Thorium- and Uranium-Mediated C-H Activation of a Silyl-Substituted Cyclobutadienyl Ligand.

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η-Cb'''')Mg(THF)] (), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between and AnCl proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η-Cb'''')AnCl(μ-Cl)Mg(THF)] (An = Th, ; An = U, ). Using a 2:1 reaction stoichiometry produces [MgCl(THF)][(η-Cb'''')An(η-CH(SiMe)-κ-(CHSiMe)(Cl)] (An = Th, [MgCl(THF)][]; An = U [MgCl(THF)][]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C σ-bond, protonation of the four-membered ring, and an η-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of and indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.