Electrochemistry of oxygenation catalysts .3. Thermodynamic characterization of electron transfer and solvent exchange reactions of Co(salen)/[Co(salen)](+) in DMF, pyridine, and their mixtures

Redox and ligand exchange reactions of the oxygenation catalyst (N,N'-bis(salicylidene)ethylenediaminato)cobalt (II), Co(salen), and its one-electron oxidation product, Co(salen)(+), are investigated in DMF, pyridine, and mixtures of these solvents. Electron transfers and solvent exchange reactions involving three neutral Co(II) and three cationic Co(III) complexes with different axially bound solvent molecules (two DMF, one DMF and one pyridine, or two pyridine molecules) form a three-rung ladder scheme. All formal potentials E-0 and equilibrium constants K in this scheme are determined from electrochemical or spectrophotometric experiments or the construction of thermodynamic cycles. The latter are also used to prove consistency of the results. Values for the E-0 and K are discussed in terms of the Co coordination geometry, solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition. Some peculiarities found in the oxygenation of flavonols and indoles are explained.