Effect of porphyrin ligands on the catalytic CH4 oxidation activity of monocationic µ-nitrido-bridged iron porphyrinoid dimers by using H2O2 as an oxidant.

ABSTRACT

The µ-nitrido-bridged iron phthalocyanine homodimer is a potent molecule-based CH4 oxidation catalyst that can effectively oxidize chemically stable CH4 under mild reaction conditions in an acidic aqueous solution including an oxidant such as H2O2. The reactive intermediate is a high-valent iron-oxo species generated upon reaction with H2O2. However, a detailed comparison of the CH4 oxidation activity of the µ-nitrido-bridged iron phthalocyanine dimer with those of µ-nitrido-bridged iron porphyrinoid dimers containing one or two porphyrin ring(s) has not been yet reported, although porphyrins are the most important class of porphyrinoids. Herein, we compare the catalytic CH4 and CH3CH3 oxidation activities of a monocationic µ-nitrido-bridged iron porphyrin homodimer and a monocationic µ-nitrido-bridged heterodimer of an iron porphyrin and an iron phthalocyanine with those of a monocationic µ-nitrido-bridged iron phthalocyanine homodimer in an acidic aqueous solution containing H2O2 as an oxidant. It was demonstrated that the CH4 oxidation activities of monocationic µ-nitrido-bridged iron porphyrinoid dimers containing porphyrin ring(s) were much lower than that of a monocationic µ-nitrido-bridged iron phthalocyanine homodimer. These findings suggested that the difference in the electronic structure of the porphyrinoid rings of monocationic µ-nitrido-bridged iron porphyrinoid dimers strongly affected their catalytic light alkane oxidation activities.