Synthesis of a monocationic µ-nitrido-bridged iron porphycene dimer and its methane oxidation activity.

Herein, we report the synthesis of a monocationic µ-nitrido-bridged iron porphycene dimer, a structural analogue of a monocationic µ-nitrido-bridged iron phthalocyanine dimer, which is known to be one of the most potent molecule-based catalysts for methane oxidation. 1H-NMR and single-crystal X-ray structural analyses showed that the porphycene complex includes two Fe(IV) ions, and the structure around the Fe-NFe core is quite similar to that of the monocationic µ-nitrido-bridged iron phthalocyanine dimer. Although methane was oxidized into MeOH, HCHO, and HCOOH in the presence of a silica-supported catalyst of this monocationic µ-nitrido-bridged iron porphycene dimer in an acidic aqueous solution containing excess H2O2, its reactive intermediate was not a high-valence iron-oxo species, as in the case of a monocationic µ-nitrido-bridged iron phthalocyanine dimer, but ˙OH. It is suggested that the high-valent iron-oxo species of the µ-nitrido-bridged iron porphycene dimer was gradually decomposed under these reaction conditions, and the decomposed compound catalyzed a Fenton-type reaction. This result indicates that the stability of the oxo-species is indispensable for achieving high catalytic methane oxidation activity using a µ-nitrido-bridged iron porphyrinoid dimer with an Fe-NFe core as a catalyst.