Sol-gel entrapped Au⁰- and Ag⁰-nanoparticles catalyze reductive de-halogenation of halo-organic compounds by BH₄⁻

This study investigated the reductive de-halogenations of toxic Br₃CCO₂⁻, Br₂CHCO₂⁻, BrCH₂CO₂⁻, CH₃CHBrCO₂⁻, CH₂BrCH₂CO₂⁻, CH₂BrCHBrCO₂⁻, Cl₃CCO₂⁻, Cl₂CHCO₂- and ClCH₂CO₂⁻ by sodium borohydride catalyzed by sol-gel silica entrapped Au⁰ and Ag⁰ nanoparticles. The results indicate that the mechanism of reduction of Br₃CCO₂⁻ differs from that of Cl₃CCO₂⁻. Calculated by DFT, the source of this difference lies in the larger bond strength of C–Cl compared to that of C–Br and the weaker M⁰–C bond strength in Au⁰-CBr₂CO₂⁻ compared to those of Au⁰-CCl₂CO₂⁻ and Au⁰-CH₂CO₂⁻. Furthermore, the de-halogenation mechanisms catalyzed by Ag⁰-NPs differ from those catalyzed by Au⁰-NPs. The latter observation is attributed to the different Ag–C and Au–C bond strengths and to the different over-potentials for H₂ release of these M⁰-NPs. In addition, product composition depends on the rate of BH₄⁻ addition. Proton labeling experiments prove that nearly all the hydrogen atoms in the products originated from the water solvent and not from the BH₄⁻. The detailed mechanistic conclusions that can be drawn from these results differ considerably from those commonly accepted for de-halogenation processes.