Dynamic Behavior of Thiolate-Protected GoldâSilver 38-Atom Alloy

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Dynamic Behavior of Thiolate-Protected Gold– Silver 38-Atom Alloy Clusters in Solution Yoshiki Niihori, Sayaka Hashimoto, Yuki Koyama, Sakiat Hossain, Wataru Kurashige, and Yuichi Negishi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02644 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

Dynamic Behavior of Thiolate-Protected Gold– Silver 38-Atom Alloy Clusters in Solution Yoshiki Niihori,† Sayaka Hashimoto,† Yuki Koyama,† Sakiat Hossain,† Wataru Kurashige,†,‡ and Yuichi Negishi†, ‡,* †

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1–3

Kagurazaka, Shinjuku-ku, Tokyo 162–8601, Japan ‡

Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki,

Noda, Chiba 278–8510, Japan

Corresponding Author Tel.: +81-3-5228–9145 E-mail: [email protected] (Y. Negishi)

ACS Paragon Plus Environment

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ABSTRACT. This study examined the dynamic behavior of butanethiolate (SC4H9)-protected gold–silver 38-atom alloy clusters ([Au38−xAgx(SC4H9)24]0) in solution using reversed-phase highperformance liquid chromatography and electrospray ionization mass spectrometry. The results revealed that [Au38−xAgx(SC4H9)24]0 synthesized by reacting [Au38(SC4H9)24]0 with an Ag(I)−SC4H9 complex, which is the most frequently used method to synthesize gold–silver alloy clusters, continue to undergo intercluster metal exchange even after the distribution of the chemical composition of the clusters looks unchanged in the mass spectrum. The frequency of this intercluster metal exchange in solution varied depending on the standing time in solution and the synthesis method of [Au38−xAgx(SC4H9)24]0. These findings should be taken into consideration in future studies to realize deeper understanding of the physical and chemical properties of thiolateprotected gold–silver alloy clusters in solution.


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INTRODUCTION In recent years, there has been remarkable progress in metal cluster research. Formerly, metal clusters could only be synthesized as fine particles with a distribution of the number of constituent atoms. However, it has now become possible to treat metal clusters as compounds with defined chemical compositions.1–4 Since 2007, the determination of the geometric structure of metal clusters via single-crystal X-ray diffraction (SC-XRD) has been carried out extensively.5–8 Using SC-XRD, the geometric structure of noble-metal clusters with more than 200 constituent atoms9– 11

and alloy clusters composed of various metal elements12–14 have been clarified. For the metal

clusters with known structures, thorough theoretical research has also been conducted,15–21 which has provided a deeper understanding of their novel physical and chemical properties. In this growing field, interesting new phenomena have been reported recently. Pradeep et al.22 discovered that when thiolate-protected gold clusters (Aun(SR)m) and thiolate-protected silver clusters (Agn’(SR)m’) were allowed to stand in solution, the clusters underwent instantaneous metal exchange while maintaining the number of constituent atoms to form gold–silver alloy clusters (Aun(SR)m + Agn’(SR)m’ → Aun−xAgx(SR)m + Agn’−xAux(SR)m’; Figure 1(a)).23 This discovery led to the establishment of a new method to synthesize alloy clusters.24,25 On the other hand, this behavior also leads us to wonder if metal exchange might continue to occur between clusters in solution even after the chemical composition of the Aun−xAgx(SR)m clusters looks unchanged in the mass spectrum (Figure 1(b)). If this is the case, the dynamic behavior26–28 of these clusters in solution must be taken into consideration to understand the origins of the catalytic activity29 and luminescence properties30 of Aun−xAgx(SR)m clusters in solution in addition to the geometrical structure in the solid state determined by SC-XRD. However, the dynamic behavior of Aun−xAgx(SR)m clusters in solution has hardly been investigated.31


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Figure 1. Comparison between (a) previous work and (b) this work.

In this study, we attempted to elucidate the dynamic behavior of [Au38−xAgx(SC4H9)24]0 in solution using reversed-phase high-performance liquid chromatography (RP-HPLC)32–34 and electrospray ionization mass spectrometry (ESI-MS). The results provided new information about the dynamic behavior of [Au38−xAgx(SC4H9)24]0.

METHODS Chemicals. All chemicals were purchased and used without further purification. Chloroauric acid tetrahydrate (HAuCl4∙4H2O) was purchased from Tanaka Kikinzoku. Silver nitrate (AgNO3), tetraoctylammonium bromide ((C8H17)4NBr), sodium borohydride (NaBH4), dichloromethane (CH2Cl2), and triethylamine were obtained from Fujifilm Wako Pure Chemical Industries. trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was purchased from Tokyo Chemical Industry Co., Ltd.. Methanol, ethanol, acetonitrile, acetone, diethylether, and toluene were sourced from Kanto Chemical Co., Inc. and 1-butanethiol (C4H9SH)


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was purchased from Aldrich. Pure Milli-Q water (18.2 MΩ·cm) was generated using a Merck Millipore Direct 3 UV system. Synthesis of Ag(I)−SC4H9 Complex. The Ag(I)−SC4H9 complex was synthesized according to the method reported by Zhu and colleagues.35 First, C4H9SH (4.52 mmol) and triethylamine (2 mL) were added to ethanol (7 mL) containing AgNO3 (4.52 mmol). After stirring for 30 min at room temperature, unreacted AgNO3 and C4H9SH were removed by washing with methanol and water to give the objective Ag(I)−SC4H9 complex. Synthesis of [Au38−xAgx(SC4H9)24]0 by Cluster−Metal Complex Reaction (CMCR). This method is called a “metal-exchange reaction21” or “anti-galvanic reaction22” in the literature. However, in this manuscript, we name this reaction the “cluster−metal complex reaction (CMCR)” to distinguish it from intercluster metal exchange discussed here. [Au38−xAgx(SC4H9)24]0 were synthesized according to the method reported by Zhu et al.35 with slight modification. First, Ag(I)−SC4H9 complex (200 µmol) was added to CH2Cl2 (1 mL) containing [Au38(SC4H9)24]0 (0.5 µmol), which was synthesized according to the method reported by Jin et al.36 with slight modification. The resulting solution was stirred for 1 min. After filtering to remove the precipitate, the filtrate was immediately evaporated using a vacuum line. The residue was washed with methanol, ethanol, and pure water. High-purity [Au38−xAgx(SC4H9)24]0 were obtained by separation using gel permeation chromatography (GPC). Synthesis of [Au38−xAgx(SC4H9)24]0 by Co-Reduction of Metal Ions (CRMI). [Au38−xAgx(SC4H9)24]0 were synthesized according to the method reported by Dass et al.37 with slight modification. First, toluene (30 mL) containing (C8H17)4NBr (0.55 mmol) and water (30 mL) containing metal salts (0.45 mmol, [HAuCl4]:[AgNO3] = 1:0.05) were mixed, which caused the metal ions to transfer to the toluene phase. Then, C4H9SH (5 mmol) was added to the toluene


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phase and the resulting solution was stirred for 30 min at room temperature. Subsequently, the solution was stirred for 30 min at 0 °C. Then, water (20 mL) containing NaBH4 (10 mmol), cooled to 0 °C, was added. The resulting solution was stirred for 3 h at 0 °C. The solvent was removed using a rotary evaporator. The residue was washed with a mixture of water and methanol (1:1) and then methanol to remove excess C4H9SH. The resulting product was further washed with acetonitrile to remove small Au–Ag bimetallic clusters. The product was dissolved in toluene (0.3 mL) and then acetone (1.0 mL) was slowly added. Then, acetonitrile (10 mL) was added with a burette at a rate of one drop every 10 s to the slowly stirred solution. The obtained precipitate (20 mg) was dissolved in toluene (1.0 mL) containing C4H9SH (0.5 mL). This solution was stirred for ~12 h at 80 °C.37 Highly pure [Au38−xAgx(SC4H9)24]0 were obtained from the crude product by GPC. HPLC/Mass Spectrometry (LC/MS). A Nexera HPLC system (DGU-20A online degasser, LC-30AD pump, CTO-20AC column oven, and SPD-M30A photodiode array (PDA) detector) or Shimadzu Prominence HPLC system (DGU-20A3R online degasser, LC-20AD pump, CTO-20AC column oven, and SPD-M20A PDA detector) was used as the HPLC system. A YMC Meteoric Core C18 column (150 mm × 4.6 mm; i.d. 2.7 µm)38 was used as a core–shell reversedphase column. The column temperature was fixed at 25 °C to maintain reproducibility. In this experiment, the mobile phase with a flow rate of 1.0 mL/min was gradually changed using a linear gradient program from a mixture of acetonitrile and diethylether (60:40) to 100% diethylether over 700 min.38 Chromatograms were monitored at 380 nm using a PDA detector. The optical absorption spectrum of each fraction was measured using the PDA detector in the range from 190 to 800 nm. ESI mass spectra were obtained using an ESI Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer (Bruker, Solarix). Mass spectrometry was performed in positive-ion


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mode. The observation of these clusters as positive ions was caused by their ionization during the ESI process, which has often been seen in the ESI MS of neutral clusters.39 Other Characterization Methods. An ESI mass spectrum of a mixture of [Au38−xAgx(SC4H9)24]0 was collected by an ESI FT-ICR mass spectrometer (Bruker, Solarix). In these measurements, a cluster solution with a concentration of ~10 μg/mL in a mixture of diethylether and acetonitrile was electrosprayed at a flow rate of 2000 μL/h. Matrix-assisted laser desorption/ionization (MALDI) mass spectra were recorded with a spiral time-of-flight mass spectrometer (JMSS3000, JEOL) equipped with a semiconductor laser (λ = 349 nm). DCTB was used as the MALDI matrix. To minimize cluster dissociation induced by laser irradiation, the cluster-to-matrix ratio was fixed at 1:1000. Ultraviolet–visible absorption spectra of the clusters in toluene solutions were measured at room temperature with a spectrometer (JASCO, V-670). The wavelength-dependent optical data (I(w)) were converted to energy-dependent data (I(E)) using the following equation, which conserved the integrated spectral areas: I(E) = I(w)/|∂E/∂w| ∝ I(w)×w2.

RESULTS AND DISCUSSION The most commonly used method to synthesize Aun−xAgx(SR)m clusters involves the reaction of an Aun(SR)m cluster with an Ag(I)–SR complex (CMCR)21,22; Figure S1(a)). Therefore, we first attempted to elucidate the dynamic behavior of [Au38−xAgx(SR)24]0 synthesized via this method. Butanethiolate (SC4H9) was used as the ligand. First, [Au38(SC4H9)24]0 was synthesized with atomic precision (Figure S2 and S3(a)). Then, [Au38−xAgx(SC4H9)24]0 (Figure S3(b)) was synthesized by reacting [Au38(SC4H9)24]0 with an Ag(I)−SC4H9 complex in dichloromethane. The obtained Au38−xAgx(SC4H9)24 was left in toluene at room temperature.


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Figure 2. Time dependence of positive-ion ESI mass spectra (+2 region) of [Au38−xAgx(SC4H9)24]0 synthesized by CMCR (Figure S1(a)); (a) 0 h, (b) 3 days, and (c) 6 days. Each cluster is observed as a cation because it ionized during the ESI process.1 Figure 2 shows the ESI mass spectrum of [Au38−xAgx(SC4H9)24]0 at each standing time (0 h, 3 days, and 6 days; Figure S4). In all mass spectra, peaks attributable to [Au38−xAgx(SC4H9)24]0 (x = 0–5 but mainly x = 0–3) were observed and the distributions of chemical compositions were similar to each other. This indicates that the distribution of the chemical composition of [Au38−xAgx(SC4H9)24]0 almost looks unchanged in the mass spectrum immediately after synthesis (Figure S5). Figure 3(a)–(c) display chromatograms of [Au38−xAgx(SC4H9)24]0 after each standing time. Over time, the chromatograms continuously changed; for example, the intensity of peak II decreased continuously and that of peak III increased continuously. This indicates that [Au38−xAgx(SC4H9)24]0 continued to change in solution even after the distribution of the chemical composition of the clusters looks unchanged in the mass spectrum (Figure S6). To clarify the origin of these changes in the chromatograms, the chemical composition(s) of the cluster(s) inducing each peak was determined by LC/MS33,38,40–42 (Figure S7) in which RPHPLC and ESI-MS were directly connected (Figure S8–S10). The results indicated that peak I, III, V, V', and VIII originated from [Au38−xAgx(SC4H9)24]0 (x = 0–3) with comparatively high purity


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Figure 3. Time dependence of chromatograms of [Au38−xAgx(SC4H9)24]0 synthesized by CMCR; (a) 0 h, (b) 3 days, and (c) 6 days. (d) Chromatogram showing [Au38−xAgx(SC4H9)24]0 synthesized by CRMI (0 h). The chemical compositions of the metal atoms estimated from mass spectra (Figure S8 and S20) are also included for major peaks. (Figure 3(a)). As shown in Figure 3(a), [Au38−xAgx(SC4H9)24]0 were eluted from the column in the order of x = 3 → 0. Under the separation conditions used in this work, metal clusters with high polarity eluted first.38 These results indicate that the polarity of [Au38−xAgx(SC4H9)24]0 increases with the number of Ag atoms (Figure S11 and S12).38 In contrast, peak II, IV, VI, and VII were derived from multiple [Au38−xAgx(SC4H9)24]0 clusters (Figure 3(a) and S8–S10). One reason for the appearance of these peaks might be the presence of structural isomers.38 [Au38−xAgx(SC4H9)24]0 (x = 1–3) may form multiple structural isomers with different Ag positions (Figure S13) and thereby different polarities. Thus, it can be interpreted that as these structural isomers were separated by RP-HPLC, peak II, IV, VI, and VII appeared. In fact, Au36Ag2(SC4H9)24 was present with relatively high purity in both peak V and V' of the sample immediately after


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Figure 4. Relationship between standing time and the frequency of metal exchange for (a) CMCR and (b) CRMI. synthesis (Figure 3(a) and S8). This indicates that Au36Ag2(SC4H9)24 possesses structural isomers that can be separated by RP-HPLC. However, Figure 3(a)–(c) also included phenomena that could not be explained only by the existence of these structural isomers. For example, only one peak should appear for [Au38(SC4H9)24]0, because structural isomers with quite different geometrical structures43 (Figure S14) could not be synthesized under the present synthesis conditions (Figure S15). Nevertheless, in Figure 3(a)–(c), [Au38(SC4H9)24]0 appears even at retention times shorter than that of [Au38(SC4H9)24]0 (Figure S16). This phenomenon can be explained if [Au38−xAgx(SC4H9)24]0 (x = 0–3) undergo mutual metal exchange as they pass through the column. For example, when two [Au37Ag(SC4H9)24]0 undergo metal exchange, [Au38(SC4H9)24]0 and [Au36Ag2(SC4H9)24]0 could be generated (Figure S17(iv)). Because the generated [Au38(SC4H9)24]0 was originally [Au37Ag(SC4H9)24]0, it should progress through the column more quickly than unreacted [Au38(SC4H9)24]0 and thereby its retention time should become shorter than that of unreacted [Au38(SC4H9)24]0 (Figure S17). The presence of [Au38−xAgx(SC4H9)24]0 in peak II, IV, VI, and VII


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Figure 5. Structural change caused by metal exchange between [Au38−xAgx(SC4H9)24]0 clusters.

(Figure 3(a)) can be explained by the occurrence of such a metal exchange process as the clusters move through the column (Figure S18). These results indicate that [Au38−xAgx(SC4H9)24]0 undergo intercluster metal exchange during the RP-HPLC process (Figure 4) and that the products generated in this way can be separated. Thus, in addition to [Au38−xAgx(SC4H9)24]0 with different number of Ag atoms (I, III, V, V’, and VIII), their structural isomers and products of intercluster metal-exchange reactions (II, IV, VI, and VII) could also be separated under the present separation conditions. In the chromatogram measured under such conditions, peak I, III, V’, and VIII became more distinct over time (Figure 3(a)–(c)). These results imply that [Au38−xAgx(SC4H9)24]0 obtained by CMCR frequently undergo intercluster metal exchange immediately after synthesis and that after standing for a long time, the frequency of intercluster metal exchange decreases (Figure 4 and S16).


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Dass et al.44 reported that there are preferential sites where Ag tends to locate in [Au38−xAgx(SC4H9)24]0 (Figure S2(b)). However, for the sample immediately after synthesis, not all [Au38−xAgx(SC4H9)24]0 clusters would necessarily have such thermodynamically stable geometrical structures.38 It can be assumed that the metastable [Au38−xAgx(SC4H9)24]0 formed by kinetic factors45 and [Au38−xAgx(SC4H9)24]0 with thermodynamically stable geometries (Figure S3(b)) are present as a mixture immediately after synthesis (Figure 5(a)).46 The metastable [Au38−xAgx(SC4H9)24]0 formed initially likely convert to thermodynamically stable geometries through intercluster metal exchange (and intracluster metal exchange47,48) in solution (Figure 5(b) and S19).44 Furthermore, the stable structures generated in this manner are less likely to undergo metal exchange than the metastable structures. For [Au38−xAgx(SC4H9)24]0 obtained by CMCR, the features in Figure 4 are considered to be observed because of this dynamic behavior in solution. Assuming that this interpretation is correct, [Au38−xAgx(SC4H9)24]0 synthesized under severe conditions, which are likely to generate thermodynamically stable clusters, should present a chromatogram similar to that in Figure 3(c) even for the sample immediately after synthesis. Then, next, [Au38−xAgx(SC4H9)24]0 (x = 0–5 but mainly x = 0–3) were synthesized by CRMI (Figure S1(b)); CRMI involves heating clusters at 80 °C for 12 h in the presence of an excessive amount of thiol.37 As depicted in Figure 3(d), [Au38−xAgx(SC4H9)24]0 synthesized by CRMI showed a chromatogram similar to that in Figure 3(c). Moreover, for [Au38−xAgx(SC4H9)24]0 synthesized by CRMI (Figure S20), the shape of the chromatogram hardly changed even after standing for a long time (Figure S21–23). These results strongly support the above interpretation of intercluster metal exchange (and intracluster metal exchange) causing the observed phenomena (Figure 4 and 5). Furthermore, these results demonstrate that [Au38−xAgx(SC4H9)24]0 clusters have different geometrical structures (and distributions) immediately after synthesis depending on the


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synthesis method (Figure 5 and S13) and thereby the dynamic behavior of Au38−xAgx(SC4H9)24 in solution also depends on the synthesis method (Figure 4).

CONCLUSIONS In conclusion, the dynamic behavior of [Au38−xAgx(SC4H9)24]0 in solution was successfully observed by combining RP-HPLC and ESI-MS. Accordingly, the following findings were obtained for [Au38−xAgx(SC4H9)24]0: (1) when the clusters are synthesized by frequently used CMCR, metastable species are also synthesized, and they undergo intercluster metal-exchange (and intracluster metal exchange) in solution to transform the geometrical structure into the thermodynamically stable one; (2) when CRMI is used for the synthesis, thermodynamically stable clusters are predominantly synthesized and intercluster metal exchange (and intracluster metal exchange) is relatively suppressed. The phenomena observed here are specific to the experimental conditions used in this study. Even if the same [Au38−xAgx(SC4H9)24]0 is used as starting cluster, the reaction rate should vary depending on the solvent, temperature, concentration, and pressure of the solution. However, these results are important because they clarify that it is necessary to consider these phenomena to obtain a deep understanding of the origin of the physical and chemical properties of [Au38−xAgx(SC4H9)24]0 and, more generally, Aun−xAgx(SR)m clusters.

ACKNOWLEDGMENT We thank Mr. Kosuke Wakamatsu and Ms. Marika Aoki for technical assistance. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant number JP16H04099, 16K21402, 16K17480, 17H05385, and 18H05178). This work was supported by the


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Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant 2018−115).

SUPPORTING INFORMATION DESCRIPTION Additional mass spectra, additional chromatogram, and additional optical absorption spectra.


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Dynamic Behavior of Thiolate-Protected GoldâSilver 38-Atom Alloy

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