Aerobic Oxidation of 2-Aminophenol Catalysed by a Series of Mononuclear Copper(II) Complexes: Phenoxazinone Synthase-like Activity and Mechanistic Study
Three mononuclear copper(II) complexes of the types [Cu(L1)(Cl)2].MeOH (1.MeOH), [Cu(L2)(Cl)2].H2O (2.H2O) and [Cu(L3)(Cl)2] (3) have been synthesized from three reduced Schiff base tridentate N3 ligands namely [H2]L1 = N-(pyridine-2-ylmethyl)quinolin-8-amine}, [H2]L2 = N-(1-methylbenzimidazol-2-ylmethyl)quinolin-8-amine, and [H2]L3 = N-(1-methylimidazol-2-ylmethyl)quinolin-8-amine}, respectively, having variable donor moieties. During metalation all three reduced Schiff base ligands undergo oxidative dehydrogenation in situ under aerobic condition to yield the corresponding Schiff base ligated mononuclear copper(II) complexes. All complexes have been characterized using various spectroscopic techniques such as IR, HRMS-ESI, UV-vis, and EPR. Structural characterization of each complex by single crystal X-ray diffraction reveals that the coordination environment around copper ion is distorted square pyramidal. The three complexes effectively catalyse the aerial oxidation of 2-aminophenol (H2AP) to 2-amino-phenoxazine-3-one (APX), thus mimic the catalytic function of the enzyme phenoxazinone synthase. Kinetic studies have been done to arrive at the following catalytic efficiency order: 3 ≫ 2.H2O > 1.MeOH. The observed trend can be attributed by considering structure-function relation in catalytic activity. Intramolecular charge distributions (valence tautomerism) within complex-substrate adduct lead to the generation of “CuI-(substrate radical)” tautomer. This phenomenon has been established by EPR spectroscopy, particularly using 2-anilino-4,6-di-tert-butylphenol (H2APPh,t-Bu, a structural analogue of H2AP) as substrate. Such “CuI-(substrate radical)” species is believed to promote the dioxygen activation. Effects of temperature and pH on reaction rates have been studied. Activation parameters (Ea, ∆H‡, ∆S‡) have been evaluated from temperature-dependent kinetic measurements. A plausible reaction pathway has been proposed on the basis of stoichiometry determination, spectroscopic data and kinetic analysis.