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

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-]x[MeN(H)OH]x[OH-], with kexp=1.6+/-0.2x10(5) M(-2) s(-1), at 25.0 degrees C, and DeltaH#=34+/-3 kJ mol(-1), DeltaS#=-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.