Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure.

The four-electron oxygen reduction reaction (4e-ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e-ORR. In this study, we investigated chemical and electrochemical 4e-ORR catalyzed by Co(μ-O) complexes bridged by xanthene () and anthracene () and by a Co(OH) complex bridged by anthraquinone (). In the chemical ORR using Fe(CpMe) as a reductant in acidic PhCN, we found that showed the highest initial turnover frequency (TOF = 6.8 × 10 s) and selectivity for 4e-ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of - have the O addition to [Co(OH)] as an intermediate in common. In the only case that complex was used as a catalyst, depended on proton concentration because the reaction rate of the O addition to [Co(OH)] was so fast as compared to that of the concerted PCET process of . Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of characterized by a slightly flexible xanthene was advantageous in capturing O and stabilizing the Co(μ-O) structure, thus increasing both the reaction rate and selectivity for 4e-ORR.