Nitrosation of N-methylhydroxylamine by nitroprusside: a kinetic and mechanistic study

Abstract

The kinetics of the reaction between aqueous solutions of Na 2[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 (pyCONH 2 = 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.