Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a non-heme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose 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 (o)C to 25 (o)C. A variety of spectroscopic methods ((119)Sn-NMR, Magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-Vis absorption, resonance Raman (rRaman), and X-ray absorption spectroscopy (XAS) ) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified based on kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The O2-binding to 1 results in the efficient formation of a stable end-on μ-1,2-peroxodicobalt(III) intermediate 2 at -50 (o)C, followed by a proton-coupled electron-transfer (PCET) reduction to complete the O(2)-to-2H2O cata-lytic conversion in an overall 4e(-)/4H(+) step. In contrast, at higher temperatures (> 20 oC) the constraints provided by the stannoxane core, makes 2 unstable against a preferential proton-transfer (PT) step, leading to the generation of H2O2 by a 2e(-)/2H(+) process. The present study provides deep mechanistic insight into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells