Metalloporphyrins electrochemical studies

Richoux, M. C., Neta, P, (1986). One- and two-electron reduction of metalloporphyrins, radiation chemical, photochemical, and electrochemical studies. Kinetics of the decay of 7C-radical anions, J. Am. Chem. Soc., 90 2462. 

M.C. Richoux, P. Neta, A. Harriman, S. Baral, and P. Hambright, One- and Two-Electron Reduction of Metalloporphyrins. Radiation Chemical, Photochemical, and Electrochemical Studies. Kinetics of the Decay of x-Radical Anions, J. Phys. Chem., 90 (1986) 2462. 

By the beginning of the 1970s, the majority of electrochemical studies on synthetic metalloporphyrins was being carried out in nonaqueous media using the technique of cyclic voltammetry. However, most utilized instrumentation was still homemade and only a handful of laboratories were actually making the measurements. An overview of the situation at this period is provided in several independent reviews [2, 6, 7, 9, 21]. 

The earliest electrochemical studies of metalloporphyrins in aprotic media... 

Tab. 1 List of solvents used in electrochemical studies of metalloporphyrins...

A list of the supporting electrolytes that have been used for studies of metal-loporphyrin electrochemistry is given in Table 2 [21]. The majority of electrochemical studies involving metalloporphyrins in... 

Various indium-metal - and thallium-metal o-bonded porphyrin complexes have been synthesized. These metalloporphyrins are represented by (P)InM (L) and (P)TIM (L) where P is OEP or TPP and M (L) is one of the following metallate ions Co(CO)4, Cr(C0)3Cp, Mn(CO)s, Mo(C0)3Cp or W(C0)3Cp. Detailed electrochemical studies involving both series of compounds have been carried out . The results of these studies indicate that the reduction of binuclear (P)MM (L) can occur by one or two electrons depending upon the specific porphyrin ring, the temperature, the nature of H and the nature of the bound metallate ion, H (L). Additional peaks are also observed for oxidation and reduction of (P)M and [(P)M] which are formed by cleavage of the reduced bimetallic species, as well as for [(P)M]+ when some decomposition of the neutral complex occurs. The generated stability of [(P)MM (L)]- and [(P)MM (L)]-2 is very low for complexes in both the T1 and In series. A low stability of electrooxidized [(P)InM (L)]+ is also observed but the singly and doubly oxidized [(P)TIM (Ii)]+ and [(P)TIM (L)] + 2 derivatives are quite stable and can be spectroscopically characterized . ... 

In recent years, much attention has been focused on electrochemical studies of metalloporphyrins, not only as mimetic compounds of the iron porphyrin unit in heme proteins but also as potential electrocatalysts . Metalloporphyrins have been found to be applicable in both homogeneous and heterogeneous catalysis - and, because oxygen can be reduced directly through a 4-electron pathway on some transition metal porphyrins, catalysis in the heterogeneous electrochemical oxygen reduction reaction has received particular attention The application of metalloporphyrins to heterogeneous electrocatalysis requires their attachment to solid electrodes which can be realized based on chemisorption, chemical reactions with previously functionalized electrodes, chemical reactions with a functionalized polymer, incorporation of the porphyrin with the polymer film and electrochemical polymerization. 

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. 

Wolberg, A., Manassen, J. Electrochemical and Electron Paramagnetic Resonance Studies on Metalloporphyrins and their Electrochemical Oxidation Products. J. Am. Chem. Soc. 92, 2982 (1970). 

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. 

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. 

Metalloporphyrins consist of porphyrin ring structures complexed to a central atom. Among them, hemin structures with central iron atoms at different oxidation states and chlorophyll pigments containing magnesium are most abundant The interest in their spectroelectrochemical studies is multiple. Thus, their adsorption and electrochemical behaviour at the electrode surface can be used not only to model their functions in a biological matrix but also to improve the practical application of porphyrin coated electrodes as catalysts or sensitizers in photoelectrochemical cells... 

Poliak KW, Leon JW, Frechet JMJ, Maskus M, Abruria HD (1998) Effects of dendrimer generation on site isolation of core moieties electrochemical and fluorescence quenching studies with metalloporphyrin core dendrimers. Chem Matia-10 30... 

The electrochemistry of metalloporphyrins at the start of the 1960s involved, in large part, measurements of standard redox potentials for naturally occurring complexes in aqueous buffered media [14], The choice of an aqueous solvent was often dictated by the biological relevance of the compounds available for study, while the choice of the measurement technique (potentiome-try or polarography at a dropping mercury electrode) was necessitated by the type of available electrochemical instrumentation, virtually all of which was homemade and... 

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