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

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 = k_exp [Fe(CN)₅NO²⁻] × [MeN(H)OH] × [OH⁻], with k_exp = 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.