Theoretical and Experimental Reactivity Predictors for the Electrocatalytic Activity of Copper Phenanthroline Derivatives for the Reduction of Dioxygen

Abstract

We have systematically studied the catalytic activity of a series of substituted bisphenanthroline Cu complexes adsorbed on glassy carbon electrodes for the oxygen reduction reaction (ORR) in aqueous media. The aim of this work is to test reactivity descriptors for ORR previously proposed for MN4 complexes. As the active site is the Cu(I) center, the foot of the wave for O 2 reduction on electrodes modified with Cu bisphenanthrolines appears at potentials very close to the [Cu(II)phen 2 ] ad ++ + e − ⇆ [Cu(I)phen 2 ] ad + redox process. Generally, the catalytic activity as (log i)E versus the E Cu(II)/(I) °redox potentials follows a linear correlation with a slope close to +0.120 V/decade, where the activity increases as the E Cu(II)/(I) °becomes more positive. This indicates that electron-withdrawing substituents on the ligand favor the reaction by shifting the E Cu(II)/(I) °to more positive values, making the Cu center more noble. This shift in the E Cu(II)/(I) °is essentially an electronic effect. However, another way to stabilize the Cu(I) state is by placing groups on the ligand that hinder the [Cu(II) square-planar ] ad ++ /[Cu(I) tetrahedral ] ad+ change. We have found that this steric effect seems to be more prominent than the electronic effects caused by electron-withdrawing groups and the increase in the catalytic activity for ORR is more pronounced.

Functionalizing Carbon Nanotubes with Bis(2,9-dialkyl-1,10-phenanthroline)copper(II) Complexes for the Oxygen Reduction Reaction

“…Closed redox potentials have been measured for the adsorbed parent complex Cu II (dmp) 2 on the EPG or GC electrode (E 1/2 = 0.73 V vs RHE). 57,63 This supports the fact that a bisphenanthroline copper intermediate participates in the 4H + /4e − at MWCNT at this potential. Previous hypotheses on the ORR mechanism in water of this family of complexes have described the participation of two copper centers in the activation of O 2 and its 4H + /4e − reduction to H 2 O.…”

Penta-coordinated transition metal macrocycles as electrocatalysts for the oxygen reduction reaction

“…In this case, the metal center not being involved in the redox process and rather at the ligand sphere [119]. The flat rigid geometry of the phthalocyanine hinder the formation of tetrahedral Cu(I) which is active for ORR [120] as well as not readily being able to present 5 coordinate systems. The poor performance of CuPc was highlighted by Ponce et al [109] when investigating ORR catalysis by FePc bound to gold surfaces via different linkers and compared it with that of CuPc.…”

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

“…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.…”