Complexation and Demetalation Reactions of Porphyrins

The mechanisms by which porphyrins form metal complexes in nature are of interest because of the importance of both the iron complexes, the active sites of hemoproteins, and the magnesium complexes, the chlorophylls. Currently there is interest in using chelated radioactive metal isotopes as diagnostic imaging and therapeutic agents; porphyrins are excellent candidates because of their extremely high stability constants with many metal ions. A great deal of effort has been invested to determine the reaction rates and examine implications of possible formation and decomposition mechanisms of metalloporphyrins: the subject has been periodically reviewed.1–5 It is my intent in this Comment to choose a few of the most salient and personally interesting major trends that have been elucidated and to indicate some of the more troublesome unsettled questions.