Synthesis and Characterization of Copper(II) Complexes Containing Sulfur/Nitrogen Donor Sets. Mimicking the Function of Phenoxazinone Synthase and Catechol Oxidase

Copper(II) complexes of the pentadentate pyridine based ligand (L1) containing two thiolate sulfur, pyridine nitrogen and two azomethines nitrogen, in addition to a series of tetradentate ligands (L2, L3 and L4) containing two thiolate sulfur, and two azomethine nitrogens have been synthesized (scheme 1). The pure isolated complexes have been characterized by elemental analyses; electrolytic conductance, magnetic susceptibility measurements, FT-IR, UV–Vis and ESR spectroscopy and their structures have been confirmed by thermal analysis (TGA and DTG) investigations. All these green colored copper(II) complexes are hydrated containing one coordinated water molecule. The solid and solution states investigations show that the complexes are neutral. The pyridine based Schiff base (L1) acts as dibasic N3S2 chelating ligand coordinating to Cu(II) center and the coordination geometry around copper(II) center of [CuL1H2O] (1) is a distorted octahedron. The five coordinated copper(II) complexes [CuLnH2O] of the dibasic tetradentate ligands L2, L3 and L3 having the coordination chromophore N2S2O exhibit structural features similarity. The coordination geometries around copper(II) centers in (2), (3) and (4) are described as square pyramidal stereochemistry based upon the EPR and crystal filed spectral results. The electrochemical properties behaviors of the chelates have been studied by cyclic voltammetry technique. Mimicking the function of phenoxazinone synthase and catechol oxidase enzymes investigations demonstrated that, the title complexes are promising functional models of these oxidase metalloproteins containing copper. Oxidative coupling of 2-aminophenol (OAPH) to 2-aminophenoxazine-3-one (APX) (phenoxazinone synthase activity) and the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCH2) to the light absorbing 3,5-di-tert-butyl-benzoquinone (3,5-DTBQ) (catechol oxidase activity), catalyzed by the title copper(II) complexes were studied. The rate constants of the above oxidation reactions showed linear dependence on catalyst concentration and saturation kinetics with respect to the corresponding substrate. Electrochemical data revealed a non-linear relationship between the complexes ability to oxidize the studied substrates (OAPH and 3,5-DTBCH2) and their redox potentials. Also, the present catalytic investigations demonstrated that the geometrical effects are only one facet of the complexes activity. Addition of Lewis base, triethylamine, in both systems exhibits dramatic effect on the rate of these catalytic aerobic oxidation reactions. The probable mechanistic implications of both catalytic systems are discussed.