Determination of Organo−Cobalt Bond Dissociation Energetics and Thermodynamic Properties of Organic Radicals through Equilibrium Studies

Two methods are described and illustrated for the measurement of organo−cobalt bond homolysis energies through reactions of tetra( p -anisyl)porphyrinato cobalt(II), (TAP)Co II •, with organic radicals of the form • C(CH 3 )(R)CN in the presence of olefins. Thermodynamic values for bond homolysis have been determined directly for (TAP)Co-C(CH 3 ) 2 CN (Δ H ° = 17.8±0.5 kcal mol - 1, Δ S ° = 23.1 ± 1.0 cal K - 1 mol - 1 ) and (TAP)Co-CH(CH 3 )C 6 H 5 (Δ H ° = 19.5 ± 0.6 kcal mol - 1, Δ S ° = 24.5 ± 1.1 cal K - 1 mol - 1 ) from evaluation of the equilibrium constants for the dissociation process (Co−R ⇌ Co II • + R • ) in chloroform. The bond homolysis enthalpy for (TAP)Co-C 5 H 9 (Δ H ° = 30.9 kcal mol - 1 ) was determined indirectly by measuring the thermodynamic values for the competition reaction (TAP)Co-C(CH 3 ) 2 CN + C 5 H 8 ⇌ (TAP)Co-C 5 H 9 + CH 2 C(CH 3 )CN (Δ H ° = 0.9 ± 0.3 kcal mol - 1 ) in conjunction with a thermochemical cycle. This indirect approach was also used to evaluate (TAP)Co-CH(CH 3 )C 6 H 5 BDE (20.5 kcal mol - 1 ) which agrees favorably with the value determined directly. When the Co−R bond homolysis enthalpies are known from independent evaluation, these equilibrium measurements provide a method for evaluating relative heats of formation of organic radicals. Application of this approach gives 40.8 kcal mol - 1 for the heat of formation of • C(CH 3 ) 2 CN in chloroform. Success of these methods is dependent on fast abstraction of H • from the organic radicals by (TAP)Co II • to form (TAP)Co-H and rapid addition of (TAP)Co-H with olefins to form organocobalt complexes. Kinetic-equilibrium simulations utilizing reaction schemes for these processes provide an accurate description of the kinetic profiles and the equilibrium concentrations of solution species when the organic radical species achieve steady state.