Improved robustness of heterogeneous Fe-non-heme oxidation catalysts: A catalytic and EPR study

• Two heterogeneous catalysts have been synthesized by covalent grafting of two homologous non-heme Fe-complexes on SiO2.
• The heterogeneneous catalysts show a remarkable robustness and improved oxidative stability vs. the homogeneous ones.
• The differences in the ligand peripheral groups correlate with catalytic reactivity and stability.
• The coordination of CH3CN, the spin states and the formation of the LS FeIII-OOH species are determinant for the catalysis.

There is currently a rarity in production and in-depth catalytic study of heterogeneous non-heme Fe catalysts. Herein, two heterogeneous catalysts have been synthesized by covalent grafting of non-heme Fe-complexes, DPEIFeIIICl and HFEIFeIIICl, on SiO2. The catalytic performance of the obtained DPEIFeIII@SiO2 and HFEIFeIII@SiO2 materials has been systematically studied for catalytic oxidation of cyclohexene. The catalytic data show that the present non-heme Fe catalysts are functional and can achieve higher activity compared to other non-heme Fe reported so far in the literature. Importantly, the heterogeneneous catalysts show a remarkable robustness and improved oxidative stability vs. the homogeneous ones. Studies by UV–vis and EPR reveal a common mechanistic pattern: CH3CN interacts with the Fe-atom promoting the formation of a Low-Spin (S = 1/2) intermediate, in the presence of H2O2, probably a FeIII-OOH hydroperoxide. The role of radical intermediates was investigated in detail by spin-trapping techniques. Finally, taking into account the nature of oxidation products, a consistent catalytic mechanism, valid for both homogeneous and heterogeneous catalysts, is discussed.

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