Key mechanistic details of paraoxon decomposition by polyoxometalates: Critical role of para-nitro substitution
Abstract
We report computational studies of (O,O-dimethyl)-(O-4-nitrophenyl)-phosphate (DMNP) and (O,O-dimethyl)-(O-phenyl)-phosphate (DMPP) decomposition by the Zr-substituted Polyoxometalate {α-PW11O39Zr(μ-OH)(H2O)}4-, which has been recently shown to be a catalytic active species in the reaction of (Et2NH2)8[{α-PW11O39Zr(μ-OH)(H2O)}2]·7H2O with nerve agents. We studied two possible mechanisms of this reaction described as "hydrolysis first" and "OH-transfer first". Both reaction pathways are initiated from the same pre-reaction complex (H2O)-(OH)-POM-(nerve agent). The "hydrolysis first" pathway starts by the concerted dissociation of the adsorbed water molecule and nucleophilic addition of the resulting OH group to the nerve agent. Conversely, the "OH-transfer first" pathway starts by nucleophilic addition of the Zr-coordinated OH ligand to the phosphorus of the nerve agent simulant. Calculations show that the "OH-transfer first" pathway exhibits a lower energy barrier for the decomposition of DMPP by ZrPOM. Thus, the presence of a hydroxo ligand in the coordination sphere of Zr(IV) introduces a mechanism switch from "hydrolysis first" [which was recently reported for the Sarin (GB) decomposition mechanism by the hexaniobate POM Cs8Nb6O19] to "OH-transfer first". These findings imply that the pH of the catalytic solution could play a critical role and potentially control the mechanism of nerve agent and simulant decomposition by polyoxometalates. We also predict and corroborate that the presence of a strong electron-withdrawing para-substituent in the substrate phenyl group accelerates this reaction: DMNP decomposition by ZrPOM occurs with a smaller rate-limiting energy barrier. The calculations reveal several factors of the DMNP decomposition by the Zr(IV)-substituted polyoxometalates that provide design elements of Zr-based materials (including MOFs and POMs) for catalytic CWA decomposition under ambient conditions.
- Publication:
-
Chemical Physics
- Pub Date:
- February 2019
- DOI:
- 10.1016/j.chemphys.2018.11.013
- Bibcode:
- 2019CP....518...30K
- Keywords:
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- Zirconium substituted polyoxometalates (POM);
- Nerve agent simulant decomposition;
- Reaction mechanism;
- Critical role of para-nitro substitution;
- Water-splitting vs OH-transfer;
- Methyl-paraoxon