Porphyrins centered oxidation

Figure 1 shows cyclic voltammograms recorded for oxidation and reduction of Ae five-coordinate Mn (in) porphyrin, Mn (TPP) Cl [6]. The left-hand side of Ae figure contains voltammograms corresponding to Ae one-electron porphyrin-centered oxidation ... 

Electrochemistry. Compounds 15a [Fen(pz)pzEts] 15b [Fen(l-MeIm)2Ets] exhibited two reversible oxidations at 0.21 and -0.06 V (vs Ag/AgCl), which were attributed to the 2 —> 3 and 3 —> 4 metal-centered oxidations. These values are 400 mV more positive than those reported for the analogous Fe TPP, suggesting that the pz stabilizes the lower oxidation state, Fe(II) better than does the porphyrin (63). This stabilization of a basic oxidation state for the pz may be attributable to the lower energy n orbitals of the pz making it a better n acceptor than the porphyrin. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

A typical electronic spectrum of a M(4-TCPyP) complex is shown in Fig. 16 (39,123,170,176,182,183). In general, the electronic transitions in the porphyrin center exhibit many similarities with those observed in the spectra of the M(4-TRPyP) species, with the Soret band typically in the range of 414- 75 nm, and the Qi and Qo o bands in the range of 557-584 and 611-645 nm, respectively. In the formal Ru(III)Ru(III)Ru(III) oxidation state, the characteristic intracluster band is observed in the 685-707-nm range, while the RusO py MLCT band can be found in the 314—351 -nm range. The spectral data of a series of M-4TCPyP derivatives are listed in Table II. 

The kinetics at 430 mn after a laser flash of the photocatalytic system (pH 8.5) in Scheme 11 are biphasic—a fast reaction on a millisecond time-scale because of formation of [(P)Fe ] + was followed by a much slower reaction on a second time-scale because of the conversion of [(P)Fe ] + to compound II, (P)Fe" =0 [168]. In general, ferric porphyrins have ligand-centered one-electron oxidation potentials at which ferric porphyrins are oxidized to ferric porphyrin n radical cations these are higher than oxidation potentials for metal-centered one-electron oxidation to ferryl porphyrins [102, 170]. Despite the smaller (by ca 0.3 eV) driving force for ligand-centered oxidation than for metal-centered oxidation [102, 170], ligand-centered oxidation of HRP occurs before metal-centered oxidation (Scheme 11). This is be-... 

The electrochemical behavior of water-soluble yS-pyrrole brominated porphyrins is more complex than that of their water insoluble analogs. The metal-centered redox reactions of (TMPyP)Mn and (TMPyPBrg)Mn are reversible while the majority of porphyrin ring-centered redox reactions of the free-base, Cu, and Mn derivatives of TMPyPBrs are irreversible The metal-centered oxidation of (TMPyPBr8)Mn is anodically shifted by 420 mV compared to 1/2 for the corresponding reaction of (TMPyP)Mn (Table 9.2). The metal-centered... 

The bismuth porphyrins studied by the groups of Kadish and Gudard undergo two macrocycle-centered oxidations, and unlike what is observed for the other Group 15 porphyrins, none of the bismuth porphyrins shows evidence for conversion from the M(III) to the M(V) form of the complex. The authors pointed out that the oxidation potentials of (P)Bi(S03Cp3) were rather insensitive to the type of porphyrin macrocycle. 

In an attempt to create biomimetic systems, there has been much research on designing dendritic molecules with porphyrins and similar molecules at their cores. Diederich etal. have synthesized dendrimers with iron and zinc-porphyrin cores in an effort to model heme and cytochrome c systems [46-49]. In these cases, the redox reactions occurring at the center of the molecule were found to be affected by the nature of the dendritic foliage. The porphyrin-centered (the Zn(II) is not electroactive) first oxidation in the zinczinc-tetraphenylporphyrin to 4-0.65 V for the largest zinc-porphyrin dendrimer (compare to the results of Kaifer [35] above). However, for the iron porphyrin dendrimers, the Fe(II/III) redox couple shifts from —0.23 V versus SCE for the smaller dendrimer to 4-0.19 V for the larger one [48]. In a different set of experiments, Diederich and coworkers demonstrated that increasing the amount... 

The reaction is Nemstian under the experimental conditions AEp equals ca. 60 mV and is independent of sweep rate. We have obtained equivalent results for ring-centered oxidations and reductions of a number of metallopotphyrins [5]. In all cases, the standard heterogeneous rate constant is larger than can be measured experimentally (ks,h > 0.1 cm s l). A large rate of electron transfer is anticipated, because the charge transferred to or from the porphyrin is distributed over a 24-atom framework necessitating little nuclear rearrangement... 

The inner-shell barrier that causes metal-centered reduction of Mn (TPP) Cl to be slow relative to its ring-centered oxidation is movement of the metal atom with respect to the porphyrin plane. X-ray crystallography [8] indicates that the Mn atom moves from a distance of 0.27 to 0.64 A above the porphyrin plane as Mn (TPP) Cl is reduced to Mn (TPP) C1 and that the Mn-Cl distance does not change. The Mn (TPP) Cl°/ couple is a good choice for assessing inner-shell effects on electron transfer kinetics, because structural information is available for both oxidation states and nuclear reorganization is limited almost entirely to out-of-plane displacement of the metal atom (distortion of the porphyrin core is neglected). 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

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