Manganese complex model

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

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). 

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

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). 

Much of the impetus for the investigation of polynuclear manganese complexes has come from a desire to produce structural and/or functional models of the manganese cluster of the OEC. Many of the results of these studies have been described in Section 5.1.2 and in reviews. ... 

Stewart s conclusion underscores the need for short-wavelength, low-temperature studies, if very high accuracy electrostatic properties are to be evaluated by Fourier summation. But, as pointed out by Hansen (1993), the convergence can be improved if the spherical atoms subtracted out are modified by the k values obtained with the multipole model. Failure to do this causes pronounced oscillations in the deformation density near the nuclei. For the binuclear manganese complex ( -dioxo)Mn(III)Mn(IV)(2,2 -bipyridyl)4, convergence of the electrostatic potential at the Mn nucleus is reached at 0.7 A" as checked by the inclusion of higher-order data (Frost-Jensen et al. 1995). 

The case study of the tetranuclear manganese complex presented above and the specific examples of structure/spectroscopy correlations have established the validity of the proposed methods and set the stage for more ambitious applications. The first such application has been the recent evaluation of several structural models of the OEC in the S2 state ( 1 3) of the Kok cycle (107). Twelve structural models were considered, 10 of which were based on Mn405Ca core topologies derived by polarized EXAFS spectra. Figure 19 shows one of the models included in the set. 

The mechanism of the epoxidation of alkenes by the cytochrome P450 model, sodium hypochlorite-manganese(III) tetraarylporphyrins, involves rate-determining formation of an active species 234 from a hypochlorite-manganese complex 233 (Scheme 6) pyridine or imidazole derivatives, as axial ligands, accelerate this step by electron donation, although the imidazoles are destroyed under the reaction conditions368. 

Several manganese complexes have recently been proposed as structural and functional models of SOD. 

Many manganese complexes decompose dihydrogen peroxide, but we limit our discussion to the functional dinuclear ones the catalase activity of bis-dinuclear (tetranuclear) photosystem II (PSII) models is discussed later. Furthermore, mainly model complexes reported in the last 5 years are discussed in detail since previous work is covered in several excellent reviews [1,2b,3a,5,8-12],... 

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. 

Proserpio, D.M., Rappe, A. K., and Gorun, S.M. (1993) Theoretical modeling of the mechanism of dioxygen activation and evolution by tetranuclear manganese complexes, Inorg. Chim. Acta 213, 319-24. 

Dinuclear Manganese Complexes as Models for the Manganese Catalase. As discussed previously, the manganese catalase has a dinuclear active site that is thought to function by cycling redox states between Mn(II)2 and Mn(III)2. Although the [Mn(IV)(salpn)(/z2-0)]2 chemistry nicely explains the alternate catalase reactions of the OEC, this system is an inappropriate model for the Mn catalase because the redox cycle in that enzyme is lower and the core structure is believed to be dramatically different. In fact, a [Mn(III/IV)(/z2-0)]2 superoxidized state of the Mn catalase has been identified and shown to be inactive. 

The oxidation of water to 02 in photosynthetic systems in plants has been greatly studied13 as have synthetic models such as those of manganese complexes the oxidation seems most likely to proceed via H202 rather than OH- radicals and probably two or four manganese centers are involved.14... 

Active Sites Iron Proteins with Dinuclear Active Sites Manganese The Oxygen-evolving Complex Models Oxidation Catalysis by Transition Metal Complexes Oxygen Inorganic Chentistry. 

Based in part on the article Manganese Oxygen-Evolving Complex Models by Lars-Erik Andreasson Tore Vanngard which appeared in the Encyclopedia of Inorganic Chemistry, First Edition. 

Manganese Inorganic Coordination Chemistry Manganese The Oxygen-evolving Complex Models. 

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