The spectroelectrochemistry of iron porphinones and their nitrosyl complexes were examined by infrared spectroscopy, as well as ferrous octaethylporphyrin nitrosyl. With the use of d(8)-THF, the solvent was transparent down to 1200 cm(-1). For the porphinones, the reduction of the macrocycle ring could be observed by the changes in the nu(CO) band and, for the nitrosyl complex, the changes in the nitrosyl ligand were directly observable from the nu(NO) band. Formation of the ferrous complexes led to a small downshift in the nu(CO) band. Further reduction to the formal Fe("I") complex led to more complex spectra which were interpreted with the help of density functional theory (DFT) calculations. The reduction of Fe(OEP)(NO) and its porphinone analogues was also examined. The reduction of the iron porphyrin and porphinone nitrosyl complexes lead to substantial decreases in the nu(NO) band from 1665 to 1670 cm(-1) to 1442-3 cm(-1). The energy of the nu(NO) band in the reduced complex was unaffected by the presence of carbonyl groups on the porphinone ring, indicating little additional delocalization of the electron density of the Fe-NO moiety because of the carbonyl groups. The identity of the nu(NO) bands was confirmed with (15)N substitution of the Fe(OEP)(NO) complex. The nu(CO) band on the porphinone ring was found to be sensitive to the degree than electron density was delocalized to the ring.

The reaction of hydroxylamine with a series of metal porphyrins was examined in methanol/chloroform media. The reductive nitrosylation reaction was observed for the manganese and iron porphyrins, leading to a nitrosyl complex that precipitated out of the solution in good isolatable yield (80-90%). This reaction could be used synthetically for the generation of iron and manganese porphyrin nitrosyl complexes and was particularly useful for making isotopically labeled nitrosyl complexes. On the other hand, Co(II)(TPP) and Cr(TPP)(Cl) did not react with hydroxylamine under anaerobic conditions. With trace amounts of oxygen, the reaction of Co(II)(TPP) with hydroxylamine led to the formation of a stable cobalt(III)-bis(hydroxylamine) complex. The infrared, resonance Raman, and proton NMR spectra were consistent with a cobalt(III)-bis(hydroxylamine) complex. The cyclic voltammetry and visible spectroelectrochemistry of this complex were examined. The one-electron reduction of Co(III)(TPP)(NH(2)OH)(2)(+) formed Co(II)(TPP), for which there was no evidence for the coordination of hydroxylamine. Further reduction led to Co(I)(TPP)(-), which reacted with the halogenated solvent to form a cobalt-alkyl complex. The difference in the reactivity of these four metal porphyrins with hydroxylamine correlated well with their E(1/2) values. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive nitrosylation reaction, while cobalt(II) and chromium(III) porphyrins are unreactive. The one-electron oxidation of the hydroxylamine complex with a M(III) porphyrin would be expected to oxidize the N-atom in the coordinated hydroxylamine. The oxidation of M(III)(NH(2)OH) with the loss of a proton would form M(II)(N(I)H(2)O)(+) by an internal electron transfer, which will eventually lead to M(NO). The relationship between the reductive nitrosyl reaction and the enzymatic interconversion of NO and hydroxylamine was discussed.

Abstract:The reaction of nitrite with ferric and ferrous porphyrins was examined using visible, infrared and NMR spectroscopy. Solutions of either ferric or ferrous porphyrin were stable in the presence of nitrite, with only complexation reactions being observed. Under voltammetric conditions, though, a rapid reaction was observed between nitrite and iron porphyrins to form the nitrosyl complex, Fe(P)(NO), where P = porphyrin. The products of the reduction of ferric porphyrins in the presence of nitrite were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)]2O. Visible, NMR and infrared spectroscopy were used to rule out the formation of Fe(P)(NO) by the iron catalyzed disproportionation of nitrite.NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.Inorganica Chimica Acta, Vol. 314, No. 1-2 (March 2001): pg. 49-57. Publisher's link. This article is © Elsevier and permission has been granted for this version to appear in e-Publications@Marquette. Elsevier does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Elsevier. 2A reaction between iron porphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction was monitored by visible spectroscopy, and the reaction was found to be firstorder with respect to Fe(OEP)(Cl) and Fe(OEP). The products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upon the concentration of nitrite, indicating that the coordinated, not the free nitrite, was the reaction species. The observed kinetics was consistent with a mixed oxidation state nitrite bridged intermediate, which carried out the oxygen transfer reaction from nitrite to the iron porphyrin. The effect of nitrite coordination on the reaction rate was examined.