Manganese cubane

Fig. 63. A manganese cubane cluster with a corner oxygen atom replaced by a chloride. [Reproduced with permission from (327). Copyright 1993 the American Chemical Society.]...

Action of chlorine trifluoride causes incandescence [1]. Manganese dioxide catalytically decomposes powerful oxidising agents, often violently. Dropped into cone, hydrogen peroxide, the powdered oxide may cause explosion [2], Either the massive or the powdered oxide explosively decomposes 92% peroxomonosulfuric acid [3], and mixtures with chlorates ( oxygen mixture , heated to generate the gas) may react with explosive violence [4], Cuban pyrolusite can be used in place of potassium dichromate to promote thermal decomposition of potassium chlorate in match-head formulations [5],... 

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. 

Riittinger, W., Yagi, M., Wolf, K., Bernasek, S. and Dismukes, G.C. (2000) O2 Evolution from the Manganese-Oxo Cubane Core MnA6 A Molecular Mimic of the Photosynthetic Water Oxidation Enzyme J. Am. Chem. Soc. 122, 10353-10357. 

Figure 52 shows the structure of [Mn4" (HL33)(L33)(NCS)4], which can also be viewed as either a dimer-of-dimers or a severely distorted butterfly. The links between the two macrocycles are two triply bridging alkoxide donors, one from each macrocycle. The wings of the butterfly have closed to bring the two outer manganese ions quite close to one another an extension of this movement would result in formation of a cubane. 

The disk-like Mn molecule, whose structure is schematically depicted in Fig. 1, has crystallographic S4 symmetry. The four manganese(iv) ions are found in a central cubane unit, which is surrounded by a ring of eight... 

There are now five examples of this metal polyhedron in the literature [2, 3, 20, 21], all containing manganese, with one example where the cental cubane contains Fe surrounded by an Mn ring. There is therefore a contrast between Mn and the other elements between Cr and Ni, but the polyhedra are related when the constituent fragments are considered. 

In pursuit of suitable models and theories for the WOC manganese cluster, Noodleman and Hendrickson 140) have used LCAO Xa density function calculations, employing the broken symmetry method (see Section III,E,3), to calculate J(Mn Mn ) and J(Mn Mn ) values in the chloro-oxo cubane [Mn Mn sOsClYlOaCCHalg]. The /a/T behavior of this complex was different in detail from the others described earlier, and good agreement with was obtained using the Xa calculations, with parameter values of = 2.02, e/(Mn - -Mn ) = -t-19.3 cm", J(Mn - -Mn ) = -46.6 cm", and Sf = f. 

The results of the XAS and EPR data, combined with knowledge from model complexes, (discussed later) allow for the formulation of structural models of the OEC. Many manganese compounds have been proposed as structural models for the OEC, ranging from butterfly arrangements to adamantanes to cubanes (26, 27,29-34, 50). Most of those proposals, however, are inconsistent with the known structural data for the OEC. Currently the most widely favored model is the dimer of dimers model (Scheme 11), first suggested by Klein,... 

---