REACTIONS OF HYDROXYLAMINE WITH METAL PORPHYRINS

The reaction of hydroxylamine with a series of metal porphyrins was ex amined in methanol/chloroform media. The reductive nitrosylation react ion was observed for the manganese and iron porphyrins, leading to a n itrosyl complex that precipitated out of the solution in good isolatab le yield (80-90%). This reaction could be used synthetically for the g eneration of iron and manganese porphyrin nitrosyl complexes and was p articularly useful for making isotopically labeled nitrosyl complexes. On the other hand, Co-II(TPP) and Cr(TPP)(Cl) did not react with hydr oxylamine under anaerobic conditions. With trace amounts of oxygen, th e 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 spectroelec trochemistry of this complex were examined. The one-electron reduction of Co-III(TPP)(NH2OH)(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 v alues. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive nitrosylation reaction, wh ile cobalt(II) and chromium(III) porphyrins are unreactive. The one-el ectron oxidation of the hydroxylamine complex with a M(III) porphyrin would be expected to oxidize the N-atom in the coordinated hydroxylami ne. The oxidation of M-III(NH2OH) with the loss of a proton would form M-II((NH2O)-H-I)(+) by an internal electron transfer, which will even tually lead to M(NO). The relationship between the reductive nitrosyl reaction and the enzymatic interconversion of NO and hydroxylamine was discussed.