Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 267

2. COBALT DIOXYGEN COMPLEXES

The majority of cobalt dioxygen complexes can be classified as of the
mononuclear superoxo type 7 .

or dinuclear

For other types the reader is referred to the review literature 1-7 .
Dioxygen complexes are often formed reversibly and can be
characterized by equilibrium constants. In catalytic systems usually enters

the catalytic cycle via equilibria of the types (1) and (2):

These reactions can be regarded as autoxidation of although
both steps (1) and (2) are substitution (complex formation) reactions. True
autoxidation with loss of an electron by occurs on protonation of the

dimer, leading to the formation of as in the case of

The cobalt-bound (activated) dioxygen exhibits higher reactivity toward
certain substrates than does free In kinetic studies the above equilibria
may be treated as rapid pre-equilibria, which maintain a near constant
concentration of the active intermediate(s).
The rate constants for binding are usually very large and kinetic
studies require the stopped-flow technique, in which a solution of the cobalt
complex prepared under an inert gas is rapidly mixed with an
solution. The reaction is then monitored spectrophotometrically. The
problem of inert storage was eliminated in the case of
cobaloxime(II), by preparing the cobalt complex in the mixing chamber
from and cobalt perchlorate 9 . This was made possible by the fact that
the formation of is much faster than its subsequent reaction

with

Recently, a new method has been reported for studying rapid
biological reactions involving dioxygen 10 . It was used to investigate the
reduction of dioxygen to water by cytochrome c oxidase. Photolysis of
a synthetic caged dioxygen
carrier, produces dioxygen in situ on a nanosecond or faster time scale. This
avoids complications due to the fate of photodissociated CO in a
conventional CO flow-flash experiment.
The kinetics of dioxygen binding to the cobalt(II) complexes of