Manganese tetranuclear

Because of the large number of carbonyl groups required to satisfy the noble gas rule and the relatively small metallic radius, serious steric limitations are present for manganese tetranuclear closed clusters12). The only known tetranuclear compounds are some mixed clusters, MnOs3(CO)16H and MnOs3(CO)13H3 (see later), which have been prepared from Os3(CO)12 and [Mn(CO)s] 161). 

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. 

The importance of manganese for bacteria, such as that of Ni and to a lesser extent Co, as we saw in the last chapter, is considerable. Of course, as we will see shortly, it is also important in the tetranuclear Mn cluster that is involved in oxygen production in photosynthetic plants, algae and cyanobacteria, as well as in a number of mammalian enzymes such as arginase and mitochondrial superoxide dismutase. Most of manganese biochemistry can be explained on the one hand by its redox activity, and on the other by its analogy to Mg2+ (reviewed in Yocum and Pecoraro, 1999). 

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

The search for inorganic compounds that can act as model systems of the tetranuclear manganese centre of photosystem II, responsible for the oxidation of water, has led to the characterization of a number of complexes of diverse nature and geometry. 

Derivatives of Diamantoidal Geometry. Figure 42 shows the tetrahedral (diamantoidal) geometry assumed by the four manganese atoms in complex [Mn406(tacn)4]4+ (tacn = 1,4,7-triazacyclononane), which was the first tetranuclear manganese complex to be structurally characterized.56,57... 

Figure 5 Tetranuclear manganese cluster core structures.

Tetranuclear manganese(III) carboxylate complexes have three different configurations for their metal centers fused open cubane, planar, or butterfly (see Figure 22). There is only one example of the first type namely [Mn4Q2(OAc)2(BSP)2] (105). The Mn Mn separations in (105) vary between 2.875(1)A and 3.122(1)A. Variable temperature magnetic measurements for (105) indicated weak antiferromagnetic interactions (/=—10.0cm and / = —3.7cm ) between the manganese(III) centers. Cyclic voltammetry of (105) in methanol shows one quasireversible oxidation wave at 0.01 V and two quasireversible reduction waves at about —0.4 V and —0.7 V vs. SCE. 

Figure 22 Core types observed for tetranuclear manganese(III) complexes.

Butterfly tetranuclear manganese(III) carboxylate complexes are monoanionic with the exception of the bpy complex. An example of a butterfly core complex is (Bu 4N)[Mn402(02CPh)7-(imac)2]-5CH3CN (108). 

Table 16 Structural parameters of selected tetranuclear manganese(III)-Schiff base complexes. ...

There are only trinuclear and tetranuclear manganese(III) complexes reported with hexadentate Schiff base ligands. Two such hexadentate ligands, bamen and Saltren (Figure 19), form unique core structures. 

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. 

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],... 

The enzyme responsible for the photolysis of water in plants is a multisubunit membrane protein (Fig. 5-22 Klein et al., 1991). Four manganese ions, probably as a tetranuclear cluster, are thought to act as a charge accumulating system and as the active site for water oxidation. Both calcium and chloride ions are also required for activity (Babcock, 1987 Ghantokakis and Yocum, 1990). The water oxidation centre (WOC) contains a total of four Mn atoms and causes the oxidative coupling of two water molecules by a currently unknown mechanism. 

The tetranuclear magnesium chelate complexes [(NH4)4n Mg4(L10 n)6 ] (29a,b) were first synthesized by reaction of dialkyl malonate 28, methylmagnesium iodide, and oxalyl chloride, followed by workup in aqueous ammonium chloride solution [102-105]. Now methyllithium/magnesium chloride instead of methylmagnesium iodide (direct method) is used, which by mere replacement of magnesium chloride by the chlorides of manganese, cobalt, and nickel also allows the synthesis of the corresponding tetranuclear complexes 29 (with Mn = Mn2+, Co2+, Ni2+) [103, 105]. 

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