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

Abstract

Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining 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 the 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 M–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 non-covalent interactions, such as intramolecular hydrogen bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage of C–H bonds by M–oxo complexes with basic oxo ligands is described.

Biological Outer-Sphere Coordination

“…In the last decade, biological OSC motifs have guided synthetic efforts. The substantial progress in this arena has been reviewed with an emphasis on dioxygen activation in a recent forum article by Shook and Borovik [123]. Examples will be presented with an emphasis on strategy, rather than a quantitative discussion of reactivity.…”

“…Chang and coworkers reported hydrogen-bond functionalized derivatives of N,N-bis(2-pyridylmethyl)-bis (2-pyridyl)methylamine (N4Py) that provide an acid-and base-stable platform for dioxygen reduction via isolable Fe II -and Fe III -OH intermediates [129]. Borovik and coworkers have explored the effects of varying the numbers of hydrogenbonding groups in tripodal metal complexes [123]. Variable efficacies of dioxygen and C-H activation have been demonstrated among families of complexes distinguished principally by the number of hydrogen bonds available to axial oxygen species.…”

Formation and Reactivity of a Biomimetic Hydroperoxocopper(II) Cryptate

“…Such second-sphere interactions have been shown to be key factors in stabilizing reactive intermediates in metal-dioxygen chemistry. [8,11] In addition, the cryptand arms are based on benzyl linkers, which mimic the benzylic position that undergoes hydroxylation in the substrate of DβH.…”

Mononuclear–Dinuclear Equilibrium of Grafted Copper Complexes Confined in the Nanochannels of MCM‐41 Silica

“…It is well known that in both metalloproteins and synthetic systems the control of the secondary coordination spheres, which are normally associated with noncovalent interactions, is also necessary to achieve highly functional metal complexes. [29][30][31][32] Because noncovalent bonds, such as hydrogen-bonding interactions, are generally weak interactions, they tend to be difficult to control in confined nanospaces. These difficulties have led to noncovalent interactions often being overlooked in the design of nano-confined metal complexes.…”