O2 with Active Sites and the Redox Mechanism

Interaction of O2 with Active Sites and the Redox Mechanism 

The predominating interactions will weaken the 0-0 bond. The metal in the complex should be in the M(II) state, so for example in alkaline solution a step will require the reduction of M(III)  

This adduct must be short lived. Otherwise it will prevent further O2 molecules from interacting with the active site. The adduct will undergo reduction as 

According to the scheme above, the total driving force of the reaction will be given by the applied potential and also by the M(III)/M(II) formal potential of the catalyst, so the catalytic activity could be correlated with the formal potential of the catalyst. Many authors have discussed this issue and it is yet not clear what sort of correlation should be expected. Reduction should occur at the potential of reduction of the M(III)02 adduct and not at the potential of the M(III)/(II) couple. The latter should only be observed if the reaction were outer sphere. For the particular case of iron phthalocyanines and other iron macrocyclics, O2 reduction starts at potentials very close to the Fe(III)/(II) couple . In contrast, for cobalt macrocyclics reduction of O2 begins at potentials much more negative than those corresponding to the Co(III)/(II) couple . Several authors have reported correlations between activity (measured as potential at constant current) and the M(III)/(II) formal potential and volcano-shaped curves have been obtained -see for 

the donor-acceptor intermolecular hardness is one-half of the difference between the ionization potential of the donor and the electron affinity of the acceptor. 

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