Electrochemistry iron porphyrins

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

The support of NIH grant DK 31038 during the writing of the original and revised article and of F. Ann Walker s research on iron porphyrins over the past 22 years is gratefully acknowledged. Professor Dennis Evans H. Evans provided very helpful conunents on the electrochemistry and thermodynamics sections. This revised article was prepared while both authors were on Sabbatical, Professor Simonis at the University of California, San Francisco, and Professor Walker at the University of Liibeck, Physics Institute, with support from an Alexander von Humboldt Senior Research Award. 

Swistak, C. and K.M. Kadish (1987). Electrochemistry of iron porphyrins under a CO atmosphere. Interactions between CO and pyridine. Inorg. Chem. 26, 405-412. 

Araki, K. and H.E. Toma (1999). Electrochemistry of a tetraruthenated iron porphyrin and its electrostatically assembled bUayered films. Electrochim. Acta 44,1577-1583. 

Ozer D, Harth R, Mor U, Bettelheim A (1989) Electrochemistry of various substituted aminophenyl iron porphyrins Part II. Catalytic reduction of dioxygen by electropolymerized films. J Electroanal Chan 266(1) 109-123... 

Other important tetra-azamacrocycle ligands displaying interesting electrochemistry particularly with metals of the iron group are phthalocyanines, porphyrins, and related species. We now discuss two examples of catalytic small molecule reduction by iron-porphyrin catalyst. 

The electrochemistry of iron porphyrins in nonaqueous media has been discussed in several reviews [2, 7, 10, 12], and only a few of the major trends of iron porphyrin electrochemistry will be summarized in the current paper. Both high and low oxidation states of the metal ion can be accessed upon reduction or oxidation of iron porphyrins and the overall electron-transfer mechanism of these metalloporphyrins is shown in Sch. 3, where [(P)Fe "]" represents the initial compound in the absence of an associated anionic ligand. 

Several iron porphyrins bound to diatomic molecules, such as CO, NO, CS, CSe, and O2, have also been examined as to their electrochemistry in nonaqueous media. Fe(II) porphyrins can coordinate CO to give mono- and his-CO derivatives [237-239], and the electrooxidation of these species by cyclic voltammetry results in irreversible waves because of a rapid loss of CO upon formation of Fe(III)[7, 30, 240). Studies of (TPP)FeCl and (TPP)FeCl04 in Py and CH2Cl2/Py mixtures under a CO atmosphere indicated that the following five types of iron(II) porphyrins could be formed (TPP)Fe, [(TPP)FeCl]-, [(TPP)Fe(CO)Clj-, (TPP)Fe(py)2, and (TPP)Fe(CO)(py), and that these could be electrochemically converted into two types of iron(I) porphyrins, namely [(TPP)Fe] and [(TPP)Fe(CO) (py)]- [240]. 

Two iron porphyrins may be bridged through a single bridging atom X to form a binuclear metalloporphyrin of the type (P)Fe-X-Fe(P), and the electrochemistry of these compounds where X = O, N, or C has been recently reviewed [7j. The formal oxidation states of the two iron atoms are 4-3 for the /t-oxo complexes, 4-3.5 for the /t-nitrido compounds and... 

A comprehensive review of o-bonded iron porphyrin electrochemistry has recently been published [12], and results on these compounds will not be discussed in the present paper. Several reviews have been published on the redox tuning of iron porphyrins over the last 20 years [2, 7, 10, 192] and this topic will also not be covered in the present paper. The exact potential for the Fe(III)/Fe(II) reaction will depend on the type of axial ligand coordinated to the Fe(III) or Fe(II) forms of the porphyrin. Axial ligands such as NO, C6H5 and 0104 will change drastically the potential at which the Fe(III)/Fe(II) redox couple is observed, but shifts of E /i for this redox reaction will occur upon solvent binding to the Fe(III) and/or Fe(II) form of the compound. The basicity of the porphyrin macrocycle will also influence E ji for the Fe(II)/Fe(III) electrode process. 

The present review describes the electrochemistry of synthetic metalloporphyrins in nonaqueous media. This work does not include metaUoporphyrin electrochemistry in aqueous media and discusses only briefly the electrochemistry of some iron porphyrins in real biological systems. We have not included the electrochemistry of porphyrin-Kke complexes such as metallocorroles, metalloporphycenes and metallochlorins, or the electrochemistry of pigments of biological relevance, such as vitamin B12 derivatives. For a coverage of these topics, the reader is referred to recent major reviews in The Porphyrin Handbook. 

The influence of pyridine binding and spin state on the spectroscopic properties and electrochemistry of aryl iron porphyrin complexes has been studied. The reduction of styrene to ethylbenzene and 2,3-diphenylbutane catalysed by [(TPP)FeCl] is proposed to proceed via a o-Fe-CHMeRi intermediate. The phthalocyanine anion Na2[FePc] reacts with ROCBr to give Na(PcFeC R] (R = Ph, Pr), and similar alkynyl anions have been prepared with a hemin-like iron centre.55... 

Iron(II) alkyl anions fFe(Por)R (R = Me, t-Bu) do not insert CO directly, but do upon one-electron oxidation to Fe(Por)R to give the acyl species Fe(Por)C(0)R, which can in turn be reduced to the iron(II) acyl Fe(Por)C(0)R]. This process competes with homolysis of Fe(Por)R, and the resulting iron(II) porphyrin is stabilized by formation of the carbonyl complex Fe(Por)(CO). Benzyl and phenyl iron(III) complexes do not insert CO, with the former undergoing decomposition and the latter forming a six-coordinate adduct, [Fe(Por)(Ph)(CO) upon reduction to iron(ll). The failure of Fe(Por)Ph to insert CO was attributed to the stronger Fe—C bond in the aryl complexes. The electrochemistry of the iron(lll) acyl complexes Fe(Por)C(0)R was investigated as part of this study, and showed two reversible reductions (to Fe(ll) and Fe(l) acyl complexes, formally) and one irreversible oxidation process."" ... 

Forshey PA, Kuwana T. 1983. Electrochemistry of oxygen reduction. 4. Oxygen to water conversion by iron(II)(tetrakis(N-methyl-4-pyridyl)porphyrin) via hydrogen peroxide. 

The reduction electrochemistry of ECP porphyrin films furthermore responds to added axial ligands in the expected ways. We have tested this (2,6) for the ECP form of the iron complex of tetra(o-amino)phenyl)porphyrin by adding chloride and various nitrogeneous bases to the contacting solutions, observing the Fe(III/II) wave shift to expected potentials based on the monomer behavior in solution. This is additional evidence that the essential porphyrin structure is preserved during the oxidation of the monomer and its incorporation into a polymeric film. 

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

The electrochemistry of dinitrogen bridging two porphyrin ligated ruthenium centers has been studied as a possible route to fixed nitrogen [45 -47]. Diazene stabilized by bonding to two iron centers in a FeS system has been advanced as a structural model of a plausible intermediate in biological nitrogen fixation [48-50]. 

A. Kitajima, M. Miyabe, T. Kobayashi, H. Koyama, O. Ikeda, K. Kijima, T. Komura, A. Uno and A. Yamatodani, Detection of nitric oxide with the iron(III) porphyrin doped Nafion/glassy carbon electrode, Electrochemistry, 1999, 67, 784-788. 

The basket-handle and picket-fence porphyrins show dramatic effects not only during metallation and binding of small molecules but in their redox and coordination chemistry. Detailed studies on the electrochemistry of the iron complex have been made paralleling the earlier electrochemical studies on the free base, magnesium and zinc complexes of 192. and 201 ... 

The reductive electrochemistry of iron-carbene porphyrins has been investigated in aprotic solvents [115]. With the vinylidene complex, there is a 2e -I- H reduction of the ligand leading to the formation of the corresponding iron(II) vinyl complex. The energies required to reduce by two electrons the other carbene complexes are quite similar [115]. The dichlorocarbene complex is an exception because the reduction is facihtated by the extreme instability of the one-electron intermediate. Formation of cr-alkyl iron(III) porphyrins has been confirmed by independent synthesis [116]. The a-alkyl iron(III) porphyrins can then be obtained by a one-electron reoxidation re-... 

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