Copper(II) complexes as catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine

•Copper(II) complexes of a bisbenzimidazolyl ligand are synthesised.•Complexes catalyse the aerobic oxidation of o-phenylenediamine.•Reaction follows first order kinetics with respect to substrate and catalyst.•Rate of oxidation shows tenfold drop in absence of catalyst.•Addition of acetate anion causes a mild inhibition of rate of reaction.

Two new mononuclear copper(II) complexes [Cu (L) (NO3)2] (1) and [Cu (L) Br2] (2) where (L = bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) are synthesized and characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV–Visible, IR spectroscopy, EPR and cyclic voltammetry. The complexes exhibit different coordination structures; the E1/2 value of the complex (1) is found to be relatively more cathodic than that of complex (2). X-band EPR spectra at low temperature in DMF supports a tetragonally distorted complex (1) while complex (2) shows three different g values suggesting a rhombic geometry. These complexes were utilized as a catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was found to be higher for the nitrate bound complex, while presence of acetate anion acts as a mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of reaction. The inhibition suggests that the generated protons are further required in another important catalytic step.

Graphical abstractCopper(II) complexes have been utilized as a catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine. Rate of oxidation in the absence of the catalyst shows a tenfold drop confirming the role of Cu(II) complex as a catalyst in this reaction.Figure optionsDownload full-size imageDownload as PowerPoint slide