Influence of Ligand Denticity and Flexibility on the Molecular Copper Mediated Oxygen Reduction Reaction

Abstract

To date, the copper complex with the tris(2-pyridylmethyl)amine ( tmpa ) ligand ( Cu - tmpa ) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the tmpa ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2′:6′,2″-terpyridine ( terpy ) and bis(2-pyridylmethyl)amine ( bmpa ) ( Cu - terpy and Cu - bmpa , respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the terpy and bmpa ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for H 2 O 2 production. The lower catalytic activity is shown to be the result of a stabilization of the Cu I state of the complex compared to the earlier reported Cu - tmpa catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate terpy and bmpa ligands compared to the tetradentate tmpa ligand. The Laviron plots of the redox behavior of Cu - terpy and Cu - bmpa indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of Cu - terpy and Cu - bmpa to form the preferred tetrahedral coordination geometry for a Cu I complex easily. Our study illustrates that both the tetradentate nature of the tmpa ligand and the ability of Cu - tmpa to form the preferred tetrahedral coordination geometry for a Cu I complex are of utmost importance for ORR catalysis with very high catalytic rates.

Influence of the spatial distribution of copper sites on the selectivity of the oxygen reduction reaction

“…Additionally, a comparison with the RDE LSV profile for ORR catalysis performed by Cu-bmpa reveals a slower increase in the reductive current for the ORR profiles of the trinuclear catalysts compared to the mononuclear catalyst above ∼0.2 V vs. the RHE. 20 This indicates that the turnover frequencies of the trinuclear catalysts are substantially lower than those of the mononuclear systems reported previously. 20 There are several possible explanations for the slow catalysis in these complexes.…”

“…For the RRDE LSV measurements the ORR onset potential of Cu 3 L1 and Cu 3 L2 has been defined as the potential at which i c /i GC > 3, in which i c is the disk current observed during ORR catalysis performed by the catalyst and i GC is the disk current observed in the absence of the catalyst. 19,20 For Cu 3 L1 and Cu 3 L2, this onset potential is located at 0.55 and 0.52 V vs. the RHE, respectively. Considering the reported ORR onset potential of 0.49 V vs. the RHE for Cu-bmpa, the overpotential for the ORR catalyzed by the trinuclear catalysts is…”

“…Dalton Transactions reproduce the concentration of O 2 in O 2 saturated 0.1 M pH phosphate buffer. 20,[70][71][72] The H 2 O 2 reduction profile obtained by RDE measurements for both Cu 3 L1 and Cu 3 L2 did not show the presence of a mass-transport limiting current plateau between 1.0 and −0.15 V vs. the RHE, which we expect to find at 400 µA according to the Levich equation (ESI 6.3 †). This absence of a mass-transport limiting current plateau confirms that the H 2 O 2 reduction by these trinuclear catalysts must be a relatively slow process, which was shown for Cu-bmpa and Cu-terpy previously as well.…”

“…This absence of a mass-transport limiting current plateau confirms that the H 2 O 2 reduction by these trinuclear catalysts must be a relatively slow process, which was shown for Cu-bmpa and Cu-terpy previously as well. 20,39 Upon addition of 1.1 mM H 2 O 2 to the Cu 3 L2 solution in aqueous phosphate buffer, a change in the color and more specifically a change in the UV-Vis spectrum were observed (Fig. S13 †).…”

“…Mononuclear Cu complexes have been explored to catalyze the ORR. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Additionally, several dinuclear complexes have been investigated to induce a cooperative effect during ORR catalysis. [35][36][37][38] We have recently shown that the mononuclear complex [Cu(tmpa)(solv)] + (Cu-tmpa, tmpa = tris(2-picolyl) amine) shows exceptionally high ORR catalytic performance with a turn-over frequency (TOF) of almost 2 million per second.…”