Mechanism of Oxidative Cleavage of Organocobalt(III) Macrocycles. Application of Electrochemical Techniques to Irreversible Electron-Transfer Processes

Article abstract of DOI:10.1021/ja00401a038

The thermally stable dimethylcobalt(III) macrocycles, trans-Me2Co(DpnH) and trans-Me2Co(TIM)ClO4, are strongly labilized upon chemical and electrochemical oxidation, leading to the selective scission of only one methyl ligand.The formation of highly labile dimethylcobalt(IV) cations as reactive intermediates is evident from the irreversibility of the cyclic voltammetric (CV) wave even at -78 deg C.The heterogeneous rates of electron transfer are obtained from the CV data and compared with the corresponding homogeneous rates measured with various polypyridineiron(III) and hexachloroiridate(IV) oxidants.The resulting linear correlation underscores a common outer-sphere mechanism for the primary steps in the electrochemical and chemical oxidations.The cleaved methyl ligand affords ethane in high yields,but it can be trapped by hydrogen atom donors (chloroform) as methane and by halogen atom donors (carbon tetrachloride and benzyl bromide) as methyl halides.The kinetic study of ethane formation by a competition method support the mechanism in Scheme III, in which the spontaneous homolytic fragmentation of the dimethylcobalt(IV) intermediate leads to a free methyl radical.The subsequent diffusive encounter of methyl radicals is in accord with the deuterium-labeling study wich establishes a pattern of random dimerization of methyl ligands.The alternative formulation involving the direct bimolecular reaction of a pair of dimethylcobalt(IV) cations is ruled out by the insensitivity of ethane formation to the ionic stren gth of the medium.Methyl radical as the active precursor also follows from the selectivity pattern (observed in the competitive trapping with halogen atom donors), which is identical with that of the methyl radical generated independently from the photolysis of acetyl peroxide.A complete kinetic analysis of this mechanistic scheme provides information about the lifetime of the metastable dimethylcobalt(IV) species at the electrode surface which is sufficient to generate a high flux of methyl radicals to ensure efficient dimerization.