Galactose oxidase models:: Tuning the properties of CuII-phenoxyl radicals

Four tripodal ligands with an N3O coordination sphere were synthesized: (2-hydroxy-3-tert-butyl-5-nitrobenzyl)bis(2-pyridylmethyl)amine (LNO2H) (2-hydroxy-3-tert-butyl-5fluorobenzyl)bis(2-pyridylmethyl)amine (LFH), (2-hydroxy-3,5-di-tert-butylbenzyl)bis(2-pyridylmethyl)amine (LtBuH) and (2-hydroxy-3-tert-butyl-5-methoxybenzyl)bis(2-pyridylmethyl)amine (LO-MeH). Their square-pyramidal copper(II) complexes, in which the phenol subunit occupies an axial position, were prepared and characterized by X-ray crystallography and UV/Vis and EPR spectroscopy The phenolate moieties of the copper(II) complexes of LtBuH and LOMeH were electrochemically oxidized to phenoxyl radicals. These complexes are EPR-active (S = 1), highly stable (k(decay)-0.008min(-1) for [Cu-II (LOMe*) (CH3CN)](2+)) and stoichiometrically oxidise benzyl alcohol. Two additional tripodal ligands providing an N2O2 coordination sphere were also studied: (2-pyridylmethyl)(2-hydroxy-3-tert-butyl-5-methoxybenzyl)- (2-hydroxy-3-tert-butyl-5-nitrobenzyl)amine (L'OMeNO2H2) and (2-pyridylmethyl)bis(2-hydroxy-3-tert-butyl-5- methoxy)benzylamine (L'OMe2H2). Their copper(II) complexes were isolated as dimers ([Cu-2(II)(L'OMe2)(2)], [Cu-2(II)(L'OMeNO2)(2)]) that are converted to monomers on addition of pyridine. The complexes were investigated by X-ray crystallography and UV/Vis and EPR spectroscopy. Their one-electron electrochemical oxidation leads to copper(II)-phenoxyl systems that are less stable than those of the N3O complexes. The N2O2 complexes are more reactive than the N3O analogues: they aerobically oxidize benzyl alcohol to benzaldehyde at a higher rate, as well as ethanol to acetaldehyde (40-80 turnovers).