Copper monooxygenase models. Aromatic hydroxylation by a dinuclear copper(I) complex containing methionine sulfur ligands

Abstract

A dinuclear copper(I) complex 1 containing a bis(imine) ligand derived from the condensation between benzene-1,3-dicarbaldehyde and two molecules of L-methionine has been prepared. When this compound reacts with dioxygen a partial aromatic hydroxylation of the ligand occurs, giving a dinuclear µ-phenoxo-µ-hydroxo-dicopper(II) complex 2, together with simple copper oxidation products. Definitive evidence of the monooxygenase activity of the present sulfur-containing model system results from the crystallographic characterisation of the dinuclear copper(II) complex 3 of the hydroxylated dicarbaldehyde, [Cu₂{C₆H₃(CHO)₂O}(ClO₄)₂], which forms upon hydrolysis of the imine groups of 2. In this complex two deprotonated 1,3-diformylphenoxide ligands bind two copper(II) ions, with di-µ-phenoxo bridges. Each copper is essentially square pyramidal, with a basal O₄ donor set, including two phenoxide and two carbonyl oxygen atoms from two 2-hydroxybenzene-1,3-dicarbaldehyde ligands. Two perchlorate oxygen atoms are bound in axial positions on opposite sides of the Cu₂O₆ plane. A minor fraction (15–20%) of 2 contains S-oxygenated methionine residues. However, oxygenation at sulfur is a secondary process, resulting from the reaction of H₂O₂, formed according to the simple copper(I) oxidation pathway, and the dinuclear copper(II) complex 2.