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

摘要

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn_6O_6 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 to 25 °C. A variety of spectroscopic methods (~(119)Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, 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 O_2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O_2 can be explained by the constraint provided by the stannoxane core that makes the O_2-binding to 1 an entropically unfavorable process. This makes the end-on μ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H_2O_2. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O_2-to-2H_2O catalytic conversion in an overall 4e~-/4H~+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.