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$^4$-Cb'''')Mg(THF)$_3$] (1), where Cb'''' is tetrakis(trimethylsilyl)-cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl$_4$ proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η$^4$-Cb'''')AnCl(μ-Cl)$_3$Mg(THF)$_3$] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg$_2$Cl$_3$(THF)$_6$][(η$^4$-Cb'''')An(η$^3$-C$_4$H(SiMe$_3$)$_3$-κ-(CH$_2$SiMe$_2$)(Cl)] (An = Th, [Mg$_2$Cl$_3$(THF)$_6$][4]; An = U [Mg$_2$Cl$_3$(THF)$_6$][5]), 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$^3$-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both
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sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C−H activation across the actinide center.