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

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

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.
Published in: Dalton Transactions, 10.1039/D0DT00475H, Royal Society of Chemistry