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 [Aux (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.
Keywords: 2e− hopping, cluster-size evolution reactions; nucleation mechanisms; thiolate-gold nanoclusters.
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