Electrochemistry of Nitrite Reductase Model Compounds. 3. Formation and Characterization of a Bis(hydroxylamine)(tetraphenylporphyrinato)Iron(II) Complex
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American Chemical Society
The reaction of hydroxylamine with FeIII(TPP)CI and FeII(TPP) (TPP = tetraphenylprphyrinate(2-)) at room temperature and at lower temperatures (<-20 °C) was studied in this work. At room temperature, hydroxylamine reacted with both FeIII- and FeII(TPP) to form Fe(TPP)(NO). But, when the temperature was reduced to -40 °C in methylene chloride, Fe(TPP)(NH2OH)2 was stable. This complex was characterized by visible and NMR spectroscopy and by electrochemistry. The visible and NMR spectra of Fe(TPP)(NH2OH)2 were similar to other low-spin six-coordinate ferrous TPP complexes. As was consistent with this low spin (S = 0) formulation, the chemical shifts of the porphyrin and hydroxylamine resonances were independent of temperature. In addition, the resonances due to the coordinated hydroxylamine were observed upfield from Me4Si. The same Fe(TPP)(NH2OH)2 complex can be formed from Fe(TPP)Cl and hydroxylamine, where the excess hydroxylamine can reduce the iron. The Fe- (TPP)-hydroxylamine complex was stable at low temperatures, but it decomposed at room temperature to Fe(TPP)(NO). Conversely, when Fe(TPP)NO and hydroxylamine were mixed together at low temperatures, Fe(TPP)(NH2OH)2 was formed. Cyclic voltammetry of this complex was carried out at -35 °C. The first reduction wave was observed with an E1/2 value of -1.22 V vs. a Ag/AgNO3 reference electrode, while the first oxidation wave occurred at an E1/2 value of -0.18 V vs. Ag/AgNO3. Coulometric oxidation of the complex at -35 °C at 0.1 V vs. Ag/AgNO3 gave a coulometric n value of 3, and the final oxidation product was Fe(TPP)(NO). The voltammetric data were consistent with an initial formation of a ferric complex, which underwent subsequent reaction to form the nitrosyl complex.