Phenoxyl radical metal complexes

A variety of phenoxyl radical metal complexes have been generated in situ by oxidation of the precursor phenol metal complex. Characterization of the resulting radical metal complexes has been mainly by EPR... 

The potentially sexadentate macrocyclic ligands l,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L H3), l,4,7-tris(3,5-di-t-butyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L"H3), and l,4,7-tris(3-t-butyl-5-methoxy-2-hydroxybenzyl)-l,4,7-triazacyclononane (L" H3) form the stable complexes of type [Mn L"]PF(5, [Mn L" ]PF(5 [M L "] [Mn L ]2(C104)3-(H30)(H20)3. These complexes were investigated by spectroelectrochemistry and were shown to undergo metal- and ligand-centered redox processes a phenoxyl radical Mn complex, [Mn L" ] +, was found to be accessible. 

The generation, stability, and function of tyrosyl radicals in ribonucleotide reductase, PGH synthase, and galactose oxidase continue to be active areas of research. The difficulties encountered in preparing and handling these proteins, as well as in probing the physical properties and reactivity of their metal-phenoxyl radical active sites, make the preparation and investigation of stable phenoxyl radical metal model complexes an attractive goal. 

Since then, chemists have attempted to synthesize model complexes that contain coordinated and uncoordinated phenoxyl radicals. This chapter reviews this literature from 1994 to early 2000. Earlier efforts describing the chemistry of extended radical-metal ion systems such as those derived from ligands shown in Fig. 1 and their metal complexes and oxidation products (3-13) have been reviewed recently by Goldberg and Lippard (14). The coordination chemistry of these interesting ligands remains to be firmly established as does its redox chemistry. 

In an attempt to establish unequivocally the spectroscopic features of coordinated (vs uncoordinated) phenoxyl radicals a series of phenolato precursor complexes containing a spectroscopically and redox-innocent Ga(III), Sc(III), or Zn(II) central metal ion were synthesized (142-148). In order to avoid metal-ligand bond dissociation in solution, the phenolate or, after one-electron oxidation, phenoxyl moieties were covalently attached to the strongly metal ion binding 1,4,7-triazacy-clononane (149) backbone. Thus a series of phenolate pendent-arm macrocyclic... 

Extensive studies of synthetic complexes comprising a phenoxyl radical coordinated to a metal ion have provided detailed insights into geometric features, spectroscopic signatures, electronic... 

The most thoroughly studied class of phenoxyl radical complexes was prepared by oxidation of first-row transition metal complexes of the type of macrocyclic phenolate ligands shown in Figure 4. Studies of those compounds that contain Ga , Sc , and have been particularly useful for differentiating and establishing the properties of coordinated vs. uncoordinated phenoxyl radicals. Due to their closed shell nature, these ions are both spectroscopically silent and redox inactive, thus enabling the properties of phenoxyl radicals in their complexes to be seen unmasked from those of the metal ion. Data for oxidized forms of the complexes [L M] ((l)-(4) are illustrative). These compounds undergo three reversible one-electron oxidations at approximately equally spaced... 

The metrical details of a coordinated phenoxyl radical have been determined through an X-ray crystal structure of [L(° 9Ci-](Q04)-3MeCN the only other metal-phenoxyl radical complex to be structurally defined by X-ray diffraction is a Cu species, which is discussed in Section... 

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