Reduction and Oxidation of Metallocorroles

The cobalt(II) corrole anion prepared as above was characterized primarily by electron spin resonance (esr) and absorption spectroscopy. When prepared via sodium film reduction, the cobalt(II) corrole oxidizes rapidly to the corresponding Co(III) corrole on exposure to air. When prepared by the other methods, it is moderately stable in air in the presence of a reducing agent. Attempts to prepare the neutral form of the initial Co(II) corrole anion, by protonation with perchloric acid, resulted in formal oxidation to the Co(III) derivative. Interestingly, further protonation of the Co(III) corrole with perchloric acid led to what appeared to be a protonated Co(III) corrole. Certainly, the absorption spectrum of this species is similar to that of the corresponding neutral nickel(II) corrole complex. However, the exact nature of this protonated material has not been fully elucidated. 

In 1974, Hush and Woolsey reported that treating the cobalt(II) corrole species discussed above with pyridine induced little in the way of spectral changes. These authors took this as an indication that the divalent cobalt corrole complex in question shows little or no tendency towards axial coordination. In 1978, however, Murakami, et al. reported a different result, stating that one pyridine molecule does in fact coordinate axially to the cobalt center of an anionic Co(II) corrole. 

The one-electron reduction of cobalt(III) corroles to cobalt(II) corroles can also be effected electrochemically. As expected, increasing the donating ability of the axially coordinating ligand decreases the ease of reduction. It also affects the extent to which the reaction can be made to be reversible under normal laboratory conditions. For instance, reductions involving pyridine cobalt(III) corroles are gen- 

Interestingly, in the cases of the Ni(II) and Cu(II) reduction products (2.188 and 2.189), the UV-vis spectra recorded proved nearly identical to those of previously described systems, prepared via treatment with hydroxide anion in DMF or with other agents such as potassium metal or The original 

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