Manganese complexes inner sphere

Collins et al. (1999a) found that Hg2+ sorbed to goethite as an iimer-sphere bidentate complex. Cheah et al. (1998) found that Cu " " sorbed to amorphous silica and Y-AI2O3 as monomeric and monodentate iimer-sphere surface complexes. However, bidentate complexes may also form on Y-AI2O3. Using polarized EXAFS, Dahn et al. (2003) determined that Ni " " sorbed to montmorillonite edge sites as an inner-sphere mononuclear surface complex. Inner-sphere surface complexes were observed with XAS for Cr " " adsorption on manganese (Manceau and Charlet, 1992) and iron oxides (Charlet and Manceau, 1992). 

The manganese(iii) oxidation of methanoP has been reported. The reaction, which takes place via an inner-sphere mechanism, is first order in both oxidant and substrate. The effects of other cations have been examined, catalysis by cobalt(n) and aluminium(iii) being exhibited with retardation of the reaction specific to manganese(n). Inner-sphere complexes have also been detected in the corresponding reaction with pinacol in acidic perchlorate media. The oxidation of a-hydroxyisobutyric acid, however,... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

At variance with the aqua ion, in most manganese(II) proteins and complexes the contact contribution to relaxation is found negligible. This is clearly the case for MnEDTA (Fig. 33), the relaxivity of which indicates the presence of the dipolar contribution only, and one water molecule bound to the complex 93). Actually the profile is very similar to that of GdDTPA (see Chapter 4), and is provided by the sum of inner-sphere and outer-sphere contributions of the same order. The relaxation rate of MnDTPA is accounted for by outer-sphere relaxation only (see Section II.A.7), no water molecules being coordinated to the complex 94). 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

A number of other metal complexes can decompose hydrogen peroxide via reactions analogous to Eqs. (45) and/or (46), including those of cerium813 b copper,823 b cobalt,833 b manganese,84 and silver.85 Many of these electron transfer reactions are thought to proceed via inner-sphere complexes of metal-hydrogen peroxides (M—OOH).84 86... 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

Manganese(ii) forms both inner- and outer-sphere complexes in hydrochloric acid solutions. (279) For 1-5mF[C1, the species Mn(H20)e and Mn(H20)6 Cl are present in solution in relative amounts 3 2. An outer sphere complex has been suggested by the spectral data for the binding of Mn with the polymeric humic acid, fulvic acid. (280) This is in contrast to the corresponding Fe complex which is of the inner sphere type. 

Finally, it can be stated that many reactivity patterns of free radical ions are equally found in oxidative and reductive transformations involving initial inner-sphere ET, such as in reactions with samarium iodide [389], low valent titanium [390] and titanocene complexes [391], manganese(III) [392], and CAN [393]. 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

Fig. 1. Manganese to phosphorus distances in inner sphere, distorted inner sphere, adjacent pyrophosphate and second sphere complexes from crystallographic and model building studies. Original references are given in (14—16). Corresponding distances in Ca2+ complexes would be 0.2 A greater and in Mg2+ complexes 0.1 A smaller...

The coupling constant is inconsistent with carboxyl coordination but consistent with carbonyl coordination 15). Similar data for -ketobu-tyrate 15) and oxalacetate (19) have been fit by exchange contributions (1/tm) and inner sphere contributions Tm and T2m)- The rates of formation of these metal bridge complexes from an outer sphere complex ( 3,4) are limited predominantly by the rate of dissociation of a water molecule from the coordination sphere of the enzyme-bound manganese (Figure 3, Table V) (15,19), as required by the Sj l-outer sphere mechanism of Eigen and Tamm (20),... 

Besides the superoxide dismutation mechanism, the reactivity of metal centers, in particular manganese complexes, toward NO is very much dependent on the possibility for binding a substrate molecule. As it will be shown later, the possibility that MnSOD enzymes and some mimetics can react with NO has been wrongly excluded in the literature, simply based on the known redox potential for the (substrate) free enzymes, mimetics, and NO, respectively. Therefore, the general fact that, upon coordination, redox potentials of both the metal center and a coordinated species are changed should be considered in the case of any inner-sphere electron-transfer process as a possible reaction mechanism. 

Thus, 1 seems to be a true catalyst rather then a new kind of free radical initiator. This behavior is in contrast to the behavior of related manganese complexes. For example, Mn(II) carboxylates are known to decompose CHP during autoxidation of cumene l dinuclear Mn(III) complexes decompose tetralin hydroperoxide during oxidation of tetralin (an inner-sphere Mn-alkyl hydroperoxide intermediate has been proposed) trinuclear, carboxylate and oxo-bridged complexes containing Mn(II) were found to decompose CHP during the catalyzed oxidation of cumene. 

Three papers have dealt with manganese(n) systems. Gale and co-workers proposed a simple method to estimate the inner-sphere hydration state of the Mn(ii) ion in coordination complexes and metal-loproteins. The method makes use of the 0 linewidth measurements for bulk water in the presence and in the absence of Mn(n), which allows the determination of transverse 0 relaxivity. Doing this as a function of temperature and finding the maximum yields a quantity which is directly proportional to the number of inner-sphere water ligands. Molnar et... 

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