Metalloporphyrins stability

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

Table 1. Demetallation reaction conditions for metalloporphyrins based on the qualitative stability constant...

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. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

The typical metalloporphyrins and metallophthalocyanines have in general very high thermodynamic stabilities. However, because it is difficult to establish conditions under which the... 

In summary, the four chemical systems described in this paper demonstrate the versatility and selectivity of electrochemical methods for synthesis and characterization of metal-carbon a-bonded metalloporphyrins. The described rhodium and cobalt systems demonstrate significant differences with respect to their formation, stability and to some extend, reactivity of the low valent species. On the other hand, properties of the electroche-mically generated mono-alkyl or mono-aryl germanium and silicon systems are similar to each other. 

M = Os) has not been established. This is very remarkable because a variety of other d6 metalloporphyrin moieties show an unusual lability at their axial coordination sites (see Sect. 3). The stability of the species [Os(OEP)L2]n (n = 0 or 1) representing the osmochrome/osmichrome system has allowed a complete physical and chemical characterization of the compounds [29a-k] listed in Table 9 and their cationic derivatives [Os(OEP)L2]PF6 (78, 80). 

Porphyrins, 21 14, 36, 135 -based manganese complexes, 46 400-402 as cobalt complex ligants, 44 284-290 compared to phthalocyanines, 7 75 complexes, 19 144, 145, 147 complex stability, 42 135-137 degeneracy lifting, 36 206 metalloporphyrins, DNA cleavage and, 45 271-283... 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

Table 7 Stability Classes of Metalloporphyrins Toward Demetallation with Acids...

The stability of a metalloporphyrin can be roughly estimated by the charge-to-radius ratio (Z/yj). Buchler et al 1 found that the stability index, SI = 100(ZAr/yj), where X is the Pauling electronegativity, predicts the stability of a metalloporphyrin to acid demetallation, and correlated the SI to the Falk-Phillips stability criteria (Table 2). 

If a metal oxidation state change occurs under the demetallation conditions, the charge (Z) and the radius (yf) should be those of the actually demetallated species. As is obvious from the definition of SI, a metalloporphyrin with the metal ion of a high charge, a high electronegativity and a small radius is stable. The SI underestimates the stability of metalloporphyrins where there are strong metal-porphyrin n interactions. 

The relative stabilities of metalloporphyrins can be compared by transmetallation reactions.18 As expected from the Zjyi ratio, monovalent metals are easily displaced by other metal ions. The order of stability is indicated as follows Cu > Zn > Cd > Hg > Pb (> Ba) > Li > Na > K. In most cases the transmetallation rate is first order both in metal ion and in metalloporphyrin, and the intermediate is considered to be the dinuclear complex [M(Por)M ] For Cu/Zn(2,4-disubstituted DPIX DME) in boiling pyridine, the substitution rate increases as the porphyrin basicity decreases, i.e. substituent = Br > CH—CH2 > CH2CH3(> Etio). 

A recent development concerns the use of polyanions of the type [XMi 039M (0H2)]". In this type, the M atom easily becomes coordinatively unsaturated by dehydration (255). The resulting dehydrated anion, [XMhOjqM ]", can be considered an inorganic metalloporphyrin analog (322, 364, 365). Oxidation catalysis by these polyanions is described in Sections VIII and IX. Here, the catalytic performance and stability are compared with that of metalloporphyrin. 

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