Electrogeneration of Oxidized Corrole Dimers. Electrochemistry of (OEC)M Where M = Mn, Co, Ni, or Cu and OEC Is the Trianion of 2,3,7,8,12,13,17,18-Octaethylcorrole

Abstract

The electrochemistry of (OEC)M where M = Mn, Co, Ni, or Cu and OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole was investigated in dichloromethane, benzonitrile, or pyridine, and the oxidized compounds were characterized by UV−visible and/or ESR spectroscopy. The first two oxidations of the Co, Ni, and Cu corroles involve the reversible stepwise abstraction of 1.0 electron per two (OEC)M units and lead to [(OEC)M]2+ and [(OEC)M]22+, which are assigned as π−π dimers containing oxidized corrole macrocycles and divalent central-metal ions on the basis of the electrochemical and spectroscopic data. The ESR spectrum of [(OEC)Cu]2+ suggests the presence of one ESR-active Cu(II) center in the singly oxidized dimer. Further bulk electrooxidation of [(OEC)Cu]2+ at potentials positive of the second oxidation results in the abstraction of a second electron from the dimeric unit and leads to a triplet ESR spectrum typical of a copper(II) dimer, from which a Cu−Cu distance of 3.88 Å is calculated. The ESR spectrum of [(OEC)Co]2+ in frozen CH2Cl2 at 77 K has a major line at g⊥ = 2.40 with a weak signal at g∥ = 1.89 and is typical of a Co(II) ion. The doubly oxidized dimer, [(OEC)Co]22+, is ESR silent in CH2Cl2 or PhCN, thus suggesting that the two unpaired electrons of the two Co(II) ions in [(OEC)Co]22+ are coupled. The absolute potential difference between E1/2 for generation of [(OEC)M]2+ and [(OEC)M]22+ can be related to the degree of interaction between the two (OEC)M units of the dimer and follows the order Co (ΔE1/2 = 460 mV) > Ni (ΔE1/2 = 260 mV) > Cu (ΔE1/2 = 140 mV). No evidence is seen for dimerization of (OEC)Mn after oxidation to its Mn(IV) form in the first electron-transfer step, and the occurrence of this metal-centered reaction may be the reason for the absence of dimerization.