Expanding Thorium Hydride Chemistry Through Th²⁺, Including the Synthesis of a Mixed-Valent Th⁴⁺/Th³⁺ Hydride Complex

The reactivity of the recently discovered Th²⁺ complex [K(18-crown-6)(THF)₂][Cp″₃Th], 1 [Cp″ = C₅H₃(SiMe₃)₂-1,3], with hydrogen reagents has been investigated and found to provide syntheses of new classes of thorium hydride compounds. Complex 1 reacts with [Et₃NH][BPh₄] to form the terminal Th⁴⁺ hydride complex Cp″₃ThH, 2, a reaction that formally involves a net two-electron reduction. Complex 1 also reacts in the solid state and in solution with H₂ to form a mixed-valent bimetallic product, [K(18-crown-6)(Et₂O)][Cp″₂ThH₂]₂, 3, which was analyzed by X-ray crystallography, electron paramagnetic resonance and optical spectroscopy, and density functional theory. The existence of 3, which formally contains Th³⁺ and Th⁴⁺, suggested that KC₈ could reduce [(C₅Me₅)₂ThH₂]₂. In the presence of 18-crown-6, this reaction forms an analogous mixed-valent product formulated as [K(18-crown-6)(THF)][(C₅Me₅)₂ThH₂]₂, 4. A similar complex with (C₅Me₄H)^1- ligands was not obtained, but reaction of (C₅Me₄H)₃Th with H₂ in the presence of KC₈ and 2.2.2-cryptand at -45 °C produced two monometallic hydride products, namely, (C₅Me₄H)₃ThH, 5, and [K(2.2.2-cryptand)]{(C₅Me₄H)₂[η¹:η⁵-C₅Me₃H(CH₂)]ThH]}, 6. Complex 6 contains a metalated tetramethylcyclopentadienyl dianion, [C₅Me₃H(CH₂)]^2-, that binds in a tuck-in mode.