COPPER MONOOXYGENASE MODELS - AROMATIC HYDROXYLATION BY A DINUCLEAR COPPER(I) COMPLEX CONTAINING METHIONINE SULFUR LIGANDS

A dinuclear copper(I) complex 1 containing a bis(imine) ligand derived from the condensation between benzene-1,3-dicarbaldehyde and two mole cules of L-methionine has been prepared. When this compound reacts wit h dioxygen a partial aromatic hydroxylation of the ligand occurs, givi ng a dinuclear mu-phenoxo-mu-hydroxo-dicopper(II) complex 2, together with simple copper oxidation products. Definitive evidence of the mono oxygenase activity of the present sulfur-containing model system resul ts from the crystallographic characterisation of the dinuclear copper( II) complex 3 of the hydroxylated dicarbaldehyde, [Cu-2{C6H3(CHO)(2)O} -(ClO4)(2)], which forms upon hydrolysis of the imine groups of 2. In this complex two deprotonated 1,3-diformylphenoxide ligands bind two c opper(II) ions, with di-mu-phenoxo bridges. Each copper is essentially square pyramidal, with a basal O-4 donor set, including two phenoxide and two carbonyl oxygen atoms from two 2-hydroxybenzene-1,3-dicarbald ehyde ligands. Two perchlorate oxygen atoms are bound in axial positio ns on opposite sides of the Cu2O6 plane. A minor fraction (15-20%) of 2 contains S-oxygenated methionine residues. However, oxygenation at s ulfur is a secondary process, resulting from the reaction of H2O2, for med according to the simple copper(I) oxidation pathway, and the dinuc lear copper(II) complex 2.