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-2(mu-O-2) complexes bridged by xanthene (1) and anthracene (3) and by a Co-2(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 x 10(2) 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 O-2 addition to [Co-2(II)(OH2)(2)](4+) as an intermediate in common. In the only case that complex 1 was used as a catalyst, k(cat) depended on proton concentration because the reaction rate of the O-2 addition to [Co-2(II)(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 O-2 and stabilizing the Co-2(mu-O-2) structure, thus increasing both the reaction rate and selectivity for 4e(-)-ORR.