Effect of Bipyridines on Catalytic Oxygen Reduction by Oxamidato-Bridged Dicopper(II) Complexes

The oxygen reduction reaction (ORR) has a crucial impact on the energy conversion efficiency and the performance of fuel cells, it is urgent to deliver low-cost efficient ORR catalysts for the aim to scale up the utilization of fuel cells. In this study, three oxamidate copper(II) complexes [Cu₂(trans-L)(H₂O)₂(ClO₄)₂]·2H₂O (Cuox), [Cu₂(cis-L)(bpy)(ClO₄)₂]·CH₃OH (Cuoxbpy), and [Cu₂(cis-L)(mebpy)(ClO₄)](ClO₄) (Cuoxmebpy) (L = N,N–bis(2–pyridylmethyl)oxamide, bpy = 2,2′–bipyridine, and mebpy = 4,4′–dimethyl–2,2′–bipyridine) were prepared and characterized. The configuration of L varied from the trans form to the cis form as the bipyridines were introduced to the dicopper complex. The redox properties of three oxamidate copper(II) complexes and their catalytic activities for the ORR were studied in neutral aqueous solutions. The ORR onset potentials by Cuox, Cuoxbpy, and Cuoxmebpy were 0.417, 0.502, and 0.481 versus RHE, respectively. Three complexes homogeneously catalyzed the ORR to predominantly generate H₂O₂. A bimolecular catalytic pathway was proposed for the ORR mediated by Cuox, whereas a single-molecular pathway was by Cuoxbpy and Cuoxmebpy. This work demonstrated that oxamidate metal complexes are promising ORR catalyst candidates provided that their electronic structures are appropriately tuned by altering the substituent on the organic ligands.