An ortho-palladated dimethylbenzylamine complex as a highly efficient turnover catalyst for the decomposition of PS insecticides. Mechanistic studies of the methanolysis of some PS-containing phosphorothioate triesters

Abstract

An ortho-palladated complex Pd(dmba)(py)(OTf) (9), or Pd(N,N-dimethylbenzylamine)(pyridine)(trifluoromethanesulfonate), was synthesized and its solution properties in methanol studied as a function of ^s_spH. In neutral solution the triflate dissociates from the complex to give a dominant form Pd(dmba)(py)(HOCH₃), and in acid the pyridine dissociates to give Pyr–H⁺ and Pd(dmba)(HOCH₃)(HOCH₃). Under basic conditions, Pd(dmba)(py)(HOCH₃) ionizes to give Pd(dmba)(py)(⁻OCH₃) from which the pyridine can dissociate to yield a mixture of a bis-methoxy-bridged dimer (Pd(dmba)(⁻OCH₃))₂ (15-dimer), and its monomer Pd(dmba)(HOCH₃)(⁻OCH₃). Kinetic studies under buffered conditions reveal that 9 is an effective catalyst for the methanolysis of fenitrothion and other PS pesticides. The active form of the catalyst is a basic one having one associated methoxide generated with an apparent ^s_spK_a of 10.8. Analysis of the change in the UV/vis spectrum as a function of ^s_spH generates a spectrophotometric ^s_spK_a of 10.8 ± 0.1. This catalytic system is shown to promote the methanolysis of fenitrothion (3), diazinon (4), quinalphos (5), coumaphos (10) and dichlofenthion (11) at 0.05 mol dm⁻³ triethyl amine buffer, ^s_spH 10.8, 25 °C, under turnover conditions where the [phosphorothioate]/[9] ratio is 48.6, 13.4, 13.4, 18.6, and 48.6 respectively. In all cases, the products were derived from displacement of the leaving group by methoxide, the second-order turnover rate constants being 36.9, 0.45, 0.12, >146.7 and 44.3 dm³ mol⁻¹ s⁻¹ respectively. An associative mechanism for the catalyzed methanolysis of the PS pesticides is proposed where a transiently coordinated SP substrate is intramolecularly attacked by the Pd^II-coordinated methoxide.