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

Four tripodal ligands with an N(3)O coordination sphere were synthesized: (2-hydroxy-3-tert-butyl-5-nitrobenzyl)bis(2-pyridylmethyl)amine (LNO(2)H), (2-hydroxy-3-tert-butyl-5- fluorobenzyl)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 (LOMeH). 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)=0.008 min(-1) for [Cu(II)(LOMe(.))(CH(3)CN)](2+)) and stoichiometrically oxidise benzyl alcohol. Two additional tripodal ligands providing an N(2)O(2) coordination sphere were also studied: (2-pyridylmethyl)(2-hydroxy-3-tert-butyl-5-methoxybenzyl)(2-hydroxy-3-tert-butyl-5-nitrobenzyl)amine (L'OMeNO(2)H(2)) and (2-pyridylmethyl)bis(2-hydroxy-3-tert-butyl-5- methoxy)benzylamine (L'OMe(2)H(2)). Their copper(II) complexes were isolated as dimers ([Cu(2II)(L'OMe(2))(2)], [Cu(2II)(L'OMeNO(2))(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 N(3)O complexes. The N(2)O(2) complexes are more reactive than the N(3)O analogues: they aerobically oxidize benzyl alcohol to benzaldehyde at a higher rate, as well as ethanol to acetaldehyde (40-80 turnovers).