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 reactions 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 were investigated in acetonitrile or dichloromethane. The homogeneous one- or two electron oxidation of the Co(III) σ-bonded complexes was 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 give the N-aryl or N-alkyl Co(II) porphyrins were characterized by ESR and UV-vis spectroscopies, while the rates of migration were determined 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 10³ s^-1) at 298 K in acetonitrile, but no significant differences were observed between E(1/2) for the first oxidation of (TPP)Co(Ph) or (TPP)Co(Bu). ESR spectra of the transient singly oxidized porphyrins indicate that the six-coordinated derivatives, represented as [(TPP)Co(R)(MeCN)]⁺ or [(TPP)Co(R)(L)]⁺ (R = Ph and Me), have a significant d⁵ cobalt(IV) character in acetonitrile, while the five- coordinate compounds, [(TPP)Co(R)]⁺, in dichloromethane can be formulated as Co(III) porphyrin π cation radicals. The formation constants for conversion of (TPP)Co(R) to (TPP)Co(R)(L) were calculated spectroscopically and the logarithmic values vary linearly with the pK(a) of the sixth axial ligand. The E(1/2) values for the first oxidation of (TPP)Co(R)(L) in dichloromethane and the migration rate constants for the R group of [(TPP)Co(R)(L)]⁺ in acetonitrile are also related 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 Co(III)-carbon bond in the neutral complex.