Reactions of hydroxylamine with metal porphyrins

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)(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 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(NH2OH) with the loss of a proton would form M-II((NH2O)-H-I)(+) 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.