Manganese complexes water

The catalytic application of clays is related closely to their swelling properties. Appropriate swelling enables the reactant to enter the interlamellar region. The ion exchange is usually performed in aquatic media because the swelling of clays in organic solvents, and thus the expansion of the interlayer space, is limited and it makes it difficult for a bulky metal complex to penetrate between the layers. Nonaqueous intercalation of montmorillonite with a water-sensitive multinuclear manganese complex was achieved, however, with the use of nitromethane as solvent.139 The complex cation is intercalated parallel to the sheets. 

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. 

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. 

It is noted that the redox and EPR spectroscopic characteristics of these dimeric manganese complexes have led them to acquire some importance in the formulation of inorganic models able to mimic the manganese centre involved in the photosynthetic oxidation of water (discussed in the following section). 

There is evidence that it is a manganese complex that acts as a mediator in supplying the electrons [through the Mn(II)/Mn(III)/Mn(IV) redox cycle] necessary to return the photo-oxidized chlorophylls back to their reduced state. The manganese centre is able to provide the four electrons produced in the oxidation of water in four successive steps. 

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

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

Manganese represents the epitome of that characteristic property of the transition element namely the variable oxidation state. The aqueous solution chemistry includes all oxidation states from Mn(II) to Mn(VII), although these are of varying stability. Recently attention has been focused on polynuclear manganese complexes as models for the cluster of four manganese atoms which in conjunction with the donor side of Photosystem(II) is believed involved in plant photosynthetic oxidation of water. The Mn4 aggregate cycles between 6 distinct oxidation levels involving Mn(II) to Mn(IV). 

Manganese complexes capable of oxidizing water have been prepared and investigated. 

Although these cobalt species do not in themselves activate dioxygen, they do exhibit similarities to the proposed structures of manganese complexes that may be involved in photosynthetic water oxidation... 

The mechanism of the reaction of the alcohol (or water) with the acyl complex to produce ester (or acid) and regenerate the cobalt hydride complex is not known. Because the reaction of the analogous manganese complex with alcohols is known to proceed through a hemiacetal-like complex, this mechanism has been written for the carboxylation reaction (equation 42). 

In order to prevent the reduction between iron(II) and formaldoxime occurring, another iron complexing agent (potassium cyanide) was used in the presence of a reductant (ascorbic acid) that reduces iron(III) to iron(II). Aluminium, titanium, uranium, molybdenum and chromium also form light-coloured complexes that normally do not interfere in the determination of manganese in water or plant material by this method. If the aluminium or titanium concentrations are higher than 40 ppm an additional masking flow of tartrate is recommended [31]. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Han G, Li J, Chen G, et al. Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes. J Photochem Photobiol B Biol 2005 81 114-20. 

Magnuson A, Liebisch P, Hogblom J, et al. Bridging-type changes facilitate successive oxidation steps at about IV in two binuclear manganese complexes - implications for photosynthetic water-oxidation. J Inorg Biochem 2006 100 1234-3. 

Beckmann K, Uchtenhagen H, Berggren G, et al. Formation of stoichiometrically lsO-labelled oxygen from the oxidation of lsO-enriched water mediated by a dinuclear manganese complex - a mass spectrometry and EPR study. Energy Environ Sci. 2008 1(6) 668-76. 

Carrell et al. (2002) XAS, ESR Manganese complex and cocomplexes in Structure and dynamics of a molecular active site + — + Water splitting... 

Meinike, C., Sole, A., Pospisil, P., Dau, H. (2000) does the structure of the water-oxidizing Photosystem II-Manganese complex at room temperature differ from its low-temperature structure A comparative X-ray absorption study, Biochemistry 39, (24)... 

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