Phenoxyl radicals copper complexes

In a later article, complexes of Ni(II), Cu(II), Pd(II), and V02+ ions with the same tetra-substituted porphyrin were reported. Stepwise oxidation of these complexes gave products for which the authors proposed quinonoid, monoradical, and diradical structures. The most prolonged oxidations yielded the diradical products, which were isolated as dark purple crystals, relatively stable in air (40). The monoradical vanadyl complex was observed to be diamagnetic, suggesting antiferromagnetic coupling between the phenoxyl radical and unpaired electron on vanadium, whereas in the copper complex no such coupling was observed. More detailed studies of these systems seem warranted. 

The catalytic mechanism of GOase has been extensively studied (Fig. 8) 63,64). The primary alcohol first coordinates to the active species A, leading to the formation of the metal-phenoxyl radical complex B. This species undergoes proton abstraction from the substrate by the axial tyrosinate (Tyr495), followed by a rapid intramolecular electron transfer from the intermediate ketyl radical anion with reduction of Cu to Cul The copper(I) species C reacts with dioxygen to form the hydroperoxo copper(II) complex D with the liberation of the aldehyde. Finally, dihydrogen peroxide is released to give back the active form of the enzyme. 

Ethanol, benzyl alcohol or 2-butanol coordinate to the proposed dinuclear copper complex (Fig. 16) to produce the active species I (Fig. 17, A). After H abstraction by one of the two phenoxyl groups, a radical is created which is oxidized to the corresponding carbonyl derivative by the second phenoxyl radical. In the case of 2-propanol and diphenylmethanol, two alcohol molecules coordinate to the dinuclear moiety resulting in species II (Fig. 17, B). The H abstractions by the two phenoxyl ligands generate two... 

To mimic the function of GAO, a large number of copper(II)-phenoxide complexes have been synthesized, and these catalyze the aerobic oxidation of alcohols to aldehydes and/or ketones via formation of intermediate Cu(II)-phenoxyl radical complexes, followed by production of H2O2 as a coproduct. Stack and coworkers reported the first functional models (Fig. 11) (116,117). Wieghardt and co-workers reported on a dinuclear Cu(II)-phenoxyl complex, which can catalyze the oxidation of primary alcohols as well as secondary alcohols to ketones (118). Futhermore, Wieghardt and co-workers have synthesized various mononuclear copper complexes with imino-, thio-, and semiquinonate ligands, which affect similar chemistry (118-120). 

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. 

Recently, a somewhat different synthetic approach has been reported. Halcrow et al. (215) synthesized a series of five-coordinate copper(II) complexes comprising a tridentate tris(pyrazolyl)borate ligand and a bidentate phenol derivative. Neutral complexes [Cun(TpPh)(bidentate phenolate)] were synthesized and structurally characterized [Tpph] = hydrido-tris(3-phenylpyrazol-l-yl)borate. The species [Cun(TpPh)(2-hydroxy-5-methyl-3-methylsulfanylbenzaldehydato)] can electro-chemically be converted to the (phenoxyl)copper(II) monocation, which has been characterized in solution by UV-vis spectroscopy. It displays two intense absorption maxima at 907 nm (e = 1.2 x 103 M 1 cm-1), and 1037 (1.1 x 103 M l cm-1), resembling in this respect the radical cofactor in GO (Fig. 7). 

Case studies concerned phenoxyl and other radicals, nitroxide radicals, the copper proteins, plastocyanin and azurin, calculation of zero field splittings in organic triplets and diradicals, and a model Mn (II) complex, Mn(acac)3 (acac acetylacetonate). In the case of plastocyanin, substantial agreement has been achieved between theory and experiment. For another copper protein, azurin, excellent results for nitrogen hyperfine constants were obtained. With regard to Mn (II), while the correct sign of D was obtained, its magnitude was too low, and theoretical reasons for this are indicated. 

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