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

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-(N-ethylpyridinium-2yl)-porphyrinate ([FeIII(TEPyP)]5+), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([FeIII(TPPS)]3−), and microperoxidase 11 ([FeIII(MP11)]) were studied for different [FeIII(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 N2 and N2O were found as gaseous, nitrogen-containing oxidation products, while NH3 was the unique reduced species detected. Different N2/N2O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [FeIII(TEPyP)]5+ and [FeIII(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [FeIII(TPPS)]3− led to the well characterized soluble intermediate, [FeII(TPPS)NO]4−. Free-radical formation was only evidenced for [FeIII(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 FeII/FeIII 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.