Nitrosyl complexes oxidative processes

The intrinsic kinetics of the reactions taking place in the scrubber, i.e. the reaction of NO with the iron chelate forming an iron nitrosyl complex (eq. 1) and the undesired oxidation reaction of the iron chelate (xanpla (eq. 2) wae deteimined in dedicated stirred cell contactors. Typical process conditions were T = 25-55 °C [Fe"(EDTA) "] = 1-100 mol/m [NO] = 1-1000 ppm pH = 5-8 and an oxygen level ranging between 1 and 20 vol%. 

In the process, the iron is reduced to the ferrous form. Ferric cytochrome c is reduced by nitric oxide through a nitrosyl intermediate to produce ferrous cytochrome c and nitrite (Orii and Shimada, 1978). The nitrosyl cytochrome c absorbs at 560 nm, which is slightly higher than the 550-nm peak observed for reduced cytochrome c. Nitric oxide may be an interference in the assay of superoxide from cultured cells by the cytochrome c method. When nitric oxide reacts with cytochrome c, there is an initial decrease in absorbance at 550 nm as the nitrosyl complex is formed followed by a rise in absorbance as the complex decomposes to nitrite and reduced cytochrome c. This is a potential artifact in studies measuring the release of superoxide from cultured endothelial cells or other cells that make nitric oxide. 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

Nitrosyl complexes have also been synthesized using hydroxylamine. Although the mechanism of this process is not well understood, hydroxylamine complexes are known.92 Therefore it is possible that oxidation or deprotonation of the coordinated NH2OH group could occur, leading to the products obtained in equation (17). 

In general, the initially formed formyl, hydroxycarbonyl or nitrito-carbonyl complexes rapidly convert to hydrido or nitrosyl complexes with elimination of CO or C02 however, occasionally intermediates have been isolated. The oxidation of coordinated CO to C02 by amine oxides (Figure 3.15) may be considered a further example. The hydroxycarbonyl example underpins a technologically important process, the water-gas shift equilibrium, involving the catalytic conversion of CO and water into CO, and hydrogen (Figures 3.22, 3.11 and 2.13). 

We have previously mentioned in Section 4.2 the chemistry developed by Selhnann etal, by using tetradentate or pentadentate S4 ligands. The use of such a ligand in nitrosyl ruthenium chemistry allowed the first conversion of a nitrosyl complex into a ruthenium HNO complex (31) by addition of NaBILi to [Ru(NO)(py S4)]Br. The formation and decomposition of HNO complexes is often invoked in many processes such as combustion of ftiels, oxidation of N2, reduction of HNO2, and so on. ... 

Moreover, the environmental systems demonstrate unique diversity and versatility of the processes depending on the actual conditions, viz. the reaction directions and rates are sensitive to many diverse parameters, sometimes even difficult to be perceived. The example may be the dependence of the photocatalytic activity of Sr—A1—Nb—0 double perovskite on the cation ordering in the oxides (267), or the effect of the in-plane twist of the quinoline-based co-ligand on the thermal stability and yield of NO photorelease from the [Ru(NO)] nitrosyl complexes (96). 

Both the chloroiron and phenyliron octaalkylcorrolates can be oxidized electrochemically by two or three one-electron processes, and reduced by two one-electron processes.One-electron reduction of each produces the simple [XFe Corr], while one-electron oxidation of each produces the [XPe Corr -J+jr-cation radicals. The nitrosyl complex of iron (III) octaethylcorrolate has been formed and both it and its 1-electron oxidized product have been characterized structurally and electrochemically. 

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