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 [(eta 4- Cb)Mg(THF)3] (1), where Cbis tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl4 proceed with intact transfer of Cbto give the half-sandwich complexes [(eta 4-Cb)AnCl(mu-Cl)3Mg(THF)3] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg2Cl3(THF)6][(eta 4-Cb)An(eta 3-C4H(SiMe3)3-kappa-(CH2SiMe2)(Cl)] (An = Th, [Mg2Cl3(THF)6][4]; An = U [Mg2Cl3(THF)6][5]), in which one Cbligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C sigma-bond, protonation of the four-membered ring, and an eta 3-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 4 and 5 indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.

Automated summary

Cyclobutadienyl complexes of the f-elements are a new and poorly understood class of organometallic compounds. In this study, we investigated the cyclobutadienyl transfer reactions towards thorium(IV) and uranium(IV) tetrachlorides, and observed the formation of half-sandwich complexes. Computational analysis of the reaction mechanism revealed the activation of C-H bonds across the actinide center.