Kinetics and mechanisms of metalloporphyrin reactions

Coordination chemistry of metalloporphyrins is a very large research field. The porphyrin macrocyclic ligand forms complexes with all transition metals, with many main group metals and many lanthanides. Metalloporphyrins easily bind various ligands in the axial positions above and below the porphyrin plane. The number of publications in the field amounts to several thousands a year. Fortunately, several review articles and monographs help manage this enormous research area.  

Such a large interest in this field is primarily caused by the biological importance of iron porphyrin complexes as oxygen carriers in hemoglobin and 

The porphyrin complexes of iron were found in the excrements of reptiles, the so-called coprolites, several tenths of millions years old, suggesting that the mechanism of oxygen transfer did not basically change during this vast period of time up to our days. 

MECHANISM OF METAL INCORPORATION INTO THE PORPHYRIN COMPLEX 

The formation of the complex between the free base (PH2) and the metal ion has been extensively studied. It was found that the complicated metal incorporation mechanism can be best interpreted for the reaction of the dipositive metal ion and the neutral chelating ligand. However, this does not mean that the anionic porphyrin species (PH and P ) are not reactive. In fact, they are present in very small concentrations, so small that they can hardly be detected. On the other hand, the cationic porphyrin species are definitively unreactive. Earlier works, of some twenty years ago, preferred the mechanism of metal incorporation which assumes a pre-equilibrium between the free base (PH2) and the dissociated protons of pyrrole nitrogens, so that the metal cation would actually react with the P dianion. Later works, however, indicate the existence of the so-called SAT (sitting-atop) complex intermediate, which was already foreseen in 1960 by Fleischer and Wang. It was later shown that the SAT metal-ion intermediate can deform porphyrin, thus 

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Lamy, J., and J. Lamy, eds. Invertebrate Oxygen-Binding Proteins. New York Dekker, 1981. Lavallee, D. K. Kinetics and mechanisms of metalloporphyrin reactions. Coord. Chem. Rev. 61... 

There is a growing interest in the involvement of metal ions and co-ordination compounds in biological systems, and this has been recognized in the United Kingdom by the inauguration of a new discussion group of the Dalton division of the Chemical Society. Some recent reviews are devoted to various aspects of this very broad field. Of relevance here are articles dealing with the kinetics and mechanism of metalloporphyrin formation, mechanisms for the reactions of molybdenum in enzymes, and a review of the chemistry of vitamin Bjg co-enzyme. A review has also appeared of the kinetics and mechanisms of substitution reactions in cobalt(ra)... 

Hydrated electrons react with certain water-soluble metalloporphyrin complexes, reducing the porphyrin ligands to pi-radical species. When the metal centers are Zn(II), Pd(II), Ag(II), Cd(II), Cu(II), Sn(IV), and Pb(II), the radical complexes are produced at diffusion-controlled rates and decay with second-order kinetics.188 Fe(III) porphyrins, on the other hand, yield Fe(II) porphyrins.189 Rather different behavior is seen in the reaction of e (aq) with [Ru(bpy)3]3 + here, parallel paths generate the well-known luminescent excited-state [ Ru(bpy)3]2 + and another reduced intermediate, both of which decay to the ground-state [Ru(bpy)3]2+, 190 In a direct demonstration of the chemical mechanism of inner-sphere electron transfer, [Coni(NH3)5L]2+ complexes where L = nitrobenzoate and dinitrobenzoate react with e (aq) to form Co(III)-ligand radical intermediates, which then undergo intramolecular electron transfer to yield Co(II) and L.191... 

Hambright et al. (1988) have also studied the kinetics of displacement of the Gd " ion from the gadolinium(III) complex of TSPP by ethylenediaminetetraacetate (EDTA) giving Gd(EDTA) and H2(TSPP) as the main products. This represents the first example of metal removal from a metalloporphyrin by a chelating ligand. A mechanism has been proposed to account for the kinetic data. The water-soluble lanthanide complexes of TMPyP also undergo demetallation in the presence of EDTA (Haye and Hambright 1991). Similar to the acid solvolysis reactions, a linear relationship between log k and the ionic radius of the metal center can be established, and complexes with smaller metal center are more stable toward demetallation by EDTA. 

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