Catalytic Four-Electron Reduction of

Catalytic Four-Electron Reduction of Dioxygen to Water... 

Co —Co OH2 complex to produce AcrH" and the Co —Co °OH2 complex. After deprotonation, the p-peroxo Co° —O2—Co complex is formed as in the case of the catalytic four-electron reduction of O2 by Fc derivatives. The heteroly tic 0—0 bond cleavage of the Co —O2—Co complex affords the high-valent Co (IV)-oxo porphyrin jr-radical cation, which is readily reduced by AcrH2 in the presence of proton to yield H2O, accompanied by formation of AcrH +. 

In the case of AcrHt-Bu, the mechanism of the catalytic four-electron reduction of O2, accompanied by the oxygenation of Bu, is modified, as shown in Scheme 30 (177). The initial ET from AcrHt-Bu to the Co(III)2 complex results in the homoly-tic C9—C bond cleavage to produce f-Bu and AcrH (178, 179). Since the homolytic C9—C bond cleavage is also the catalytic rate-determining step, the... 

Cytochrome c oxidase is the terminal oxidase in both prokaryotic and eukaryotic cells and is responsible for the generation of cellular energy via oxidative phosphorylation [127]. It couples the catalytic four-electron reduction of O2 to... 

Many chemical and electrochemical analyses were carried out in the solution after this reaction with had occured. None was able to detect any trace of H2O2 Also, no chemical reaction occurred when H2O2 was added to the solution containing [ Fe H2 Thus, the observed reaction of O2 with [ Fe H2 seems to be an homogeneous, catalytic four electrons reduction of molecular oxygen by [ Fe H2, according the scheme ... 

Fe L H2, with molecular oxygen to regenerate the starting complex [ Co L X2. Thus the homogeneous catalytic four electrons reduction of O2 may also be achieved with this cobalt(II) complex. ... 

The catalytic mechanism of the four-electron reduction of oxygen with formic acid is shown in Scheme 4.11. The hydride complex, which is produced by the reaction of lr -OH2 with HCOO, deprotonates to generate the low-valent complex Ir. The Ir complex reacts with O2 to produce an iridium(v)-0x0 complex (B) and water via formation of the iridium(iii)-peroxo complex (A). The formation of an iridium-peroxo complex by the reaction of a low-valent iridium complex with O2 has been well established. The formation of an iridium(v)-oxo complex with cleavage of the 0-0 bond of an iridium(iii)-peroxo complex has also been reported. The 0x0 complex (B) reacts with HCOOH to reproduce Ir -OH2. Each intermediate in the catalytic cycle in Scheme 4.11 has been detected by stopped flow measurements. Because formic acid is used as a reductant for the catalytic four-electron reduction of O2, the water-soluble Ir catalyst can remove dissolved O2 with formic acid completely at an ambient temperature. 

Three new criteria were proposed [Collman et al., 2003a] to establish that the four-electron reduction of O2 by a catalytic film represents the selectivity of an individual molecule of the catalyst (i) independence of the i /i ratio on the catalyst surface coverage (ii) much higher stabUity of the catalyst in the ORR compared with reduction of H2O2 under conditions that reproduce the concentration of H2O2 in the film that would be generated if ORR proceeded by a step-wise mechanism and (iii) the nature of the turnover-determining step. 

The laccases, classed as polyphenol oxidases, catalyze the oxidation of diphenols, polyamines, as well as some inorganic ions, coupled to the four-electron reduction of oxygen to water see Fig. 12.4 for the proposed catalytic cycle. Due to this broad specificity, and the recognition that this specificity can be extended by the use of redox mediators [27], laccases show promise in a range of applications [28], from biosensors [29-32], biobleaching [27, 33-35] or biodegradation [36], to biocatalytic fuel cells [1-3, 18, 26, 37-42]. 

Figure 13 Catalytic cycle of VO complexes in four-electron reduction of 02.

Catechol oxidase catalyzes the oxidation of catechols to the respective quinones through a four-electron reduction of dioxygen to water. Whereas the exact mechanism of the enzymatic conversion remains uncertain, the commonly accepted mechanism is that proposed by Krebs and co-workers [3, 21] (Figure 5.4). The catalytic cycle begins with the met form of catechol oxidase, which is the resting form of the enzyme. The dicopper(II) center of the met form reacts with one... 

In neutral medium, on the other hand, the neutral form undergoes preferential four-electron reduction of the bond systems C3—C4 and C5— C6, with the formation of the tetrahydro derivative, but the 2e-intermediate (5,6-dihydrocytosine) was also found. In neutral medium the reduction products undergo only partial (60 %) deamination, in agreement with data on the catalytic reduction of cytosine 86). The scheme for electroreduction of cytosine would therefore be as follows (Scheme 8 a, b)1,84). 

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. 

The reactions described in Equation (20) are, however, only known for heterogeneous catalysts. For example, formaldehyde was formed selectively by a four-electron reduction of CO2 by an aluminum amalgam in the presence of titanium trichloride, even at room temperature and atmospheric pressure. The reaction is catalytic wiih respect to titanium [117,118]. 

Bimetallic catalysis also plays a vital role in the four-electron reduction of dioxygen to water, which is the reverse process of photosynthesis, to maintain the life of an aerobic organism by the respiration (3,157,158). Cytochrome c oxidases (CcOs), the last protein in the electron transport chain, are responsible for catalyzing the reduction of dioxygen to water by the soluble electron carrier, cytochrome c(3,157,158). The X-ray structures of CcOs revealed that the catalytic... 

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]... 

Scheme 28. The catalytic mechanism of the four-electron reduction of O2 by ferrocene with a cofacial cobalt porphyrin (168).

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