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

Abstract

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 [CoII 2(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 O2 addition to [CoII 2(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.

Reversible Dioxygen Binding to Co(II) Complexes with Noninnocent Ligands

“…A commonality across the series of cobalt­(III)-alkylperoxo complexes is the lack of prominent absorption features in the visible region (Figure S13), which appears to be the defining spectroscopic trait of this class of Co/O 2 adducts. In contrast, cobalt­(III)-superoxo species (like CoATP O2 ) and dicobalt­(III)-μ-peroxo species ,, generally display intense absorption bands in the visible region. Thus, our results provide a helpful spectroscopic marker to distinguish between the various types of structures that arise from reversible O 2 coordination to Co complexes.…”

Macrocycle-Induced Modulation of Internuclear Interactions in Homobimetallic Complexes

“…Slow diffusion of pentane into this solution at room temperature afforded compound 4 [Cu] as large, green block crystals. (993 mg, 85% (100), 333 (5000), 318 (8600), 304 (9800), 292 (11,000), 279 (18,000). IR (KBr, cm −1 ): 1600 s (CN stretch).…”

“…Homometallic, dinuclear complexes are becoming increasingly important due to their ability to offer new avenues toward important chemical transformations. Cooperative bimetallic interactions are credited with the unique ability of metalloenzymes to activate small molecules, , and model complexes are being created to probe the fundamental principles of these biological systems. − In addition, the exploitation of bimetallic interactions is inspiring new strategies toward small molecule activation, − unique polymerization outcomes, − sensing applications, , and mimicking metallic surface processes, − and they are increasingly understood to be important in organic transformations historically defined by mononuclear strategies. − …”

Tuning Active Species in N-Doped Carbon with Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction Reaction

“…The oxygen reduction reaction (ORR) plays a pivotal role in emerging electrochemical energy technologies such as fuel cells and metal–air batteries. − At present, platinum (Pt)-based materials are the state-of-the-art ORR catalysts because of their low overpotentials. Unfortunately, their inherent drawbacks of prohibitive cost, small reserve, poor stability, and low methanol tolerance have severely hampered their widespread applications.…”