Three-Coordinate Copper(I) Amido and Aminyl Radical Complexes

Abstract

Electron transfer (ET) through proteins often utilizes copper-containing active sites as efficient one-electron relays. The type-1 active sites of the blue copper proteins are prominent examples. It is generally thought that high ET rates through type-1 redox sites occur because the protein environments enforce unusual trigonally distorted coordination spheres to allow for minimal structural reorganization during ET. Though large Cu^II/Cu^I self-exchange ET rate constants (kS) in the range observed for type-1 sites have been achieved in certain synthetic monocopper systems using geometries distinct from trigonal environments, ET studies have yet to be conducted in a synthetic system featuring isolated, trigonally disposed copper centers. The simplest such systems would contain a trigonal planar geometry. Here we report the structural characterization of a trigonal planar system featuring formally Cu^II and Cu^I amido complexes related by a reversible one-electron redox event. We find in this system that ET is extremely rapid and is accompanied by a small degree of structural reorganization during redox. We propose that this structural rigidity in the absence of secondary coordination sphere effects results from significant covalency of the copper-amide linkages. In fact, a CuI-aminyl radical description of the formally Cu^II-amide complex may be most appropriate.

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