Activation of C-H/H-H bonds by rhodium(II) porphyrin bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(11) bimetalloradical with H-2, and with the methyl C-H bonds for a series of substrates CH3R (R = H, CH3, OH, C6H5) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M.... X... Y.... M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C-H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (DeltaG(double dagger)) decreases regularly as DeltaGdegrees becomes more favorable. An absolute Rh-H bond dissociation enthalpy of 61.1 +/- 0.4 kcal mol(-1) is directly determined, and the derived Rh-CH2R BDE values increase regularly with the increase in the C-H BDE.