Abstract
In this study, nickel(II) complexes with a series of linear tetradentate N4 ligands consisting with a cyclic diamine framework and (2-pyridyl)methyl or (2-pyridyl)ethyl sidearms were synthesized and characterized, and the oxidation of alkanes with m-CPBA catalyzed by the nickel(II) complexes was investigated. In this system, all nickel(II) complexes showed efficient catalytic activity (yield ∼ 80 %), where the nickel(II) complex of 6-membered cyclic diamine with (2-pyridyl)methyl sidearms (Ni(L6Pym2)(AN)2) showed the highest yield (88 %) and high alcohol-product selectivity (A/K = 9.8) for cyclohexane oxidation. Kinetic analysis showed that the rate-determining step in this catalytic reaction is the CH bond activation from the substrate. Counter anion of complexes affected the initial reaction rate, indicating that NiII-m-CPBA adduct is a key intermediate in catalytic cycle. In this system, ArC(O)O• (aroyloxyl radical), which is generated by homolytic cleavage of OO bond of the m-CPBA adduct intermediate, abstracts hydrogen atom from the substrate. However, radical trapping experiment using CCl4 showed that a Ni-based oxidant is also involved in addition to ArC(O)O•.
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