Pyridylalkylamine copper complex-mediated oxygen reduction: effect of alkyl chain and substituents on catalytic performance.

The oxygen reduction reaction (ORR) plays an essential role in many biological and artificial systems related to energy conversion. Modifying the organic scaffolds of metal complexes can adjust their catalytic performances toward the ORR. In this study, we have synthesized and characterized three pyridylalkylamines bearing different chains and substituents and their copper complexes, namely [Cu(tmpmen)](ClO4)2 (Cu-tmpmen), [Cu(tpmpn)](ClO4)2 (Cu-tpmpn), and [Cu(etOHtpen)](ClO4)2·CH3CN (Cu-etOHtpen) (tmpmen = N-methyl-N,N',N'-tris[(3-methyl-2-pyridinyl)methyl]-1,2-ethanediamine, tpmpn = N-methyl-N,N',N'-tris(2-pyridylmethyl)propane-1,3-diamine, and etOHtpen = 2-[[2-[bis(2-pyridinylmethyl)amino]ethyl](2-pyridinylmethyl)amino]ethanol). The catalytic activities of the three copper complexes toward the ORR were studied using cyclic voltammetry, rotating ring disk electrode measurements and controlled potential electrolysis, which revealed the product selectivity of oxygen reduction to H2O2 (>70%) and the high catalytic stability of the three copper complexes in neutral aqueous solutions. The ORR onset potentials mediated by Cu-tmpmen, Cu-tpmpn, and Cu-etOHtpen were 0.459, 0.465 and 0.480 V vs. RHE, respectively. Meanwhile, the corresponding turnover frequencies (TOFs) for the 2e-/2H+ ORR were 2.5 × 105, 8.3 × 104 and 3.4 × 102 s-1, respectively, which were related to the ligand flexibility and distortion of the coordination sphere of the copper center.