Electrochemistry and Spectroelectrochemistry of Cobalt Porphyrins with π‑Extending and/or Highly Electron-Withdrawing Pyrrole Substituents. In Situ Electrogeneration of σ‑Bonded Complexes

A series of cobalt porphyrins with π-extending or highly electron-withdrawing β-pyrrole substituents were investigated as to their electrochemistry, spectroscopic properties, and reactivity after electroreduction or electroxidation in nonaqueous media. Each porphyrin, represented as PorCo (where Por = TPP­(NO₂)­Y₂ or TPP­(NO₂)­Y₆ and Y = phenyl, phenylethynyl, Br, or CN) was shown to undergo multiple redox reactions involving the conjugated π-ring system or central metal ion which could exist in a Co­(III), Co­(II), or Co­(I) oxidation state under the application of an applied oxidizing or reducing potential. Thermodynamic half-wave potentials for the stepwise conversion between each oxidation state of [PorCo]ⁿ (where n ranged from +3 to −3) were measured by cyclic voltammetry and analyzed as a function of the compound structure and properties of the electrochemical solvent. UV–visible spectra were obtained for each oxidized or reduced porphyrin in up to six different oxidation states ranging from [PorCo]^3– to [PorCo]³⁺ and analyzed as a function of the compound structure and utilized electrochemical solvent. Chemically or electrochemically generated Co­(I) porphyrins are known to be highly reactive in solutions containing alkyl or aryl halides, and this property was utilized to in situ generate a new series of methyl carbon-bonded cobalt­(III) porphyrins with the same π-extending or highly electron-withdrawing substituents as the initial Co­(II) derivatives. The electrosynthesized carbon-bonded Co­(III) porphyrins were then characterized as to their own electrochemical and spectroscopic properties after the addition of one, two, or three electrons in nonaqueous media.