Holistic Assessment of Actinide-Borohydride Covalency Using NMR and X-ray Absorption Spectroscopies

The influence of metal-ligand covalency on actinide electronic properties and reactivity continues to be a topic of intense interest given its implications for nuclear and radiochemical applications. Extensive work has focused on developing spectroscopic methods that can be used to quantify metal-ligand covalency within actinide complexes for benchmarking and comparison with theoretical efforts. Ligand K-edge X-ray absorption spectroscopy (XAS) and NMR spectroscopy are two physical methods that have dominated these efforts, but surprisingly, they have not been used together on the same systems to see how the methods compare. Herein, we report the first comparative assessment of actinide-ligand covalency using B K-edge XAS and multinuclear NMR spectroscopy. The B K-edge XAS spectra of An(κ3-BH4)(NR2)3 (R = SiMe3) with An = Th (1) and U (2) reveal a dominant feature assigned to B 1s → An-H-B π* MOs due to covalent three-center, two-electron bonding with the actinide 6d orbitals, as revealed by comparison with time-dependent density functional theory (TDDFT) calculations. By contrast, the data indicate that 5f-orbital contributions to the bonds are significantly smaller. These findings were corroborated by 1H and 11B NMR studies of diamagnetic 1 and DFT calculations used to quantify the impact of orbital mixing via spin-orbit coupling on the chemical shifts. These results represent the first instance of using B K-edge XAS and 11B NMR spectroscopy to assess covalency in actinide complexes. Moreover, they demonstrate how ligand K-edge XAS, which provides rich orbital-specific information relevant to chemical bonding, and NMR spectroscopy, which is routinely available in most chemical research settings, can be used as complementary techniques to provide a holistic assessment of covalency in actinide-ligand bonds.