Metalloporphyrins electrochemistry

Despite a large variety of known compounds, or perhaps because of it, previous reviews on metalloporphyrin electrochemistry have concentrated for the most part on describing the behavior of simple model compounds with octaethyl-porphyrin (OEP) or tetraphenylporphyrin (TPP) [2-10] macrocycles, and were most often arranged according to a specific element (Fe, for example), group of elements, the Periodic Table of the elements or the nature of the metal-ligand bond examples in the latter case include porphyrins with metal-carbon bonds [3, 4, 11, 12] and those with metal-metal bonds [4, 11, 13]. Unfortunately, these approaches and perspectives are truly useful only if the reader is made aware of how changes in structure... 

The impetus for this study was threefold. First, we sought to place the Mn capped porphyrin within the general framework of metalloporphyrin electrochemistry as recently reviewed by Kadish . Second, we hoped to discover and exploit the unique reactivity of the Mn capped porphyrin afforded by the presence of the nondisplaceable steric blocking group on one side of the macrocycle. Third, we wished to explore the chemistry of high valent Mn capped porphyrin. 

Electrosynthesis and Electrochemistry of Metalloporphyrins Containing a Metal—Carbon cr-Bond... 

For a review of electrochemistry as applied to porphyrin and metalloporphyrin systems, see (B-75MI30707). 

The only reactions that do occur with these metalloporphyrins follow oxidations of the metals or the porphyrin in rings. They have been known for a long time and summarized in reviews on the primary redox processes of metalloporphyrins [292, 293] or electrochemistry in nonaqueous media [294]. 

The enormous research activity devoted to metalloporphyrins, and in special, to the noble metal porphyrins is governed by the idea of biomimesis, i.e. the desire of chemists to understand and imitate nature with its ingenious biocatalytic pathways. In this respect, two important neighbouring disciplines to synthetic chemistry will be touched on in this article electrochemistry (Sect. 5.1) and catalysis (Sect. 5.2). However, in order to keep the article within certain limits, only some notes will be given on these fields, which sometimes are linked by the term electrocatalysis and bear some relevance to material science. Further... 

As such, they are much more resistant to acid- or base-induced dissociation or hydrolysis than are coordination complexes [e.g., Fen(TPP) is stable in the presence of 1 M H30+ or 1 M HO", whereas Feu(NH3)4+ is not]. An especially useful review of the electrochemistry of metalloporphyrins in nonaqueous media is available,3 which is complemented by a review of the electrochemistry of metalloproteins.4... 

In summary, the electrochemistry of organometallic and metalloporphyrins is dominated by synergistic electron transfer of extramolecular solution components (H20, 02, electrophiles, and nucleophiles). This provides a convenient means for evaluation of the molecular activation (catalytic) properties of these important metal-centered systems. Only in the case of iron (II)- and cobalt(II)-... 

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,... 

The group of Van Berkel [40-42] studied electrochemical processes in the ESI source, e.g., by demonstrating that the radical cations observed in the ESI mass spectra of metalloporphyrins and polycyclic aromatic hydrocarbons have an electrochemical origin [40], and by proposing strategies to convert analytes into electrochemically-ionizable derivatives [42]. In a review on analytical applications of on-line electrochemistry-ESl-MS, Diehl and Karst [43] discussed topics related to the electrochemical processes in ESI as well. 

Radish, Karl M., The Electrochemistry of Metalloporphyrins in Nonaqueous Media. 34 435... 

Electrochemistry and Spectroelectrochemistry of the Free Porphyrin Ligands H2Por and Metalloporphyrins MPor (M = Mg, Zn, Cd)... 

There is great interest in studying the electrochemistry of gold and silver porphyrins in aqueous and nonaqueous solutions80 -96. The redox reactions for metalloporphyrins include changes in the oxidation state of the porphyrin nucleus and, in some cases, changes in the oxidation state of the metal. The porphyrin ring tends to stabilize the... 

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... 

Fuhrhop, J.-H. (1975a). Reversible reactions of porphyrins and metalloporphyrins and electrochemistry. Porphyrins and Metalloporphyrins (Smith, K. M., ed.), Elsevier Scientific Publishing Company, New York, p. 593. 

Fuhrhop, J.-H., Subramanian, J. (1976). The chemical and physical behavior of metalloporphyrins as compared to other tetrapyrrole pigment metal complexes. Reactivity, spectra, electrochemistry, model calculations and biological implications, Phil. Trans. Roy. Soc., B 273 335. 

The electrochemistry of metalloporphyrins with o-bonded alkyl or aryl groups has been reported for complexes with nine different central metals. These include transition metal complexes of cobalt rhodium , and iridium , and... 

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