Formation of Methane versus Benzene in the Reactions of (C 5 Me 5 ) 2 Th(CH 3 ) 2 with [CH 3 PPh 3 ]X (X=Cl, Br, I) Yielding Thorium-Carbene or Thorium-Ylide Complexes

The reaction of (C Me ) Th(CH ) with the phosphonium salts [CH PPh ]X (X=Cl, Br, I) was investigated. When X=Br and I, two equivalents of methane are liberated to afford (C Me ) Th[CHPPh ]X, rare terminal phosphorano-stabilized carbenes with thorium. These complexes feature the shortest thorium-carbon bonds (≈2.30 Å) reported to date, and electronic structure calculations show some degree of multiple bonding. However, when X=Cl, only one equivalent of methane is lost with concomitant formation of benzene from an unstable phosphorus(V) intermediate, yielding (C Me ) Th[κ -(C,C')-(CH )(CH )PPh ]Cl. Density functional theory (DFT) investigations of the reaction energy profiles for [CH PPh ]X, X=Cl and I showed that in the case of iodide, thermodynamics prevents the production of benzene and favors formation of the carbene.