Paramagnetic Organometallic Molecules. 18.¹ Redox Chemistry of the Flyover Complexes R₆C₆Co₂(CO)₄

The redox chemistry of a series of “flyover” complexes R₆C₆Co₂(CO)₄has been investigated by electrochemical and ESR techniques in a variety of solvents. The compounds all undergo reversible one-electron reduction to a radical anion, the stability of which is dependent on the solvent system. In acetone or dichloromethane, the radical anion (CF₃)₆C₆Co₂(CO)4^-. decomposes by ring closure of the flyover bridge to give hexakis(trifluoromethyl)benzene, (CF₃)₆C₆, via the radical anion (CF₃)₆C₆^-. In contrast the radical anion (CF₃)₆C₆Co₂(CO)₄^-• shows exceptional stability in THF solvent. A second reduction step producing an inherently unstable dianion is also observed in all solvents. Unusual scan rate dependence of this second step under conditions of cyclic voltammetry may be consistent with a homogeneous electron-transfer reaction between the dianion and the neutral “flyover” complex. In acetonitrile, rapid electron-transfer-catalyzed substitution of the radical anions leads to the formation of acetonitrile complexes. The ESR spectra of electrolytically generated radical anions of several of the flyover complexes were observed in THF solutions. The isotropic spectra show coupling to two equivalent cobalt nuclei. Isotropic ESR spectra of (CF₃)₆-C₆Co₂(CO)₄^-. in acetone solution again showed coupling to two equivalent cobalt nuclei, but slow decomposition of the radical anion of the flyover complex led to the observation of the radical anion (CF₃)₆C₆^-.