Organic-ligand oxygen-transfer

The main problem to be overcome with these organic-ligand oxygen-transfer processes is the lifetime of the carrier, which is measured in weeks and not months or years, although improvements are continuing. 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

As described below, the metal insertion into the salen ligand with acetic acid as solvent was determined to be fast and not the rate-limiting step of the global reaction. The activation step was then studied to determine if a pure chemical resistance limited the reaction rate or if oxygen transfer from the gas phase into the liquid mixture controlled the reaction. The kinetic rate of oxidation should be independent of the rate of oxygen introduction into the organic liquid phase under chemical control, but directly related to the rate of oxygen introduction under mass transfer limitations. 

One of the problems with much of the work on P450 models is that reactions are performed in organic solvents in which it is not possible to obtain detailed information about the reaction mechanisms involved. This is because the proton activity in organic solvents is not easily determined. It is only in aqueous solution that the conditions necessary for oxygen transfer, such as ionic strength, acidity, and ligand species concentration, are best controlled, and data (e.g. electrochemical and kinetic) are best interpreted. To that end, water-soluble iron and manganese tetraarylporphyrins have been prepared by Bruice et al. and their reactions with hydroperoxides studies. ... 

The factors responsible for the enormous variations in rate when bifunctional organic ligands mediate in electron-transfer reactions are by no means understood, but in the case of simple monocarboxylato-ligands the details of the adjacent-attack mechanism seem particularly well worked out. Sykes has reviewed the contribution that dinuclear complexes have played in this. In all simple monocarboxylato-com-plexes of the type (5) the point of Cr + attack is the carbonyl oxygen and its accessibility is determined by the size of R. The conformation (5) is more strongly demanded... 

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