Aerobic oxidation of alcohol by model complexes relevant to metal site galactose oxidase: role of copper(I) intermediate, evidence for the generation of end-on copper(II)-OOH species and catalytic promiscuity for oxidation of benzyl alcohol, catechol and o-aminophenol

摘要

Tridentate ligands having meridional NNO donor centres were designed and synthesized mimicking the copper coordination in the metal site of galactose oxidase enzyme. Mononuclear copper complexes [Cu(L-1)Cl] (1) ((LH)-H-1 = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L-2)Cl] (2) ((LH)-H-2 = (E)-4-methyl-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L-3)Cl] (3) ((LH)-H-3 = (E)-1-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)naphthalen-2-ol), [Cu(L-4)Cl] (4) ((LH)-H-4 = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L-5)Cl] (5) ((LH)-H-5 = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), and [Cu(L-6)Cl] (6) ((LH)-H-6 = (E)-2,4-di-tert-butyl-6-(((pyridin-2-ylmethyl)imino)methyl)phenol) were synthesized and characterized. Molecular structure of complex 3 was determined by single crystal X-ray crystallography. Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium nitrate (CAN), and the radical complexes were characterized by UV-Vis-NIR spectrophotometer. DFT calculations were performed at B3LYP level to optimize the ground-state molecular geometry of the complexes. To understand the electronic properties and absorption spectra of the complexes, TD-DFT calculations were executed for phenoxyl radical complexes considering triplet as well as singlet spin states. Alcohol oxidation was examined utilizing complexes 1-6 as catalyst, and importance of stabilization of Cu(I) intermediate was scrutinized and generation of Cu(II)-OOH was examined. Catalytic promiscuity for catechol oxidase and phenoxazinone synthase activity by complexes (1-5) was investigated. Theoretical calculations and ESI-MS spectral studies were performed during oxidation chemistry of benzyl alcohol, catechol and o-amino phenol to support the proposed mechanism.