Thin-layer yielding mechanism

Close inspection of the solution phase redox peaks for the four species under analysis obtained in a thin layer cell in deaerated non-aqueous media (not shown here) yielded values for the onset of the reduction of the first Co site very similar to one another. Hence, it can only be surmised that the differences in specificity are related to the presence of the peripheral substituent and seemingly unrelated to the redox properties of the metal sites. Evidence that the differences in the specificity between the meso-substituted and non-meso-substituted materials are due to subtle electronic effects was provided by quantum mechanical calculations. These showed that upon addition of the weso-substituents, the electronic charge density associated with the HOMO (highest occupied molecular orbital) localized on the dioxygen moiety is markedly reduced, thereby weakening its Lewis acid character and hence its ability to coordinate a proton (see Figure 3.67). 

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