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Heme and non-heme manganese complexes can efficiently catalyze various biological and chemical oxidations in presence of some non-redox metal ions and proton acids, however, the intrinsic roles of the non-redox metal ions and proton acids in the oxidation processes still remain elusive. This work demonstrates that non-redox metal ions as Lewis acid and some proton acids can sharply improve the olefin epoxidation efficiency of a manganese(II) catalyst bearing with non-heme ligand.
Promotion effect by the addition of proton acid was studied using [MnII(TPA)Cl2] as the catalyst in olefin epoxidation, and compared with that of Lewis acid. Experiment results of EPR, FT-IR, UV-Vis and CV demonstrated that strong proton acid such as trifluoromethanesulfonic acid (HOTf) can induce the dissociation of mixed valent di-μ-oxo-bridged diamond Mn(III)(μ-O)2Mn(IV) core and formed the active moiety Mn(IV)=O, thus accelerated the catalytic epoxidation of olefin. But some proton acid, such as HOAc, showed limited promotion effect because it cannot dissociate the diamond Mn(III)(μ-O)2Mn(IV) core. Even strong proton acid such as HOTf demonstrated less effective promotion effect than non-redox metal ions (Mn+) as Lewis acids, such as Mg2+. This is related with difference of proton and Mn+ in Lewis acidity.