds and further demonstrates the value of solid-state NMR spectroscopy for interrogating actinide-ligand bonding.