Oxidized iron species

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

Another interesting example of the exploitation of the relative inertness of the carbon surface is provided by the use of Fe/C catalysts in the hydrogenation of carbon monoxide. In this case, the inertness of the carbon surface facilitates the presence of zero-valent iron in the catalyst [32,33], which is more difficult for other supports, such as alumina, on which the reduction of the oxidized iron species is hindered. Vannice et al. carried out an extensive study of carbon-supported iron catalysts using different carbons and preparation methods and concluded that highly dispersed Fe/C catalysts could be prepared on high-surface-area carbons, due to the weak chemical interactions between oxidized iron precursors and the carbon surface [32-34]. 

The iron-modified ferrierite was prepared by impregnation method, followed by calcination [10S3]. The cationic Fe and oxidic iron species were identified. Monomeric and dimeric iron ions have been obtained, being active in oxidative dehydrogenation of ethane to ethene. The iron oxide nanoclusters are active too and overoxidize ethane and/or ethene to C, CO, and CO2. 

As for the effect of precipitation temperature, it has been reported that amorphous or smaller particles of a-FeOOH are stable in colloidal solutions at temperatures below 55 °C, while large (50-60nm) particles of hematite, c -Fe202, are formed above 85 °C (ref. 21). It seems likely that smaller particles of oxidized iron species in the precursor are reduced more easily, resulting in... 

Mechanistically, in approximately neutral solutions, solid state diffusion is dominant. At higher or lower pH values, iron becomes increasingly soluble and the corrosion rate increases with the kinetics approaching linearity, ultimately being limited by the rate of diffusion of iron species through the pores in the oxide layer. In more concentrated solutions, e.g. pH values of less than 3 or greater than 12 (relative to 25°C) the oxide becomes detached from the metal and therefore unprotective . It may be noted that similar Arrhenius factors have been found at 75 C to those given by extrapolation of Potter and Mann s data from 300°C. 

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

Fenton chemistry comprises reactions of H2O2 in the presence of iron species to generate reactive species such as the hydroxyl radical OH. These radicals ( = 2.73 V) lead to a more eflident oxidation chemistry than H2O2 itself (E° = 1.80 V). 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

The UV/Vis, Mossbauer, EXAFS, and EPR spectroscopic data suggest a rather complicated picture regarding the speciation of oxidized TAML species derived from 1 and various oxidants in aqueous solution (Scheme 5). Peroxides ROOH have the capacity to function as two-electron oxidants and usually do. In cases where prior coordination occurs, they can oxidize metal ions via one-electron processes where the 0-0 bond is cleaved homo-lytically or two-electron processes where it is cleaved hetero-lytically. The two-electron oxidation of 1 presumably would give the iron-oxo intermediate 6, two electrons oxidized above the iron(III) state (see below). Before 6 was actually isolated, there... 

Eqs. (19) and (20) were derived applying the steady-state approximation to the oxidized Fe-TAML species and using the mass balance equation [Fe-TAML] = 1 + [oxidized Fe-TAML] ([Fe-TAML] is the total concentration of all iron species, which is significantly lower than the concentrations of H2O2 and ED). The oxidation of ruthenium dye 8 is a zeroth-order reaction in 8. This implies that n[ED] i+ [H202]( i+ m). Eq. (19) becomes very simple, i.e.,... 

The ratio of ferrous to ferric species represents a redox state considerably less oxidizing than suggested by the dissolved oxygen content. The measured Eh falls between these values. Because the values vary over a range of more than 500 mV, this water clearly is not in redox equilibrium assuming that it is gives an incorrect distribution of iron species. 

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

In summary, the results of this investigation indicated that the formal oxidation of the nickel sites in a composite nickel-iron oxyhydroxide modifies the electronic and structural properties of the ferric sites yielding a more d-electron deficient iron species. Although it may be reasonable to suggest that the elec-trocatalytic activity of this composite oxide for oxygen evolution may be related to the presence of such highly oxidized iron sites,... 

Iron and its various oxidation state species are common components of the environment. In addition to the oxides FeO and Fe203, it is found in minerals such as hematite, goethite, and ferrihydrite, and in a number of hydroxy and oxy compounds. Because of its common occurrence in the environment in general, and in soil in particular, the total iron content of soil is usually not a useful piece of information. 

It is important to point out that D. vulgaris hydrogenase contains three multinuclear iron clusters and each cluster may exist in equilibrium between two different oxidation states in each sample. Consequently, the raw Mossbauer spectra are complex, consisting of overlapping spectra originating from different iron sites of these various clusters. For clarity, we present only the deconvoluted spectra of the H cluster. These spectra were prepared by removing the contributions of other iron species from the raw spectra. Details of the analysis are available (Pereira et al. 2001). 

Other examples of oxidant-iron(III) adducts as intermediates in iron porphyrin-catalyzed reactions have been published as listed in references 54a-k. Competitive alkene epoxidation experiments catalyzed by iron porphyrins with peroxy acids, RC(0)00F1, or idosylarenes as oxidants have been proposed to have various intermediates such as [(porphyrin)Fe (0-0-C(0)R] or [(porphyrin)Fe (0-I-Ar)]. Alkane hydroxylation experiments catalyzed by iron porphyrins with oxidant 3-chloroperoxybenzoic acid, m-CPBA, have been proposed to operate through the [(porphyrin)Fe (0-0-C(0)R] intermediate. J. P. CoUman and co-workers postulated multiple oxidizing species, [(TPFPP )Fe =0] and/or [(TPFPP)Fe (0-I-Ar)] in alkane hydroxylations carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where TPFPP is the dianion of me50-tetrakis(pentafluorophenyl)porphyrin. ... 

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