Improvements in the Ligand-Exchange Reactivity ... - ACS Publications

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Improvements in the Ligand-Exchange Reactivity of Phenylethanethiolate-Protected Au25 Nanocluster by Ag or Cu Incorporation Yoshiki Niihori,† Makoto Eguro,† Ayano Kato,† Sachil Sharma,† Bharat Kumar,† Wataru Kurashige,† Katsuyuki Nobusada,‡,§ and Yuichi Negishi*,†,∥,⊥ †

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ‡ Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan ∥ Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan ⊥ Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan S Supporting Information *

ABSTRACT: This study reports improvements in the ligand-exchange reactivity of phenylethanethiolate-protected Au25 cluster (Au25(SC2H4Ph)18) following Ag or Cu incorporation. Following the synthesis of Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu, or Pd), we determined the ligand-exchange reaction rates, using octanethiol (C8H17SH) as the exchange ligand, by employing mass spectrometry. The results show that incorporating Ag and Cu enhances the ligand-exchange reactivity of the clusters. On the basis of density functional theory calculations, it is concluded that the elevated reactivity of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) results from the more highly positive charge density of metal atoms in the staple upon Ag or Cu substitution. In addition, in the presence of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu; x ≠ 0), an improvement in the ligand-exchange reaction rate was also observed for Au25(SC2H4Ph)18, even though this cluster does not include a heteroatom. This unexpected behavior is attributed to the contribution of a chemical reaction between clusters. These findings are expected to deepen our understanding of ligand-exchange reactions, and lead to design guidelines for the creation of Aun(SR)m clusters exhibiting new chemical compositions and functions, using this reaction.

1. INTRODUCTION Thiolate-protected gold clusters (Aun(SR)m) represent some of the most extensively studied metal clusters, and exhibit sizespecific physical and chemical properties not observed in bulk gold, such as photoluminescence, catalytic activity, and redox behavior.1−6 Furthermore, their geometrical structures can be elucidated, allowing exploration of the relationship between the structures and the material properties of these clusters.7−15 In addition, these clusters are highly stable both in the solution and in the solid state. Owing to these multiple factors, Aun(SR)m clusters currently attract significant attention as structural units for functional nanomaterials in a wide range of areas, ranging from basic research to practical applications.16−22 With regard to these Aun(SR)m clusters, the solubility can be adjusted using ligand-exchange reactions,23−26 in which the original surrounding ligands are swapped with other groups. It is also possible to impart specific functions, such as molecular recognition ability or responsiveness to external stimuli, through ligand-exchange reaction. In recent years, ligand© XXXX American Chemical Society

exchange reactions have also been frequently used in the sizeselective synthesis of specific Aun(SR)m clusters that are difficult to obtain by direct synthesis.27,28 Thus, ligand-exchange reactions are an extremely efficient means of creating Aun(SR)m clusters with new chemical compositions and functions. Our group has searched for methods to facilitate such ligandexchange reactions, and in this regard, we have investigated the effect of heteroatom substitution on the speed of these reactions. In a previous study, Pd-substituted Au24Pd(SC12H25)18 exhibited higher ligand-exchange reactivity than Au25(SC12H25)18.29 In the present work, we examined this substitution effect using both Ag and Cu. Specifically, we synthesized Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu, or Pd) clusters and compared the reaction rates within this series of clusters during ligand-exchange reactions with octanethiol Received: April 14, 2016 Revised: June 13, 2016

A

DOI: 10.1021/acs.jpcc.6b03812 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Negative-ion MALDI mass spectra of the products obtained by ligand-exchange reaction of C8H17SH with (a) Au25(SC2H4Ph)18, (b) Au25−xAgx(SC2H4Ph)18 (xave. = 2.7), (c) Au25−xCux(SC2H4Ph)18 (xave. = 2.7), and (d) Au24Pd(SC2H4Ph)18 clusters. The new peaks appearing on the right of the Au 25−x M x (SC 2 H 4 Ph) 18 (M = Au, Ag, Cu, or Pd) spectra are assigned to the ligand-exchanged products, Au25−xMx(SC2H4Ph)18−y(SC8H17)y.

previous work by our own group and others has established synthetic methods for Ag, Cu, and Pd substituted Au 25−x M x (SC 2 H 4 Ph) 18 (M = Ag, Cu or Pd; Figure S1).26,30−33 Therefore, in this study, we examined the impact of heteroatom substitution on the rates of ligand-exchange reactions using Au25(SC2H4Ph)18. The upper parts of Figure 1 provide the matrix-assisted laser desorption ionization (MALDI) mass spectra of the Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu, or Pd) used in this work. As shown in Figure 1a,d, Au25(SC2H4Ph)18 and Au24Pd(SC2H4Ph)18 clusters synthesized with atomic precision were employed for the reactions.26,30 In contrast, in the cases of the Au25−xAgx(SC2H4Ph)18 and Au25−xCux(SC2H4Ph)18 (Figure 1b,c), the clusters used in the reactions exhibited a distribution in the number of substituting atoms (x).31,32 The chemical composition of the latter two clusters will hereinafter be expressed using the average number of substitutions (xave.) (see Figure S2a and the Experimental Section). The ligand-exchange reactions were conducted in dichloromethane by reacting each cluster with C8H17SH at a [C8H17SH]/[Au25−xMx(SC2H4Ph)18] ratio of 500. Figure 1

(C8H17SH). The results show that the ligand-exchange reactivity of the clusters is improved by the incorporation of Ag and Cu. The substitution with several Ag or Cu atoms enhances the ligand-exchange reactivity of the cluster to approximately the same extent as does Pd substitution. On the basis of density functional theory (DFT) calculations, the high reactivity of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) clusters is attributed to the increasingly positive charge density of the metal atom in the staple upon Ag or Cu substitution. In addition, we observed an unexpected phenomenon: in samples containing Au25−xMx(SC2H4Ph)18 (M = Ag or Cu; x ≠ 0), the ligand-exchange reaction rate also increased for Au25(SC2H4Ph) 18, despite these clusters being without heteroatom substitution. This phenomenon is likely related to the exchange of ligands, metal atoms, or metal−thiolate complexes between clusters.

2. RESULTS AND DISCUSSION 2.1. Ligand-Exchange Reaction. SC2H4Ph is one of the most frequently used ligands in cluster synthesis. In addition, B


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The Journal of Physical Chemistry C Table 1. Values of yave. for Each Reaction

presents the MALDI mass spectra of the reaction products obtained from Au25(SC2H4Ph)18, Au25−xAgx(SC2H4Ph)18 (xave. = 2.7), Au25−xCux(SC2H4Ph)18 (xave. = 2.7), and Au24Pd(SC2H4Ph)18, respectively. In each case, a peak attributable to ligand-exchanged Au25−xMx(SC2H4Ph)18−y(SC8H17)y was observed in the mass spectrum following the reaction. The number of exchanged ligands (y) was observed to continuously increase with the reaction time, and these results indicate that the ligand-exchange reactions proceeded under these experimental conditions. With regard to the products obtained in this manner, the value of y exhibited a distribution, as was also observed in previous work.29,34 In section 2.2, we compare the ligand-exchange reactivity of the clusters based on comparisons of the average number of exchanged ligands (yave.) (see Figure S2b,c and the Experimental Section). 2.2. Ligand-Exchange Reactivity of Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu, or Pd). Figure 2a

cluster yave.(120 min) Au25(SC2H4Ph)18 Au25−xAgx(SC2H4Ph)18 Au25−xCux(SC2H4Ph)18 Au24Pd(SC2H4Ph)18

xave. xave. xave. xave.

= = = =

1.4 2.7 0.6 2.7

1.58 2.82 8.22 3.43 9.52 7.54

Figure 2b shows the variations in yave. over time during the reactions of Au25−xCux(SC2H4Ph)18 (xave. = 0.6 or 2.7) and Au25(SC2H4Ph)18 (Figure S5). The value of yave. showed a further increase over time with increasing Cu substitution number such that, following 120 min, the value of the Au25−xCux(SC2H4Ph)18 (xave. = 2.7) was 6.03 times that of the Au25(SC2H4Ph)18 (Table 1). This increase by Cu substitution was also confirmed to have reproducibility (Figure S4). These results indicate that Cu substitution is even more effective at improving the ligand-exchange reactivity of the cluster than Ag substitution. Figure 2c provides a comparison of the time dependence of yave. when reacting the Au25(SC2H4Ph)18, Au25−xAgx(SC2H4Ph)18 (xave. = 2.7), Au25−xCux(SC2H4Ph)18 (xave. = 2.7), and Au24Pd(SC2H4Ph)18 clusters. As noted above, previous work29 revealed that Pd substitution of Au25(SC12H25)18 improves the reactivity of the cluster. Figure 2c demonstrates that this effect also occurs when SC2H4Ph is used as the ligand. Au25−xAgx(SC2H4Ph)18 (xave. = 2.7) and Au25−xCux(SC2H4Ph)18 (xave. = 2.7) exhibited the same rate of ligand-exchange reaction as the aforementioned Au24Pd(SC2H4Ph)18. This result demonstrates that an improvement in the ligand-exchange reactivity of the cluster equivalent to that obtained with Pd substitution can be generated by substitution with a plurality of Ag or Cu atoms. 2.3. Origin of Improvements in the Ligand-Exchange Reactivities of the Clusters Following Ag or Cu Incorporation. Au25(SR)18 has a geometrical structure in which six −S(R)−[Au−S(R)−]2 staple units cover an Au13 core (Figure S1a). The ligand-exchange reaction is presumably initiated by the attack of the nucleophilic incoming ligand on an Au atom in a staple of this structure.25,26 Previous research has shown that Ag replaces Au on the surface of an Au13 core (Figure S1b), while Cu substitutes for Au in the −S(R)−[Au− S(R)−]2 staple covering the Au13 core (Figure S1c).31,35−38 To examine the impact of heteroatom substitution on the charge density of metal elements in the staple, we estimated the Mulliken populations of metal elements based on the results of DFT calculations for [Au25(SCH3)18]−, [Au24Ag(SCH3)18]−, and [Au24Cu(SCH3)18]− (Figure 3).32,39 Figure 3b shows the Mulliken populations of the metal elements in the staples of [Au25(SCH3) 18]−, [Au24Ag(SCH 3 ) 18 ] − , and [Au 24 Cu(SCH 3 ) 18 ] − . Compared with [Au25(SCH3)18]−, the average Mulliken population of the metal atoms in the staple is positively polarized in [Au24Ag(SCH3)18]− (Table 2). As described above, in the case of [Au24Ag(SCH3)18]−, Ag, having a lower electronegativity than Au (1.93 versus 2.54), is substituted at the metal core surface. It is believed that the ligand-exchange reactivity of the cluster is improved by Ag substitution (Figure 2a) because the charge density of the metal atoms in the staple is modified even when the Ag substitution takes place on the surface (Figure 3b).

Figure 2. Comparison of the time dependence of the average number of exchanged ligands (yave.) between (a) Au25(SC2H4Ph)18 and Au25−xAgx(SC2H4Ph)18 (xave. = 1.4 or 2.7), (b) Au25(SC2H4Ph)18 and Au 25−x Cu x (SC 2 H 4 Ph) 18 (x ave. = 0.6 or 2.7), and (c) Au 2 5 (SC 2 H 4 Ph) 1 8 , Au 2 5 − x Ag x (SC 2 H 4 Ph) 1 8 (x a v e . = 2.7), Au25−xCux(SC2H4Ph)18 (xave. = 2.7), and Au24Pd(SC2H4Ph)18.

summarizes the time dependence of yave. during reactions using Au 2 5 − x Ag x (SC 2 H 4 Ph) 1 8 (x a v e . = 1.4 or 2.7) and Au25(SC2H4Ph)18 (Figure S3). The value of yave. is seen to increase over time with increases in the Ag substitution number. After 120 min, the Au25−xAgx(SC2H4Ph)18 (xave. = 2.7) had a yave. value 5.20 times that of the Au25(SC2H4Ph)18 (Table 1). This increase by Ag substitution was confirmed to have reproducibility (Figure S4). These results indicate that Ag substitution is effective at improving the ligand-exchange reactivity of the cluster. C


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Figure 4. Time dependence of average number of exchanged ligands (yave.) of nonoxidized and oxidized Au25−xCux(SC2H4Ph)18 (xave. = 0.6) and Au24Pd(SC2H4Ph)18.

confirmed by comparing the ligand-exchange reaction rates of clusters having different numbers of substituents (x). As an example, Figure 5 summarizes the time dependence of yeach ave. Figure 3. (a) Optimized structures for [Au25(SCH3)18]−, [Au24Ag(SCH3)18]−, and [Au24Cu(SCH3)18]− (refs 32, 39) and (b) Mulliken populations of metal atoms in the staples of the clusters. The position of the atom at each index in (b) is depicted in (a).

Table 2. Average Mulliken Populations of Metal Atoms in the Staples of Each Cluster cluster

ave. of Mulliken charge −

[Au25(SCH3)18] [Au25−xAgx(SCH3)18]− [Au25−xCux(SCH3)18]−

+0.178 +0.187 +0.193

Regarding [Au24Cu(SCH3)18]−, Cu, which also has a lower electronegativity than Au (1.90 versus 2.54), is instead substituted in the staple (Figure S1c). The average Mulliken population of the metal atoms in the staple is more positive in [Au24Cu(SCH3)18]− than in [Au24Ag(SCH3)18]− (Table 2). It is considered that these effects are attributed to a further improvement of the rate of the ligand-exchange reaction by Cu substitution (Figure 2a). Previous studies demonstrated that the rate of a ligandexchange reaction also depends on the charge state of the cluster.29,40 As an example, the ligand-exchange reaction rate can be increased by oxidizing Au25(SC12H25)18 such that it transitions from a negatively charged cluster to a neutral cluster.29 It has been suggested that Au25−xAgx(SR)18 and Au25−xCux(SR)18 are also prepared in the form of the negatively charged [Au25−xAgx(SR)18]− and [Au25−xCux(SR)18]−.31,32,38 Thus, a similar effect of oxidation was also anticipated for the Au25−xAgx(SC2H4Ph)18 and Au25−xCux(SC2H4Ph)18 (Figure S6). In fact, an increase in reaction rate was observed upon subjecting the Au25−xCux(SC2H4Ph)18 to oxidation. Figure 4 presents a diagram comparing the ligand-exchange reaction rate of Au25−xCux(SC2H4Ph)18 (xave. = 0.6) before and after oxidation together with that of Au24Pd(SC2H4Ph)18, in which the oxidized Au25−xCux(SC2H4Ph)18 (xave. = 0.6) exhibits an even higher ligand-exchange reaction rate than the Au24Pd(SC2H4Ph)18. These results show that improved ligandexchange reactivity can be achieved by combining Cu substitution with oxidation of the clusters. 2.4. Ligand-Exchange Reactivity of Au25(SC2H4Ph)18. As detailed above, Ag or Cu substitution improves the ligandexchange reactivity of the cluster. This effect can also be

Figure 5. Time dependence of yeach ave. of (a) x = 0−3 clusters for Au25−xAgx(SC2H4Ph)18 (xave. = 1.4) and (b) x = 0−2 clusters for Au25−xCux(SC2H4Ph)18 (xave. = 0.6).

for the x = 0−3 clusters of Au25−xAgx(SC2H4Ph)18 (xave. = 1.4) and that for the x = 0−2 clusters of Au25−xCux(SC2H4Ph)18 (xave. = 0.6), respectively (Figures S7 and S8). Over the same reaction time, the ligands are exchanged slightly faster in the case of the Au25−xMx(SC2H4Ph)18, which has a greater number of substituting atoms (Table 3). These results are consistent with the above understanding regarding the improved ligandexchange reactivity induced in clusters by Ag or Cu substitution. This appraisal of the ligand-exchange reaction rates of clusters having various values of x further revealed that the reaction rate of Au25(SC2H4Ph)18 depends on the nature of the coexisting clusters. Figure 6a shows the reaction time depend ence of y e a c h a v e . for Au 2 5 (SC 2 H 4 Ph ) 1 8 in Au25−xAgx(SC2H4Ph)18 (xave. = 1.4 or 2.7) and for pure Au25(SC2H4Ph)18 (Figure S9). The ligand-exchange reaction rate of the Au25(SC2H4Ph)18 is seen to slightly increase in the order of pure Au25(SC2H4Ph)18 < Au25−xAgx(SC2H4Ph)18 (xave. = 1.4) < Au25−xAgx(SC2H4Ph)18 (xave. = 2.7). This phenomenon is more clearly observed with the Au25−xCux(SC2H4Ph)18 clusters (Figure 6b and Figure S10). These results indicate that D


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The Journal of Physical Chemistry C Table 3. Values of yeach ave. for Au25(SC2H4Ph)18−y(SC8H17)y in Each Reaction cluster yeach ave.(120 min) Au25(SC2H4Ph)18 Au25−xAgx(SC2H4Ph)18

xave. = 1.4

xave. = 2.7

Au25−xCux(SC2H4Ph)18

xave. = 0.6

xave. = 2.7

x x x x x x x x x x x x x x x

= = = = = = = = = = = = = = =

0 1 2 3 0 1 2 3 0 1 2 0 1 2 3

1.58 2.18 2.78 3.12 3.60 4.22 5.83 7.61 8.15 3.13 3.90 4.15 7.82 9.20 9.33 10.22

Figure 7. Proposed mechanism for improvement of ligand-exchange reactivity of Au25(SC2H4Ph)18: (a) ligand exchange, (b) metal exchange, and (c) exchange of metal−thiolate complex between Au25−xMx(SC2H4Ph)18 (M = Ag or Cu; x ≠ 0) and Au25(SC2H4Ph)18.

thiolate complexes does indeed occur between the clusters, we mixed Au 25−x M x (SC 2 H 4 Ph) 18 (M = Ag or Cu) and Au25(SC2H4Ph)18 to prepare Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) (Figure 8), having a different distribution of the number

Figure 6. Comparison of time dependence of number of exchanged ligands (yeach ave.) for Au25(SC2H4Ph)18−y(SC8H17)y between (a) pure Au25(SC2H4Ph)18−y(SC8H17)y and Au25−xAgx(SC2H4Ph)18−y(SC8H17)y (xave. = 1.4 or 2.7) and (b) pure Au25(SC2H4Ph)18−y(SC8H17)y and Au25−xCux(SC2H4Ph)18−y(SC8H17)y (xave. = 0.6 or 2.7).

the ligand-exchange reaction rate of Au25(SC2H4Ph)18 is increased in the presence of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu; x ≠ 0), even without heteroatom substitution. 2.5. Origin of Improvements in the Ligand-Exchange Reactivity of Au25(SC2H4Ph)18 by Ag or Cu Incorporation. Previous studies have demonstrated that ligand-exchange reactions also occur between clusters in solution.26,29 Thus, Au25(SC2H4Ph)18 is likely to exchange ligands even with Au25−xMx(SC2H4Ph)18−y(SC8H17)y (M = Ag or Cu; x ≠ 0) in solution (Figure 7a). This ligand exchange with clusters having a large number of exchanged ligands could contribute to the unexpected phenomenon described above (Figure 7a). In addition, a recent study41 reported that an exchange of metal atoms (Figure 7b) or metal−thiolate complexes (Figure 7c) could also occur between clusters in solution. To investigate whether this exchange of metal atoms or metal−

Figure 8. Time dependence of mass distribution of (a) Au25−xAgx(SC2H4Ph)18 and (b) Au25−xCux(SC2H4Ph)18. In these experiments, the initial samples (0 h) were prepared by mixing two cluster specimens having different mass distributions (Figures S11 and S12).

of substituents compared to the cluster immediately after preparation. After allowing this solution to stand, we tracked the changes in the distributions of the number of substituents (Figures S11 and S12). Figure 9 shows the changes in the distribution of the number of substitutions for Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) obtained from these experiments. The abundance ratio of Au25−xMx(SC2H4Ph)18 (x E


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can also be achieved by combining such substitution with oxidation of the clusters. The origin of the enhanced reactivity of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) can be attributed to the reduced charge density of the metal atoms in the staple owing to the incorporation of Ag or Cu. Furthermore, this work observed an unexpected phenomenon; in the presence of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu; x ≠ 0), the ligandexchange reaction rate of Au25(SC2H4Ph)18 is also increased, even without heteroatom substitution. These data offer new insights into ligand-exchange reactions, and it is hoped that they will also provide new methods of designing Aun(SR)m clusters having useful compositions and functions.

4. EXPERIMENTAL SECTION 4.1. Chemicals. All chemicals were commercially obtained and used without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) was purchased from Tanaka Kikinzoku. Silver nitrate (AgNO3), palladium(II) sodium chloride trihydrate (PdCl2·2NaCl·3H2O), tetraoctylammonium bromide ((C8H17)4NBr), sodium tetrahydroborate (NaBH 4 ), 1-dodecanethiol (C 12 H 25 SH), 1-octanethiol (C8H17SH), and dichloromethane were obtained from Wako Pure Chemical Industries. Copper(II) acetylacetonate (Cu(C5H7O2)2) was obtained from Aldrich. Methanol, acetonitrile, toluene, acetone, tetrahydrofuran (THF), and hexane were obtained from Kanto Kagaku, while 2-phenylethanethiol (PhC2H4SH) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene]malononitrile (DCTB) were purchased from Tokyo Kasei. Deionized water with a resistivity of >18 MΩ cm was used in all experimental work. 4.2. Synthesis of Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu, or Pd). Au25(SC2H4Ph)18 was synthesized by methods previously reported in the literature30 with slight modifications. HAuCl4·4H2O (0.406 mmol) was dissolved in 30 mL of THF containing (C8H17)4NBr (0.470 mmol) at room temperature. After stirring for 15 min, PhC2H4SH (2.04 mmol) was added to this solution and the resulting mixture was stirred for 30 min. A cold aqueous solution (10 mL) of NaBH4 (4.06 mmol) was then rapidly added to the solution followed by additional stirring at room temperature. After 6 h, the THF was removed by evaporation and the remaining reddish-brown powder was washed with methanol to remove excess thiol and other byproducts. The Au25(SC2H4Ph)18 product was extracted from the dried sample using acetonitrile. Au25−xAgx(SC2H4Ph)18 (xave. = 0.6 or 2.7) was synthesized by methods reported in the literature30 with a slight modification. HAuCl4·4H2O (0.726 mmol) was dissolved in 25 mL of THF containing (C8H17)4NBr (0.76 mmol) at room temperature. After stirring for 15 min, PhC2H4SH (4.7 mmol) was added to this solution and the mixture was stirred for 15 min. Subsequently, a AgNO3 aqueous solution (0.024 mmol) was added. The initial molar ratio of [HAuCl4]:[AgNO3] was varied between 24.2:0.8 and 23.5:1.5. A cold aqueous solution (5.8 mL) containing NaBH4 (8.7 mmol) was then rapidly added to the solution with additional stirring at room temperature. After 6 h, the THF solvent was evaporated and the remaining reddish-brown powder was washed with methanol to remove excess thiol and other byproducts. The Au25−xAgx(SC2H4Ph)18 was extracted from the dried sample using acetonitrile. Au25−xCux(SC2H4Ph)18 (xave. = 0.6 or 2.7) was synthesized by methods reported in the literature30 with a slight modification. HAuCl4·4H2O and Cu(C5H7O2)2 (total of 0.260 mmol) were dissolved in 10 mL of THF containing

Figure 9. Time dependence of relative intensities of peaks for each x in the mass spectra of (a) Au25−xAgx(SC2H4Ph)18 (x = 0−5; Figure 8a) and (b) Au25−xCux(SC2H4Ph)18 (x = 0−5; Figure 8b). The value of xave. at each reaction time is also plotted.

= 0 or 3−5) near both ends of the distribution evidently decreased with the passage of time. In contrast, the abundance ratio of Au25−xMx(SC2H4Ph)18 (x = 1, 2) in the intermediate region of the distribution remained unchanged or increased. After 4 h, the distribution of the number of substitutions was closer to the distribution normally seen immediately after preparation of the clusters (Figure 8).31,32 In these experiments, the average substitution number did not change substantially (Figure 9). These results confirm that Au25−xMx(SC2H4Ph)18−y(SC8H17)y (M = Ag or Cu) clusters are subject to exchange of metal atoms (Figure 7b) or metal− thiolate complexes (Figure 7c) in solution. Au25−xMx(SC2H4Ph)18−y(SC8H17)y, having a large number of exchanged ligands, could be transformed into Au25(SC2H4Ph)18−y(SC8H17)y by this exchange (Figure 7b,c). In addition, if the Au of Au25(SC2H4Ph)18−y(SC8H17)y is temporarily replaced with Ag or Cu, the rate of the ligandexchange reaction of the cluster could be provisionally accelerated. Although further studies are required to elucidate the details of this mechanism, it is considered that this exchange of metal atoms or metal−thiolate complexes could represent at least one factor causing the unexpected phenomenon described above.

3. CONCLUSIONS In conclusion, this study revealed that the substitution of Au25(SC2H4Ph)18 by Ag or Cu improves the ligand-exchange reactivity of the cluster. Even better ligand-exchange reactivity F


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The Journal of Physical Chemistry C (C8H17)4NBr (0.263 mmol) at room temperature. The initial molar ratio of [HAuCl4]:[Cu(C5H7O2)2] was set to 22.0:3.0 or 21.0:4.0. After stirring for 15 min, PhC2H4SH (1.57 mmol) was added to this solution and the mixture was stirred for 15 min, after which a cold aqueous solution (2.0 mL) of NaBH4 (3.01 mmol) was rapidly added to the solution followed by stirring at room temperature. After 3 h, the THF was evaporated and the remaining reddish-brown powder was washed with methanol to remove excess thiol and other byproducts. The Au25−xCux(SC2H4Ph)18 was extracted from the dried sample using acetonitrile. Au24Pd(SC2H4Ph)18 was synthesized by a method similar to that in the literature.26 First, Au24Pd(SC12H25)18 was obtained using a technique previously reported by our group,39 which is based on the Brust method.42 Subsequently, all Au24Pd(SC12H25)18 ligands were replaced with SC2H4Ph by reacting Au24Pd(SC12H25)18 with PhC2H4SH in dichloromethane. This was accomplished by repeating two procedures several times: a ligand-exchange reaction and removal of excess PhC2H4SH and byproducts using a 1:4 water:methanol solution. 4.3. Ligand-Exchange Reaction. In this study, C8H17SH was used as the incoming thiol because there is sufficient mass difference (∼8 Da) between SC2H4Ph and SC8H17 to allow monitoring of the exchange of the ligand via mass spectra. In these experimental trials, 0.1 μmol of Au25−xMx(SC2H4Ph)18 (M = Au, Ag, Cu or Pd) was dissolved in 600 μL of dichloromethane. To this solution, 50 μmol of C8H17SH was added and the solution was left to sit at room temperature. The concentration of the cluster solution was estimated by measuring the weight of cluster to be solved. At specific reaction times, 10 μL aliquots of the solution were taken and characterized by MALDI mass spectrometry. 4.4. Oxidation of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu). Oxidation of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) was performed by passing the clusters through a silica gel chromatography column using dichloromethane as the eluent, similar to a procedure reported in the literature.9 4.5. Characterization. MALDI mass spectra were collected using a spiral time-of-flight mass spectrometer (JEOL Ltd., JMS-S3000) with a semiconductor laser (wavelength 349 nm) with DCTB34 as the matrix. The cluster-to-matrix ratio was set to 1:1000. UV−vis absorption spectra of the clusters were recorded in toluene at ambient temperature with a spectrometer (JASCO, V-630). 4.6. Estimation of Average Number of Substitution Atoms (xave.). For Au25−xMx(SC2H4Ph)18 (M = Ag or Cu), the average number of the substitution atoms (xave.) was estimated by the following equation (Figure S2a). xave.

yeach ave. ⎛

18

=

∑ yeach ⎜⎜

I(yeach )

⎝ I(yeach = 0) + I(yeach = 1) + ... + I(yeach

y=0

⎞ ⎟ = 18) ⎟⎠ (2)

Here, yeach is the number of exchanged ligands in each doped cluster Au25−xMx(SC2H4Ph)18−y(SC8H17)y (M = Au, Ag, Cu, or Pd). For Au25−xMx(SC2H4Ph)18−y(SC8H17)y (M = Ag or Cu), the average number of exchanged ligands was initially estimated for each doped cluster (yeach ave.) using eq 2. Subsequently, the average number of exchanged ligands (yave.) was estimated by the following equation (Figure S2c). 18

yave. =

∑ yeach ave.

⎛ ⎞ I(x) yeach ave. ⎜ ⎟ ⎝ I(x = 0) + I(x = 1) + I(x = 2) + ... ⎠ =0 (3)

5. CALCULATIONS DFT calculations were performed for [Au25(SCH3)18]−, [Au24Ag(SCH3)18]−, and [Au24Cu(SCH3)18]−.32 Geometry optimizations of the clusters were carried out starting from an initial estimated structure based on single-crystal X-ray data for Au25(SC2H4Ph)18.7 The TURBOMOLE package of ab initio quantum chemistry programs 43 was utilized in all the calculations. Geometry optimizations within Ci molecular symmetry ([Au25(SCH3)18]−) or without symmetry ([Au24Ag(SCH3)18]−, [Au24Cu(SCH3)18]−) were performed employing the hybrid functional PBE0.44 The double-ζ valence quality plus polarization basis in the TURBOMOLE basis set library was adopted in the calculations along with a 60-electron (28electron) relativistic effective core potential45 for the gold (silver) atom.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03812. Geometric structure of each cluster, details of analysis, mass and optical spectra of products, time dependence of the mass distribution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-3-5228-9145. Notes

The authors declare no competing financial interest.

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⎛ ⎞ I (x ) = ∑ x⎜ ⎟ ⎝ I(x = 0) + I(x = 1) + I(x = 2) + ... ⎠ x=0

ACKNOWLEDGMENTS The authors wish to thank Mr. Masaki Yamaguchi, Mr. Yoshihiro Kikuchi, and Mr. Daisuke Shima for providing technical assistance. This work was supported by JSPS KAKENHI Grant Number JP15H00763. Funding from the Canon Foundation, the Kajima Foundation, the Nippon Sheet Foundation for Materials Science and Engineering, the Sumitomo Foundation, the Suzuki Foundation, and the Sasagawa Foundation is also gratefully acknowledged. K.N. acknowledges support by Grant-in-Aid (No. 25288012), MEXT, and by the MEXT program “Elements Strategy

(1)

Here, x is the number of substitution metal atoms and I(x) is the ion intensity (height) of Au25−xMx(SC2H4Ph)18 (M = Ag or Cu) having each x, which was estimated from mass spectral data. 4.7. Estimation of Average Number of Exchanged Ligands (yave.). For Au25(SC2H4Ph)18−y(SC8H17)y and Au24Pd(SC2H4Ph)18−y(SC8H17)y, the average number of the exchanged ligands was estimated by the following equation (Figure S2b). G


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Initiative to Form Core Research Center” (since 2012). The computation was partly performed at RCCS, Okazaki, Japan.

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