Manganese redox chemistry

Johnson, K. S. 2006. Manganese redox chemistry revisited. Science 313 1896-1897. 

Previous studies (1-3) have shown that manganese redox chemistry in the oxygen evolving center (OEC) can be probed by flash-induced enhancements of the nmr relaxation rate of solvent protons (nmr-PRE). We report here an analysis of temperature profiles of the 1 and 2 flash nmr-PRE amplitudes at two field strengths, 0.48 and 1.08 Tesla. Results on model Mn complexes are also reported. 

A series of complexes have been reported of type [Mn2L(R C0A]C104, where LH2 is (211) (with R = OMe, H, F, Br for R = Me and R = Br for R = CF3). The electrochemical redox chemistry of these species was shown to be effectively controlled by the nature of R and R. The X-ray structure of the derivative with R = R = Me shows that each manganese(II) ion has a highly distorted octahedral geometry. 

Reaction of the manganese tropocoronand complex [Mn(tc-5,5)(NO)] with [Fe(tc-5,5)] results in complete transfer of the NO to the [Fe(tc-5,5)]. Other nitric oxide complexes appear in the sections on nitroprusside (Section S.4.2.2.6 above), on phthalocyanines (Section 5.4.3.7.4 above), and on polynuclear iron-sulfide complexes (Roussin s salts Section 5.4.5.9.2 below) Fe-por-phyrin-NO redox chemistry has been mentioned in Section 5.4.3.7.2 above. 

The redox chemistry of manganese is dealt with volt-equivalent diagrams and a description of the small amount of aqueous chemistry of Tc and Re follows. A volt-equivalent diagram for the oxidation states of Mn is... 

The first member of this family, manganese, exhibits One of the most interesting redox chemistries known thus it has already been discussed in detail above. Technetium exhibits the expected oxidation states, and associated with these are modest emf values. All of the isotopes of technetium are radioactive but "Tc has a relatively long half-life (2.14 k 10s years) and is found in nature in small amounts because of the radioactive decay of uranium. Oxidation slates of rhenium range from +7 to - 3, with some species ReOj and Re3+) unstable with respect to disproportionation. 

Finally, pyrazole binding to manganese(l) has been achieved by photoinduced substitution of CO, and the adjacent N4-coordination pocket can accommodate a second metal ion (Equation 12). The heterodinuclear MnZn complex 55 was characterized crystallographically and its redox chemistry investigated by spectroelectrochemical methods <2002JOMII3, 2005IC3863>. 

When the overall oxidation state of a system is desired, unless a water is obviously anaerobic (e.g., it has an H2S odor) one should first attempt to measure dissolved oxygen as an index of system redox state. Eh measurements are unlikely to be stable and thermodynamically meaningful in surface-waters, except in acid waters (where ferrous and ferric species are usually present). Eh measurements may be stable and meaningful in anaerobic sediments or groundwaters, when species of iron, sulfur, and manganese dominate the redox chemistry, but otherwise are of qualitative value only. 

Figure 6.5 Electron transfer reactions for sediments (Ruddy, 1993). CHjO.N.P (organic matter) is transformed by bacterial decomposition reactions to bicarbonate in the pore-waters. This is the primary source of electrons (and therefore energy) for the remainder of the sediment redox chemistry. Most of the primary flux of electrons may pass through the sulphur, iron and manganese cycles, but will eventually react with oxygen. Only a small part of the total electron flux will ultimately be buried as reduced minerals.

Each of the complexes described in this section have been synthesised, at least initially, by the serendipitous assembly of manganese salts or pre-formed clusters of Mn with a combination of one or more flexible organic bridging ligands. These species represent the vast majority of SMMs reported to date. Mn cluster chemistry involves complicated processes in which the crystallographically characterised products are almost certainly not the only complexes present in solution at any one time and where the synthesis also involves the protonation/deprotonation, redox chemistry and structural rearrangement of many species simultaneously. Thus, mechanistic, kinetic and any other detailed studies of the reaction processes are almost impossible. 

Only with iron has the redox chemistry in the porphyrin series been worked out in more detail than with manganese. It is mainly the groups of M. Calvin and L. J. Boucher who have investigated these metalloporphyrins, and extensive reviews are available from both [Boucher [11), Calvin [22)]. The interest of these complexes lies in the fact that manganese is always found in photosynthetically active plant material and seems to be active in the oxidation of water [Park [138)]. Because of... 

The mechanisms by which manganese complexes and manganese superoxide dismutase react with superoxide radicals are of interest as knowledge of the kinetic parameters and the reaction pathways may allow the synthesis of model compounds with specific chemical features. These compounds may then have clinical application or may allow the control of specific redox chemistry in catalytic processes. 

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