Metal Coordinated Phenoxyl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 281

8

Metal Coordinated Phenoxyl Radicals

Fabrice Thomas
University of Grenoble, Department of Molecular Chemistry, Grenoble, France

8.1 Introduction

The term “phenoxyl” radical was first introduced in 1914 by Pummerer 1 to designate species involved in
the oxidation of naphthols and phenanthroles. It was not until the 1960s that the existence of phenoxyls was
demonstrated by Electron Paramagnetic Resonance (EPR). The term “stable” is commonly used to designate
radicals such as nitroxides, trityls, and so on. Although the term “stable” had been associated with some
phenoxyl radicals in 1967, 2 it must be realized that phenoxyl radicals exhibiting stabilities comparable
to those of nitroxides were rather rare at this time. During the 1970s, Reichard et al . 3 demonstrated
that radicals related to phenoxyls could also arise from mono-electronic oxidation of the phenolic side
chain of tyrosines in some proteins, and thus could be involved in biological processes. The new term
“tyrosyl” was then introduced to designate these residues, and many other biological systems involving
such radicals have been described. 4,5 One of the more important recent advances in tyrosyl radical history
was achieved in the 1990s with the characterization of the galactose oxidase (GO) active site. 6-8 For the
first time it was demonstrated that tyrosyls could exist coordinated to a metal ion. To better understand
this association, chemists have subsequently developed many complexes involving coordinated phenoxyl
radicals. 9-13 Elucidation of the properties of coordination compounds involving redox active ligands,
especially those involving sterically hindered phenolates, is one of the most recent and fascinating topics
of interest in bioinorganic chemistry. The challenge in this kind of chemistry is the right description of the
electronic structure of the M n + - OPh entity (where OPh represents a phenolate group) once it has been
oxidized by one electron. In principle, either the M ( n + 1 ) + - OPh, that is the oxidation is metal based, or
the M n + - • OPh form, that is the oxidation is ligand based, could be obtained. The right description is thus
not obvious and, as will be shown below, it strongly depends on the nature of the metal ion, the denticity
of the ligand, the substituents of the phenolate precursor and even the temperature. Coordination is also
found to improve the radical stability, and exerts an extraordinary control on their magnetic properties and
reactivity (regio- and stereoselective oxidations are promoted by a radical).