Oxidation of pinacyanol chloride by H₂O₂ catalyzed by Fe^III complexed to tetraamidomacrocyclic ligand: Unusual kinetics and product identification

Oxidative degradation of pinacyanol chloride (PNC) dye by H ₂O₂, 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 = (c ₁[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])/(b ₃ + [PNC]). The kinetic data has been rationalized using the adopted mechanism of catalysis by Fe^III-TAML activators, which involves the reaction between 1 and H₂O₂ to form reactive, oxidized TAML (k_I, k_-I) followed by its reaction with the dye (k_II). 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 (k_ID, 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 × 10⁵ (mol L^-1)^-1 s^-1 at pH 11 and 25°C.