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 pi-extending or highly electron-withdrawing beta-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)Y-2 or TPP(NO2)Y-6 and Y = phenyl, phenylethynyl, Br, or CN) was shown to undergo multiple redox reactions involving the conjugated pi-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 n-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.