Zero‐Order Catalysis in TAML‐Catalyzed Oxidation of Imidacloprid, a Neonicotinoid Pesticide

Bis‐sulfonamide bis‐amide TAML activator [Fe{4‐NO 2 C 6 H 3 ‐1,2‐( N COCMe 2 N SO 2 ) 2 CHMe}] − ( 2 ) catalyzes oxidative degradation of the oxidation‐resistant neonicotinoid insecticide, imidacloprid (IMI), by H 2 O 2 at pH 7 and 25 °C, whereas the tetrakis‐amide TAML [Fe{4‐NO 2 C 6 H 3 ‐1,2‐( N COCMe 2 N CO) 2 CF 2 }] − ( 1 ), previously regarded as the most catalytically active TAML, is inactive under the same conditions. At ultra‐low concentrations of both imidacloprid and 2 , 62 % of the insecticide was oxidized in 2 h, at which time the catalyst is inactivated; oxidation resumes on addition of a succeeding aliquot of 2 . Acetate and oxamate were detected by ion chromatography, suggesting deep oxidation of imidacloprid. Explored at concentrations [ 2 ]≥[IMI], the reaction kinetics revealed unusually low kinetic order in 2 (0.164±0.006), which is observed alongside the first order in imidacloprid and an ascending hyperbolic dependence in [H 2 O 2 ]. Actual independence of the reaction rate on the catalyst concentration is accounted for in terms of a reversible noncovalent binding between a substrate and a catalyst, which usually results in substrate inhibition when [catalyst]≪[substrate] but explains the zero order in the catalyst when [ 2 ]>[IMI]. A plausible mechanism of the TAML‐catalyzed oxidations of imidacloprid is briefly discussed. Similar zero‐order catalysis is presented for the oxidation of 3‐methyl‐4‐nitrophenol by H 2 O 2 , catalyzed by the TAML analogue of 1 without a NO 2 ‐group in the aromatic ring.