Alkane hydroxylation by m-chloroperbenzoic acid catalyzed by nickel(II) complexes of linear N4-tetradentate ligands

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 similar to 80 %), where the nickel(II) complex of 6-membered cyclic diamine with (2-pyridyl)methyl sidearms (Ni(L6(Pym2))(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 C-H bond activation from the substrate. Counter anion of complexes affected the initial reaction rate, indicating that Ni-II-m-CPBA adduct is a key intermediate in catalytic cycle. In this system, ArC(O)O center dot (aroyloxyl radical), which is generated by homolytic cleavage of O-O 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 center dot.

Automated summary

In this study, a series of nickel(II) complexes were synthesized and used for catalyzing the oxidation of alkanes with m-CPBA. It was found that the nickel(II) complex with a 6-membered cyclic diamine and (2-pyridyl)methyl sidearms showed the highest catalytic activity and selectivity.