Dicobalt porphyrins

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. 

Dicobalt species have also been shown to result in cobalt s possessing characteristically unstable oxidation states. A binary cofacial dicobalt porphyrin species has been shown to undergo a pair of one-electron oxidation steps as well as a pair of one-electron reduction steps [49] the two cobalt centers behave essentially independently of one another, so that mixed-valence complexes can be generated. Brooker and coworkers [50] investigated a pyridazine Schiff-base macrocycle with two cobalt centers (4), each of which could undergo a separate one-electron oxidation or reduction to produce a species capable of existing in five different combinations of redox states. Most interesting about this complex is the formation of cobalt... 

One such reaction that has been studied is the electrocatalytic reduction of oxygen directly to water.25,27 The electrocatalysts for this process are often based on metal porphyrins and phthalo-cyanins. Thus a graphite electrode whose surface was modified by the irreversible adsorption of a cofacial dicobalt porphyrin dimer was able to reduce oxygen under conditions where the reduction did not occur on the bare electrode itself. Similarly, a catalytic chemically modified electrode for the oxidation of chloride to chlorine has been prepared28 where the active catalyst was reported to be a ruthenium dimer, [(bipy)2(OH)RulvORuvO(bipy)2]4+, which was reduced to the corresponding [Rum-RuIV] dimer during the reaction. 

The catalytic reduction of O2 by ferrocene (Fc) derivatives (one-electron reductants like cytochrome c) with a monomer cobalt porphyrin and cofacial dicobalt porphyrins was examined in the presence of perchloric acid (HCIO4) in PhCN (168). This finding allowed for the difference between the single and bimetallic systems to be clarified. 

When a cofacial dicobalt porphyrin, Co2(DPX), is used as a catalyst instead of a single cobalt porphyrin, Co(OEP), the concentration of [Fe(C5H4Me)2]formed... 

According to Scheme 27, the rate of formation of ferricenium ions is given by Eq. 28, where the catalytic rate constant (/ cat) corresponds to 2A et- The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] (DPA = bisporphyrin with anthrocene spacer, DPB = bisporphyrin with biphenylene spacer, DPD = bisporphpyrin with dibenzofuron spacer) also catalyze the reduction of O2 by Fe(CsH4Me)2, but the amount of [Fe(C5H4Me)2] formed is <4 equiv of O2 (168). Thus, the clean four-electron reduction of O2 by [Fe-(C5H4Me)2]... 

Thus, the four-electron reduction of O2 by AcrHR catalyzed by cofacial dicobalt porphyrins occurs efficiently via ET from AcrHR to cofacial dicobalt porphyrins, followed by the C9 H and C9 C bond cleavage of AcrHR" depending on the... 

The electrochemical behavior of both these species in the form of coatings was very similar to that found for the original most active dicobalt porphyrins. This is illustrated in Figure 3.62A [110], which compares 1,8-anthryldiporphyrin (solid lines) and Co-Co4 (dashed lines). In particular, the values of E1/2 were found to be in between those observed for Co-Co4 and Co2FTF 4—2,1N-H and the amount of hydrogen peroxide detected at the ring was about 8%. 

Figure 3.64 Dicobalt porphyrin dimers with wide and narrow bites.

Collman, J.P, M. Marrocco, P. Denisevich, C. Koval, and F.C. Anson (1979). Potent catalysis of the electroreduction of oxygen to water by dicobalt porphyrin dimers adsorbed on graphite electrodes. J. Electroanal. Chem. 101, 117-122. 

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. 

Cofacial Dicobalt Porphyrin and Porphyrin-Corrole Dyads... 

When monomeric cobalt porphyrins are replaced by cofacial dicobalt porphyrin dyads and porphyrin-corrole dyads, the four-electron reduction of O2 by ferrocene derivatives occurs efficiently, depending on the type of spacer (Sp in Figure 4.18), in the presence of HCIO4 in PhCN. " ... 

When a cofacial dicobalt porphyrin Co2(DPX) is used as a catalyst instead of a single cobalt porphyrin CoOEP, the concentration of Me2Fc formed in the Co2(DPX)-catalyzed reduction of O2 by Me2Fc is four times that of the O2 concentration. Thus, the four-electron reduction of O2 by Me2Fc occurs efficiently in the presence of a catalytic amount of Co2(DPX) and HCIO4 in PhCN. It was confirmed that no H2O2 was formed in the catalytic reduction of O2 by Me2Fc. ... 

Based on the detailed kinetic comparison of the catalytic reactivities of cofacial dicobalt porphyrins and a single cobalt porphyrin together with the detection of the reactive intermediates by EPR, the catalytic mechanism of four-electron reduction of O2 by ferrocene derivatives is summarized in Scheme 4.7. The initial electron transfer from ferrocene derivatives to the Co -Co ... 

Thus, the interaction of two cobalt nuclei with an active form of oxygen is essential for the four-electron reduction of O2. The p-superoxo species of cofacial dicobalt porphyrins are produced by the reactions of cofacial dico-balt(ii) porphyrins with O2 in the presence of a bulky base (l- er -butyl-5-phenylimidazole) and the subsequent one-electron oxidation of the resulting peroxo species by iodine.The superhyperfine structure due to two equivalent cobalt nuclei is observed at room temperature in the EPR spectrum of the... 

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