Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study?

Dalton Transactions Pub Date: 2008-07-29 DOI: 10.1039/B805329D

Abstract

The kinetics of the reaction between aqueous solutions of Na2[Fe(CN)5NO]·2H2O (sodium pentacyanonitrosylferrate(II), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling (15NO), in the pH range 7.1–9.3, I = 1 M (NaCl). The main products were N-methyl-N-nitrosohydroxylamine (MeN(NO)OH) and [Fe(CN)5H2O]3?, characterized as the [Fe(CN)5(pyCONH2)]3? complex (pyCONH2 = isonicotinamide). No reaction occurred with Me2NOH (N,N-dimethylhydroxylamine, Me2HA) as nucleophile. The rate law was: R = kexp [Fe(CN)5NO2?] × [MeN(H)OH] × [OH?], with kexp = 1.6 ± 0.2 × 105 M?2 s?1, at 25.0 °C, and ΔH# = 34 ± 3 kJ mol?1, ΔS# = ?32 ± 11 J K?1 mol?1, at pH 8.0. The proposed mechanism involves the formation of a precursor associative complex between SNP and MeHA, followed by an OH?-assisted reversible formation of a deprotonated adduct, [Fe(CN)5(N(O)NMeOH)]3?, and rapid dissociation of MeN(NO)OH. In excess SNP, the precursor complex reacts through a competitive one-electron-transfer path, forming the [Fe(CN)5NO]3? ion with slow production of small quantities of N2O. The stoichiometry and mechanism of the main adduct-formation path are similar to those previously reported for hydroxylamine (HA) and related nucleophiles. The nitrosated product, MeN(NO)OH, decomposes thermally at physiological temperatures, slowly yielding NO.

Graphical abstract: Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study
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