Mechanism Of Water Oxidation Catalysis By A Mononuclear N5-based Nickel Complex: A Theoretical Study

In nature,plants are capable of splitting water to produce dioxygen through photosynthesis,with the release of protons and electrons for the reduction of carbon dioxide to biomass.In order to construct artificial photosynthetic systems,more and more scientists have been devoted to the development of transition metal complexes for water oxidation catalysis.With the synthesis of a large number of water oxidation catalysts,and the rapid development of modern experimental techniques and quantum chemical methods,the detailed reaction mechanism can be uncovered.In this thesis,a mononuclear nickel water oxidation catalyst Ni-PY5 was studied.Through theoretical calculations,possible reaction pathways under the electro-catalytic condition were analyzed,followed by prediction of the activity of the manganese,iron,cobalt and copper complexes.The specific research contents are as follows:Density functional calculations were used to elucidate the reaction mechanism of water oxidation catalyzed by this nickel complex.Redox potentials,O-O bond formation pathways,and the whole potential energy profiles were explored.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⁺.Under the experimental condition with an applied potential of 1.5 V,the single-electron oxidation of Ni^V/Ni^IV has a potential of 2.17 V.As a consequence,the generation of[O=Ni^V-PY5]³⁺can be safely excluded.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 HPO₄^2-as the external general base,with a total barrier of only 11.5 kcal mol^-1.The resulting intermediate[HOO-Ni^II-PY5]²⁺then undergoes a proton-coupled electron transfer and a one electron oxidation to afford the Ni^II-O₂ intermediate.Finally,the release of O₂ molecule and the binding of another water molecule regenerate the starting catalyst[OH₂-Ni^II-PY5]²⁺.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.We also predicted the catalytic activity of other PY5-based first-row transition metal complexes,namely,involving Mn,Fe,Co and Cu.The calculation results show that under the same experimental conditions,the formation of O-O bond is induced by M^IV=O（M:Mn,Fe,Co and Cu）,and the Mn,Fe,Co complexes have higher barrier for water oxidation.However,the Cu complex has lower barrier,which is 6.5 kcal mol^-1,and higher water oxidation activity compared with the Ni complex.