Redox potentials porphyrins

The distorted octahedral species [10] and [11] are the essential part of the cytochromes acting as redox catalysts. In these, a very specific porphyrin redox potential may have been adjusted by an appropriate choice of the axial ligands exerting a cis effect on the porphyrin system transmitted through the iron atom. In cytochrome c, these axial ligands are the imidazole of a histidine and the thioether function of methionine, as in [10], and in the cytochromes a or b5 they are presumably two imidazoles of histidine, as in [77] (7). 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

Table 5.6 Properties of three typical photoredox-active molecules. bpy denotes 2,2 bipyridine, TMPP is tetra N-methylpyridine porphyrin Amax is the wavelength of the absorption maximum, e is the extinction coefficient at Amax, cpT is the quantum yield of the formation of the excited triplet state, r0 is its lifetime, and E0 are standard redox potentials...

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... 

Fig. 3 illustrates schematically the structures of some well characterized hemoproteins. In the cytochrome series the desired range of redox potentials is achieved by variations in the axial ligands, contributed by the porphyrin moiety, as well as by substitutions around the periphery of the tetrapyrrole nucleus with groups of differing electron-attracting ability. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

An important class of porphyrins is that constituted by confor-mationally distorted porphyrins, which mimic the non-planar geometry of the porphyrins present in photosynthetic systems.89 Obtainment of such non-planar distortions is associated with the introduction into the macrocyclic frame of proper crowding substituents, which therefore not only cause structural distortion but also affect, through their electronic effects, the redox potentials. A typical case is that constituted by [Cun(OETPP)] (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), the saddle-distorted molecular structure of which is illustrated in Figure 56.102... 

The different cytochromes display rather similar redox activity, i.e. the reversible Fe(III) zzt Fe(II) process, but their redox potentials change with the peripheral substituents of the Fe(III)-porphyrin (particularly, as a function of the axially coordinated groups). 

Table 9. Comparison of the wavelengths (A.a) and wavenumbers (va) of the or-bands, the chemical shifts (8) of the porphyrin meso-protons, and the Os /Os111 redox potentials (E1/2) in a series of osmochromes Os(OEP)L2 ]29a-k).

The linear correlations of the data of the complexes [29a-29e] and [29i] displayed in Table 9 and Fig. 6 then may illustrate the alteration of the a-donor-rr-acceptor balance within the axial ligands, which is fully transmitted to the porphyrin orbitals via the metal. Linear correlations between the redox potentials and the energy of optical absorption maxima are well-known in organic molecules (95). In this case, a metal is strongly conjugated with the porphyrin system. The cis influence... 

The extremely negative redox potential of Os(OEP)(l-MeIm)2 [29f] is therefore with confidence attributed to an internal effect which is in our opinion the additional 7r-donor effect invoked for the imidazole moiety in Sect. 5.4. Obviously this ligand induces an additional electron density at the Os11 ion which is not transmitted to the porphyrin ring because the a-band of [29f] falls between [29e] and [29i] which both have higher redox potentials than [29f itself. 

Table 11. Comparison of the wavenumber of X=Z vibrations ( x=z)> the wavelength of the a-band the chemical shift of the meso-porphyrin protons (6) and the Os /Os -redox potential (Ej ) of various dinitrogen or nitrosyl octaethylporphinatoosmium(II) complexes Os(OEP)(XZ)L [31 a-3 If]...

Fig. 9. Correlation of the Osn/Osffl redox potentials and the energy of the a-bands of osmium porphyrins Os(OEP)X(L) (ligands X/L indicated in the drawing for further information, see Tables 10 and 11)...

Table 13. Further series (a—h) of iron or ruthenium porphyrins showing cis effects exerted by the axial ligands X or L on the wavelength of the a-band (or (3-band in Series d, e and g) and the chemical shift (6) of the porphyrin meso-protons (Series c, h o-proton in c). The metal II/III-redox potentials (Ej/2) are also given. For abbreviations, see Table 2... 

The metal (II/III) redox potentials, E, 2, do not show a systematic metal effect they fall in the series Ru > Fe > Os. Whitten and Meyer (129) have pointed out that this unexpected behavior may be caused by two properties with opposite metal increments, namely the Mn/Mni ionization potential, decreasing in the series Fe >Ru> Os, and the metal-to-porphyrin backbonding, increasing in the opposite sense Fe < Ru < Os, and thus the Ru11 state becomes the most stable of the three. 

From these redox potentials it is seen that Os11 is more closely related to Fe11 than Ru11, another reason for our preferential investigations of Os11 porphyrins (see Sect. 6.1). 

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