Porphyrin complexes with cobalt

Studies on the electrocatalytic activity of metal porphyrins are limited in comparison with those on other classes of macrocyclic metal complex. Among the few porphyrin complexes tested, cobalt porphyrins have been demonstrated to be efficient electrocatalysts for the reduction of C02 to CO... 

Although the cofacial diporphyrins represent a vibrant and innovative direction in dioxygen activation, simple porphyrins and their derivatives also remain an important research area. The dichlorophenyl-substituted porphyrin tdcpp [5,10,15,20-tetrakis(2,6-dichlorophenyl)-porphyrin] forms a complex with cobalt(II), [Co(tdcpp)], and catalyzes the oxidation of conjugated olefins to (after experimental workup) ketones in the presence of dioxygen and triethylsilane (80) a hydroperoxide intermediate has been isolated from these reactions (81). 

There is little support for the mechanism expressed by eqs 12 and 13. MMA is able to form a -complex with cobalt porphyrins,160 but the chain-transfer constant for its formation (1.8 L/mol s) is not high and is much smaller than the observed CCT chain-transfer constants. If the mechanism of eqs 12 and 13 is correct, then reduced concentrations of monomer should disfavor formation of LCoM, resulting in a decrease in the rate of CCT. The chain-transfer constant of the chain transfer is independent of the concentration of monomer.14,52 The mechanism expressed by eqs 12 and 13 will not be considered further. 

Metal porphyrins act as catalysts for chain transmission in radical polymerisation [55] and for electrochemical oxidation of sulphur dioxide [64], as well as for oxidation of a wide range of organic substrates [65]. Cobalt and nickel complexes with LI5 are effective in the electrocatalytic reduction of carbon dioxide [66]. Porphyrin complexes of cobalt and tetraazamacrocyclic complexes of nickel catalyse reduction of the proton [67-71]. 

Figure 5.9 Radical interchange for organo-cobalt porphyrin complexes with solution polymeric radicals from solution.

Oganova et a/. observed that certain cobalt (II) porphyrin complexes reversibly inhibit BA polymerization presumably with formation of a cobalt (111) intermediate as shown in Scheme 9.27. Thus, it seemed reasonable to propose these species may function as initiators in living radical polymerization.250 259... 

Very recently, a series of trihalomethyl cobalt porphyrin complexes Co(OEP)CX was prepared either by the reaction of Co(OEP) with CBrCl, CBr4 or CI4, or by the reaction of fCo(OEP)] with CCI4 or CB14. The dihalomethyl complexes, formed in small amounts in these reactions, were prepared in larger amounts from lCo(OEP)P with In a similar fashion, R SnH reacted with Co(OEP)CH ,... 

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. " " ... 

Experiments have been carried out to compare nanoscale catalysts composed of Fe-, Ni-, and Co-complexes of several porphyrins or cyanocobalamin (Dror et al. 2005). A cobalt-porphyrin complex and cyanocobalamin in the presence of Ti(III)citrate reduced the initial concentrations of tetrachloromethane and tetrachloroethene by —99.5%, and the porphyrin was equally effective with trichloroethene. The advantage of using heterogeneous catalysts was shown by experiments in repetitive cycling of tetrachloromethane. Zero-valent metals degrade vicinal dichlorides such as tetrachloroethene by a-elimination to produce dichloroacetylene and hnally acetylene (Roberts et al. 1996). 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

In the case of iron, the Fe(ni) alkyl complex is reduced, in a successive step, into the Fe(ii) alkyl complex by the starting iron(i) porphyrin, whereas this reaction does not occur with cobalt. This diflference is related to the fact... 

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