Manganese complexes electron-transfer reactions

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

A number of other metal complexes can decompose hydrogen peroxide via reactions analogous to Eqs. (45) and/or (46), including those of cerium813 b copper,823 b cobalt,833 b manganese,84 and silver.85 Many of these electron transfer reactions are thought to proceed via inner-sphere complexes of metal-hydrogen peroxides (M—OOH).84 86... 

Hou, J.-M., Boichenko, V.A., Diner, B.A., Mauzerall, D. (2001) Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers Volume change, enthalpy, and entropy of electron transfer reactions in manganese-depleted photosystem II core complexes, Biochemistry 40(24), 7117-7125. 

Manganese compounds are used as oxidant and catalyst in the various electron-transfer reactions in vivo involving the Mn(II), Mn(III), or Mn(IV) oxidation states. In particular, manganese complexes play an important role as oxygen carrier, similar to iron and copper porphyrins such as hemoglobin and hemocyanin. Manganese compounds have a pecuhar nature for the reaction with molecular oxygen. 

Iron, copper, and manganese can shuttle between different cationic forms and participate in electron transfer reactions that form the core of bioenergetics. For instance, because the configuration of Fe is [Ar]3d 4s, the Fe and Fe cations have the configurations [ Ar]3d and [Ar]3d respectively. These are the oxidation states adopted by the iron ions bound to the protein cytochrome c as it transfers electrons between complexes II and IV in the mitochondrial electron transport chain (Section 5.10). 

Generation of the carbon based radical in these processes involves the prior formation of a complex between manganese(lll) and the enol of the carbonyl reactant. Intramolecular electron transfer occurs within this complex. Addition to the olefin then takes place within the co-ordination sphere of manganese. When manganese is present in catalytic amount, the relative values of the equlibrium constants between manganese and both the carbonyl compound and the alkene arc important. If the olefm is more strongly complexed then no radical can form and reaction ceases. Reactions are usually carried out at constant current and the current used must correspond to less than the maximum possible rate for the overall chemical steps involved. Excess current caused the anode potential to rise into a region where Kolbe reaction of acetate can occur and this leads to side reactions [28]. 

The electrochemistry of oxo-bridged manganese complexes in aqueous solution is characterized by coupled electron and proton-transfer reactions. The cyclic voltammetric behavior of [Mn2 02(phen)4] + in aqueous pH 4.5 phosphate buffer is illustrated in Fig. 12 [97]. It is of interest to compare this result with that obtained for the same complex dissolved in CH3CN (Fig. 9). Two one-electron reactions are observed in each case. However, these correspond to Mn(IV,IV) Mn(IV,III) and Mn(IV,III) Mn(III,III) reductions in the nonaqueous solvent and to Mn(IV,III) Mn(III,III) and Mn(III,III) Mn(III,II) reductions in... 

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