Synthesis, crystal structures and electrochemical characterization of dinuclear paddlewheel copper(II) carboxylates

Two new dinuclear copper(II) complexes 1 and 2, viz., [pyCu(mu(2)-OOCCH2C6H4R)(4)Cupy] where R = para-NO2(1) and para-CH3(2) and py = pyridine, have been synthesized and characterized using FT-IR, single crystal XRD and electrochemical solution studies. In both complexes, the two copper centers are linked by four carboxylate ligands in bridging bidentate bonding fashion. Each copper(II) ion is penta-coordinated with distorted square pyramidal geometry. Both complexes have typical paddlewheel structures with the apical positions occupied by pyridine molecules. The electrochemical behavior of the complexes was investigated by cyclic voltammetry in which the complexes gave rise to metal centered single electron irreversible electro-activity, corresponding to Cu(II)/Cu(III) oxidation and Cu(II)/Cu(I) reduction processes. A reduction wave in relatively higher negative potential range (-1.3 to -1.4 V) was observed for the nitro group of complex 1. From the slope value of log i versus -log v, the redox processes were found predominantly diffusion controlled. The order of diffusion coefficient and heterogeneous rate constant for oxidation is: 1 > 2 owing to the more stable redox products in case of 1 while for reduction the order is: 2 > 1 due to overriding solvent effects and the larger molecular mass of 1 compared to 2. The charge transfer coefficient was also calculated and found typical of the irreversible redox processes. (C) 2013 Elsevier Ltd. All rights reserved.