Phenoxyl radicals zinc complexes

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... 

The key feature of the [Cu(II)(L)] (L = dianionic tetradentate ligand) complexes is their distorted non-square-planar structure, deduced from an x-ray structure and solution EPR spectra. One-electron oxidation (i.e., using N0 BF4 ) leads to EPR silent Cu(II)-phenoxyl radical complexes, as corroborated by Cu K-edge x-ray absorption spectra and EPR spectra on analogous one-electron oxidized zinc(II) phenoxyl radical complexes. The radical cation complexes [Cu(II)L] bind alcoholates in a 1 1 stoichiometry to form pentacoordinate [Ci II)L( OR)] alkoxide species, and for R = PhCHg, anaerobic conversion to benzaldehyde and a copper(I) complex occurs. 

Two phenoxyl radical complexes [Cu (2 )N03] and [Zn (2 )N03] oxidize benzyl alcohol to benzaldehyde and have been studied as models for the enzyme galactose oxidase (GO). GO contains a dipeptide unit (3) in which a tyrosine residue is covalently bound to an adjacent cysteine residue and which is similar to (2), the tyrosyl radical in (3) also being bound to the Cu centre (see Figure 1). Second-order kinetics were observed with respect to [Zn°(2 )N03]+ and there was no evidence of redox reaction at the zinc site, suggesting that a dimeric form of the complex is active however, the reaction of [Cu H2 )N03]+ with benzyl alcohol is first order in the metal complex and [Cu (2H)]+ is identified as a product, suggesting a formal 2e /2H+ mechanism in which the monomeric form coordinates the alcohol in the manner believed to operate for G0. 2... 

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