Activation of methane and toluene by rhodium(II) porphyrin complexes

Article abstract of DOI:10.1021/ja00014a025

Thermodynamic and kinetic-mechanistic studies are reported for reactions of (tetramesitylporphyrinato)rhodium(II) monomer, (TMP)Rh^., and (tetraxylylporphyrinato)rhodium(II) dimer, [(TXP)Rh]₂, with methane that produce hydride and methyl derivatives. A Rh^II-Rh^II bond energy of ～ 12 kcal mol^-1 in [(TXP)Rh]₂ was determined by ¹H NMR line broadening and found to dominate differences in the thermodynamic and kinetic parameters for reactions of methane with Rh(II) porphyrins. The sum of the Rh-H and Rh-CH₃ energies is found to be ～ 117 kcal in both the (TMP)Rh and (TXP)Rh derivatives. Rate laws, activation parameters, and deuterium isotope effects suggest that a four-centered linear transition state (Rh-H₃C-H-Rh) provides a relatively low activation enthalpy route for methane reacting with two metalloradicals. Comparative studies demonstrate that rhodium(II) porphyrins react with toluene exclusively at the benzylic C-H bond, and kinetic studies suggest that this reaction proceeds through a transition state related to that for the methane reactions. Aromatic C-H bond reactions are kinetically excluded for rhodium(II) porphyrins due to steric effects in the transition state.