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

The solid-state N-15 NMR powder spectra of the thorium nitride complex, [K(18-crown-6)(THF)(2)][(R2N)(3)Th(mu-N-15)Th(NR2)(3)] ([K][1-N-1(5)], R = SiMe3), and the thorium amide complex, [Th(NR2)(3)((NH2)-N-15)] (2-N-15), were recorded. The spectrum for [K] [1-N-15] represents the first reported solid-state N-15 NMR data for an actinide complex. The experimentally measured tensor spans are Omega = 847 ppm for [K][1-N-15] and Omega = 237 ppm for 2-N-15. Both shielding tensors exhibit axial symmetry, which for [K] [1-N-15] is consistent with a local rotational symmetry of its N-15-labeled nitride ligand. For 2-N-1(5), 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-15], 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-1(5)] vs 2-N-1(5) is primarily a consequence of more pronounced covalency of the sigma(N-Th) bonds and large spin-orbit coupling due to significant Th Sf 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 Sf orbitals in Th-N multiple bonds and further demonstrates the value of solid-state NMR spectroscopy for interrogating actinide-ligand bonding.