Catalytic oxidation of nitric oxide and nitrite mediated by water-soluble high-valent iron porphyrins at an ITO electrode

A stepwise catalytic oxidation for nitric oxide and nitrite by water-soluble iron(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (FeIII(4-TMPyP)) was first revealed by using an ITO electrode. Electrochemical and spectroelectrochemical studies characterized the formation of oxoiron(IV) porphyrin and oxoiron(IV) porphyrin π-cation radical in the oxidation of FeIII(4-TMPyP). OFeIV(4-TMPyP) showed selective catalytic oxidation towards NO against NO2− with a normalized catalytic current of 3.4. The electrogenerated OFeIV(4-TMPyP) π-cation radical effectively catalyzed the oxidation of NO2− to NO3− via nitrogen dioxide (NO2) intermediate. Product analysis clarified that nitrite was the sole product in the catalytic oxidation of NO by OFeIV(4-TMPyP) electrogenerated at 0.85 V vs. Ag|AgCl, but nitrate was the final product after electrolysis at 1.1 V vs. Ag|AgCl. The different chemical reactivities of high-valent iron porphyrins electrogenerated at the separated potentials indicated the possibility of selective electrochemical detection of NO. A mechanism of typical chemical catalysis was proposed for the catalytic oxidations of NO and NO2− by oxoiron(IV) porphyrin complexes.