Metalloporphyrins reduction potentials

The first general principle is that the type of metalloporphyrin incorporated into a designed protein or natural cytochrome offers a method to adjust the reduction potential of the heme. Sharp et al. (149) have demonstrated that a synthetic heme, l-methyl-2-oxomesoheme XIII, incorporated into a designed heme protein has a reduction potential 90 mV higher than the same protein with heme b. Additionally, Gib-ney et al. (148) have illustrated that heme a has a reduction potential 160 mV higher than heme b in the identical protein scaffold. This heme-dependent effect provides protein designers with a predictable modulation of the heme reduction potential in a synthetic protein. 

The metal-centered reduction of iron and cobalt porphyrins [(por)Afn] yields metalloporphyrin anions [Eq. (13.13)]. The reduction potential for this reaction is 13, and is equivalent to the N- value for the oxidation of the metal-centered nucleophile [(por)uM-]. The one-electron reduction of alkyl halides yields the... 

Reduction potentials of a large number of peripherally substituted metal-free porphyrins have been measured, and substituent partial potentials have been derived the reduction potentials can also be correlated with porphyrin basicity (p/ ) and reactivity (80MI30701). For a review of electrochemistry as applied to porphyrin and metalloporphyrin systems,... 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

The limited extent of charge transfer interaction between the exciplex components in the nonpolar benzene solutions is Indicated by plots of the logarithm of kq, the observed rate constant for the quenching of the metalloporphyrin triplet, versus the quencher electrochemical reduction potential. Although the slopes of such plots were linear for the nitroaromatics and for the chloro compounds, the slopes differed for the two groups of... 

Several investigations of the redox properties of various free base hydroporphyrins and their metal derivatives have been reported. As is typical of many porphyrins and metalloporphyrins, these hydroporphyrins generally show two oxidations and one or more reductions. The reversibility of these redox reactions depends on the nature of the hydroporphyrin and its stereochemistry. For example, the cyclic voltammograms of ris-H2(OEC) and frans-H2(OEC) were superficially alike, although substantial differences existed in the stability of the cation radicals and dications of the cis and trans isomers [85]. The first oxidation of rrans-H2(OEC) was reversible whereas ds-H2(OEC) was not reversible. However, the notable features observed in the redox chemistry of hydroporphyrins is the shift of both oxidation and reduction potentials of hydroporphyrins towards more negative values compared to porphyrins, i.e., they are more easier to oxidise and difficult to reduce [78]. A significant trend was observed in the electrochemistry of free base octaethyl- [86, 87] and tetraphenyl [88,89] hydroporphyrins (Table 2). The porphyrin and chlorin of each series... 

A plot of the midpoint one-electron oxidation and reduction potentials against each other yield linear correlation, indicating that metalloporphyrins that are easy to oxidize are difficult to reduce and vice versa (Figure 6.4.3) (Fuhrhop, Kadish, et al., 1973). 

Figure 6.4.3 Plot of oxidation and reduction potentials of various metalloporphyri-nates. Metalloporphyrins that easily form cation radicals with weak oxidants cannot be chemically reduced (e.g., the Zn- and Mg-porphyrinates). Metalloporphyrins that easily form anion radicals upon reduction [e.g., Sn(IV)], on the other hand, cannot be chemically oxidized. Only electrochemical reactions are possible at potentials far above 1 V or far below -1 V...

Extending the studies on the reductive electron transfer from reduced metalloporphyrin states to [60]fullerene into aqueous media requires employment of a water-soluble form of [60]fullerene. Therefore, a micellar assembly consisting of the fullerene incorporated into Triton X-100 was investigated. The electron transfer does indeed occur across the interface of the micellar assembly. The rates exhibit a considerable slow-down compared to the homogeneous systems but, on the other hand, show a clear dependence on the reduction potential of the water-soluble metalloporphyrin. 

Oxidatfon reduction potentials are tabulated for 241 hemoproteins and metalloporphyrins at various temperatures. 

The electronic structure of metalloporphyrins is similar to that of a metal surface in that the metal atom is supported by a buffering supply of delocalized electrons and the 3d-states are comparatively localized. Iron atoms chelated by porphyrins form the active sites of hemeproteins. The vibrational frequencies - vco and vpec of liganded CO, and the fer-ric/ferrous reduction potential, Eq, respond to the electronegativity of peripheral substituents of the heme (ironporphyrin) plane as well as to perturbations via the protein-porphyrin interface [4],... 

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... 

The coverage in this chapter is not comprehensive and no tabulation of rate data has been attempted. A more extensive account of the material will appear in Volume 9 of the series the presentation of the material also differs slightly from previous volumes. There have been many important developments in this general area. A comprehensive review of one-electron reduction potentials for nonmetallic substrates has appeared. Detailed kinetic studies have been reported for several important reactions where metal ions serve as catalysts in the transformation of organic substrates.Catalytic oxidation reactions involving metal complexes and macrocyclic metal complexes have been reviewed. Continued interest centers on catalysis by metalloporphyrins, and the role of metal complexes in electrocatalytic reductions has been reviewed. ... 

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. 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

At the end of this first stage of the chemical catalysis process, the metalloporphyrin is left under the form of a metal(III) bromide. The reaction that closes the catalytic loop is thus the reduction of this species into the metal(I) complex by means of two successive electron transfers from the electrode. This is a fast process since the electrode potential is adjusted so as to reduce rapidly the metal(II) complex. 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

---