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

The electrochemistry of (OEC)M where M Mn, Co, Ni, or Cu and OEC is th e trianion of 2,3,7,8, 12,13,17,18-octaethylcorrole was investigated i n dichloromethane, benzonitrile, or pyridine, and the oxidized compoun ds 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](2)(2+), which are assigned as pi-pi dimers containing oxidized corrole macrocycles and divalent central-metal io ns on the basis of the electrochemical and spectroscopic data. The ESR spectrum of [(OEC)Cu](2)(+) suggests the presence of one ESR-active C u(II) center in the singly oxidized dimer. Further bulk electrooxidati on of [(OEC)Cu](2)(+) at potentials positive of the second oxidation r esults in the abstraction of a second electron from the dimeric unit a nd leads to a triplet ESR spectrum typical of a copper(II) dimer, from which a Cu-Cu distance of 3.88 Angstrom is calculated. The ESR spectr um of [(OEC)Co](2)(+) in frozen CH2Cl2 at 77 K has a major line at g(p erpendicular to) = 2.40 with a weak signal at g(parallel to) = 1.89 an d is typical of a Co(II) ion. The doubly oxidized dimer, [(OEC)Co](2)( 2+), is ESR silent in CH2Cl2 or PhCN, thus suggesting that the two unp aired electrons of the two Co(II) ions in [(OEC)Co](2)(2+) are coupled . The absolute potential difference between E-1/2 for generation of [( OEC)M](2)(+) and [(OEC)M](2)(+) can be related to the degree of intera ction between the two (OEC)M units of the dimer and follows the order Co (Delta E-1/2 = 460 mV) > Ni (Delta E-1/2 = 260 mV) > Cu (Delta E-1/ 2 =; 140 mV). No evidence is seen for dimerization of (OEC)Mn after ox idation to its Mn(IV) form in the first electron-transfer step, and th e occurrence of this metal-centered reaction may be the reason for the absence of dimerization.