One-electron reduction and demetallation of copper porphyrins

Abstract

Reduction of water-soluble Cu{sup II} porphyrins has been studied by {gamma}-radiolysis and pulse radiolysis at various pH values. In neutral and alkaline solutions, the transient species produced by one-electron reduction were identified as the {pi}-radical anions, based on their broad absorption spectra at 600-800 nm, and the two-electron reduction products were either Cu{sup II} phlorins or Cu{sup II} chlorins, depending on pH and on porphyrin structure. Reduction in acidic solution, however, often led to demetallation of these otherwise stable complexes. The pH at which demetallation became apparent was <4 and dependent on phorphyrin structure. Under these conditions, pulse radiolytic reduction gave the spectra of the products of protonation of Cu{sup I} porphyrin. It is concluded that the {pi}-radical anions of Cu{sup II} porphyrins have some Cu{sup I} character, which in acidic solutions results in protonation and demetallation due to the large ionic radius of Cu{sup I}. For comparison, a study of the acid-catalyzed solvolysis of a Mg porphyrin was carried out both by pulse radiolysis and by classical methods. 25 refs., 5 figs.