Formation of Methane versus Benzene in the Reactions of (C5Me5)2Th(CH3)2 with [CH3PPh3]X (X=Cl, Br, I) Yielding Thorium-Carbene or Thorium-Ylide Complexes.

The reaction of (C₅Me₅)₂Th(CH₃)₂ with the phosphonium salts [CH₃PPh₃]X (X=Cl, Br, I) was investigated. When X=Br and I, two equivalents of methane are liberated to afford (C₅Me₅)₂Th[CHPPh₃]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 (C₅Me₅)₂Th[κ²-(C,C′)-(CH₂)(CH₂)PPh₂]Cl. Density functional theory (DFT) investigations of the reaction energy profiles for [CH₃PPh₃]X, X=Cl and I showed that in the case of iodide, thermodynamics prevents the production of benzene and favors formation of the carbene. [ABSTRACT FROM AUTHOR]