Binuclear copper (II) complexes containing pyN4O moiety as a model for catechol oxidase; synthesis, characterization and catechol oxidase activity

A new series of copper (II) complexes, [(MepyN4O)Cu2II(??SO4)]2SO4 (1), [(MepyN4O)Cu2IIBr2]Br (2), [(MepyN4O)Cu2II(NO3)2]NO3 (3), [(BrpyN4O)Cu2II(??SO4)]2SO4 (4), [(BrpyN4O)Cu2IIBr2]Br (5), [(BrpyN4O)Cu2II(NO3)2]NO3 (6) and [(BrpyN4O)Cu2IICl2]Cl (7) were synthesized and completely characterized. Catalytic activity towards the oxidation of catechol and 4-tert-butylcatechol by copper complexes 1-7 has been investigated and correlated with the structure. The complexes bearing MepyN5OH moiety have less catechol oxidase activity in respect to complexes containing BrpyN4OH moiety. The rate of oxidation of substrate depends strongly on the anionic ligands. For instance, the activity of complexes containing BrpyN5OH moiety was found to be in the following order: Br- > NO3- > SO4-2 > Cl-. For complexes containing MepyN5OH moiety, the catalytic activity follows the order of NO3- > SO4-2 > Br- for oxidation of 4-tert-butylcatechol, whereas, the order of activity for oxidation of catechol is Br- > SO4-2 > NO3-. In a general, complexes 5 was found to be the most efficient catalysts due to the perfect match between the copper (II) centers and the incoming substrate (catecholate) to get bound effectively for subsequent redox reaction. The oxidation of 4-tert-butylcatechol is easier than catechol because of the presence of the electron-donating tert-butyl group in the 4-tert-butylcatechol.