Reaction mechanisms into iron complexes

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

This behavior, as well as complementary observations, can be explained on the basis of the reaction mechanism depicted in Scheme 5.3. The main catalytic cycle involves three successive forms of the enzyme in which the iron porphyrin prosthetic group undergoes changes in the iron oxidation state and the coordination sphere. E is a simple iron(III) complex. Upon reaction with hydrogen peroxide, it is converted into a cation radical oxo complex in which iron has a formal oxidation number of 5. This is then reduced by the reduced form of the cosubstrate, here an osmium(II) complex, to give an oxo complex in which iron has a formal oxidation number of 4. 

In reality, the oxidation of pyrite and other Fe(II) sulfides typically involves several intermediate reactions, which may be enhanced by microbial activity or various chemical species, such as bicarbonate (HCO3-) (Welch et al., 2000 Evangelou, Seta and Holt, 1998). The exact mechanisms of each intermediate reaction are often very complex and poorly understood (Rimstidt and Vaughan, 2003). Mostly likely, sulfide oxidizes in pyrite before iron. Fe(II) is then released into solution as shown by the following reaction involving oxygen and water (Gleisner and Herbert, 2002, 139-140) ... 

Chem. 29 (1967), pp. 1637-1642. Spectroscopic studies of die reaction of hexa-cyanoferrate(III) in water and ethanol. 3.3x 10 4 M Fe(NC>3)3 were exposed with a cyanide excess of likewise 3.3x10 mol l"1. With pH values of approximately 10, all the Fe2[Fe(CN)6] was converted into Iron Blue within 48 hours. Cyanate, die anticipated product of die oxidation of die CN-, could not, however, be proven. Perhaps this is further oxidized directly into C02. If this mechanism is assumed, die result, purely stoichiometrically, is that an alkaline environment must be favorable. This finding is supported by die known fact that hexacyan-oferrate(III) is a strong oxidation agent in alkaline medium and is even able to oxidize divalent chrome to hexavalent, therefore, that is, CN ions must have oxidized very quickly J.C. Bailar, Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, Oxford 1973, p. 1047. An overly alkaline environment would, however, disturb die complexing of the Fe3+- ion by cyanide, which is then displaced by OH- (Fe(OH)3 then occurs as a by-product) and/or the latter can hardly be displaced from die iron. 

Imhof et al. [22] studied the reaction mechanism of the [2+2+1] cycloaddition reactions of diimines, CO, and ethylene catalyzed by iron carbonyl complexes on the basis of density functional theory (Scheme 4). The catalytic reaction does not start when CO dissociates from 10 followed by the addition of ethylene, but instead the associative pathway to 11 is proposed. In addition, it can be concluded that the insertion of CO in 11 takes place into a C-Fe bond but not... 

Methyl ethyl ketone, for example is oxidized under mild conditions in the presence of a number of metal complexes in aqueous solution [271-274]. The products of reaction are acetaldehyde and acetic acid. Komissarov and Denisov [272-274] have shown that an iron(III)-o-phenanthroUne complex [272] and a copper(II) pyridine complex [274] catalyze this reaction. In the proposed reaction mechanisms [272, 274] it is suggested that the enolate ion from the ketone is incorporated into the coordination sphere of the metal complex where electron transfer occurs to yield a radical which is attacked by dioxygen, equation (188). In the absence of molecular oxygen, aqueous iron(III) is capable of further oxidizing the radical to form butane 23-dione, equation (189) [271]. 

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