Formation of Methane versus Benzene in the Reactions of (C5 Me5 )2 Th(CH3 )2 with [CH3 PPh3 ]X (X=Cl, Br, I) Yielding Thorium-Carbene or Thorium-Ylide Complexes

Abstract

The reaction of (C₅ Me₅ )₂ Th(CH₃ )₂ with the phosphonium salts [CH₃ PPh₃ ]X (X=Cl, Br, I) was investigated. When X=Br and I, two equivalents of methane are liberated to afford (C₅ Me₅ )₂ Th[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 (C₅ Me₅ )₂ Th[κ² -(C,C')-(CH₂ )(CH₂ )PPh₂ ]Cl. Density functional theory (DFT) investigations of the reaction energy profiles for [CH₃ PPh₃ ]X, X=Cl and I showed that in the case of iodide, thermodynamics prevents the production of benzene and favors formation of the carbene.

Keywords: carbenes; multiple bonding; organometallics; thorium; ylides.