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 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.