Manganese chemistry

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). 

We begin this overview of manganese biochemistry with a brief account of its role in the detoxification of free radicals, before considering the function of a dinuclear Mn(II) active site in the important eukaryotic urea cycle enzyme arginase. We then pass in review a few microbial Mn-containing enzymes involved in intermediary metabolism, and conclude with the very exciting recent results on the structure and function of the catalytic manganese cluster involved in the photosynthetic oxidation of water. 

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As the oxidation state of manganese increases, the basicity declines, eg, from MnO to Mn20y. Oxyanions are more readily formed ia the higher valence states. Another characteristic of higher valence-state manganese chemistry is the abundance of disproportionation reactions. 

Another important line of investigation concerned the carbonyl insertion reaction, which was best defined in manganese chemistry (75, 16) and extended to acylcobalt tetracarbonyls by Heck and Breslow. The insertion may be through three-membered ring formation or by nucleophilic attack of an alkyl group on a coordinated CO group. 

Sun, L. Hammarstrom, L. Akermark, B. Styring, S. 2001. Towards artificial photosvthesis Ruthenium-manganese chemistry for energy production. Chem. Soc. Rev. 30 36 19. 

CHRISTOU AND VINCENT Oxide-Bridged Manganese Chemistry... 

A second structural motif in binuclear manganese chemistry is the triply bridged structure in which one /x-oxo unit has been replaced by two bridging carboxy-lates (8 in Sect. 16.1.4). The remaining coordination sites on each metal are commonly occupied by a tridentate tripod or a macrocyclic ligand such as HB(pz)3 ... 

Hsiung, T.-M. The Manganese Chemistry at Lake Richard B. Russell. Dissertation, Clemson University, Clemson, SC, 1990. 

The relative positions of the H20—02 boundary and the Mn2+—Mn02 boundaries for both 10"3 and 10"7M activities of aqueous Mn2+ indicate that for pH values greater than about 4, Reaction 58 is spontaneous. Similarly, the pure solid phases MnCOa and Mn(OH)2 are unstable with respect to oxidation to MnO >. Extensive interpretations of manganese chemistry in terms of the thermodynamic properties of the oxides and on other solid phases and solution species of manganese can be found in the recent literature (3,14, 24). 

Hammarstrom L (2003) Towards artificial photosynthesis ruthenium-manganese chemistry mimicking photosystem II reactions. Curr Opin Chem Biol 7 666-673... 

Thus, in its broad features, manganese chemistry reflects its position in the middle of the transition series. 

The story of the coordination chemistry of manganese is very incomplete it is only at an early stage of development compared with most of the other transition metals and there is an obvious need for X-ray structural techniques to be used much more routinely in those studies of manganese chemistry that rely on the isolation and characterization of solid compounds for their purpose. 

The marked lability of manganese(II) species is undoubtedly one of the reasons for the neglect of manganese chemistry. This lability, often coupled with low stability of species (see Irving-Williams Series)29 ensures that there is little correlation between solutions and solids, and adds an uncertainty to this area of chemistry that is largely absent from the chemistry of inert compounds. 

We do not distinguish the various ideal polyhedra. Constraints within the molecules usually ensure that some intermediate shape is adopted. bThe tris(ethylnitrosolato)manganese(II) anion (P. Gouzerh, Y. Jeannin, C. Rocchiccioli-Deltcheff and F. Valentini, J. Coord. Chem., 1979, 6, 221) has been described as a low spin species but the details given are unique in manganese chemistry, and need further confirmation. 

L.C. Sun, L. Hammarstrom, B. Akermark, S. Styring, Towards Artificial Photosynthesis Ruthenium-Manganese Chemistry for Energy Production , Chem. Soc. Rev., 30,36 (2001)... 

Laxen D. P. H., Davison X., and Wook C. (1984) Manganese chemistry in rivers and streams. Geochim. Cosmochim. Acta 48, 2107-2111. 

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