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Journal of Coordination Chemistry
Year: 2010  |  Volume: 63  |  Issue: 14  |  Page No.: 2605 - 2618

Oxidation of pinacyanol chloride by H2O2 catalyzed by FeIII complexed to tetraamidomacrocyclic ligand: unusual kinetics and product identification

Douglas A. Mitchell, Alexander D. Ryabov, Soumen Kundu, Arani Chanda and Terrence J. Collins    

Abstract: Oxidative degradation of pinacyanol chloride (PNC) dye by H2O2, as catalyzed by the 1 FeIII-TAML activator (TAML = tetraamidomacrocyclic ligand), occurs rapidly in water, goes to completion, and exhibits noticeably complex kinetics at pH 11. The reaction achieves a carbon mineralization of 51%. The detected products are acetate, formate, oxalate, maleate, 2-nitrobenzoate, nitrite, and nitrate. The catalytic reaction is a first-order process in 1 and the reaction rate has a Michaelis dependence in hydrogen peroxide (H2O2). The reaction rate increases sharply with increasing PNC concentration, reaches a maximal value, and then declines as the PNC concentration is further increased. The initial rate (v) versus [PNC] profile has been quantified in terms of the equation: v = (c1[PNC] + c2[PNC]2)/(c3 + c4[PNC] + [PNC]2) which accounts for the maximum and further rate decline. Kinetic analysis at a more acidic pH (9 vs. 11) revealed that there is no initial rate increase and only the hyperbolic retardation by PNC is observed, in accord with the rate law v = (b1 + b2[PNC])/(b3 + [PNC]). The kinetic data has been rationalized using the adopted mechanism of catalysis by FeIII-TAML activators, which involves the reaction between 1 and H2O2 to form reactive, oxidized TAML (kI, k-I) followed by its reaction with the dye (kII). The minimalistic addition to the scheme to account for the PNC case is the assumption that 1 may rapidly and reversibly associate with PNC (K), and the associated complex reacts also with H2O2 (kID, k-ID) to form also the oxidized TAML. Spectral evidence for this association is presented. The optimization of PNC structure by density functional theory rules out coordination of PNC to 1 via the formation of a Fe-N bond. The kinetic data indicate that the rate constant kII exceeds 1 times 105 (mol L-1)-1 s-1 at pH 11 and 25°C.

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