Rate constants and activation parameters for organo-cobalt porphyrin bond homolysis from NMR relaxation times

H-1 NMR line broadening is found to be an effective complimentary method to chemical trapping for determining the rates and activation parameters for organo-metal bond homolysis events that produce freely diffusing radicals. Application of this method is illustrated by measurement of bond homolysis activation parameters for a series of organo-cobalt porphyrin complexes ((TPP)Co-C(CH3)(2)CN (Delta H-not equal = 19.5 +/- 0.9 kcal mol(-1), Delta S-not equal = 12 +/- 3 cal degrees K-1 mol(-1)), (TMP)Co-C(CH3)(2)CN (Delta H-not equal = 20 +/- 1 kcal mol(-1), Delta S-not equal = 13 +/- 2 cal degrees K-1 mol(-1)), (TAP)Co-C(CH3)(2)CO2CH3 (Delta H-not equal = 18.2 +/- 0.5 kcal mol(-1), Delta S-not equal; = 12 +/- 2 cal degrees K-1 mol(-1)), (TAP)Co-CH(CH3)C6H5 (Delta H-not equal = 22.5 +/- 0.5, Delta S-not equal = 17 +/- 2 cal degrees K-1 mol(-1))). The line broadening method is particularly useful in determining activation parameters for dissociation of weakly bonded organometallics where the rate of homolysis can exceed the range measurable by conventional chemical trapping methods. (C) 1998 Elsevier Science S.A.