Mechanism porphyrin metallation

The kinetics and mechanism of metal ion insertion into porphyrins has been the subject of a considerable number of studies. As expected, relative to the formation of complexes of flexible macrocycles, such reactions are slow. For the overall reaction M2+ + LH2 ML + 2H+... 

Figure 3.4 Mechanism of porphyrin metallation. (a) Out-of-plane saddle structure in which two pyrrole rings with unprotonated nitrogens (blue spheres) point upwards, while the other two, protonated (blue and white spheres) point downwards, (b) Steps in the mechanism for incorporation of the metal ion (red) into the porphyrin (pyrrole rings in green), described in the text. (From Al-Karadaghi et al., 2006. Copyright 2006, with permission from Elsevier.)...

The four-orbital model of the porphyrin spectra as applied by Gouterman (49-50) also serves to explain the mechanism of metal-to-porphyrin backbonding in metalloporphyrins of the spectral hypso type. This backbonding has been invoked for these complexes by Williams (45) and Falk (20). 

The present paper describes the mechanism of porphyrin metalation in the presence of catalyst with special emphasis on the reaction intermediates including heterodinuclear metalloporphyrin, mixed-valence metalloporphyrin, and sandwich-metalloporphyrin with 18-crown-6. 

A.G. Cochran and P.G. Schultz. 1990. Antibody-catalyzed porphyrin metallation Science 249 781-783. (PubMed) Enzyme kinetics and mechanisms... 

Molecular mechanics and molecular dynamics simulations of porphyrins, metal-loporphyrins, heme proteins, and cobalt corrinoids 02CCR(225)123. 

Fig. 7 (a) Porphyrin metallation catalyzed by DNAzymes. (b) Proposed mechanism for the peroxidation reaction catalyzed by the DNAzyme/hemin complex [97]... 

Studies on water soluble and insoluble porphyrins have elucidated aspects of the mechanisms of metal ion incorporation into porphyrins to form metalloporphyrins (Bailey Hambright, 2003 Hambright et al, 2001 Lavallee, 1987 Funahashi et al., 2001). 

It was found while studying the mechanism of electron transfer (ET) across the interface between two immiscible liquids that specific anion adsorption occurs at the octane-water interface in the presence of metalloporphyrins. This adsorption increases in the order of Cl , Br , I , and is caused by coordinative bonding of the anions as ligands of the porphyrin metal atoms. 

Bain-Ackerman and Lavallee " have measured the rate parameters for the entry of five bivalent metal ions into A-methyltetraphenylporphyrin in dmf solution. All reactions follow a second-order rate law and with the exception of Mn(II), the order of the porphyrin metalation rate constants (Cu(II) > Zn(II) > Co(II) > Ni(II)) coincides with that of solvent exchange at the metal ions. The values of k for this deformed porphyrin (Table 6.4) are all larger than for the corresponding metal ion reacting with planar porphyrins. The authors favor a mechanism which involves solvent dissociation from the metal ion as an important rate-determining factor but they also point out that porphyrin deformation... 

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... 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 reduction by these porphyrins. At present, it remains to be seen if the strategy of coordinating metal complexes on the periphery of a metalloporphyrin can be exploited in the rational design of new ORR catalysts. 

The MM calculations show (77) that for a metal ion the size of five-coordinate high-spin Fe(II) in a porphyrin, there is a very small difference in energy between whether the Fe(II) rises up out of the plane or remains in the plane of the porphyrin. Thus, a small modification of the bell-pull mechanism (80) of hemoglobin may be that the high-spin Fe(II) with no oxygen coordinated to it does not rise out of the... 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

The organometallic complexes with d-metals are considered as promising electrocatalysts for oxygen electroreduction in air-metal electrochemical cells. Obviously, the first idea was to employ the catalytic mechanism of the oxygen reduction with porphyrin-like metal complexes [1] found in living beings (Figure 1). 

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