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)(2)Th(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)(2)Th[CHPPh3]X, rare terminal phosphorano-stabilized carbenes with thorium. These complexes feature the shortest thorium-carbon bonds (approximate to 2.30 angstrom) 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)(2)Th[kappa(2)-(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.