Nucleophilic addition reactions of the nitroprusside ion - The case of O-methylhydroxylamine

Abstract

The kinetics of the reaction between aqueous solutions of [Fe(CN) 5NO] 2- and NH 2OCH 3 has been studied by means of UV/Vis spectroscopy and complementary solution techniques, FTIR/ATR spectroscopy, mass spectrometry, and isotopic labeling ([Fe(CN) 5 15NO] 2-), in the pH range 6.0-9.3, I = 1 M (NaCl). The main products were nitrous oxide (N 15NO), CH 3OH, and [Fe II(CN) 5H 2O] 3-, characterized as the [Fe II(CN) 5(pyCONH 2)] 3- complex (pyCONH 2 = isonicotinamide). In excess NH 2OCH 3, the kinetic runs indicated pseudo-first-order behavior, with corresponding rate constants, k obs [s -1], that correlated linearly with the concentration of NH 2OCH 3. The rate law is: R = k exp[Fe(CN) 5NO 2-][NH 2OCH 3], with k exp = (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 k exp is much lower than that for similar addition reactions of NH 2OH 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-. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.