Alkane hydroxylation catalyzed by a series of mononuclear nonheme iron complexes containing 4-nitropyridine ligands

We have prepared a series of mononuclear nonheme iron(II) complexes supported by TPA (TPA = tris(2-pyridylmethyl)amine) and its derivatives that have one to three nitro-groups at 4-position of the pyridine ligand, and evaluated the effect of the nitro-groups on the catalytic oxidation activity of these complexes. The introduction of nitro-group to the TPA ligand proved to decrease the product yield of cyclohexane oxidation with hydrogen peroxide as well as the product ratio of alcohol to ketone. During the catalytic reaction, a significant amount of Fe(II) species exists in cases of nitro-substituted complexes, while the parent Fe–TPA complex was rapidly oxidized to Fe(III) state. These results suggest that the introduction of nitro-groups prevents from the generation of cis-oxo-hydroxo-iron(V) species, but induces homolysis of the FeO bond of Fe(III)OOH species to initiate radical oxidation of cyclochexane.

Graphical abstractCatalytic oxidation of cyclohexane with H2O2 was examined by using a series of iron(II) complexes supported by tris(2-pyridylmethyl)amine and its nitro-substituents as a catalyst. The introduction of nitro-groups retarded the generation of cis-oxo-hydroxo-iron(V) species, leading to a decrease in turnover number, and decreased the A/K ratio close to 1, probably through homolysis of the FeO bond of Fe(III)OOH species.Figure optionsDownload full-size imageDownload as PowerPoint slide