Iron complexes reduction rates

Relative reduction rates of ferrioxamine, hexacyanoferrate(III) and iron(III) complexes of edta and citrate by ferri-reductase have been established. ... 

There are only a few cases where the dissolution of an iron oxide by all three types of processes under comparable conditions has been investigated. Banwart et al. (1989) found that at pH 3, the rate of dissolution of hematite increased in the order, protonation < complexation < reduction with a factor of 350 between the extremes. A similar factor (400) was found for goethite (Zinder et al, 1986) (Fig. 12.15). Hematite dissolution processes were also compared in the pH range similar to that found in neutral environments (Fig. 12.16). Again, dissolution by simple protonation was extremely slow, whereas reduction, especially when aided by Fe complexing ligands, was particularly effective (Banwart et al, 1989). It can, thus, be concluded that reduction, particularly when assisted by protonation and complexation will be the main mechanism for Fe transport in global ecosystems. 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Porphyrin Basicity. The reduction rate is decreased significantly as the porphyrin is made more basic (Figure 6). The bis-cyanide complex of ferric octaethyl porphyrin, the more basic porphyrin, is reduced the slowest while the bis-cyanide complex of ferric TPP, the least basic (16), is reduced the fastest. Increasing the porphyrin basicity places more electron density on the iron, making it more difficult to accept another electron. 

Rate constants have been determined for the reduction of hydrogen peroxide by iron(II) and a number of iron(II) complexes. These rate constants have been compiled in Table 2. It is immediately clear that there is not much agreement between the results of various groups. However, there is a discernable trend metal complexes with more water-accessible coordination sites react faster. Graf et al. [117] have commented upon the importance of coordinated water molecules for the Fenton reaction. It is also clear that the rate of the Fenton reaction for a chelated complex near neutral pH is much faster than that of aqueous iron(II) at low pH. The use of the low-pH value of 16M ] s l in a recent calculation [118] of the flux of hydroxyl radicals in a cell gives an estimate that is at least two orders of magnitude too low. 

We envisioned that at higher 1-MeIm concentration the hyponitrite hgand may be displaced in which case the EPR active (OEP)Fe(l-MeIm)2 complex could form. Quite interestingly, the latter complex was not observed in the EPR spectrum. Thus, it is likely that the presence of the N-donor histidine at the axial position of the heme iron complex may play a role in speeding up the rate of reduction of NO to N2O in NORs. 

The rate of reduction of cytochrome c by the cobalt-substituted analogue of rubredoxin is a factor of 2.25 lower than the rate with rubredoxin itself. Both proteins mediate the reduction of cytochrome c in the presence of NADH and the decrease in the rate is attributed to the decreased efficiency in oxidation of cobalt(ii) compared with iron(ii). Reduction of cytochrome c by NADPH is catalysed by an adrenodoxin reductase-adrenodoxin complex in which the rate-determining step is electron transfer from the flavin (FAD) of the reductase to the FeaS2 centre of adrenodoxin. The pH dependence of the rate shows a pATa of 6.75, with the high-pH form 27.5 times more reactive than the low-pH form. Both NADPH reduction of the complex and cytochrome c oxidation of the complex were faster than the catalytic rate. Catalytic roles of four iron-sulphur centres in trimethylene dihydrogenase and ferredoxin nitrite reductase have also been examined. Synthetic analogues of four iron ferredoxins have also attracted much attention. - ... 

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