Oxidation reactions superoxo intermediates

Oxygen uptake studies show that the conversion of 39 to 40 requires 0.6 (1) mol of 02, indicating that four ferrous atoms are oxidized per consumed 02 (63). This stoichiometry is similar to that reported for the autoxidation of ferrous porphyrins. The proposed mechanism, shown in Scheme 5, suggests that the autoxidation is initiated by the interaction of 02 with monomeric units of 39 that are either coordinatively unsaturated or contain a labile ligand such as solvent. The reaction of a second monomeric unit with the transient superoxo ferric complex gives rise to a (fi-peroxo)diferric species similar to that reported for [Fe HB(3,5-iPr2pz)3 (OBz)(CH3CN)] (67). This species is then somehow reduced by two electrons from 39 to yield two molecules of 40. No intermediates in the autoxidation of 39 have yet been detected even at low temperatures (—80 °C), unlike the porphyrin systems in which intermediate species such as [(Por)Fe2+02], [(Por)Fe3+-00-Fe3+(Por)], and [(Por)-Fe4+ = 0] have been spectroscopically identified (68). 

In another class of reactions, NO is oxidized to N03 or its equivalent by metal-(02/02 ) complexes. In these reactions, NO first adds to the 02 moiety to form a peroxynitrito intermediate that can decompose in various ways.203 The analogy with the direct reaction of NO with free superoxide to form peroxynitrite and its isomerization to nitrate is clear. An important example is the oxidation of NO to NO3 by oxyhemoglobins, where the Fe-02 reactants have considerable Fem-(02 ) character.204 In other examples, NO reacts with superoxo complexes of Rh(III) and Cr(III) to produce peroxonitrito intermediates that decompose in complicated ways.205,206... 

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