Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

The reaction of nitrite with ferric and ferrous porphyrins was examined usi ng visible, infrared and NMR spectroscopy. Solutions of either ferric or fe rrous porphyrin were stable in the presence of nitrite, with only complexat ion reactions being observed. Under voltammetric conditions, though, a rapi d reaction between nitrite and iron porphyrins was observed to form the nit rosyl complex, Fe(P)(NO), where P = porphyrin. The products of the reductio n of ferric porphyrins in the presence of nitrite were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)](2)O. Visible, NMR and infrared spectroscopy were used to rule out the formation of Fe(P)(NO) by t he iron-catalyzed disproportionation of nitrite. A reaction between iron po rphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction were monitored by visible s pectroscopy, and the reaction was found to be first-order with respect to F e(OEP)(CI) and Fe(OEP). The products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upo n the concentration of nitrile, indicating that the coordinated, not the fr ee nitrite, was the reaction species. The kinetics observed were consistent with a mixed oxidation state nitrite-bridged intermediate, which carried o ut the oxygen transfer reaction from nitrite to the iron porphyrin. The eff ect of nitrite coordination on the reaction rate was examined. (C) 2001 Els evier Science B.V. All rights reserved.