Copper Takes the Lead: First-Row Transition Metals and Alloys as Catalysts for Halo-Acetic Acids Reduction by Borohydride

The M⁰-NPs of the earth-abundant first-row transition metals, Fe, Co, Ni, and Cu, catalyze the reduction/dehalogenation of halo-acetic acids with BH₄^-. M(III/II)@ORMOSIL and M(III/II)/M’(II)@ORMOSIL precatalysts were prepared by ion-exchanging metal cations inside the organically modified silica matrices (ORMOSIL), using the sol-gel method. These were reduced in situ by BH₄^- to form M⁰-NPs@ORMOSIL and M⁰/M′⁰-NPs@ORMOSIL that catalyze the dehalogenation processes. The order of reactivity of the catalysts, as measured for the reduction of CClH₂CO₂^- is Cu⁰-NP@ORMOSIL > (Ni/Co)⁰-NP@ORMOSIL > (Cu/Fe)⁰-NP@ORMOSIL > Ni⁰-NP@ORMOSIL > Co⁰-NP@ORMOSIL > (Cu/Co)⁰-NP@ORMOSIL > Fe⁰-NP@ORMOSIL > (Fe/Co)⁰@ORMOSIL. The same order of reactivities has been found for the reduction of CX₃CO₂^-, X = Br or Cl, by M⁰@ORMOSIL. However, not for the M⁰/M′⁰@ORMOSIL alloys. Furthermore, the ratio of the product concentrations in the dehalogenation of CX₃CO₂^-, e.g., [fumarate + bromo-fumarate]/[acetate], changed into other orders. The exceptional reduction capability of Cu in the case of CClH₂CO₂^- underscores its superior reducing power compared to other metals, as the dehalogenation of CX₃CO₂^- involves additional radical-mediated reactions. This is attributed to the effect of the M⁰-NP on the M-C bond strength and the diffusion of the alkyl radicals on its surface.