Nitrooctaethylporphyrins: synthesis, optical and redox properties

The reaction of zinc octaethylporphyrin with N2O4 in dichloromethane gives, in a stepwise reaction, the zinc complexes of mono-, di-, tri-, and tetra-nitrooctaethylporphyrins. Demetallation, under acidic conditions, gives the corresponding free base porphyrins. The meso-nitro groups exert both steric and electronic effects on the porphyrin macrocycle. Thus N-protonation of the nitrated species causes a distortion of the ring and gives an optical spectrum similar to that of protonated meso-aryl-substituted porphyrins. In addition, the nitro groups make the oxidation of the porphyrin ring more difficult and at the same time facilitate the ring reductions.