Metalloporphyrins oxidation potentials

Another spectroscopic technique that should be able to quantify (Po is transient absorption of the photosensitizer radical cation. This was attempted in combination with a peculiar approach to lower the value of the oxidation potential of the photosensitizer, and hence to increase the energy of the HOMO and make HOMO-LUMO gap smaller than the triplet state energy. As shown in Table I, this can be achieved with Zn rather than Pd metalloporphyrins derivatives. In fact, the zinc complex of the bacteriopurpurinimide derivative identified in Fig. 8, ZnBPur, has F t = 1-4 eV, F i/2° = 0.45 V, and F i/2 =-0.81V versus SCE (81), and the following sequence of reactions becomes exothermic in polar solvents... 

The next important information on the product is derived from the electronic absorption spectrum. This can usually be obtained by electrolysis of a dilute solution of the metalloporphyrin at a constant potential or by oxidometric titrations directly in the absorption cell. Most redox reactions with metalloporphyrins give good isosbestic points when absorption spectra are taken at various stages of oxidation, and they are fully reversible when no chemical addition reaction to the porphyrin ligand has occurred. Ten typical absorption spectra of metalloporphyrins are given below and correlated with the various metalloporphyrin oxidation states. 

Metallo-chlorins and bacteriochlorins have lower oxidation potentials than the corresponding metalloporphyrins. The replacement of a pyrrole by a hydrogenated pyrroline ring lowers the potentials by about 100 mV. Furthermore, the oxidation potentials are generally somewhat lower in polar solvents or in micelle and vesicle membranes than in apolar solutions because the charged oxidation and reduction products become more solvated in polar media. 

The interest in the properties of metalloporphyrin radical cations in water relates to their potential use as oxidants in solar energy storage schemes. In particular, the oxidation of water to oxygen by highly-oxidized metalloporphyrins has been considered as a model for photosynthesis. The pulse radiolytic studies outlined above provided good understanding of the factors that influence the stability of the radical cations in water. A critical point is the correlation between stability of a porphyrin radical cation and the redox potential for oxidation of the porphyrin. Consequently, porphyrins with a sufficiently high oxidation potential to be useful oxidants of water have short-... 

In Eqs. (9-6), (9-7), (9-8), and (9-9), M(II)P and M(III)P represent metalloporphyrins composed of metal(II) ions and metal(III) ions, respectively. In addition to a five-coordinate metalloporphyrin formation and its appropriate oxidation potential (as mentioned above), the following requisites for reversible oxygen-binding have been elucidated to suppress the side-reactions of an oxygen adduct or irreversible oxidation of a metalloporphyrin, as represented by Eqs. (9-7) through (9-9) [10,23]. 

Table 1 summarizes the reduction potentials in MeCN for atomic oxygen 2 various iron-oxene species of this model system, the oxene adducts of other metalloporphyrins, and the oxidation potentials for OH adducts of metal porphyrins. The shift in the two-electron reduction potential for 0(g) (from +0.63 V vs NHE in a neutral unbuffered solution to +2.94 V in acidic media) is analogous to that observed when protons are... 

Akiba investigated the electrochemical behavior of a variety of phosphorus octaethylporphyrin derivatives all compounds showing a single reversible oxidation wave [91]. The absolute difference in potential between the first ring-centered oxidation and reduction varies from 2.19 to 2.36 V in dichloromethane. These values are within the range of the HOMO-LUMO gap observed for most metalloporphyrins. 

These metalloporphyrins are unique among Fe and Co porphyrins in their high catalytic efficiency of electroreduction of H2O2 (at potentials <0.75 V vs. NHE at pH 7), as well as disproportionation and oxidation of H2O2 (at potentials >0.8 V). 

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

A prime example of this is the crucial ease of such oxidation in the magnesium(II) complex of the chlorin system present in chlorophyll during photosynthesis. Table 8 shows the half-wave potentials for the first ring oxidation of some metalloporphyrins, the examples chosen being based on 5,10,15,20-tetraphenyl porphyrin, TPP (2) and on 1,2,7,8,12,13,17,18-octaethylporphyrin, OEP (16). 

The first oxidation occurs at potentials more negative than those reported for the oxidations of similar metalloporphyrinates as shown graphically in... 

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