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 sspH. 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 sspKa of 10.8. Analysis of the change in the UV/vis spectrum as a function of sspH generates a spectrophotometric sspKa 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, sspH 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 PdII-coordinated methoxide.