Two Major Pathways in TAML-Catalyzed Degradation of Carbendazim by H₂O₂: Elimination of a Nitrogen-Rich Benzimidazole Fungicide and Its Parallel Nitration

Abstract

Oxidative catalysis is becoming a leading trend in fighting environmental pollution created, inter alia, by numerous pesticides. Among them are carbendazim (Cbz), a benzimidazole fungicide and the metabolite of other fungicides, including thiophanate methyl (Tpm). We investigated the oxidative degradation of Cbz and Tpm by H2O2 catalyzed by TAML activators including a bis-amide bis-sulfonamide variant [Fe{4-NO2C6H3-1,2-(NCOCMe2 NSO2)2CHMe}(OH2)]− (1). Studies were performed in water at pH 7.0 and 25 °C, i.e., under the conditions that correspond best to environmental demands. Two major parallel catalytic pathways were identified for Cbz, viz. typical of TAMLs’ deep oxidative destruction and rare catalytic nitration of Cbz by nitrite liberated on the oxidation of Cbz. Catalyst 1 is 4.6 times more reactive than its tetra-amide rival [Fe{4-NO2C6H3-1,2-(NCOCMe2 NCO)2CF2}(OH2)]− (2b). Kinetic, fluorescence, DFT methods all were used to understand the advantages of 1, which are discussed in detail. Several degradation products of Cbz were identified. Tpm is rapidly transformed into Cbz by 1/H2O2, mimicking metabolic processes. Mechanisms of Cbz oxidation and nitration, as well as of Tpm to Cbz conversion, are discussed.