Ligand Exchange Reactions in Thiolate-Protected Au25 Nanoclusters

Article pubs.acs.org/JPCC

Ligand Exchange Reactions in Thiolate-Protected Au25 Nanoclusters with Selenolates or Tellurolates: Preferential Exchange Sites and Effects on Electronic Structure Sakiat Hossain,† Wataru Kurashige,‡ Shota Wakayama,‡ Bharat Kumar,‡ Lakshmi V. Nair,‡ Yoshiki Niihori,‡ and Yuichi Negishi*,†,‡ †

Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278−8510, Japan Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1−3 Kagurazaka, Shinjuku-ku, Tokyo 162−8601, Japan

‡

S Supporting Information *

ABSTRACT: Ligand exchange reactions can introduce new ligands onto clusters to afford new physical/chemical properties and functions. Many studies on the ligand exchange reactions of thiolate-protected gold clusters using other chalcogenates (i.e., selenolates or tellurolates) as exchange ligands have been conducted in recent years. However, there is limited information on the preferential exchange sites and electronic structure of the exchanged products. In this study, we investigated the geometric and electronic structures of the products obtained by reacting [Au25(SC2H4Ph)18]− with PhSeH or (PhTe)2 by single-crystal X-ray structural analysis, differential pulse voltammetry, and optical absorption spectroscopy. The results revealed that these exchange reactions preferentially produce products containing substituted ligands close to the gold core. In addition, we quantitatively determined the changes in the redox potentials and optical transition energies induced by continuous ligand exchange. This systematic investigation revealed that exchange with SePh induces nonlinear changes in the electronic structure of the clusters with the number of exchanged ligands. These findings are expected to lead to the improved design guidelines to produce clusters with new functions by ligand exchange with other chalcogenates.

1. INTRODUCTION Thiolate-protected gold clusters (Aun(SR)m) with small metal cores have a geometrical/electronic structure different from that of the corresponding bulk metal.1−7 Regarding the geometric structure of Aun(SR)m clusters, a special atomic arrangement with restrained surface energy for a given volume, such as an icosahedral structure, appears in addition to the close-packed structure of bulk gold. Furthermore, Aun(SR)m clusters possess a discrete electronic structure instead of a continuous bulk structure. Because of these differences from bulk gold, small Aun(SR)m clusters exhibit size-specific physical and chemical properties, such as photoluminescence,8,9 redox behavior,3,10 and catalytic activity.11,12 In addition, these properties and functions vary considerably depending on the number of constituent metal atoms and geometrical structure of the metal core.3,12 Aun(SR)m clusters with these characteristic features have attracted much attention as new functional nanomaterials in a variety of fields ranging from basic research to applications.13−15 For these Aun(SR)m clusters, the solubility and optical properties can be modified via ligand exchange reaction with SR bearing different functional groups.16−18 In addition, it is possible to provide a cluster with specific functions such as molecular recognition ability19 or sensitivity to external stimuli.20 Furthermore, these ligand exchange reactions have © XXXX American Chemical Society

also recently been used for size-selective synthesis of Aun(SR)m clusters that are difficult to produce by direct synthesis.10,21 Thus, ligand exchange reactions are an effective method to generate Aun(SR)m clusters with new chemical compositions and functions. These ligand exchange reactions have also been extensively studied for the exchange site. Initial research used nuclear magnetic resonance spectroscopy and mass spectrometry as the main analytical techniques.22−25 These studies revealed that preferential sites exist in these ligand exchange reactions. In recent years, many investigations using other techniques, such as single-crystal X-ray structure analysis,26,27 density functional theory calculations,28−30 and reverse-phase high-performance liquid chromatography (RP-HPLC),31,32 have also been performed, providing deeper understanding of the exchange sites in ligand exchange reactions. In addition, detailed knowledge has been obtained with regard to the effect of the ligand exchange process on the electronic structure of clusters in these reactions.33,34 In addition to the introduction of SR with different functional groups, ligand exchange reactions also enable the Received: August 26, 2016 Revised: October 12, 2016

A

DOI: 10.1021/acs.jpcc.6b08636 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C synthesis of clusters containing chalcogenate ligands such as selenolates (SeR) and tellurolates (TeR).35−37 Selenium (Se) and tellurium (Te) belong to the same group as sulfur (S) in the periodic table. However, because of the different atomic radii and electronegativities of these elements, gold clusters containing SeR or TeR ligands typically form gold−ligand bonds different from those in Aun(SR)m clusters.36−38 Thus, different physical and chemical properties from those of Aun(SR)m clusters are expected for these clusters.39 For clusters containing SeR or TeR as ligands, Au 25 (SePh) 18 , 35 Au38(SeC12H25)24,36 and Au25(SC8H17)18−x(TePh)x (x = 1− 18)37 have been synthesized by ligand exchange reactions. However, there is limited information on the preferential exchange sites and the electronic structure37 of the products obtained by ligand exchange reactions involving SeR and/or TeR chalcogenates. Among Aun(SR)m clusters, [Au25(SC2H4Ph)18]− (Figure S1; cluster 1)40,41 protected by phenylethanethiolate (SC2H4Ph) has been studied the most extensively. Furthermore, cluster 1 has been used as a precursor to synthesize [Au25(SePh)18]−.35 Therefore, in this study, [Au25(SC2H4Ph)18]− was used as the precursor to study the ligand exchange reaction of Aun(SR)m clusters with PhSeH or (PhTe)2. The single-crystal X-ray structure analysis of the product revealed that the main products were the clusters in which ligand exchanges occur at the sites close to the metal core (core site; Figure S1). Furthermore, the differential pulse voltammetry (DPV) and optical absorption spectroscopy elucidated how the redox potentials and the optical transition energies vary by the continuous ligand-exchange.

Figure 1. Negative-ion ESI mass spectra of the products prepared by reaction of cluster 1 with PhSeH or (PhTe)2 in dichloromethane at room temperature for 6 h with ratios of (a) [PhSeH] to [Au25(SC2H4Ph)18] of 0.30 (cluster 2) and (b) [(PhTe)2] to [Au25(SC2H4Ph)18] of 0.75 (cluster 3). xave is the average number of substitution ligands in each cluster estimated from the mass spectra.

the ligands studied herein.40−42 Then, the substitution site was determined by single-crystal X-ray structure analysis in this work. Single crystals were grown by the vapor diffusion method.43−45 The clusters were dissolved in toluene, and ethanol was used as an antisolvent to promote crystallization. After standing for 2−6 days at room temperature, brown needle-like crystals were obtained (Figure S4). The structural analysis of the crystals showed that the counterion of the clusters ([(C8H17)4N]+) was disordered. However, the presence of [(C8H17)4N]+ was confirmed in clusters 2 and 3, indicating that they are negatively charged [Au25(SC2H4Ph)18−x(SePh)x]− and [Au25(SC2H4Ph)18−x(TePh)x]−, respectively. Figure 2a shows the framework structure of cluster 2 determined by single-crystal X-ray crystallography (Figure S5a and Tables S1−S5). Among the 18 ligand sites in [Au25(SC2H4Ph)18]−, 16 were found to be fully occupied by SC2H4Ph. The remaining ligand site was shared by both SC2H4Ph and SePh (Figure 2a). The occupancy ratio of SePh at each ligand site is summarized in Table 2. The chemical composition of the crystallized cluster estimated from the occupancy ratio of each ligand was [Au25(SC2H4Ph)17.83(SePh)0.17]− (Table S2). In the obtained structure, SePh was located close to the metal core (core site; see Figure S1). This result demonstrates that the main product in the reaction of cluster 1 with PhSeH is the cluster formed by substitution of ligands at the core site. Similar results were also obtained for the reaction between cluster 1 and (PhTe)2. Figure 2b presents the framework structure of cluster 3 (Figure S5b and Tables S1−S3 and S6 and S7). The occupancy ratio of TePh at each ligand site is summarized in Table 2. The chemical composition estimated from the occupancy ratio of each ligand was Au25(SC2H4Ph)17.58(TePh)0.42 (Table S2). In this structure, TePh was also located at the core site (Figure 2b). This indicates that the main product in the reaction of cluster 1 with (PhTe)2 is also a cluster in which the incoming ligands substitute ligands at the core site. This preference for ligand exchange at the core site has also been observed for ligand exchange reactions using RSH as a substituting ligand.27,29 However, in this last case, prolonged standing at room temperature resulted in a change of the ratio

2. RESULTS AND DISCUSSION 2.1. Preferential Exchange Sites. The ligand exchange reaction was performed by reacting cluster 1 and PhSeH or (PhTe)2 in dichloromethane. The molar ratio of [PhSeH] to [cluster 1] was set to [PhSeH]/[Au25(SC2H4Ph)18] = 0.30 (products; cluster 2) or [(PhTe)2]/[Au25(SC2H4Ph)18] = 0.75 (products; cluster 3) to determine the exchange site in low ligand-exchanged products. After 6 h of reaction (Figure S2), the excess ligand (PhSeH or (PhTe)2) and byproducts were removed from the mixture (see Section 4 for details). Figures 1a and 1b show ESI mass spectra of clusters 2 and 3, respectively. Peaks ascribed to Au25(SC2H4Ph)18−x(SePh)x (x = 1 or 2) or Au25(SC2H4Ph)18−x(TePh)x (x = 1 or 2) appeared in the mass spectra in addition to that of Au25(SC2H4Ph)18. This confirms that ligand exchange occurred in both reactions, with the number of substituting ligands limited to two at most. The average chemical compositions of clusters 2 and 3 were estimated to be Au 2 5 (SC 2 H 4 Ph) 1 7 . 5 3 (SePh) 0 . 4 7 and Au25(SC2H4Ph)17.38(TePh)0.62, respectively (Table 1; see Section 4 for details). The ligand substitution sites were examined for the synthesized clusters 2 and 3. In our previous studies of the reaction between Au24Pd(SR1)18 and R2SH (R1 and R2 are different functional group), the coordination isomers in the product were separated with high resolution by RP-HPLC, and the substitution site was determined by estimating the relative abundance ratio of the coordination isomers.31 However, because the two ligands used in this work have similar polarity, it was difficult to separate the different coordination isomers of the clusters by RP-HPLC (Figure S3). Thus, it was difficult to determine the substitution site of clusters 2 and 3 by RPHPLC. Fortunately, the cluster can be readily crystallized using B


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The Journal of Physical Chemistry C

Table 1. Average Numbers of Exchanged Ligands, Redox Potentials, and Optical Transition Energies of Clusters 1−23 redox potentialsb xave

cluster

a

Red 1

Ox 1

Ox 2

−2.92 × 10−1 −

−1.91 −

1 2

0 0.47

3

0.62

4 5 6 7 8 9 10 11 12 13 14

1.76 2.72 5.03 5.31 6.43 9.78 10.69 13.96 14.69 14.90 18

−1.85 −1.82 −1.75 −1.74 −1.75 −1.64 −1.56 −1.48 −1.48 −1.47 −1.35

−2.74 −2.66 −2.14 −2.19 −2.23 −1.70 −1.59 −1.30 −1.31 −1.24 −0.99

15 16 17

1.04 3.58 5.02

−1.87 −1.78 −1.71

−2.82 × 10−1 −2.84 × 10−1 −2.49 × 10−1

18 19 20 21 22 23

3.86 4.65 6.11 7.43 10.55 10.86

−1.81 −1.80 −1.74 −1.72 −1.64 −1.63

−2.43 −2.38 −2.13 −2.02 −1.52 −1.57

−

− × × × × × × × × × × ×

× × × × × ×

optical transition energiesc

10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1

10−1 10−1 10−1 10−1 10−1 10−1

Ox 3

[Au25(SC2H4Ph)18‑x(SePh)x] −1.48 × 10−2 7.20 × − − [Au25(SC2H4Ph)18‑x(TePh)x] − − [Au25(SC2H4Ph)18‑x(SePh)x] −1.79 × 10−2 7.15 × −1.57 × 10−2 7.17 × 0.37 × 10−2 7.28 × 0.16 × 10−2 7.28 × 0.04 × 10−2 7.29 × 1.71 × 10−2 7.39 × 1.21 × 10−2 7.36 × 0.94 × 10−2 7.28 × 1.59 × 10−2 7.33 × 0.64 × 10−2 7.20 × −9.86 × 10−2 6.80 × [Au25(SC2H4Ph)18‑x(TePh)x] −2.69 × 10−2 6.96 × −5.81 × 10−2 6.42 × −4.67 × 10−2 6.44 × [Au25(SC2H4Ph)18‑x(SPh)x] 0.93 × 10−2 7.42 × 1.08 × 10−2 7.48 × 2.32 × 10−2 7.61 × 3.58 × 10−2 7.82 × 6.33 × 10−2 8.02 × 6.23 × 10−2 8.09 ×

A

B

C

D

1.58 −

1.83 −

2.74 −

3.08 −

−

−

−

−

10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1 10−1

1.57 1.57 1.59 1.59 1.56 1.53 1.57 1.51 1.49 1.48 1.44

1.81 1.82 1.83 1.82 1.82 1.79 1.80 1.76 1.73 1.73 1.70

2.72 2.71 2.67 2.67 2.65 2.59 2.57 2.52 2.50 2.49 2.43

3.08 3.07 3.06 3.05 3.05 3.01 2.98 2.95 2.93 2.92 2.87

10−1 10−1 10−1

1.58 1.54 1.47

1.82 1.78 1.72

2.71 2.58 2.56

3.09 3.06 3.06

10−1 10−1 10−1 10−1 10−1 10−1

1.55 1.57 1.58 1.56 1.51 1.54

1.80 1.81 1.82 1.81 1.78 1.79

2.73 2.73 2.72 2.72 2.71 2.70

3.03 3.03 3.00 2.99 2.96 2.95

10−1

a

These values were estimated from ESI mass spectra of the products (see section 4). bRedox potentials (V) are averages of reduction and oxidation peak potentials in the DPV potential scans (Figures 3 and S7). cThe energy (eV) of each peak in the optical absorption spectra of the clusters (Figures 5 and S8S13).

exchange between or within the products in solution at room temperature.31 In contrast, although the products described herein were kept for an additional 2−6 days in solution to allow crystallization after reaction for 6 h, the incoming ligand was observed only at the core site for clusters 2 and 3 (Figure 2, parts a and b). This indicates that when SePh or TePh is the substituting ligand, the probability of final products with substituted core sites is higher than that when RSH is the substituting ligand. This result implies that the core-sitesubstituted products are largely more stable than that substituted at an apex site, or the release of the ligand or Au−ligand complex46 is inhibited after the generation of the core-site-substituted product for [Au25(SC2H4Ph)18−x(SePh)x]− and [Au25(SC2H4Ph)18−x(TePh)x]−.31 2.2. Effects of Ligand Exchange on the Electronic Structure of the Clusters. DPV and optical absorption spectroscopy were used to investigate the effect of continuous ligand exchange reactions with SePh or TePh on the electronic structure of the clusters. With the aim of monitoring the continuous changes of their electronic structure, clusters with a large number of substituting ligands were prepared (Table 1). The reactions were performed for a sufficiently long time (>12 h). The ESI mass spectra of the formed [Au 2 5 (SC 2 H 4 Ph) 18 − x (SePh) x ] − (clusters 4−14) and [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 15−17) are summarized in Figure S6. In the ESI mass spectra, with the exception of [Au25(SePh)18]− (cluster 14), a distribution was observed in

Figure 2. Framework structures of clusters (a) 2 and (b) 3 determined by single-crystal X-ray structure analysis. The full structures including the phenyl groups are provided in Figure S5. Yellow and orange balls represent gold atoms and green balls represent sulfur atoms. Magenta and cyan balls represent the anchor atoms of the ligand that are replaced with those of SePh or TePh with an occupancy ratio of over 4% (Table 2 and Supporting Information).

Table 2. Occupancy of SePh or TePh in Clusters 2 and 3 occupancy in each ligand site (%)a cluster

1/1′

2/2′

3/3′

4/4′

5/5′

6/6′

7/7′

8/8′

9/9′

2 3

0 0

0 0

0 12.8

0 8.1

0 0

0 0

8.5 0

0 0

0 0

a

See Figures 2 and S5.

of the core site to an apex site into a relative abundance ratio of 2:1 (proportional to the site number; Figure S1) because of C


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−2/−1, −1/0, 0/+1, and +1/+2, respectively.47−49 The peak potentials for clusters 1, 4−17 are summarized in Table 1. Figure 4 shows the dependence of the Red 1 and Ox 1−3 peak potentials of clusters 1, 4−17 on xave. The results obtained

the chemical composition of the clusters,37 as in the case of clusters 2 and 3 (Figure 1). Thus, the average number of substituting ligands (xave; see Section 4 for details) was considered to be the number of exchanged ligands in the following discussion (Table 1). Study by DPV. We examined the effect of ligand exchange on the redox potentials of the clusters by DPV. Figures 3a and 3b illustrate the DPV curves of [Au25(SC2H4Ph)18−x(SePh)x]− (clusters 1, 4−14) and [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 1, 15−17), respectively. The arrow in each curve represents the solution open-circuit potential.47 In these DPV curves, the peaks labeled as Red 1, Ox 1, Ox 2, and Ox 3 could be assigned to the redox reactions between the charge states of

Figure 4. Relationships of the peak energies of (a) Red 1, (b) Ox 1, (c) Ox 2, and (d) Ox 3 (Figures 3 and S7 and Table 1) with xave for [Au25(SC2H4Ph)18]− (cluster 1; black point), [Au 25 (SC 2 H 4 Ph) 18−x (SePh) x ] − (clusters 4−14; red points), [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 15−17; cyan points), and [Au25(SC2H4Ph)18−x(SPh)x]− (clusters 18−23; green points).

for [Au25(SC2H4Ph)18−x(SPh)x]− (0 < xave ≤ 10.9) produced by exchange between cluster 1 and SPh are also illustrated for comparison (clusters 18−23; Figure S7 and Table 1). The potential of the redox peaks changed as a function of xave for both [Au25(SC2H4Ph)18−x(SePh)x]− and [Au 25 (SC 2 H 4 Ph) 1 8− x (TePh) x ] − . Previous studies on [Au25(SR)18]− demonstrated that the redox potential of [Au25(SR)18]− varies with the ligand functional group (R).50 Indeed, changes in the redox potential were observed during the continuous exchange process from SC2H4Ph to SPh (green points in Figure 4). Figure 4 demonstrates that similar variations of redox potential also occur during continuous exchange with SePh or TePh.37 On the other hand, the shift of redox potential observed here does not necessarily coincide with that obtained during continuous exchange with SPh. The redox potentials of Red 1 and Ox 1−3 all shifted to the positive side during the exchange process from SC2H4Ph to SPh (green points in Figure 4). In the substitution of SC2H4Ph with SePh or TePh, the redox potentials of Red 1 and Ox 1 also shifted to the positive side (Figure 4, parts a and b). However, the redox potentials for Ox 2 and Ox 3 shifted to the negative side during the final stage of ligand exchange (around x = 18) (Figure 4, parts c and d).

Figure 3. DPV curves of (a) [Au25(SC2H4Ph)18−x(SePh)x]− (clusters 1, 4−14) and (b) [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 1, 15−17). In these experiments, each cluster was dissolved in 0.1 M tetrabutylammonium perchlorate in CH2Cl2. DPV was performed at room temperature. The arrow in each curve represents the solution open-circuit potential. The Ox 2 peak is presumed to overlap with the Ox 1 peak in DPV curve of [Au25(SePh)18]− (cluster 14). D


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The Journal of Physical Chemistry C These results demonstrate that substitution with SePh or TePh affects the redox potential differently to that of SPh. These systematic experiments further revealed that in the case of exchange with SePh, the redox potential changes nonlinearly with xave. As shown in Figure 4, parts c and d, the shift of the redox potentials of Ox 2 and Ox 3 changed around xave = 10. This means that the dependence of the redox potential on xave changes during exchange with SePh. The atomic radii of the anchor atoms of SC2H4Ph and SePh are different (S, 0.88 Å; Se, 1.03 Å). Therefore, the exchange of SC2H4Ph with SePh is expected to distort the framework structure of the cluster,51 which affects the orbital energies.52 The influence of framework distortion on redox potential is weak at low xave and might become considerable when xave exceeds 10. Another plausible alternative is that an increase in xave above xave = 10 might decrease distortion of the cluster framework structure. The above-mentioned preferential site for substitution might also be related to the nonlinearity of the variation of xave. Regardless of the main factor behind it, even larger nonlinearity of the change with xave is expected for the substitution with TePh, which involves Te (radius of 1.23 Å) as the anchor atom. However, it was difficult to synthesize TePhexchanged clusters with xave value larger than 5.0 (Figure S6b) in this study.37,39 It is considered that clusters with large xave are destabilized during exchange with TePh because the generated distortion is too large,37,39 so synthesis of such clusters is energetically unfavorable. Study by Optical Absorption Spectroscopy. We next examined the effect of continuous ligand exchange on the optical absorption properties of the clusters obtained by optical absorption spectroscopy. Figure 5 shows the optical absorption spectra of [Au25(SC2H4Ph)18]− (cluster 1), [Au 25 (SC 2 H 4 Ph) 18−x (SePh) x ] − (clusters 4−14), and [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 15−17) (Figure S8). In these optical absorption spectra, the optical absorption up to 2.0 eV is attributed to the transition from the orbitals around the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).41,53−56 Absorbance above 2.0 eV also includes the transitions from orbitals strongly influenced by the ligands.41,56 In the absorption region below 2.0 eV, two peaks (labeled A and B) are observed in all cases (Figure 5).57 Recent studies revealed that these two peaks are related to the difference in orbital energies of three orbitals around the HOMO.48,55,56 We estimated the maximum values of these two peaks (Figures S9− S11) by curve fitting (see Supporting Information). The xave dependence of the peak values obtained (Table 1) is presented in Figure 6a. The results obtained for [Au25(SC2H4Ph)18−x(SPh)x]− (clusters 1, 18−23) are also included in Figure 6a for comparison (Figures S12 and S13). Figure 6a reveals that both peaks shift to lower energy with increasing xave. This indicates that ligand exchange with SePh or TePh narrows the HOMO−LUMO gap of the cluster.37,51 This narrowing of the HOMO−LUMO gap occurred slightly even during the exchange from SC2H4Ph to SPh (Figure 6a).37,52 Therefore, the narrowing of the HOMO−LUMO gap caused by ligand exchange with SePh or TePh is considered to be related to the effects of both the replacement of functional groups and the exchange of the anchor atom. In contrast, the energy difference between A and B remained almost constant (∼0.25 eV) regardless of xave (Figure 6a). This indicates that ligand exchange with SePh or TePh hardly influences the difference in energies of the three orbitals around the HOMO.

Figure 5. Optical absorption spectra of (a) [Au 25 (SC 2 H 4 Ph) 18−x (SePh) x ] − (clusters 1, 4−14) and (b) [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 1, 15−17).

Similar changes in peak positions were also observed in the absorption region above 2.0 eV. Figure 6b depicts the dependence of the positions of peak C and D on xave (Figure 5, parts a and b). These peaks shifted upon exchange with SePh or TePh. The magnitudes of the shift of peak C to lower energy were greater than that in the case of exchange with SPh. Therefore, these observed shifts to lower energy are thought to be caused by the effects of both the replacement of functional groups and the exchange of the anchor atom. Similarly, exchange with SePh or TePh shifted peak D to lower energy, although the magnitudes of the shifts were smaller compared with that induced by SPh. It is considered that the exchange of anchor atoms causes peak D to shift to higher energies, resulting in a smaller shift during exchange with SePh or TePh compared with that of SPh. Figure 6 also shows that the absorption peaks vary in a nonlinear manner with xave during SePh exchange, similar to the redox potentials (Figure 4). For example, the HOMO−LUMO gap of the cluster decreases only slightly until xave ∼ 10, whereas a more pronounced decrease is observed at xave exceeding 10 (Figure 6a). A similar nonlinear change of peak E


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(PhSeH) and benzenethiol (PhSH) were purchased from Tokyo Kasei, and 2-phenylethanethiol (PhC2H4SH) from Aldrich. Deionized water with a resistivity of >18 MΩ cm was used. 4.2. Synthesis of Cluster 1 ([Au25(SC2H4Ph)18]−) and 14 ([Au25(SePh)18]−). Cluster 1 was synthesized by a method similar to that in the literature.58 First, (C8H17)4NBr (0.76 mmol) was added to THF (25 mL) containing HAuCl4·4H2O (0.75 mmol). The resulting solution was stirred for 15 min at room temperature and then PhC2H4SH (4.7 mmol) was added. The solution changed from clear red to colorless. After stirring for 15 min at room temperature, cold water (5.8 mL) containing NaBH4 (8.7 mmol) was quickly added to the solution. The solution rapidly became black, and was kept stirring at room temperature. After 12 h, the THF was evaporated and the remaining red-brown oily product was washed with a mixture of water and methanol, and then pure methanol to remove excess (C8H17)4NBr, PhC2H4SH, and other byproducts. Highly pure cluster 1 was obtained by acetonitrile extraction of the dried product. Cluster 14 was synthesized according to the method reported by Zhu and colleagues.35 First, PhSeH (1.6 mmol) was added to cluster 1 (35 mg or 4.5 μmol) in toluene (20 mL) and the resulting mixture was stirred at room temperature. After 12 h, the red-brown precipitate was collected by centrifugation. The precipitate was washed with methanol to remove excess PhSeH and other byproducts. Highly pure cluster 14 was obtained by dichloromethane extraction of the dried product. 4.3. Preparation of Clusters 2−13 and 15−23 by Ligand Exchange Reactions. All clusters used in this work except clusters 1 and 14 were prepared by ligand exchange reactions between cluster 1 and PhSeH, (PhTe)2, or PhSH in dichloromethane under atmospheric conditions.37 Clusters 2 and 3 were used for crystallization and the other clusters (clusters 1 and 4−23) were used to investigate electronic structures. In the experiments, cluster 1 (30 mg) was first dissolved in dichloromethane (20 mL). Then PhSeH, (PhTe)2, or PhSH was added to the solution with a molar ratio of [PhSeH] to [cluster 1] of 0.30 (2), 1.5 (4), 2.5 (5), 4.5 (6), 5.0 (7), 6.0 (8), 11 (9), 14 (10), 20 (11), 40 (12), or 45 (13), or a molar ratio of [(PhTe)2] to [cluster 1] of 0.75 (3), 1.5 (15), 4.5 (16), or 6.0 (17), or a molar ratio of [PhSH] to [cluster 1] of 4.0 (18), 5.0 (19), 14 (20), 18 (21), 40 (22), or 100 (23). After stirring for 12 h at room temperature, the dichloromethane was evaporated and then the dried powder was washed with methanol to remove any byproducts. The dried products were again dissolved in acetone or acetonitrile, and only the soluble products were filtered. We also conducted similar experiments while increasing the ratio of ligand to cluster 1 in attempts to synthesize [Au25(TePh)18]− and [Au25(SPh)18]− in which all ligands are substituted with incoming ligands. However, these clusters could not be synthesized by this approach. 4.4. Estimation of the Average Number of Exchanged Ligands (xave). For clusters 1−23, the average number of the exchanged ligands was estimated by using eq 1.59 Here, x is the number of the exchanged ligands and S(x) is the peak area of clusters 1−23 with each x in its electrospray ionization (ESI) mass spectrum.

Figure 6. Relationships of the peak energies of (a) peaks A and B, and (b) peaks C and D (Figures 5 and S12 and Table 1) with xave for [Au25(SC2H4Ph)18]− (cluster 1; black point), [Au 25 (SC 2 H 4 Ph) 18−x (SePh) x ] − (clusters 4−14; red points), [Au25(SC2H4Ph)18−x(TePh)x]− (clusters 15−17; cyan points), and [Au25(SC2H4Ph)18−x(SPh)x]− (clusters 18−23; green points).

D with xave was also observed (Figure 6b). These results demonstrate that ligand exchange with SePh causes nonlinear change of both optical absorption peaks and redox potential with xave.

3. CONCLUSIONS In this study, the geometrical/electronic structures of the products obtained by reacting [Au25(SC2H4Ph)18]− with PhSeH or (PhTe)2 were examined. The results revealed that the products with ligands exchanged at the core site are preferentially produced in these reactions. In addition, the changes in the redox potentials and HOMO−LUMO gaps of the clusters induced by continuous ligand exchange were quantitatively elucidated. This systematic investigation revealed that exchange with SePh induced nonlinear changes in the electronic structure of the clusters as xave varied. These findings deepen our understanding of ligand exchange reactions, and may lead to design guidelines for the synthesis of gold clusters that exhibit new functions through ligand exchange reactions with other chalcogenates. 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. Tetraoctylammonium bromide ((C8H17)4NBr), sodium tetrahydroborate (NaBH4), dichloromethane, dry dichloromethane, and diphenyl ditelluride ((PhTe)2) were obtained from Wako Pure Chemical Industries. Methanol, toluene, acetonitrile, ethanol, and tetrahydrofuran (THF) were sourced from Kanto Kagaku. Benzeneselenol F



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⎛

18

xave =

⎞ S(x ) ⎟ ⎝ S(x = 0) + S(x = 1) + ··· + S(x = 18) ⎠

∑ x⎜ x=0

*(Y.N.) E-mail: [email protected]. Telephone: +81-35228-9145. Notes

4.5. Crystallization. Clusters 2 and 3 were crystallized by vapor diffusion at room temperature. To grow crystals, ∼10 mg of each cluster was dissolved in toluene (500 μL) in an inner vial and ethanol was used as the antisolvent in the outer vial. Very sharp needle-like crystals (Figure S4) were obtained within 2−6 days. We also attempted to crystallize clusters 4−13 and 15−17. However, we could not obtain crystals for these clusters. 4.6. Characterization. ESI mass spectrometry was performed using a Fourier transform ion cyclotron resonance mass spectrometer (Bruker, Solarix). In these measurements, a cluster solution with a concentration of 1 mg/mL in toluene/ acetonitrile (1:1, v:v) was electrosprayed at a flow rate of 200 μL/h. Diffraction data of the crystal samples were recorded by a Bruker SMART APEX 2 Ultra equipped with an Apex II CCD diffractometer. Data evolution, indexing, integration, absorption correction (multiscan), and space group P-1 (based on intensity statistics and systematic absences) were determined using the Bruker APEX 2 software package.60 The X-ray crystal structures of the clusters were solved by SHELXT using the intrinsic phasing method61 and refined by SHELXL-2014/762 using the Olex 2 platform.63 UV−Vis absorption spectra of the clusters were acquired in dichloromethane solutions at ambient temperature with a spectrometer (JASCO, V-630). The wavelength-dependent optical data, I(w), were converted to energy-dependent data, I(E), using eq 2, which conserved the integrated spectral areas. I (E ) ∂E ∂w

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Yoshihisa Sei for providing technical assistance. This work was supported by JSPS KAKENHI Grant Nos. JP15H00763, 15H00883, and JP16H04099. Funding from the Canon Foundation, the Nippon Sheet Foundation for Materials Science and Engineering, the Sumitomo Foundation, Takahashi Industrial and Economic Research Foundation, Tanaka Kikinzoku Memorial Foundation, and the Sasagawa Foundation is also gratefully acknowledged.

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REFERENCES

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∝ I(w)w 2 (2)

DPV was performed at room temperature using an electrochemical analyzer (BAS, ALS610D). Cluster solutions were prepared by adding 5−10 mg of each cluster into dry dichloromethane (3 mL) containing tetrabutylammonium perchlorate (100 mg). A glassy carbon working electrode and Pt wire counter electrode were used, and ferrocene was employed as an internal reference. Measurements were conducted under an Ar atmosphere. After obtaining DPV data for each cluster, ∼5 mg of ferrocene was added to the solution and the potential of the observed peaks was used as the reference.

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AUTHOR INFORMATION

Corresponding Author

(1)

I (E ) =

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08636. Peak analysis of optical absorption spectra, mass spectra of clusters 1 and 4−23, photographs of the crystals of clusters 2 and 3, full structures of clusters 2 and 3, crystal data for clusters 2 and 3, and other supporting figures (PDF) The checkCIF/PLATON report (PDF) Cif file (CIF) G


Article

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Ligand Exchange Reactions in Thiolate-Protected Au25 Nanoclusters

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