Bimetallo-Radical Carbon−Hydrogen Bond Activation of Methanol and Methane

Carbon−hydrogen bond cleavage reactions of CH3OH and CH4 by a dirhodium(II) diporphyrin complex with a m-xylyl tether (·Rh(m-xylyl)Rh·(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH3OH) = 1.45 × 10-2 M-1 s-1) and methane (k(CH4) = 0.105 M-1 s-1) show a clear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH3OH)/k(CD3OD) = 9.7 ± 0.8, k(CH4)/k(CD4) = 10.8 ± 1.0, T = 296 K), consistent with a rate-limiting step of C−H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (ΔH ⧧ = 15.6 ± 1.0 kcal mol-1; ΔS ⧧ = −14 ± 5 cal K-1 mol-1) and methane (ΔH ⧧ = 9.8 ± 0.5 kcal mol-1; ΔS ⧧ = −30 ± 3 cal K-1 mol-1) are reported.