Cobalt porphyrin reduction potentials

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Cobalt porphyrins have been some of the most studied catalysts for oxygen reduction, due in part to their strong interaction with molecular oxygen and the ease with which they catalyze the reduction at low potentials. Anson and coworkers [97] performed a study of the simplest of cobalt porphyrins, cobalt porphine, in the hope of gaining a baseline for the observed reactions of other porphyrins. Instead, they observed a very different process from most other monomeric cobalt porphyrins. 

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... 

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. 

An obvious choice for the construction of artificial oxygen binding mimics is the use of oxidisible transition metal (particularly iron) complexes of porphyrin ligands such as 12.23, as in the natural system. In such cases, the O2 affinity is governed essentially by the reduction potential of the metal centre (Ej) as in the reaction + e = M +. Iron(ll) and cobalt(ll) are particularly favourable in... 

With Co-facial cobalt porphyrins (Figure 2.5) adsorbed on pyrolytic graphite both cobalt centers are electrochemically active . However if one of the cobalt centers is replaced by iron, this metal shows no redox activity. This cofacial porphyrin with two different metal centers shows catalytic activity for the reduction of O2 and the foot of the reduction is observed at a potential where presumably the Fe(III)/(II) should appear. The fact that the Fe center is electrochemically silent is not clear . 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

Catalysis is required in order to achieve the direcL four-electron reduction of O2 to H2O at potentials near the thermodynamically allowed values and metalloporphyrins are frequently employed as catalysts for this purpose. Cobalt porphyrins, which are active at reasonably positive potentials, typically do not carry the reduction beyond the two-electron step to H2O2. 

Oxygen reduction at ITIES has recently been revisited. Using decamethylferro-cene as the electron donor, Su et al. have shown that the potential control of the proton pumping at the interface can be used to control the overall reaction [234]. Also, Samec et al. have shown that cobalt porphyrins and free-base porphyrins can be used to reduce oxygen at the interface in the presence of lipophilic donors [235]. 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

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