Electrocatalysis of the Oxygen Reduction Reaction by Copper Complexes with Tetradentate Tripodal Ligands

The tetradentate tripodal ligand scaffold is capable of supporting the expected geometries of the copper ion during the oxygen reduction reaction (ORR) catalysis. As such, we probed the reactivity of copper complexes with these types of ligands by electronically and structurally tweaking the tris(pyridin 2-ylmethyl)amine (tmpa) scaffold by progressively replacing the terminal pyridines with carboxylate donors. This work shows that systems with one carboxylato donor (bpg = bis(pyridin-2-ylmethyl)glycine), (bpp = (3-(bis(pyridin-2-ylmethyl)amino)propanoic acid)) are active in electrocatalyzing the homogeneous ORR under circumneutral aqueous conditions. Turnover frequencies in the range from 10(5) to 10(6) s(-1), on par with that for Cu-tmpa under identical conditions, were obtained. It is noteworthy that the Cu-II/Cu-I redox potentials for the Cu-bpg, Cu-bpp, and Cu-tmpa systems in phosphate-buffered water (pH 7, under Ar) are similar at -0.409, -0.375, and -0.401 V vs Ag/AgCl, respectively. This is rationalized by the influence of the Lewis acidity of the copper ions on the water coligand. Corroborating this are pK a values for [Cu(tmpa)(H2O)](2+), Cu(bpg)(H2O)](+), and [Cu(bpp)(H2O)]+ of 6.6, 8.8, and 10.2, respectively. Thus, the overall charge of the solution species for all three complexes will be +1 at pH 7 and this will be an important determinant for the redox potentials and, in turn, the catalytic overpotentials, which are also similar. A cis carboxylato donor offers H-bonding possibilities for exogenous resting state water and intermediate hydroperoxo coligands. This is reflected by the higher pK( a )values for Cu-bpp and Cu-bpg compared with that for Cu-tmpa, with the Cu-bpp system furnishing the least strained H-bonding.

Automated summary

This study investigates the catalytic activity of copper complexes in the oxygen reduction reaction by modifying the structure of the ligands. The results show that copper complexes with one carboxylate donor exhibit high activity in neutral aqueous conditions. The turnover frequencies are comparable to traditional copper complexes. Additionally, the study reveals that the Lewis acidity of copper ions also influences other ligands in the solution.