On the mechanism of reduction of M(H2O)mn+ by borohydride: the case of Ag(H2O)2+

Abstract

The redox potentials of M(H₂O)_mⁿ⁺/M⁰_(atom) couples are often far too negative to enable the formation of M⁰_(atom) by most reducing agents. Therefore, one has to reconsider the mechanism of formation of M⁰-NPs by the bottom-up procedure. A deep and detailed theoretical analysis of the reduction of Ag(H₂O)₂⁺ by BH₄⁻ points out that silver cations act mainly as catalysts of the reactions BH₄⁻ + 4H₂O → B(OH)₄⁻ + 4H₂. However, the transition states of the catalyzed process differ from those of the un-catalyzed process. The formation of (H₂O)Ag–H, which is the starting stage for the formation of intermediates with Ag–Ag bonds, is only a side reaction in the process. Experimental evidence of the complexity of the process is presented, by stopped-flow; at least four processes are observed prior to the formation of Ag⁰-NPs. The spectra of these intermediates differ from those of Ag⁰_atom and Ag₂⁺_aq. Though DFT calculations were performed only for silver cations, it is believed that analogous mechanisms are involved in the reductions of other cations.