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.