Dioxygen evolution

In the absence of electron donors, complexes 1 display a catalase-like activity (Eq. (17)). Dioxygen evolution can be visually observed at [H202] >0.01M. This catalytic feature has been studied kinetically by monitoring the initial rates of 02 formation with a Clark electrode (53). 

Clearly, the S0 state must have one Mn(II), one Mn(III) and two Mn(IV) while the S4 must have three Mn(IV) and one Mn(V)—but where they are located in the tetra-Mn centre and how their four-electron reduction is coupled with water splitting and dioxygen evolution remains to be established. (For recent reviews see Goussias et al., 2002 Ferreira et al., 2004 Rutherford and Boussac, 2004 Iverson, 2006.)... 

Scheme 8 Proposed dioxygen evolution mechanism for Mnm complexes of LI 1C and formal analogy to Kok states. The (CH2) group is the variable backbone link referred to in the text. (From Ref. 163b.)...

Fig. 8. The application of vesicles for photocatalytic water decomposition in sacrificial systems (a) — dihydrogen evolution in the vesicle cavity. Pt metal catalyst is anchored to the inner membrane // water interface (b) — dioxygen evolution in the bulk solution. Manganese oxide catalyst is anchored to the outer membrane // water interface of the vesicle...

In the vesicle suspension of Fig. 8 it was possible to isolate the centers for dihydrogen and dioxygen evolution and thus to avoid cross reactions of S+ and A- with the catalysts for H2 and 02 evolution, respectively. However, it turned out that 02 evolution gradually inhibits the H2 evolution, because oxygen evolved in the outer volume permeates across the membranes and destroys the apparatus for dihydrogen evolution located inside the vesicles. Note, that such a problem also arises for biological systems adapted to provide simultaneous evolution of H2 and Oz [275, 276],... 

This is, by nature, a very complex reaction. The elementary steps in the detailed mechanism are far from known. However, the kinetics can be improved by surface modification. For instance by surface coating with an efficient catalyst for dioxygen evolution, e.g. Ru02 [74], or increasing the surface area, which facilitate a lower current density which in... 

The results of catalytic epoxidation of various olefins, using f-BuOOH as the terminal oxidant and Mn(Me2EBC)Cl2 as the catalyst, are summarized in Table 3.4. The color of the reaction mixture turns to purple upon addition of f-BuOOH and the ultraviolet-visible spectrophotometry shows that the manganese is present predominantly in the tetravalent state, and no dioxygen evolution was observed. 

Thus, most of the recent experimental data have been rationalized in terms of an active site that contains binuclear- or tetranuclear-manganese clusters with coordinated hydroxide ligands. The majority of the mechanisms for dioxygen evolution that are based on such models involve (1) sequential... 

Studies involving calcium and chloride have shown these ions to be critical to the proper functioning of the OEC (35, 146, 188), although their exact roles have yet to be elucidated. Without either of these, dioxygen evolution is halted and the OEC cannot complete one full cycle. It has recently been reported, for example, that the OEC can reach S2 without chloride, but cannot be further oxidized to S3 or pro-... 

Scheme 12. A mechanism for dioxygen evolution by the OEC as proposed by Babcock that employs the hydrogen atom abstraction concept. [Reproduced with permission from (182). Copyright 1998, the American Chemical Society.]...

Scheme 13. A mechanism for dioxygen evolution by the OEC involving hydrogen atom abstraction as proposed by Pecoraro.

The Co(II) bis(azooxo) complexes (III, Ilia, VI, Via, VII, Vila) evolve quantitatively dioxygen v hen treated with chloroform, for half hour, at 293 K, with stirring. The percent dioxygen content is given in Table 2. Dioxygen evolution takes place with the formation of the parent compounds, confirmed by elemental analysis and IR absorption spectra. 

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