Oxygen activation by metal complexes

Article By:

Bakac, Andreja Department of Chemistry, Iowa State University, Ames, Iowa.

Last reviewed:2015

DOI:https://doi.org/10.1036/1097-8542.YB150549

Molecular oxygen (or dioxygen, O₂) is a powerful oxidant. It is also safe and abundant and would therefore seem to be a perfect candidate as an oxidant in both industry and the laboratory. However, oxidations of organic substrates by O₂ are extremely slow. In fact, the very existence of life on Earth depends on oxygen's inability to utilize its thermodynamic power efficiently to burn (that is, oxidize) organic matter. The reason for the discrepancy between large thermodynamic potential and slowness of the reaction lies in the fact that the overall four-electron reduction of O₂ to water must take place in a series of smaller steps, the first of which is the least facile and therefore slow. The use of molecular O₂ as an oxidant for practical purposes is feasible only if the kinetic barriers are lowered. This can be accomplished in various ways, but the most successful approach is based on the use of transition-metal complexes (TMCs) as O₂ activators. In the process of activation, TMCs interact with O₂ and generate new compounds that are more reactive than O₂ itself. TMCs cover an enormous range of substitution rates and reduction potentials, and many have multiple oxidation states that can be utilized in the reaction with oxygen or with various intermediates generated along the way to final products, as described below.

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