Capturing the Role of Phosphate in the Ni-PY5 Catalyzed Water Oxidation

摘要

The mononuclear Nickel complex Ni-PY5 [PY5=2, 6-bis(1,1-bis(2-pyridyl)ethyl)pyridine] has been disclosed to catalyze water oxidation electrochemically with an applied potential of 1.5 V at pH 10.8 in aqueous phosphate buffer solution. Density functional calculations were used to elucidate the reaction mechanism of water oxidation catalyzed by this nickel complex and to capture the role of the phosphate. The calculations demonstrated that the oxidations of the starting [OH2-Ni-II-PY5](2+) complex by two sequential proton-coupled electron-transfer processes lead to the formation of a key intermediate [O=Ni-IV-PY5](2+). O-O bond formation then takes place through a water nucleophilic attack on the high-valent Ni-IV=O moiety of the catalyst, facilitated by a hydrogen phosphate anion, with a total barrier of 11.5 kcal mol(-1). The calculated barrier agrees very well with the experimental turnover frequency of about 2000 s(-1), which corresponds to a barrier of 12.9 kcal mol(-1). The calculated deuterium kinetic isotope effect of 1.99 is also in excellent agreement with the experimental value of 2.06. Finally, we also predicted the catalytic activity of other PY5-based first-row transition metal complexes, namely, involving Mn, Fe, Co and Cu. The calculations showed that the Mn, Fe, Co complexes have higher barrier for water oxidation, while the Cu complex has lower barrier and higher water oxidation activity compared with the Ni complex.