Cobaltacycles in three oxidation states: Redox properties and reaction routes

Article abstract of DOI:10.1021/om00150a012

The electrochemical reduction and oxidation of the Co(III) metallacycle Cp(PPh₃)CoC₄Ph₄ (1) and its analogues have been studied. Oxidation is reversible (E° = +0.35 V vs. SCE) in dichloromethane to give a persistent cation radical which was characterized by ESR spectroscopy. A second oxidation (E° = +1.17 V) gives a more reactive dication which ultimately decomposes by disruption of the metallacycle. Reduction goes by fast one-electron transfer to a 19-electron complex which rapidly loses phosphine to give a transient 17-electron coordinatively unsaturated anion. In the presence of phosphines PR₃ which are more basic than PPh₃, efficient substitution of PPh₃ by PR₃ accompanies the reduction, in a partially electrocatalytic process. In the absence of added ligands, the 17-electron anion undergoes protonation and subsequent reduction via a ECEC mechanism to give a π-butadiene complex. The electrochemical results are interpreted in terms of previously published molecular orbital calculations on this class of molecules.