Methane Metathesis at a Cationie Indium Center

摘要

Recently, Amdtsen and Bergman reported low-temperature hydiocarbon bond activation by a cationic indium complex. In this reaction, the complex activates a hydrocarbon C--H bond, transfers the hydrogen atom to a methyl ligand, and ejects methane as a reaction product. The net result is a σ-bond metathesis reaction between M--CH_3 and R--H, where M is Cp*Ir(P(CH_3)_3)(CH_3)~+ and Cp* = C_5(CH_3)_5. Two specific mechanisms were proposed; (1) The metathesis proceeds through a four-center adduet betweenM--CH_3 and R--H, in which the hydrogen atom transfers from R to CH_3 while maintaining a long distance from indium, a mechanism which has been observed for early transition metals. (2) The hydiocarbon undergoes oxidative addition to the indium atom, sothat R and H are both directly bonded to the metal atom before the reductive elimination of methane, a mechanism which cycles the oxidation state of the indium atom from Ir(III) to Ir(V) and back to Ir(IH) Both mechanisms are illustrated in Scheme. Thesetwo mechanisms are explored theoretically for the specific case where R--H is methane and M--CH_3 is CpIrPH_3CH_3~+ (Cp = CsHs), a eationjc complex which is a model system for the cationic complex studied by Arndtsen and Bergman. The theoretical data show that the oxidative-addition mechanism is a low-energy pathway and that the fom-center adduet mechanism is doubtful even at higher energies.