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 Co2(μ-O2) complexes bridged by xanthene (1) and anthracene (3) and by a Co2(OH)2 complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)2 as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOFinit = 6.8 × 102 s-1) 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 1-3 have the O2 addition to [CoII2(OH2)2]4+ as an intermediate in common. In the only case that complex 1 was used as a catalyst, kcat depended on proton concentration because the reaction rate of the O2 addition to [CoII2(OH2)2]4+ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O2 and stabilizing the Co2(μ-O2) structure, thus increasing both the reaction rate and selectivity for 4e--ORR.