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IJC-A Vol.48A(05) [May 2009] >

Title: Kinetics and mechanism of uncatalyzed oxidation of hydrazine with superoxide coordinated to cobalt(III)
Authors: Mondal, Amit
Banerjee, Rupendranath
Keywords: Kinetics
Issue Date: May-2009
Publisher: CSIR
IPC CodeInt. Cl.8 CO7B33/00
Abstract:  Hydrazine, in deoxygenated aqueous solutions at moderate pH 3.8-6.1, quantitatively reduces [(NH3)5CoIII( μ-superoxo)CoIII(NH3)5]5+ (I) to the corresponding peroxo complex, [(NH3)5CoIII-peroxo)CoIII(NH3)5]4+ (II) which, instead of reacting with another molecule of hydrazine quickly decomposes to Co2+ according to the overall stoichiometry: 4(I) + N2H5+ + 35H+ = 8Co2+ + 40NH4+ + N2 + 4O2. The reaction is dramatically catalyzed by Cu2+, to the degree that impurity levels of Cu2+ are sufficient to mask the direct reaction such that the direct oxidation process is inaccessible, though the reaction is not catalyzed by any of the other usual transition metal centers. Dipicolinic acid is a very effective inhibitor of this trace metal ion catalysis, and with 0.10 mM dipicolinic acid, the direct oxidation can be studied, which reveals an inverse [H+] dependence of reaction rate for the direct reaction. This has been ascribed to the formation of a conjugate base, [(NH3)4(NH2)CoIII-superoxo)CoIII(NH3)5]4+ as the kinetically active species. Change in buffer concentration has no effect on the rate, neither is there any observable kinetic solvent isotope effect. Enthalpy of activation has a moderate value while the entropy of activation is moderately negative. A reaction scheme consistent with these kinetic and stoichiometric observations has been proposed.
Page(s): 645-649
ISSN: 0376-4710
Source:IJC-A Vol.48A(05) [May 2009]

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