Equilibria and catalytic properties of a chloro-bridged diimine copper(II) complex in the N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) oxidation

Abstract

Some new copper(II) complexes with a tridentate imine ligand, bimpy (or 2-(benzimidazolyl)methylene-2-amino-1-ethylpyridine), were prepared, and had their catalytic activity in the N,N,N′,N′-tetramethyl-p- phenylenediamine (TMPD) oxidation compared. Mononuclear species, [Cu(bimpy)(H2O)2](NO3)2, (1) and [Cu(bimpy)Cl2]·H2O, (2), as well as the dinuclear species containing chloro-bridges, [Cu2(bimpy)2Cl 2]X2·nH2O as perchlorate 3, nitrate 4 and chloride 5 salts, respectively, were obtained and characterized by different techniques (UV/Vis, IR, EPR and X-ray crystallography). In non-aqueous solvents, the dinuclear species predominates; however, in aqueous solution or solvents containing water, equilibrium between dinuclear and correlated mononuclear species 1 was verified. Kinetic studies, in methanol solution, on the catalyzed TMPD oxidation indicated a first-order dependence of the initial rate with the catalyst concentration, and a saturation effect with increasing concentrations of the substrate. The dinuclear species was observed to be more reactive than the mononuclear one, and further experiments showed that the counter-ion chloride can significantly affect the kinetic parameters, mostly by shifting the mono-/dinuclear equilibrium, and stabilizing an ion-pair with the substrate.