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

摘要

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-(N-ethylpyridinium-2yl)-porphyrinate ([Fe-III(TEPyP)](5+)), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([Fe-III(TPPS)](3-)), and microperoxidase 11 ([Fe-III(MP11)]) were studied for different [Fe-III(Porph)]/[HA] ratios, under anaerobic conditions at neutral pH. Efficient catalytic processes leading to the disproportionation of HA by these iron(III) porphyrinates were evidenced for the first time. As a common feature. only N-2 and N2O were found as gaseous, nitrogen-containing oxidation products, while NH3 was the unique reduced species detected. Different N-2/N2O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [Fe-III(TEPyP)](5+) and [Fe-III(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [Fe-III(TPPS)](3-) led to the well characterized soluble intermediate, [Fe-II(TPPS)NO](4-). Free-radical formation was only evidenced for [Fe-III(TEPyP)](5+), as a consequence of a metal centered reduction. We discuss the catalytic pathways of HA disproportionation on the basis of the distribution of gaseous products, free radicals formation, the nature of porphyrinic intermediates, the Fe-II/Fe-III redox potential, the coordinating capabilities of each complex, and the kinetic analysis. The absence of NO2- revealed either that no HAO-like activity was operative under our reaction conditions, or that NO2-, if formed, was consumed in the reaction milieu.