Electrochemical and Spectroelectrochemical Studies on Oxygen Reduction Mediated by Cu(II) Complexes Containing the Alkylamine Ligand N,N-Dimethylethylendiamine

In this study, Cu(II) complexes containing the bidentate ligand N,N-dimethylethylendiamine (dmen), i.e., [CuII(dmen)2(CH3COO)2], [CuII(dmen)2(NO3)2], and [CuII(dmen)2Cl2], were developed to explore molecular catalysis for the oxygen reduction reaction (ORR). Cyclic voltammetry and UV–vis spectroelectrochemical and electrochemical impedance spectroscopy experiments were performed in the absence and presence of oxygen. The UV–vis spectroscopy results suggested that the aforementioned Cu(II) complexes present an octahedral geometry in the solid state; meanwhile, they show a square pyramidal geometry in an aqueous solution. It is proposed that the chemical species [CuI(dmen)2H2O]+ reacts with O2, exhibiting an outer-sphere electron transfer mechanism. The same UV–vis spectroelectrochemical response obtained with and without O2 indicated a direct electron transfer from Cu(II) to Cu(I), with the regeneration of catalyst and the absence of other intermediate species. Among the reported compounds, [Cu(dmen)2(NO3)2] exhibited the highest catalytic rate (TOF = 1.3 × 104 s−1). The impedance spectroscopy results showed that the resistance charge transfer (Rct) of the redox pair CuII|CuI decreased in the presence of O2 from 36.391 kΩ to 5.54 kΩ. For a better understanding of the effect of aliphatic amines on the ORR, a comparison with the complex [Cu(1,10-phen)2NO3]NO3 is also presented.