ELECTRON-TRANSFER MECHANISM OF ORGANOCOBALT PORPHYRINS - SITE OF ELECTRON-TRANSFER, MIGRATION OF ORGANIC GROUPS, AND COBALT-CARBON BOND-ENERGIES IN DIFFERENT OXIDATION-STATES

The chemical or electrochemical oxidations and coupled chemical reacti ons of (TPP)Co(R) and (TPP)Co(R)(L) where R = Bu, Et, Me, or Ph, L = a substituted pyridine, and TPP = the dianion of tetraphenylporphyrin w ere investigated in acetonitrile or dichloromethane. The homogeneous o ne- or two-electron oxidation of the Co(III) sigma-bonded complexes wa s accomplished using [Fe(phen)(3)](3+) (phen = 1,10-phenanthroline) as an oxidant. The products of the initial oxidation as well as that of the subsequent R group migration from (TPP)Co(R) or (TPP)Co(R)(L) to g ive the N-aryl or N-alkyl Co(II) porphyrins were characterized by ESR and UV-vis spectroscopies, while the rates of migration were determine d using stopped flow kinetics. The rate constants of R group migration from the metal to nitrogen in [(TPP)Co(R)](+) were found to vary by 6 orders of magnitude upon going from (TPP)Co(Ph) (1.3 x 10(-3) s(-1)) to (TPP)Co(Bu) (1.2 x 10(3) s(-1)) at 298 K in acetonitrile, but no si gnificant differences were observed between E-1/2 for the first oxidat ion of (TPP)Co(Ph) or (TPP)Co(Bu). ESR spectra of the transient singly oxidized porphyrins indicate that the six-coordinated derivatives, re presented as [(TPP)Co(R)(MeCN)](+) or [(TPP)Co(R)(L)](+) (R = Ph and M e), have a significant d(5) cobalt(IV) character in acetonitrile, whil e the five-coordinate compounds, [(TPP)Co(R)](+), in dichloromethane c an be formulated as Co(III) porphyrin pi cation radicals. The formatio n constants for conversion of (TPP)Co(R) to (TPP)Co(R)(L) were calcula ted spectroscopically and the logarithmic values vary linearly with th e pK(a) of the sixth axial ligand. The E-1/2 values for the first oxid ation of (TPP)Co(R)(L) in dichloromethane and the migration rate const ants for the R group of [(TPP)Co(R)(L)](+) in acetonitrile are also re lated to the ligand pK(a). Cobalt-carbon bond dissociation enthalpies and entropies were determined for (TPP)Co(R) and [(TPP)Co(R)](+), and a comparison of these two sets of data reveals that the Co(IV)-carbon bond in the singly oxidized species is significantly weaker than the C o(III)-carbon bond in the neutral complex.