Cobalt electrooxidation

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

Allylic acetoxylation of cyclohexene (96) at 80 °C affords 3-acetoxycyclohexene (97) in 67% yield (Scheme 36). It was found that the catalytic double-bond isomerization of 3-phenylpropene proceeds by the action of an electrochemically generated 17-electron Co(II) species [132]. The cobalt(III)-mediated electrooxidative decomposition of chlorinated organics, that is, l,3-dichloro-2-propanol, 2-monochloro-propanol, and so on, has been performed... 

The electrooxidation of hydrazine is also catalysed by metal compounds other than oxides, as shown by Hou et al. using an FI manifold. They employed a cobalt tetraphenylporphyrin-modified glassy carbon electrode to oxidize hydrazine at +0.5 V vj Ag/AgCl and pH 2.5, and achieved a detection limit of 0.1 ng [194]. 

Electroreduction and electrooxidation of salene (7V,N -bis(salicylidene)-ethyledi-amine) complexes of cobalt and copper studied by Kapturkiewicz and Behr [147] in eight aprotic solvents obey these conditions. These authors were the first to demonstrate experimentally the significant influence of the dielectric relaxation time of solvents on the electrode kinetics. They found earlier [171] that the mechanism of electrode reactions of salene complexes is independent of the solvents applied. No correlation with the prediction of the Marcus theory was found, but the kinetic data correlated well with the viscosity of the solvents and their dielectric relaxation time. However, because the ohmic drop was not well compensated, their rate constants are likely to be too low, as was shown in DMSO by Lasia and coworkers [172]. 

Cp—Cp coupling occurs, probably via the first-formed palladium phenolate (315) to give the bisquinone methide (316), and the latter spontaneously undergoes intramolecular Diels-Alder reaction to the natural lignan carpanone (317) in 46% yield, with stereocontrol at five chiral centers. High yields, up to 94%, have been recorded using oxygen as oxidant with a metal(II)-salen complex as catalyst, e.g. cobalt(II) salen. A low yield of carpanone was also obtained in electrooxidation. 8... 

Another approach uses a cobalt tetrakis(o-aminophenyl)porphyrin polymer film, prepared by electrooxidative polymerization of the monomer on top of the electrode as conductive film-mediator-couple [19]. 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

Komorsky-Lovric, S., M. Lovric, and F. Scholz (1997). Sulfide ion electrooxidation catalysed by cobalt phthalocyanine microcystals. Microchim. Acta 127(1-2), 95-99. 

Gnlppi, M., F. Bedioui, and J.H. Zagal (2001). Overoxidized polypyrrole/cobalt ter-tasnlfonated phthalocyanine modified ultramicro-carbon-fiber electrodes for the electrooxidation of 2-mercaptoethanol. Electroanalysis 13(13), 1136-1139. 

Lelj, F., G. Morelli, G. Ricciardi, and A. Rosa (1990). The electrochemical behaviour and electrooxidative polymerization of tetraazannulenic alkyl- and aryl-cobalt complexes. Inorg. Chim. Acta 176, 189-194. 

Scheme 9 Electrooxidation of cobalt(ll) porphyrins in coordinating media.

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

The electrochemistry of cobalt porphyrins has been examined under both CO [36,114,115] andO2 atmospheres.The studies under O2 were to examine the application of cobalt porphyrins as electrocatalysts for the four-electron reduction of O2 to H2O [314, 316, 317, 322]. The oxidation of (P)Co under a CO atmosphere has been reported for P = TPP [115], OEP [114] and Br TPP (jc = 0 to 8) [29]. (OEP)Co is converted to [(OEP)Co "(CO)]+ in CH2CI2 under CO [114] and these results contrast with what was observed for oxidation of the same compound under N2, where a Co(II) porphyrin tt -cation radical was proposed as the singly oxidized product. The electrooxidation of (Br TPP)Co under CO leads to both mono and bis-CO complexes in solution with the ratio of mono/bis adducts depending on the number of Br groups on the porphyrin macrocycle [29]. 

Cobalt is used to promote CO oxidation in reformers [284, 285], suggesting PtCo alloys may be useful catalysts for H2 oxidation in the presence of CO. PtCo alloys have been proposed as improved methanol oxidation catalysts [286] because cobalt may assist with CO removal (CO is an intermediate in meflianol electrooxidation) through a mechanism analogous to the PtRu bifunctional mechanism. PtCo alloys have also been studied as improved ORR catalysts [200, 287, 288]. In addition to their improved ORR kinetics, these alloys have been shown to be more tolerant to methanol crossover in direct methanol fuel cells (DMFCs), again possibly through improved CO removal kinetics [289]. However, Stevens et al. [235] observed no impact on CO-stripping with the addition of eobalt to Pt, and explained this as due to surface cobalt dissolving away. 

Reaction (48) is considered to be 5n2, like that of the cobalt analogs. The alkyliron(II) compounds can be electrooxidized to alkyliron(III) and alkyl-iron(IV) species, the last being rather less stable with life times of the order of a millisecond. ... 

Many different compounds have been used as electron mediators for hydrazine electrooxidation, including catechin film [10], hydroquinone salophen derivatives [11], nickel hexacyanoferrate [12], zinc pentacyanonitrosylferrate film [13], nickel pentacyanonitrosulferrate film-modified aluminum [14], hybrid hexacyanoferrates of copper and cobalt films [15], carbon nanotubes [16], pyrogallol red [17], DNA [18], chlorogenic acid/carbon ceramic composite [19], and N4 metallomacrocyclic compounds (MN4) [20-24]. 

The kinetic parameters are slightly dilferent for iron N4-macrocyclic complexes, compared to cobalt complexes. In previous investigation of the electrooxidation of hydrazine catalyzed by FeN4 macrocyclics, the proposed mechanism involved adduct formation between Fe and the hydrazine molecule, prior to the rate determining step [46]. It is evident that the formation of a bond between the metal active site and the hydrazine molecule is a crucial step in electrocatalysis phenomena [47-50]. The electrooxidation of hydrazine on iron N4 macrocyclic complexes results in a Tafel plot with slope of around 0.040 V/decade, instead of 0.060 V/decade. The order in hydrazine is still one, but the order with respect to OH is two, so a reaction mechanism was proposed as follows [44, 45] ... 

The first mechanistic study involving electrooxidation of a-bonded cobalt porphyrins was reported for (TPP)Co(R) complexes where R was C2H5 and C2Hs02C . The reversible formation of [(TPP)Co(R)]+ and... 

---