Dioxygen cofacial porphyrins

Bolze, R, C.P. Gros, P.D. Harvey, and R. Guilard (2001). Luminescence properties of a cofacial dipalladium porphyrin dimer under argon and in the presence of dioxygen. J. Porphyrins Phthalocyanines 5(7), 569-574. 

Collman JP, Hendricks NH, Leidner CR, Ngameni E, L Her M. 1988. Multilayer activity and implications of hydrogen peroxide in the catal3dic reduction of dioxygen by a dicobalt cofacial bis(porphyrin) (C02FTF4). Inorg Chem 27 387. 

Fukuzumi S, Okamoto K, Gros CP, Guilard R. 2004. Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins. J Am Chem Soc 126 10441. 

The discussion of porphyrin complexes in the activation of dioxygen in solution can be readily subdivided into discussions of monomeric systems and dimeric systems. The dimeric systems (cofacial metallodi-porphyrins) have recently been reviewed by Collman, Wagenknecht, and Hutchison (73). That review highlights the significant amount of research stimulated by the initial discovery by Collman and co-workers (74), confirmed later by Chang and co-workers (75), that dicobalt cofacial diporphyrins can promote the direct four-electron reduction of dioxygen to water. 

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). 

A number of iridium-porphyrin systems, including mononuclear and cofacial diporphyrins, have been adsorbed on pyrolytic edge-plane graphite electrodes and tested for their ability to reduce dioxygen to water (82). The original system investigated, [Ir(oep)H], where oep = 2,3,7,8,12,13,17,18-octaethylprophyrinato, was unique in that, while monomeric, the complex was still active in acidic solutions at potentials of +0.72 V vs NHE at pH 1 (82a). The [Ir(oep)H] did become inactive at potentials less than +0.2 V vs NHE, unlike the cofacial dicobalt diporphyrin systems. In the more recent report of these systems (82b),... 

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. 

Collman, and co-workers (346-352) also demonstrated that pyrolitic graphite electrodes modified with cofacial bis(porphyrinato cobalt) complex (CoFTF4) can promote the four-electron reduction of O2 to H2O. According to the proposed model, the dioxygen molecule is activated by the simultaneous coordination to two cobalt porphyrin sites, but electronic factors and/or the lack of electrons can also limit the four-electron process. 

Liu H, Weaver M, Wang C, Chang C. Dependence of electrocatalysis for dioxygen reduction by adsorbed cofacial dicobalt porphyrins upon catal5 st structure. J Electroanal Chem 1983 145 439-47. 

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