ACTIVATION OF METHANE AND TOLUENE BY RHODIUM(II) PORPHYRIN COMPLEXES

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 approximately 12 kcal mol-1 in [(TXP)Rh]2 was determined by H-1 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 approximately 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.