Department of Chemistry, College of Science, Rikkyo University, 3-34-1, Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan.
Inorg Chem. 2021 Jul 5;60(13):9402-9415. doi: 10.1021/acs.inorgchem.1c00293. Epub 2021 May 14.
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.
四电子氧还原反应（4e-ORR）是化学能转换的主要途径。阐明影响催化剂反应速率和选择性的因素对于开发更高效的 4e-ORR 催化剂非常重要。在这项研究中，我们研究了由香豆素（）和蒽桥连的 Co(μ-O) 配合物（）以及蒽醌桥连的 Co(OH) 配合物（）催化的化学和电化学 4e-ORR。在使用 Fe(CpMe) 作为还原剂、在酸性 PhCN 中的化学 ORR 中，我们发现三个配合物中具有最高的初始周转频率（TOF = 6.8×10 s）和对 4e-ORR 的选择性（96%）。详细的动力学分析表明，- 的催化循环中的决速步骤（RDS）具有共同的中间体 [Co(OH)] 的 O 添加。在仅使用配合物作为催化剂的情况下，依赖于质子浓度，因为 O 添加到 [Co(OH)] 的反应速率比协同 PCET 过程的反应速率快。通过 X 射线、拉曼和电化学分析以及化学计量反应，我们发现了由略微柔性的香豆素特征的面对面结构有利于捕获 O 并稳定 Co(μ-O)结构，从而提高了 4e-ORR 的反应速率和选择性。
