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

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+), [Co-2(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 (1.0mmol L (1)1), 2 showed higher selectivity (99%) and reaction rate (k(cat) = 6.0 (c) 10(3)M(1)1 s(1)1) for 4e(-)-ORR than the bpy analog 1 (k(cat) = 6.2 (c) 10M(1)1 s(1)1) and 4DHBP analog 3 (k(cat) = 1.5 (c) 10(2)M(1)1 s(1)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 mu-O2 ligand. Hydroxy groups at the 6,6'-position of bpy would deliver protons to the mu-O2 ligand of the intermediate, thereby promoting O-O bond cleavage in the proton-coupled electron transfer (PCET) process.