Porphyrins electrochemistry

Another level of permeability Is that of axial bases, such as pyridine. As noted above, we have seen the porphyrin electrochemistry to respond to addition of axial bases to the contacting solutions this implicitly demonstrates that bases such as pyridine are able to penetrate the films to the axial porphyrin sites in its interior. 

Today, UV-visible spectroelectrochemistry has become an essential tool in the study of redox processes, particularly when the processes are reversible and the reagents or products of electrolysis are colored. For example, the technique is of enormous value in the study of the electrochemistry of porphyrins and ph-thalocyanines, though it took some time to catch on. A review of porphyrin electrochemistry from 1986 barely mentions the technique, now an essential, even routine, tool in this field [4]. Another area of chemistry in which spectroelectrochemistry has been extremely important is conducting polymers, a field that had barely started 20 years ago. Spectroelectrochemistry has also been developing rapidly as the technology of electronic and optical detection has advanced. For example, we shall see later how time-resolved spectroelectrochemistry is now able to provide a wealth of information about electrochemical processes. 

This present review will not attempt to provide a comprehensive description of all known porphyrin electrochemistry in non-aqueous media, but will concentrate in part on specific types of porphyrin macrocycles, in part on specific groups of metalloporphyrins, and in part on guiding the reader through the vast array of elechon-transfer mechanisms that can exist for a related series of compounds under a given set of experimental conditions. It is hoped that this approach will answer the majority of the reader s questions as to what has been done in the past, while at the same time enabling the reader to utilize the data in the literature to predict what might be observed in future studies involving the electrochemistry of yet-to-be synthesized metalloporphyrin complexes. 

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. 

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. 

K. M. K. acknowledges support of the Robert A. Welch foundation for continuous support of his research on porphyrin electrochemistry over the last 25 years under Grant E-680. 

Guilard R, Lecomte C, Kadish KM (1987) Synthesis, Electrochemistry, and Structural Properties of Porphyrins with Metal-Carbon Single Bonds and Metal-Metal Bonds. 64 205-268... 

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. ... 

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. 

Radish KM, Shao J, Ou Z, Zhan R, Burdet E, Barhe J-M, Gros CP, Guilard R. 2005. Electrochemistry and spectroelectrochemistry of heterobimetalUc porphyrin-corrole dyads. Influence of the spacer, metal ion, and oxidation state on the pyridine binding ability. Inorg Chem 44, 9023-9038. 

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. 

Electrochemistry and spectroscopy of the tt cation radical of meso-tetraalkylchlorin (tetra-methyl) and various porphyrins (tetramethyl, tetraethyl, and tetra-ra-propyl) indicate that these do not convert to Nim at low temperatures.280 Optical evidence reveals, however, that oxidation of the tt cation radical of [Ni(pEt2N)(TPP)] leads to a Ni111 cation radical which can be further oxidized to a Ni111 porphyrin dication. Similar studies have been carried out for various other derivatives of me.so-tetraarylporphyrins such as /V-oxides of TPP and 5,10,15,20-tetramesitylpro-phyrin (TMP). Addition of trifluoroacetic acid (TFA) to the /V-oxide of [NinTMP] at —25 °C in CH2C12 results in [Nim(TMP)]+ with a rhombic EPR spectrum, g = 2.40, 2.12, and 2.04.281... 

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