Proton Relay Effects on Oxygen Reduction Reaction Catalyzed by Dinuclear Cobalt Polypyridyl Complexes with OH Groups on Bipyridine Ligands

Abstract Four-electron oxygen reduction reaction (4e−-ORR) is the foundation of both natural and artificial energy conversion systems. Mechanism studies and catalysis improvements of 4e−-ORR are important research for the actualization of a sustainable society. In this study, we present a dinuclear cobalt complex containing mono-deprotonated forms of 6,6′-dihydroxy-2,2′-bipyridine (6DHBP-H+), [Co2(OH)2(6DHBP–H+)2(btpyxa)](PF6)2 (2) is a highly active 4e−-ORR catalyst in a low acid concentration solution. When ferrocene (Fe(Cp)2) was used as a reductant in PhCN containing a low concentration of perchloric acid (1.0 mmol L−1), 2 showed higher selectivity (99%) and reaction rate (kcat = 6.0 × 103 M−1 s−1) for 4e−-ORR than the bpy analog 1 (kcat = 6.2 × 10 M−1 s−1) and 4DHBP analog 3 (kcat = 1.5 × 102 M−1 s−1). A high catalytic current in the cyclic voltammetry (CV) of 2 indicates a high reaction rate for electrochemical ORR under low acid concentration conditions. Moreover, X-ray crystallography of the corresponding monomeric analog [Co(OH2)(6DHBP–2H+)(trpy)](PF6) (4, 6DHBP–2H+: a doubly deprotonated form of 6DHBP) suggests that OH groups of 2 can form hydrogen bonds with a μ-O2 ligand. Hydroxy groups at the 6,6′-position of bpy would deliver protons to the μ-O2 ligand of the intermediate, thereby promoting O–O bond cleavage in the proton-coupled electron transfer (PCET) process.