The Central Transition Metal Effect

Depending on the nature of the central metal, the oxygen reduction reaction takes place either via a 2-electron transfer pathway to peroxide, or a 4-electron transfer pathway to water, or a mixed pathway of 2- and 4-electron transfers. The central metal ion of a macrocycle seems to play a decisive role in the ORR mechanism. Most Fe complexes can reduce oxygen directly to water through a 4-electron 

The formation of dimetal face-to-face macrocycles can effectively promote a 4-electron transfer pathway from oxygen to water. For example, such catalysts include dicobalt face-to-face porphyrins [15], and pillared dicobalt cofacial porphyrins [16], as well as other binuclear and polynuclear Co phthalocyanines [17]. In this catalyst, two Co centers can provide two adsorption sites for O2 to form a bridge adsorption, facilitating a 4-electron transfer process, as suggested by Anson et al. [18, 19]  

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