Use of 77Se and 125Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3]− (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners

Reaction of [Th­(I)­(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K­(18-crown-6)]2[Se4] or [K­(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K­(18-crown-6)]­[Th­(η2-E2)­(NR2)3] (E = Se, 2; E = Te, 3), respectively. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K­(18-crown-6)]­[Th­(E)­(NR2)3] (E = Se, 4; E = Te, 5), respectively. Both 4 and 5 were characterized by X-ray crystallography and were found to feature the shortest known Th–Se and Th–Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with 77Se and 125Te NMR spectroscopy accompanied by detailed quantum-chemical analysis. We also recorded the 77Se NMR shift for a U­(VI) oxo-selenido complex, [U­(O)­(Se)­(NR2)3]− (δ­(77Se) = 4905 ppm), which features the highest frequency 77Se NMR shift yet reported, and expands the known 77Se chemical shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both 77Se and 125Te NMR chemical shifts of given chalcogenide ligands were identified as quantitative measures of the An–E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide–ligand bonding for uranium­(VI) complexes in contrast to those involving thorium­(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and density functional theory (DFT) calculations provide convincing evidence for the actinide–chalcogen multiple bonding in the title complexes. Larger An–E covalency is observed in the [U­(O)­(E)­(NR2)3]− series, which decreases as the chalcogen atom becomes heavier.