Electronic Properties of the Porphyrin Ligands

Woodward has pointed out that porphyrins contain pyrrole units that are on average one half of an electron away from a stable 6-ji-electron configuration [Woodward (214)]. From the presumed tendency to become aromatic pyrrole units one can deduce a metalloporphyrin resonance structure like (VIII), where the methine bridge carbons have lost n- 

The Redox Behavior of Free Porphyrin Bases, their Protonated Dications and Deprotonated Dianions 

Usually the chemical oxidation of porphyrins and their deprotonated anions and protonated cations is not reversible. Poor reductants and oxidants do not induce any reaction, whereas the action of powerful reagents usually leads to irreversible decomposition. There are, however, some reports on unresolved, low-percentage radical esr signals in solutions of free-base porphyrins and phthalocyanines [George (81), Gibson (52)], and there are reports of polarographic redox potentials of some non-metallic porphyrin derivatives, which correspond to reversible reactions (Tables 2 and 3). 

The general rule that can be extracted from these tables is that porphyrins of higher electron density are easier to oxidize and more difficult to reduce than the more electropositive porphyrins. This charge effect is most pronounced in the photochemistry of the cations [Umrik-hina (187), Mauzerall (130)] and anions [Fischer (65)]. Introducing substituents which disturb the planarity of the porphyrin ring tends to facilitate reduction and impede oxidation. 

Note The hydrogen atoms on the nitrogens of the neutral porphyrin bases have been omitted. 

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Secondly, there is an indication that metal(III)-peroxo side-on complexes are in general in equilibrium with corresponding metal(II)-superoxo end-on species. The position of such equilibrium could depend on various factors as structural and electronic properties of the porphyrin ligand, coordination of an axial ligand trans to peroxide/superoxide, solvent medium, temperature and involvement of coordinated peroxide/superoxide in possible hydrogen bonding or electrostatic interactions. These are interesting questions which should be addressed in future studies. 

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