Phenoxyl Radical–Metal Complexes

Abstract

Abstract The redox chemistry of metal complexes has been widely developed in recent years, affording deep insights into the reaction mechanisms for many useful homogeneous catalytic reactions and enzymatic reactions at the active site of metalloenzymes. During the course of the studies, a large number of novel complexes have been born and well characterized; species formed as a result of redox reactions such as organic and other radicals are especially important not only in inorganic but also in other fields of chemistry. In the early 1990s, it was discovered that the phenoxyl radical can bind with a metal ion as an open‐shell ligand performing a role of an organic radical cofactor in a metalloenzyme, galactose oxidase (GO), which is a single copper oxidase catalyzing two‐electron oxidation of the primary alcohol to the aldehyde. After the discovery of the radical coordination in the active center, many metal?phenoxyl radical complexes have been synthesized and characterized for understanding the detailed mechanism of GO and the properties of the metal complexes with the coordinated phenoxyl radical. This chapter focuses on the synthesis, structures, properties and reactivity of almost all of the metal‐phenoxyl radical complexes. The first few sections describe historical aspects and general properties of the metal‐phenoxyl radical complexes, including a comparison of their properties with those of the high valent metal‐phenolate complexes which are iso‐electronic with the phenoxyl radical complexes. In later sections, the properties of the metal coordinated phenoxyl radical are described in the order of the periodic table.