Methylviologen-pendant iron porphyrins as models of a reduction enzyme: six-electron reduction of nitrobenzene to aniline

Methylviologen-pendant iron porphyrins, in which methylviologen was introduced to the meso-phenyl group through an amido-bridge at either the p- or m-position, were newly synthesized, in expectation that these methylviologen-pendant iron porphyrins would be good functional models of a multi-electron reductase such as nitrite reductase since the methylviologen-pendant can play the role of an electron-trapping and storage unit like the iron-sulfur cluster of the nitrite reductase. These iron porphyrins were successfully applied to the six-electron reduction of nitrobenzene to aniline, which is a model reaction of nitrite reduction to ammonia catalyzed by nitrite reductase. Both p- and m-methylviologen-pendant iron porphyrins give somewhat larger yields of aniline in the reduction of nitrobenzene and much larger yields of p- methoxyaniline in the reduction of p- nitroanisole than does normal iron tetraphenylporphyrin. Though normal iron tetraphenylporphyrin can not catalyze well the reduction of p- nitroanisole in the presence of the dioxygen molecule, these methylviologen-pendant iron porphyrins can catalyze well the reduction of p- nitroanisole and give a considerably larger yield of p- methoxyaniline even in the presence of the dioxygen molecule. These methylviologen-pendant iron porphyrins give a much larger yield of aniline in the reduction of nitrosobenzene and somewhat larger yield of aniline in the reduction of phenylhydroxylamine than does iron tetraphenylporphyrin. m-Methylviologen-pendant iron porphyrin exhibits higher catalytic activity than does the p- pendant one. The role of methylviologen moiety is discussed, based on cyclic voltammograms and UV-VIS spectra of these iron porphyrins.