Cobalt-oxygen complexes

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

But one must not be too facile with sweeping generalizations concerning concentration dependences or their absence. For example, consider the reaction between the cobalt(II) complex known as Co(sep)2+ and molecular oxygen. With these reagents only, the two-step reaction in acidic, aqueous solution is5... 

Although coordination of the heterocyclic nitrogen does not occur, two cobalt(II) complexes of 3-hydroxy-5-hydroxymethyl-2-methyl-4-formylpyridine have been isolated with stereochemistry [Co(34-H)A] 2H2O (A = NO3, OAc) [173], For both complexes coordination is ONS (deprotonated phenolic oxygen), but magnetic or electronic spectral data are not included. 

It has been noted (2) that many oxygen-carrying cobalt(II) complexes involve ligands that provide four donor atoms lying in the same plane as the cobalt atoms, such as salen , porphyrin and dimethylglyoxime, DMG, and related ligands. 

A calorimetric study has been made (97) of the oxygenation of the cobalt (II) complexes formed with histidine, histamine and ethylenediamine in aqueous media at 25 °C. 

We have seen, in the previous section (and section III.A), that cobalt (Salen) and its active derivatives normally form diamagnetic peroxo type I dioxygen adducts. However, a pyridine solution of Co(3-methoxy Salen) has been shown 137) to take up dioxygen with 1 1 (Co O2) stoichiometry it was found 137) that there was no significant IR absorption band, attributable to the 0—0 stretch, for the oxygenated complex, and this suggested that the dioxygen is symmetrically bonded in an unidentate manner,... 

The given structure shows two molecules of TTA to have reacted with a cobalt ion to form the cobalt-TTA complex, in which the cobalt atom forms a valence bond solid lines) with one, and a coordinate bond (broken lines) with the other, oxygen atom of each TTA molecule. Thus, in the cobalt-TTA complex there is a six-membered ring formed by each TTA molecule with the cobalt atom. Metal chelate complexes of this type have good stability, they are nonpolar and soluble in the organic phase. The usefulness of the chelating extractants in solvent extraction is therefore obvious. 

In isolation the electron distribution in the trivalent chromium (III) ion consists of three unpaired electrons in the d shell, as indicated in line (a) of Table 5.1. In line (b) the six electron pairs donated to the central chromium atom by oxygen atoms of water molecules give rise to sp3d2 hybridisation. This is characteristic of an octahedral structure. A similar situation arises with the trivalent cobalt(III) complex in line (e), where each of the three t2g levels is doubly occupied by an electron pair from each cyano ligand. 

Many cobalt(II) complexes are very reactive towards dissolved oxygen. The reasonably complex kinetic behavior of these systems can usually be interpreted in terms of a simple two step mechanism (8, 10) ... 

Electron paramagnetic resonance (continued) cobalt-thermolysin complex, 28 334, 335 exchange reactions, 31 106-107 glutamine synthetase, 28 358-364 invisible oxygen species, 31 94-95 metalloenzymes, 28 324, 326 metal particle size distribution, 36 99-100, 104... 

Cobalt catalyst precursors are cobalt(III) or cobalt(II) salts ligated to nitrogen and eventually oxygen-containing polydentate molecules like B12, salen, C2(DO)(DOH)p . The III/II electroreduction occurs at around OV vs SCE. Further reduction at ca. — 1 V vs SCE corresponds to the formation of cobalt(I) complexes which are the reactive species involved in the reactions mentioned below. 

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