Divalent Metal Ion-Catalyzed Hydrolysis of Phosphorothionate Ester Pesticides and Their Corresponding Oxonates

The divalent metal ion-catalyzed hydrolysis of thionate (PS) and oxonate (PO) organophosphorus pesticides has been examined in light of three possible catalysis mech anisms: (1) metal ion coordination of the thionate sulfur or oxonate oxygen to enhance the electrophilicity of the phosphorus electrophilic site; (2) metal ion coordination and induced deprotonation of water to create a reactive nucleophile; and (3) metal ion coordination of the leaving group to facilitate its exit. The effect of the following metals at a concentration of 1 mM was examined: CoII, NiII, CuII, ZnII, and PbII. These metal ions were chosen for their ability to complex organic ligands and inorganic nucleophiles. Of these metal ions, CuII possesses properties most suitable for all three catalytic mechanisms and serves as the most effective catalyst for the five thionate esters (chlorpyrifos-methyl, zinophos, diazinon, parathion-methyl, and ronnel) and the two oxonate esters (chlorpyrifos-methyl oxon and paraoxon) included in this study. A decrease in the degree of CuII catalysis at high pH arises from solubility limitations. PbII nearly matches CuII as a catalyst for oxonate esters, but is a less effective catalyst for thionate esters. Catalysis by CoII, NiII, and ZnII is negligible. Phenolate product analysis indicates that metal catalysis in some instances shifts hydrolysis from alkyl carbon-centered pathways to phosphorus-centered pathways.