Synthesis and characterization of copper(II) complexes with multidentate ligands as catalysts for the direct hydroxylation of benzene to phenol

Four copper(II) complexes with multidentate ligands, 1 ([CuL1Cl2]), 2([Cu(HL2)Cl-2]), 3 ([Cu-2(L-2)(2)](ClO4)(2)) and 4 ([CuL3(HOCH3)ClO4]) {L-1 = N,N-bis((pyridin-2-yl)methyl) prop-2-yn-1-amine, HL2 = 2-((((1-methyl-1H-imidazol-2-yl)methyl)(pyridin-2-ylmethyl) amino) methyl)phenol and HL3 = 2-((((1-methyl-1H-imidazol- 2-yl) methyl)(pyridin-2-ylmethyl)amino)methyl)-2-t-butyl-phenol} are reported. The complexes were characterized by UV-vis spectroscopy, elemental analysis and electrochemical analysis. Complexes 1 and 3 were further characterized by X-ray single crystal diffraction analysis. The catalytic performances of these complexes were evaluated in the direct hydroxylation of benzene to phenol with hydrogen peroxide as an oxidant in aqueous acetonitrile media. Under optimized reaction conditions, complex 4 with the most negative reduction potential exhibited the highest conversion without considering the dinuclear complex 3. A correlation between the catalytic efficiency and the reduction potentials of these complexes was observed, that is the more negative the reduction potential, the higher the benzene conversion. A radical mechanism for the catalysis was confirmed by the fact that addition of radical scavengers such as TEMPO into the reaction mixture could severely suppress the catalysis.