Electrochemistry of nitrite reductase model compounds. 3. Formation and characterization of a bis(hydroxylamine)(tetraphenylporphyrinato)iron(II) complex

Abstract

The reaction of hydroxylamine with Fem(TPP)Cl and Fen(TPP) (TPP = tetraphenylporphyrinate(2-)) at room temperature and at lower temperatures (<-20 °C) was studied in this work. At room temperature, hydroxylamine reacted with both Femand Fen(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 £1/2 value of-1.22 V vs. a Ag/AgN03 reference electrode, while the first oxidation wave occurred at an E¡,2 value of -0.18 V vs. Ag/AgN03. Coulometric oxidation of the complex at -35 °C at 0.1 V vs. Ag/AgNQ3 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.

Disproportionation of hydroxylamine by water-soluble iron(III) porphyrinate compounds

“…Differing from previous results with [Fe III (TPP)] + in organic solutions [17,36] 5À after dissociation (k off = 6.3 Â 10 À4 s À1 ) [37], and reoxidation (Fig. 2).…”

Insight into Solvent Coordination of an Iron Porphyrin Hydroxylamine Complex from Spectroscopy and DFT Calculations

“…It is the aim of this report to provide a more detailed spectral characterization. Bis(hydroxylamine) complexes of ferrous porphyrins have been previously characterized at reduced temperatures . At room temperature, the bis(hydroxylamine) ferrous porphyrin complex will undergo reductive nitrosylation to form Fe(OEP)(NO) .…”

Disproportionation of O‐Benzylhydroxylamine Catalyzed by a Ferric Bis‐Picket Fence Porphyrin Complex

“…However, [Fe(TPP)(NH 2 OH) 2 ] is only marginally stable at –30 °C in dichloromethane and was never isolated. This complex was characterized in solution at –30 °C by 1 H‐NMR and UV/Vis spectroscopy and Cyclic Voltammetry 8. Interestingly, Feng and Ryan found that the reaction of excess NH 2 OH with the ferric porphyrin complexes [Fe(TPP)(X)] (X = Cl – , NO 3 – ), [Fe(OEP)(Cl)], and [Fe(PPDME)(Cl)] (OEP 2– = dianion of octaethylporphyrin, PPDME 2– = dianion of protoporphyrin(IX) dimethyl ester), and with the ferrous porphyrin [Fe(TPP)] at room temperature generates in each case a ferrous heme‐nitrosyl complex in over 80 % yield as the final product (both in dichloromethane8 and in chloroform/methanol mixtures6c).…”