The chemistry of porphyrin pi-cations.

The redox properties of metalloporphyrins have been studied extensively, and particular attention has been paid to the changes in oxidation states of the metaL t-3 In general, the most stable metalloporphyrins are those in which the metal is in the + 2 oxidation state. However, most of the transition metalloporphyrins show a variety of oxidation states. Thus Co(ll) porphyrins can, like the corresponding cobalt-containing vitamin B12 , be oxidized to the Co(IIl),4 or be reduced to the Co(I) complex; and like the Co(I) containing vitamin Bt2(Bt2 s), the Co(I) porphyrin is nucleophilic and undergoes oxidative addition with alkyl halides to give the corresponding alkyl cobalt porphyrins. 5 Manganese porphyrins show + 2, + 3, and + 4 oxidation states,6 and it has recently been shown 7 that Pb(ll) porphyrins can be oxidized to Pb(IV) while the more stable Sn(IV) can be reduced to Sn(Il) systems. 8, 9 A particularly striking example of the stability of the divalent complexes is that of the Ag(Il) porphyrins. to Few examples of divalent silver complexes are known. but the planar tetradentate porphyrin ligand stabilized this unusual oxidation state which can, however, be oxidized to the trivalent state. Z5 Although the redox properties of metalloporphyrins are fascinating and important in their own right, the initial focus and interest in the oxidation states of these systems stems from the redox properties of the cytochromes (which are enzymes containing iron porphyrins, and function catalytically via theFe(II)4±Fe(Ill)couple) and the function of hemoglobin (an Fe(Il) porphyrin which. unlike simple Fe(Il) porphyrins, is not oxidized by oxygen to Fe(lII) but reversibly binds oxygen at the Fe(ll) oxidation level) as well as from the incompletely determined role of valence changes of iron in the peroxidase/catalase class of heme enzymes. The central role of the iron in these naturally occurring systems and the considerable efforts that have been expended on elucidating the roles of metals in these and other metalloporphyrins resulted in the widely held assumption that the macrocyclic porphyrin ligand serves merely to modify the redox potentials of the metals, and to act as a convenient bridge between the metal and the protein-an assumption which is far from true.