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

Abstract

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][1-¹⁵N], R = SiMe₃), and the thorium amide complex, [Th(NR₂)₃(¹⁵NH₂)] (2-¹⁵N), were recorded. The spectrum for [K][1-¹⁵N] represents the first reported solid-state ¹⁵N NMR data for an actinide complex. The experimentally measured tensor spans are Ω = 847 ppm for [K][1-¹⁵N] and Ω = 237 ppm for 2-¹⁵N. Both shielding tensors exhibit axial symmetry, which for [K][1-¹⁵N] is consistent with a local rotational symmetry of its ¹⁵N-labeled nitride ligand. For 2-¹⁵N, 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][1-¹⁵N], 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][1-¹⁵N] vs 2-¹⁵N 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.