Biomimetic copper complexes

Abstract

A series of mono-, bi-, tri- and tetranuclear copper complexes have been synthesized, developing a general method for the preparation of multidentate ligands, potentially useful for modelling multicopper protein active sites. Complexation to Cu(I) was accomplished for several cases, yielding copper(I) complexes that have been characterized in solution by NMR spectroscopy and conductivity, and in the solid state by elemental analysis, mass spectrometry, and/or X-ray crystallography. The dicopper(I) complex [Cu2(MeL66)O2]2+(where MeL66 is the hexadentate ligand 3,5-bis-bis-[2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino-methylbenzene) reacts reversibly with dioxygen at low temperature to form a ¹-peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from -80 to -55 ±C yielded the activation parameters for the forward and reverse reaction, respectively, and the binding parameters of O2. The hydroxylation of a series of p-substituted phenolate salts by [Cu2(MeL66)O2]2+studied in acetone at -55 ±C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant ½=-1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 ±C for a range of sodium p-cyanophenolate concentrations. The rate plots were hyperbolic and enabled to derive the activation parameters for the monophenolase reaction and the binding parameters of the phenolate to the ¹-peroxo species. Thus, the complete set of kinetic and thermodynamic parameters for the two separate steps of O2 binding and phenol hydroxylation have been obtained for [Cu2(MeL66)O2]2+.