Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 322

The chemistry of cobalt-dioxygen complexes of hydrotris(pyrazolyl)-
borate ligands (Tp(R)) has been reviewed 204 with special reference to the
formation of low-valent metal-peroxo and high-valent metal-oxo species,
such as Co(II)-superoxo, alkylperoxo and dinuclear
complexes. In

the hydrotris(3,5-diisopropyl- 1 -pyrazolyl)borate ligand

system oxygenation of the proximal isopropyl substituents on

is mediated by the

and the alkylperoxocobalt

species.

17. CONCLUSIONS

The cobalt(II) species most widely used as catalysts in oxidations by
are complexes with salen, porphyrin, phthalocyanine, acac, dimethyl-
glyoximato, amine, pyridine, cyclidene and carboxylato ligands.
Low-spin cobalt(II) complexes are inherently reactive toward dioxygen
due to their d 7 electron configuration. Many of the 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)