Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 266

1. INTRODUCTION
Various cobalt (II) complexes, usually referred to as synthetic dioxygen
carriers, are known to interact reversibly with dioxygen under ambient
conditions, affording predominantly dioxygen complexes of different types,
which have been extensively reviewed 1-6 . The ranges of O-O bond lengths in
the most common structural types are shown in Table 1 6 .
In its triplet ground state the reactivity of dioxygen, having two
unpaired electrons in its degenerate antibonding orbitals, is controlled by the
rules of spin conservation. The kinetic barrier imposed by these conditions is
sufficiently large to ensure survival of all living organisms despite their
thermodynamic instability toward oxidation in an atmosphere containing
dioxygen.
The kinetic inertness of dioxygen can be overcome by coordination to a
metal center, which eliminates the restrictions of spin conservation. The
interaction of with a transition metal ion surrounded by suitable ligand(s),
i.e. dioxygen complex formation, can be broadly regarded as of two major
types, viz., (i) reversible binding (oxygenation), and (ii) activation of
dioxygen. Reversibly bound can be removed from the complex by
pumping or flushing with an inert gas ( Ar), and this cycle can be
repeated many times over. However, there is always a gradual loss of
reversibility, as demonstrated for cobalt(II)-based synthetic dioxygen
carriers. This is due to irreversible oxidation of the central metal ion
and/or the ligand(s) surrounding the metal. In the presence of a
suitable external substrate, its catalytic oxidation may take place, leading to a
catalytic cycle, in which the activated dioxygen species is the key
intermediate. Substrate oxidation and catalyst regeneration should occur in
the successive cycles. The catalytic activity usually decreases with time and
is ultimately lost due to metal centered and/or ligand based irreversible
oxidations.