The electrochemistry of a stable Zn isoporphyrin

Abstract

Recently, Xie and Smith [l] described the synthesis of a stable isoporphyrin with Z&I) as the central metal species (Scheme 1). Previously, similar compounds had been formed in solution by several techniques including electro-oxidation [2], but they had not been isolated. Isoporphyrins are especially interesting from an electrochemical point of view because of their unusual redox properties with respect to those of normal porphyrins. Chemically, the macrocyclic conjugation in the isoporphyrin is interrupted at position 5 owing to the presence of a saturated mesocarbon atom [l]. This has interesting consequences which may be possible with respect to the kinetics of electron transfer if the electron is transferred to the ring rather than the central metal ion. Although much is known about the redox chemistry of porphyrins [31, the kinetics of electron transfer to or from these systems have rarely been studied. An important question is whether electron transfer involves the central metal ion or the surrounding ring [41. If electron transfer takes place on the ring, it is expected to be fast [4,5], the magnitude of the rate constant depending on the extent of electron delocalization. Other factors affecting the rate of electron transfer are the spin state of the central metal ion [6] and structural changes accompanying the reaction [7]. The purpose of the present study was to determine the effect of the interruption of conjugation in the isoporphyrin ring on the kinetics of electron transfer. In order to examine this question the kinetic parameters for oxidation and reduction of the zinc isoporphyrin (ZnIP) are compared with those for zinc octaethylporphyrin (ZnOEP), a system known to undergo fast electron transfer [8]. Since the possible extent of delocalization involves the same area in the two molecules, this