Ligands Modulate Reaction Pathway in the Hydrogenation of 4‐Nitrophenol Catalyzed by Gold Nanoclusters

Abstract

Protective ligands are key components of ligand‐protected metal nanoclusters (NCs), as they offer good stability and multiple functionalities to the metal NCs in solution. Herein, we demonstrated that the ligand landscape on the NC surface could also be used to modulate the catalytic active sites of metal NCs in solution thus to direct the catalytic reaction towards desirable products through a different pathway. We found that thiolate ligands, namely, p‐mercaptobenzoic acid (p‐MBA), in Au25(p‐MBA)18 NCs could influence the reaction pathway in the catalytic hydrogenation of 4‐nitrophenol in solution. In particular, the well‐defined ligand structure of Au25(p‐MBA)18 NCs in solution provided a unique environment for the coadsorption of two substrate molecules (4‐nitrophenol) on the Au NC surface, and this ligand modulation could activate a reaction pathway involving the formation of azobenzene intermediates from the two adsorbed substrate molecules, which are missing in nanogold catalysts without protective ligands. The discovery of this alternative reaction pathway highlights the importance of ligands on nanogold catalysts in modulating their active sites and directing their catalytic reaction pathways in solution and provides good opportunities to tailor the selectivity of ligand‐protected metal NC catalysts.

The Effect of Poly(vinylpyrrolidone) (PVP) on the Au Catalyzed Reduction of p–nitrophenol: The Fundamental Role of NaBH4.

“…As mentioned above, stage β is characterized by an isosbestic point indicating the attainment of a ′quasi‐steady‐state′ condition, in agreement with the stage B depicted by Gu et al . However, the temporal evolution of the peak located at 496 nm would indicate that 4,4’‐dihydroxyazobenzene could be forming during the reaction through the so‐called condensation route, against the evidence of Gu et al., that point to the formation of p‐hydroxylaminophenol as stable intermediate. Although this option had been previously discarded under the argument that concentrations of nitroso‐ and hydroxylamino compounds were not sufficient for formation of azoxybenzenes (due to the strong adsorption of p‐hydroxylaminophenol on the particle surface), new evidences pointing to the formation of these compounds were recently reported by Nasaruddin et al.…”

“…The evolution of absorbance measured at 496 nm (depicted in Figure .c) shows that this peak grows during the α stage, remains constant during the β stage and slowly decreases along the δ stage, suggesting the formation of an intermediate product during the first stage, the achievement of a ′quasi‐steady‐state′ condition during the β stage and its consumption during the third stage. The intermediate product, based on previously reported results, could be assigned to 4,4’‐dihydroxyazobenzene produced by the incomplete hydrogenation or semi‐hydrogenation of p‐NP . Besides, as it is also shown in Figure .c, the absorbance intensity at 300 nm increases during the α and β stages attains its maximum value in the transition β‐δ and then slowly decreases as the reaction progresses in the δ stage.…”

“…After Au NPs addition, absorbance at 400 nm progressively decreases and a new peak at around 300 nm appears associated with the reaction progress. Non‐deprotonated p‐NP, possible intermediate products and p‐AmP may contribute to this peak . In some cases, the reaction only occurs after an induction time of several minutes.…”

“…A second point to consider in this reaction is the effect of the reagents mixing sequence. Only a few reports considering the effect of this variable have been recently published, even when it seems to exert a profound influence on the reaction pathway of catalytic reduction of p‐NP ,…”

“…The influence of the nature of the ligand on the reduction rate and length of the induction time has been discussed in several papers regarding the catalytic reduction of p‐NP ,,. Main effects attributed to the presence of the ligands are related with partial blocking of active sites,,, withdrawal or injection of electron density from/to the metal core, and ligand‐induced facilitated adsorption of one specific reagent on the catalyst surface (e. g. affinity of p‐NP by aromatic rings present in capping ligands) . In some cases, the presence of stabilizing ligands have shown to be beneficial for increasing the reduction rate whereas in other cases it has shown to be deleterious.…”