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

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.

Stacking of a Cofacially Stacked Iron Phthalocyanine Dimer on Graphite Achieved High Catalytic CH4 Oxidation Activity Comparable to That of pMMO.

Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a µ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity. The activity is almost 50 times higher than that of other potent molecule-based methane oxidation catalysts and comparable to those of certain MMOs, in an aqueous solution containing H2O2. It was demonstrated that the graphite-supported µ-nitrido-bridged iron phthalocyanine dimer oxidized methane, even at room temperature. Electrochemical investigation and density functional theory calculations suggested that the stacking of the catalyst onto graphite induced partial charge transfer from the reactive oxo species of the µ-nitrido-bridged iron phthalocyanine dimer and significantly lowered the singly occupied molecular orbital level, thereby facilitating electron transfer from methane to the catalyst in the proton-coupled electron-transfer process. The cofacially stacked structure is advantageous for stable adhesion of the catalyst molecule on the graphite surface in the oxidative reaction condition and for preventing decreases in the oxo-basicity and generation rate of the terminal iron-oxo species. We also demonstrated that the graphite-supported catalyst exhibited appreciably enhanced activity under photoirradiation owing to the photothermal effect.

High catalytic methane oxidation activity of monocationic µ-nitrido-bridged iron phthalocyanine dimer with sixteen methyl groups.

Herein, we report the highly potent catalytic methane oxidation activity of a monocationic µ-nitrido-bridged iron phthalocyanine dimer with 16 peripheral methyl groups. It was confirmed that this complex oxidized methane stably into MeOH, HCHO, and HCOOH in a catalytic manner in an acidic aqueous solution containing excess H2O2 at 60 °C. The total turnover number of the reaction reached 135 after 12 h, which is almost seven times higher than that of a monocatinoic µ-nitrido-bridged iron phthalocyanine dimer with no peripheral substituents. This suggests that the increased number of peripheral electron-donating substituents could have facilitated the generation of a reactive high-valent iron-oxo species as well as hydrogen abstraction from methane by the reactive iron-oxo species.

Application of µ-Nitrido- and µ-Carbido-Bridged Iron Phthalocyanine Dimers as Cathode-Active Materials for Rechargeable Batteries.

µ-Nitrido- and µ-carbido-bridged iron phthalocyanine dimers, when used as cathode-active materials for rechargeable lithium batteries, showed four stable redox waves in cyclic voltammetry studies in solution and a stable discharge capacity of approximately 60 mAh g-1 after 200 cycles. These results indicate that µ-heteroatom-bridged iron phthalocyanine dimers are good platforms for designing novel phthalocyanine-based electrode-active materials.

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.