Nucleophilic Addition Reactions of the Nitroprusside Ion - The Case of O-Methylhydroxylamine

• Published in: European journal of inorganic chemistry Vol. 2012; no. 28; pp. 4433 - 4438
• Main Authors: Gutiérrez, María M., Olabe, José A., Amorebieta, Valentín T.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.10.2012; WILEY‐VCH Verlag
• Subjects: Amines; Density functional calculations; Iron; Kinetics; Nucleophilic addition
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201200387

The kinetics of the reaction between aqueous solutions of [Fe(CN)5NO]2– and NH2OCH3 has been studied by means of UV/Vis spectroscopy and complementary solution techniques, FTIR/ATR spectroscopy, mass spectrometry, and isotopic labeling ([Fe(CN)515NO]2–), in the pH range 6.0–9.3, I = 1 M (NaCl). The main products were nitrous oxide (N15NO), CH3OH, and [FeII(CN)5H2O]3–, characterized as the [FeII(CN)5(pyCONH2)]3– complex (pyCONH2 = isonicotinamide). In excess NH2OCH3, the kinetic runs indicated pseudo‐first‐order behavior, with corresponding rate constants, kobs [s–1], that correlated linearly with the concentration of NH2OCH3. The rate law is: R = kexp[Fe(CN)5NO2–][NH2OCH3], with kexp = (4.1 ± 0.4) × 10–4 M–1 s–1 at 25 ± 0.2 °C, ΔH# = 27 ± 1 kJ mol–1, and ΔS# = –220 ± 5 J K–1 mol–1, at pH 7.1. The value of kexp is much lower than that for similar addition reactions of NH2OH and of the N‐substituted methyl derivative. In addition, the latter reactions exhibit a third‐order rate law with a linear dependence of R on the concentration of OH–. The reaction profile has been modeled by density functional theoretical methodologies, providing mechanistic evidence on the different reaction steps, namely, the adduct formation and subsequent decomposition processes, with valuable comparisons with the reactivity of other nucleophiles with [Fe(CN)5NO]2–. The kinetics of the reaction comprising the reversible addition of NH2OCH3 to [Fe(CN)5NO]2– and the subsequent decompositions to [Fe(CN)5H2O]3–, CH3OH, and N2O have been studied. The results provide theoretical evidence on the formation of an η2‐N2O intermediate.