Mechanistic Investigations into the Selective Reduction of Oxygen by a Multicopper Oxidase T3 Site-Inspired Dicopper Complex

Abstract

Understanding how multicopper oxidases (MCOs) reduce oxygen in the trinuclear copper cluster (TNC) is of great importance for development of catalysts for the oxygen reduction reaction (ORR). Herein, we report a mechanistic investigation into the ORR activity of the dinuclear copper complex [Cu 2 L(μ-OH)] 3+ (L = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This complex is inspired by the dinuclear T3 site found in the MCO active site and confines the Cu centers in a rigid scaffold. We show that the electrochemical reduction of [Cu 2 L(μ-OH)] 3+ follows a proton-coupled electron transfer pathway and requires a larger overpotential due to the presence of the Cu-OH-Cu motif. In addition, we provide evidence that metal−metal cooperativity takes place during catalysis that is facilitated by the constraints of the rigid ligand framework, by identification of key intermediates along the catalytic cycle of [Cu 2 L(μ-OH)] 3+ . Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu 2 L(μ-OH)] 3+ differ from the ones found for analogous mononuclear copper catalysts. In addition, the metal−metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70% because reaction intermediates can be stabilized better between both copper centers. Overall, the mechanism of the [Cu 2 L(μ-OH)] 3+ -catalyzed ORR in this work contributes to the understanding of how the cooperative function of multiple metals in close proximity can affect ORR activity and selectivity.

Scaling Relation between the Reduction Potential of Copper Catalysts and the Turnover Frequency for the Oxygen and Hydrogen Peroxide Reduction Reactions

“…These correlations are in perfect agreement with a potential-determining reduction of Cu II to Cu I , followed by rate-limiting binding of O 2 for the entire series of copper species investigated here. In the case of Cu-tmpa, these potential- and rate-determining steps were already identified on basis of kinetic studies discussed in previous reports. ,, It is important to note that also the data previously obtained for Cu-bmpa ( E 1/2 = 0.30 V vs RHE, TOF max for ORR = 2.4 × 10 4 s –1 ) correlates very well with the ORR TOF max versus E 1/2 trend reported here, while catalysts that show very sluggish and therefore rate-determining Cu­(II) reduction kinetics do significantly underperform as one would expect. ,,, …”

“…22,23,62 It is important to note that also the data previously obtained for Cu-bmpa (E 1/2 = 0.30 V vs RHE, TOF max for ORR = 2.4 × 10 4 s −1 ) correlates very well with the ORR TOF max versus E 1/2 trend reported here, 39 while catalysts that show very sluggish and therefore rate-determining Cu(II) reduction kinetics do significantly underperform as one would expect. 39,47,63,64 Displacement of water for peroxide presumably takes place prior to the rate-determining step of the HPRR, and with similar energetics for all copper complexes, unlike the binding of dioxygen. The relative insensitivity of H 2 O 2 versus H 2 O binding to these copper sites is to be expected, as both species bind to the copper site in a very similar manner.…”

Iron phthalocyanine integrated with boron-doped reduced graphene oxide for highly selective four-electron oxygen reduction: an experimental study

“…32 The small peak potential difference indicates a faster electron transfer, which is beneficial for the ORR. 32–34…”

Enhancing oxygen reduction activity of dinuclear copper complexes loaded on an N-doped carbon support via a low-temperature pyrolysis strategy

“…22–24 A dinuclear copper complex [Cu 2 L(μ-OH)] 3+ was reported to show a different ORR mechanism from the analogous mononuclear copper catalyst, where the oxygenated intermediates could be better stabilized between the two copper centres, and the metal–metal cooperativity greatly improved the selectivity of four-electron reduction in the ORR. 25 And a dinuclear copper complex containing a pyrene moiety loaded on carbon nanotubes (CNTs) via π–π stacking 26 demonstrated enhanced ORR activity and stability due to the improved conductivity.…”