Electrochemistry of Nitrite Reductase Model Compounds. 2. Formation of an Iron Bis-Nitro Porphyrin Complex
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American Chemical Society
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The reaction of FeIII(P)CI and FeIII(P)(NO,) (where P = tetraphenylporphyrin (TPP), tetraphenylchlorin (TPC), and octaethylporphyrin (OEP)) with nitrite was studied by UV/visible, proton NMR, and infrared spectroscopy and by electrochemistry. Quantitative analysis of the visible spectra generated by the titration of ferric-porphyrin complexes with nitrite showed the appearance of mono- and bis-nitrite complexes. The red shift of the Soret band for the bis-nitrite complexes of FeIIIPP and -TPC is consistent with an anionic porphyrin complex. If a stronger axial ligand such as chloride was present, it was difficult to observe the mono-nitrite intermediate. Proton NMR of the titration of Fe(TPP)(NO3) with nitrite in DMF showed that both the mono and bis complexes were low spin, while the infrared spectrum was consistent with a Fe-N-bonded nitro complex. The cyclic voltammograms at slow scan rates of Fe(TPP)(NO3) in DMF and Me2SO in the presence of nitrite showed reversible waves, and the shift in the E1/2 values was consistent with the formation of mono- and bis-nitro complexes, with no coordination of the ferrous complex with nitrite. The formation constants calculated voltammetrically were consistent with the spectroscopically obtained values. The presence of the mono- and bis-nitro complexes was also seen for high concentrations of nitrite (about 20 mM) where, at high scan rates, a second reduction wave, which grew at the expense of the wave seen at low scan rates, was observed to be about 150 mV more negative than the original ferric/ferrous wave. Thus, the reduction must proceed by dissociation of the bis-nitrite complex to the mono-nitrite complex prior to reduction. At high scan rates, the dissociation of the bis-nitrite complex is too slow for the reduction to proceed exclusively through that mechanism, and a new wave is seen for the direct reduction of the bis-nitrite complex. A single oxidation wave was observed at all scan rates. For Fe(OEP)(NO3), there is evidence that one of the nitrite ligands remains coordinated in the ferrous oxidation state. In methylene chloride, the ferric/ferrous wave was irreversible, and the details of the electrochemistry are still under investigation. For all porphyrins and solvents studied, a new wave was seen at about -0.9 V vs. Ag/AgCl when nitrite was present. At this potential, one expects either NO or Fe(L)(NO) to reduce. At this time, the evidence points to the origin of this wave being due to NO and not Fe(L)(NO) when the solvent is DMF. During prolonged electrolysis, as in coulometry, though, a significant amount of Fe(L)(NO) was eventually formed.