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Zn2+-Catalyzed Methanolysis of Phosphate Triesters:  A Process for Catalytic Degradation of the Organophosphorus Pesticides Paraoxon and Fenitrothion

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journal contribution
posted on 18.10.2004, 00:00 by William Desloges, Alexei A. Neverov, R. S. Brown
The methanolyses of two neutral phosphorus triesters, paraoxon (1) and fenitrothion (3), were investigated as a function of added Zn(OTf)2 or Zn(ClO4)2 in methanol at 25 °C either alone or in the presence of equimolar concentrations of the ligands phenanthroline (4), 2,9-dimethylphenanthroline (5), and 1,5,9-triazacyclododecane (6). The catalysis requires the presence of methoxide, and when studied as a function of added NaOCH3, the rate constants (kobs) for methanolysis of Zn2+ alone or in the presence of equimolar 4 or 5 maximize at different [-OCH3]/[Zn2+]total ratios of 0.3, 0.5, and 1.0, respectively. Plots of kobs vs [Zn2+]total either alone or in the presence of equimolar ligands 4 and 5 at the [-OCH3]/[Zn2+]total ratios corresponding to the rate maxima are curved and show a nonlinear dependence on [Zn2+]total. In the cases of 4 and 5, this is explained as resulting from formation of a nonactive dimer, formulated as a bis-μ-methoxide-bridged form (L:Zn2+(-OCH3)2Zn2+:L) in equilibrium with an active monomeric form (L:Zn2+(-OCH3)). In the case of the Zn2+:6 system, no dimeric forms are present as can be judged by the strict linearity of the plots of kobs vs [Zn2+]total in the presence of equimolar 6 and -OCH3. Analysis of the potentiometric titration curves for Zn2+ alone and in the presence of the ligands allows calculation of the speciation of the various Zn2+ forms and shows that the binding to ligands 4 and 6 is very strong, while the binding to ligand 5 is weaker. Overall the best catalytic system is provided by equimolar Zn2+, 5, and -OCH3, which exhibits excellent turnover of the methanolysis of paraoxon when the substrate is in excess. At a concentration of 2 mM in each of these components, which sets the pH of the solution at 9.5, the acceleration of the methanolysis of paraoxon and fenitrothion relative to the methoxide reaction is 1.8 × 106-fold and 13 × 106-fold, respectively. A mechanism for the catalyzed reactions is proposed which involves a dual role for the metal ion as a Lewis acid and source of nucleophilic Zn2+-bound -OCH3.