Probing the Electronic Structure of a Thorium Nitride Complex by Solid-State ¹⁵N NMR Spectroscopy

The solid-state ¹⁵N NMR powder spectra of the thorium nitride complex, [K(18-crown-6)(THF)₂][(R₂N)₃Th(μ-¹⁵N)Th(NR₂)₃] ([K][1-N], R = SiMe₃), and the thorium amide complex, [Th(NR₂)₃(¹⁵NH₂)] (2-N), were recorded. The spectrum for [K][1-N] represents the first reported solid-state ¹⁵N NMR data for an actinide complex. The experimentally measured tensor spans are Ω = 847 ppm for [K][1-N] and Ω = 237 ppm for 2-N. Both shielding tensors exhibit axial symmetry, which for [K][1-N] is consistent with a local rotational symmetry of its ¹⁵N-labeled nitride ligand. For 2-N, the axial asymmetry can be rationalized by a quasi-free Th-NH₂ bond rotation in the solid-state. Density functional theory calculations overestimate the tensor span somewhat for [K][1-N], 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 [K][1-N] vs 2-N 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.