Oxygen reduction mediated by tetrapyridylalkylamine Cu(II) complexes with diimines.

• Redox properties of tetrapyridylalkylamine Cu(II) complexes with diimines. • Oxygen reduction catalysis by tetrapyridylalkylamine Cu(II) complexes with diimines. • Catalytic oxygen reduction mechanism by tetrapyridylalkylamine Cu(II) complexes with diimines. Oxygen reduction reaction (ORR) is important in artificial energy conversion systems such as fuel cells. However, it is kinetically sluggish. It is thereby highly desirable to develop ORR catalysts to improve the efficiency of energy conversion. In this work, three copper(II) complexes based on the hexadentate tetrapyridylalkylamine N,N,N',N'-tetrakis(2-pyridyl-methyl)-1,4-butanediamine (tpbn), namely [Cu 2 (tpbn)(CH 3 OH) 4 (ClO 4) 2 ](ClO 4) 2 (Cu-tpbn), [Cu 2 (tpbn)(bpy) 2 ](ClO 4) 4.2CH 3 COCH 3 (Cu-tpbn-bpy, bpy = 2,2′-bipyridine), and [Cu 2 (tpbn)(Me 2 bpy) 2 ](ClO 4) 4.3CH 3 CN (Cu-tpbn-Me 2 bpy, Me 2 bpy = 4,4′-dimethyl-2,2′-bipyridine), were prepared and their catalytic properties for ORR studied. All three complexes were homogeneous electrocatalysts for ORR in neutral phosphate buffer solutions. The ORR mediated by Cu-tpbn-Me 2 bpy had the most positive onset potential of 0.461 V vs RHE. In the applied potential window of -0.19 – 0.50 V vs RHE, Cu-tpbn boosted the stepwise four-electron ORR to mainly produce H 2 O, whereas Cu-tpbn-bpy and Cu-tpbn-Me 2 bpy mediated the ORR to mainly generate H 2 O 2. The rate constants for the ORR to H 2 O promoted by Cu-tpbn, Cu-tpbn-bpy and Cu-tpbn-Me 2 bpy were 1.2 × 103 s−1, 3.2 × 10 s−1 and 7.8 × 10 s−1, respectively. The foot-of-the-wave analysis showed that the turnover frequencies for the reduction of O 2 to H 2 O 2 mediated by Cu-tpbn, Cu-tpbn-bpy and Cu-tpbn-Me 2 bpy were 1.5 × 104 s−1, 7.5 × 10 s−1 and 2.7 × 102 s−1, respectively. Meanwhile, complexes Cu-tpbn-bpy and Cu-tpbn-Me 2 bpy were more stable than Cu-tpbn. This work unveiled that the catalytic activity and the product selectivity of the ORR were greatly associated with the denticity, the steric effect and the electronic property of the organic ligands. Three tetrapyridylalkylamine copper(II) complexes were homogeneous catalysts for oxygen reduction reaction (ORR). The main product of ORR mediated by the Cu-tpbn systems was directed from H 2 O to H 2 O 2 via the incorporation of the diimine ligands. [Display omitted] [ABSTRACT FROM AUTHOR]