Cobalt complexes atom-transfer substitution reactions

The possibility that substitution results from halogen-atom transfer to the nucleophile, thus generating an alkyl radical that could then couple with its reduced or oxidized form, has been mentioned earlier in the reaction of iron(i) and iron(o) porphyrins with aliphatic halides. This mechanism has been extensively investigated in two cases, namely the oxidative addition of various aliphatic and benzylic halides to cobalt(n) and chromiumfn) complexes. 

Reaction 11 involves hydrogen atom transfer as proposed by Halpern et al. (13) in the mechanism of formic acid oxidation by cobalt (III) in aqueous solutions. In this reaction one could consider that as peracetic acid approaches the coordination sphere of Co111 and transfers the hydrogen atom to the coordinated acetate, the Co111 atom is transformed into a Co11 complex of peracetoxy radical (or Co111 complex of peracetate anion). Complexes of free radicals with metal ions have been postulated by Kochi (16). The substitution rate in this complex could be intermediate between the rate of substitution of cobalt (III) and cobalt (II) complexes owing to the contribution of the resonance structures ... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Most reactions of coordination compounds can be classified as either substitutions or oxidation-reductions. The classic book by Basolo and Pearson19 discusses both types in detail. The oxidation-reductions can occur either by simple electron transfer or by atom transfer. Taube s work on the reduction of cobalt(IJI) complexes by Cr11 is especially important in this regard. Among the many reactions which he has studied, the best known, perhaps, is20... 

Low-spin cobalt(ll) complexes are inherently reactive toward dioxygen due to their d electron configuration. Many of the O2 binding reactions are very fast, therefore, few rate constants have been determined and special techniques such as stopped-flow or flash photolysis had to be employed in kinetic work. The superoxocobalt(III) species formed are in most cases the active intermediates in catalytic reactions, which occur readily with substituted phenol or aniline derivatives, producing quinone type dehydrogenation products. The key step in catalytic cycles is often H-atom abstraction or electron transfer to the superoxo complex from the substrate, which is converted to a free radical, possibly reacting further with cobalt(III)... 

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