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.