Exploration of Formation and Size-Evolution Pathways of Thiolate-Gold Nanoclusters in the CO-Directed [Au₂₅(SR)₁₈]⁻ Synthesis

An intermolecular association and decarboxylation mechanism is proposed to understand the experimental evidence of the stepwise 2e⁻ hopping in the reductant-assisted thiolate-gold cluster synthesis. Based on the newly proposed intermolecular reaction mechanism, a total of 19 molecular-like reaction equations are deduced to account for the bottom-up formation of 2e⁻–8e⁻ gold nanoclusters in the CO-directed [Au₂₅(SR)₁₈]⁻ synthesis. With these established reaction equations, atomic pathways of three prototype cluster-size evolution reactions are comprehensively explored in the course of [Au₂₅(SR)₁₈]⁻ synthesis, namely, the conversion of 0e⁻ homoleptic Au^(I)-SR complexes to the 2e⁻ intermediate Au₁₅(SR)₁₃ cluster, the size-evolution of 2e⁻ Au₁₅(SR)₁₃ cluster to the 4e⁻–8e⁻cluster (stepwise 2e⁻-hopping), and the isoelectronic addition reaction of [Au₂₃(SR)₁₆]⁻ to the [Au₂₅(SR)₁₈]⁻. The studies reveal that the CO can combine with the Au(I)-complex to form [Au_x(SR)_x-COOH]⁻ species in the alkaline condition, which acts as the active precursors in the 2e⁻ hopping cluster-size evolution process. Lastly, as a conceptual extension of the mechanistic studies of the CO-reduction system, a similar intermolecular reaction mechanism is proposed for the 2e⁻ reduction in the conventional “NaBH₄ reduction” system.