Catalytic dioxygen reduction mediated by a tetranuclear cobalt complex supported on a stannoxane core.

Anirban Chandra,Peter Hildebrandt,Subrata Kundu,Holger Dau,Kallol Ray,Uwe Kuhlmann,Stefan Mebs

doi:10.1039/d0dt00475h

Abstract

The synthesis, spectroscopic characterization (infrared, electron paramagnetic resonance and X-ray absorption spectroscopies) and density functional theoretical calculations of a tetranuclear cobalt complex Co4L1 involving a nonheme ligand system, L1, supported on a stannoxane core are reported. Co4L1, similar to the previously reported hexanuclear cobalt complex Co6L2, shows a unique ability to catalyze dioxygen (O2) reduction, where product selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 30 °C. Detailed mechanistic insights were obtained on the basis of kinetic studies on the overall catalytic reaction as well as by low-temperature spectroscopic (UV-Vis, resonance Raman and X-ray absorption spectroscopies) trapping of the end-on μ-1,2-peroxodicobalt(iii) intermediate 1. The Co4L1- and Co6L2-mediated O2-reduction reactions exhibit different reaction kinetics, and yield different ratios of the 2e-/2H+ and 4e-/4H+ products at -50 °C, which can be attributed to the different stabilities of the μ-1,2-peroxodicobalt(iii) intermediates formed upon dioxygen activation in the two cases. The deep mechanistic insights into the transition-metal mediated dioxygen reduction process that are obtained from the present study should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.

Highlights

In the present manuscript we report the synthesis, characterization and X-ray structure of a tetranuclear stannoxane based non-heme ligand system (L1), and a detailed kinetic study of the catalytic dioxygen reduction reaction mediated by the corresponding cobalt complex Co4L1
Sn4O2 core supports the four metal-binding sites (Fig. 1: top). This is in contrast to the situation reported earlier for the hexanuclear non-heme ligand system L2, where six metal-binding sites were located in a wheel-like arrangement around a central Sn6O6 prismane core (Fig. 1: bottom)
In our previous study[8] we reported the synthesis and characterization of a hexanuclear cobalt complex Co6L2 involving a nonheme ligand system, L2, supported on a Sn6O6 stannoxane core (Fig. 1: bottom), whose cobalt complex acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e−/4H+ dioxygen-reduction to a 2e−/2H+ process only by increasing the temperature from −50 to 25 °C

Summary

Introduction

In the present manuscript we report the synthesis, characterization and X-ray structure of a tetranuclear stannoxane based non-heme ligand system (L1), and a detailed kinetic study of the catalytic dioxygen reduction reaction mediated by the corresponding cobalt complex Co4L1. The concentration of Fc*+ formed in the complex Co4L1-catalyzed reduction of O2 by Fc* is dependent on the temperature at which the reactions were performed (Fig. 4 bottom, S7a–c†).

Results

Conclusion