Electrochemistry of Nitrite Reductase Model Compounds 6. Voltammetric and Spectroelectrochemical Studies of Iron(II) Nitrosyl Complexes with Porphyrins, Hydroprophyrins and Porphinones
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Inorganica Chimica Acta
The effect of hydroporphine and porphinone macrocycles on the voltammetry and spectroscopy of Fe(P)(NO) was examined. Two oxidation and two reduction waves were observed for all the complexes examined. The variation in the half-wave potentials fell into two categories: those where the E12 of the complex varied linearly with the E12 of the free-based, and those where the relationship was non-linear. Linear relationships were observed where the electron transfer was primarily macrocycle-centered, while non-linear relationships were observed for metal-centered processes. Of the four waves observed, the first reduction and second oxidation of Fe(P)(NO) was non-linear, while the second reduction and first oxidation waves were linear. Comparison with the voltammetry of Fe(P)(Cl) showed the same relationships. The first reduction and second oxidation waves were non-linear while the second and third reductions and the first oxidation waves were linear. For the linear relationships, where the E12 of the complex was proportional to the E12 of the free-based, the energy of the LUMO or HOMO controlled the redox potential so that the porphyrin molecular orbital was involved substantially in the redox product, if not itself oxidized or reduced. This would indicate that the first reduction of Fe(P)(NO) was centered on the Fe(NO) moiety, while the second reduction led to the formation of a porphyrin radical anion. The variation in oxidation redox potentials was consistent with previous formulations of Fe(P)(NO)+ as a π-cation radical (except for Fe(OEP)(NO)+). The variations in the E12 of the second oxidation wave would indicate that the oxidation was centered on the Fe(NO) moiety. The variation in the half-wave potentials of Fe(X-TPP)(NO), where X was a substituent at the three- or four-position on the phenyl ring, also confirmed this interpretation. The slope of the E12 values for the first wave was 0.045 V, while a value of 0.072 V was observed for the second wave. It has been previously shown that slopes of about 0.068 V were indicative of porphyrin reduction, while lower values were generally observed for metal reduction. The formation constants for pyridine with Fe(P)(NO) were also determined. For the porphyrins and hydroporphyrins, the values were small (about 0.3 to 1.1), which were consistent with other iron nitrosyl porphyrins and chlorins. The porphinone and porphinedione complexes, though, showed a much greater affinity for pyridine, with formation constants of 4.8±0.2 and 88±3. These values are one to two orders of magnitude larger than the values observed for other iron-porphyrins nitrosyl complexes, and indicate a chemical advantage for the porphinedione (heme d1) in dissimilatory nitrite reductases, where the heme is coordinated by an imidazole in the fifth position. A weak complex (K = 9) was observed between Fe(2,4-OEiBC)(NO)+ and pyridine.