Phenoxyl radicals electronic transitions

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

It is well known that benzophenone generates a biradical through n-ir electronic transition under irradiation ( 340 nm). Irradiation of a mixture of 1,4-benzoquinone (34) and aromatic aldehydes in the presence of benzophenone generates 2-aroyl-l,4-dihydroxybenzene (35) [47-49]. This reaction comprises of the abstraction of a formyl hydrogen atom of an aromatic aldehyde by the oxygen-centered radical of the benzophenone biradical to form an aroyl radical and a 1,1-diphenylhydroxymethyl radical, and addition of the nucleophilic aroyl radical to 1,4-benzoquinone (34) to form a phenoxyl radical derivative, which finally abstracts a hydrogen atom from an aromatic... 

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... 

As expected, the potentials decreased (phenolates more readily oxidized) when methoxy substituents were present (R = OMe, Table 1). Successive generation of species with one, two, and three phenoxyl radicals was supported by UV-vis spectroscopy on MeCN solutions generated by controlled potential coulometry at —30°C. Intense bands between 400-430 nm were observed that are particularly diagnostic of phenoxyl radicals (tt tt transition). Proof for coordination of the phenoxyl radical(s) to the metal ion in these compounds was obtained through EPR spectroscopy. For example, X-band EPR spectra of the 5 = 1/2 1-electron oxidized species [(L M]+ (1+) and (3" ), M = Sc and Ga respectively) and analogs deuterated at the benzylic positions showed hyperfme coupling to the metal ion nuclei 7=3/2, 0.23/0.29 mT " Sc, 7=7/2, 0.12mT). ... 

Cellobiose dehydrogenase (CDH) preferentially oxidizes cellobiose, the product of cellulose hydrolysis by cellulases. A variety of substrates can be used as electron acceptors, including quinones, transition metals, phenoxyl radicals, and O2 (Figure 1C, 14). A possible secondary reaction is the production of Fenton reagent resulting in the hydroxyl radical. 

Alkoxyl and phenoxyl (RO ) radicals are generated in vivo from complexed transition metals (M) and organic hydroperoxides (e.g. Upid hydroperoxides) via catalysed electron transfer reactions ... 

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