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)[Se-4] or [K(18-crown-6)](2)[Te-2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(eta(2)-E-2)(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 Se-77 and Te-125 NMR spectroscopy accompanied by detailed quantum-chemical analysis. We also recorded the Se-77 NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)(3)](-) (delta(Se-77) = 4905 ppm), which features the highest frequency Se-77 NMR shift yet reported, and expands the known Se-77 chemical shift range for diamagnetic substances from similar to 3300 ppm to almost 6000 ppm. Both Se-77 and Te-125 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.