Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C₅H₃(SiMe₃)₂]₃Th}^1- anion containing thorium in the formal +2 oxidation state

Reduction of the Th³⁺ complex Cp''3Th, 1 [Cp'' = C₅H₃(SiMe₃)₂], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp''3Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC₈ in the presence of 18-crown-6 generates the analogous Th²⁺ compound, [K(18-crown-6)(THF)₂][Cp''3Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M^-1 cm^-1, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 Å Th-(Cp'' ring centroid) distances, respectively, equivalent to the 2.520 Å distance measured in 1. Density functional theory analysis of (Cp''3Th)^1- is consistent with a 6d² ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp''2Th(C₈H₈), 4, and [K(18-crown-6)]Cp''.