Role of the Secondary Coordination Sphere in Metal-Mediated Dioxygen Activation

Alfred Werner proposed nearly 100 years ago that the secondary
coordination sphere has a role in determining the physical properties
of transition-metal complexes. We now know that the secondary coordination
sphere impacts nearly all aspects of transition-metal chemistry, including
the reactivity and selectivity in metal-mediated processes. These
features are highlighted in the binding and activation of dioxygen
by transition-metal complexes. There are clear connections between
control of the secondary coordination sphere and the ability of metal
complexes to (1) reversibly bind dioxygen or (2) bind and activate
dioxygen to form highly reactive metal−oxo complexes. In this
Forum Article, several biological and synthetic examples are presented
and discussed in terms of structure−function relationships.
Particular emphasis is given to systems with defined noncovalent interactions,
such as intramolecular H-bonds involving dioxygen-derived ligands.
To further illustrate these effects, the homolytic cleavage of C−H
bonds by metal−oxo complexes with basic oxo ligands is described.