Complex Water oxiding

Fig. 7. A schematic view of Nafion membrane showing the microheterogeneous environment. A hydrophobic fluorocarbon phase B hydrophilic sulfonate ionic clusters C interfacial region formed between A and B and Ru adsorbed ruthenium complex water oxidation catalyst...

Photosystem II Inhibitors. The PSII complex usually is assumed to be that stmctural entity capable of light absorption, water oxidation, plastoquiaone reduction, and generation of transmembrane charge asymmetry and the chemical potential of hydrogen ions (41). The typical PSII complex... 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

The electrochemistry of single-crystal and polycrystalline pyrite electrodes in acidic and alkaline aqueous solutions has been investigated extensively. Emphasis has been laid on the complex anodic oxidation process of pyrite and its products, which appears to proceed via an autocatalytic pathway [160]. A number of investigations and reviews have been published on this subject [161]. Electrochemical corrosion has been observed in the dark on single crystals and, more drastically, on polycrystalline pyrite [162]. Overall, the electrochemical path for the corrosion of n-EeS2 pyrite in water under illumination has been described as a 15 h" reaction ... 

The membrane-bound catalyst for water oxidation to O2 can be prepared via oxidation of Mn(Il) and Co(ll) salts to Mn(IV) and Co(Ill) hydroxides, respectively, in the presence of lipid vesicles. Using these catalysts and photogenerated Ru(bipy)j complex as an oxidant, it is possible to oxidize water to O2 in vesicle systems. One of such systems for O2 evolution is schematically represented in Fig. 4. 

In 1979, the first isolation of the hydrido(hydroxo) complex by oxidative addition of water to an electron-rich platinum(O) complex was accomplished by Yoshida and Otsuka [22]. Highly coordinatively unsaturated bis(triisopropylphosphine)platinum (24b) can activate water very easily at room temperature to give the hydrido(hydroxo)... 

Scheme 6-2 Preparation of hydrido(hydroxo) and hydrido (hydroxide) complexes by oxidative addition of water...

In contrast with former opinions about the reaction mechanism in KF titration, more recent investigations by Verhoef and co-workers146 have shown that neither S02 nor a pyridine-S02 complex is oxidized by iodine in the presence of water, but the monosulphite ion ... 

In contrast the oxo-ruthenium complex c ,c -[ (bpy)2Runl(0H2) 2(//-0)]4+ and some of its derivatives are known to be active catalysts for the chemical or electrochemical oxidation of water to dioxygen.464-472 Many studies have been reported473 181 on the redox and structural chemistry of this complex for understanding the mechanism of water oxidation. Based on the results of pH-dependent electrochemical measurements, the basic structural unit is retained in the successive oxidation states from Rum-0 Ru111 to Ruv O Ruv.466... 

Several strategies to immobilize the p-oxo catalysts on an electrode surface or in a membrane have been employed. However, no available data about their efficiency as modified electrodes for water oxidation have been given.482-486 It should be noted that [ (bpy)2RuIII(OH2) 2(M C))]4+ is also an excellent electrocatalyst for oxidation of chloride to chlorine (better than for the oxidation of H20 into 02) at 1.20 V vs. SCE in 0.05 M HC1 solution,487 or at a modified electrode prepared by incorporation of the complex by ion-exchange into polystyrene sulfonate or Nafion films.482,4 8... 

The solution chemistry and water oxidation of a trinuclear complex [(bpy)2(H20)RuIIIORuIV (bpy)20Ruln(OI I2)(bpyh]6+ 489 and of a (nonisolated) binuclear Ru -terpyridine complex [ (terpy)(H20)2Rum 20] f+ 490 have also been reported. However, these complexes, as well as mononuclear [Ru(bpy)2(H20)2]2+,491 arenotcatalystsasaconsequenceoftheirfastdecomposition. 

Huang, R Kurz, R Styring, S. 2007. EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II. Appl. Magn. Reson. 31 301-320. 

Asymmetric alkylation. Deprotonation of (-)-l provides exclusively an (E)-enolate, which is alkylated to provide a single diastereomeric product. De-complexation by oxidation [Br, I2, Ce(IV)] in the presence of water provides the corresponding acid with the same configuration. This sequence has been used for synthesis of the drug (- )-captopril (3). In this case liberation of the acyl group in the presence of the amine provides the amide 2. 

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

Figure 38 Structural models of the manganese complex which constitutes the active site responsible for the water oxidation in WOC...

This behaviour does not allow the catalytic processes of water oxidation to be detected on the time scale of cyclic voltammetry. However, during exhaustive electrolysis at the potential of the second process (Ew— +1.38 V), catalytic generation of gaseous oxygen is observed with restoration of the original complex.55... 

Tetranuclear Manganese Complexes Modelling the Photosynthetic Water Oxidation Site... 

The starting material is an 18 electron nickel zero complex which is protonated forming a divalent nickel hydride. This can react further with alkenes to give alkyl groups, but it also reacts as an acid with hard bases to regenerate the nickel zero complex. Similar oxidative addition reactions have been recorded for phenols, water, amines, carboxylic acids, mineral acids (HCN), etc. 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

I have presented an overview of the current state-of-the-art in studies of the Mn complex in photosystem II. There are many unresolved questions and a clear picture of the structure and function of Mn in photosynthetic water oxidation is still not available. One useful approach to help determine the structure of the Mn complex in photosystem II involves the synthesis and characterization of Mn model complexes for comparison with the properties of the Mn complex in photosystem II. Recently, several tetrameric high-valent Mn-oxo complexes have been reported (see the chapter in this volume by G. Christou). Further characterization of existing and new high-valent tetrameric Mn-oxo model complexes, especially EPR and EXAFS measurements, will no doubt help clarify the present uncertain picture of the structure of the Mn complex in photosystem II. 

Mn, Mn S, 2Mn, 2Mn. Additionally, the putative S i level (28-30), accessible under certain conditions but not participating in the water oxidation cycle, would contain 2Mn, 2Mn. The synthetic Mn402 complexes would therefore have the... 

Two proposals for the water oxidation cycle Involving Mn/0 assemblies established in synthetic complexes have been published to date. These have attempted to describe the structural rearrangements of the Mn core and concomitant substrate binding... 

Figure 11. Proposed mechanistic scheme for water oxidation employing the Mn402 and Mn403 cores established in model complexes.

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