Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in

Oct 1, 2014 - The preferential locations of Ag and Cu atoms in the initial stage of doping into [Au25(SC2H4Ph)18]− were studied by X-ray absorption ...
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Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in [Au25−xMx(SC2H4Ph)18]− (x ∼ 1) As Determined by Extended X‑ray Absorption Fine Structure and Density Functional Theory Calculations Seiji Yamazoe,†,‡ Wataru Kurashige,§ Katsuyuki Nobusada,‡,∥ Yuichi Negishi,§ and Tatsuya Tsukuda*,†,‡ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan § 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 ‡

S Supporting Information *

ABSTRACT: The preferential locations of Ag and Cu atoms in the initial stage of doping into [Au25(SC2H4Ph)18]− were studied by X-ray absorption spectroscopy and density functional theory computations. The extended X-ray absorption fine structure (EXAFS) spectra of [Au23.8Ag1.2(SC2H4Ph)18]− at the Ag K-edge were reproduced using a model structure in which the Ag dopant occupied a surface site in the icosahedral Au13 core that was computationally the most stable site. In contrast, the Cu K-edge EXAFS spectra of [Au23.6Cu1.4(SC2H4Ph)18]− indicated that the Cu dopant was preferentially located at the oligomer site that was computationally less stable than the surface site. This discrepancy between the Cu location experimentally determined and that theoretically predicted was explained in terms of variations in the stability of the Cu dopant at the two sites against aerobic oxidation. These results demonstrate that the mixing patterns of bimetallic clusters are determined not only by the thermodynamic stability but also by the durability of the mixed structure under synthetic and storage conditions.

1. INTRODUCTION Bimetallic clusters exhibit properties that are both superior to and more widely varied than those of their monometallic counterparts. This occurs because the properties of such clusters may be adjusted by varying the particular elements that make up their compositions as well as the mixing ratio (or chemical composition) and mixing pattern (or chemical ordering).1,2 The mixing patterns, including random alloy (solid solution) and phase-segregated structures, are typically governed by several factors, such as the atomic radius, surface energy, cohesive energy, and electronegativity of the constituent elements. The balance between the surface and cohesive energies determines whether a given element is preferentially located at the surface or the interior of the cluster. A difference in the electronegativity of the metals can induce the formation of heteropolar (ionic) bonding and thus promotes mixing of the two elements. In intermetallic compounds, such as AuCu and PtFe, atomically ordered structures have been observed to form at certain compositions.3−7 For example, it is theoretically predicted that Ag favors the surface sites in Au−Ag bimetallic clusters8 because the surface energy of Ag(111) (1.25 J m−2) is less than that of Au(111) (1.55 J m−2).9 The Au−Ag mixture is energetically © XXXX American Chemical Society

favored due to the ionic nature of the Au−Ag bond and the similar sizes of Ag (1.45 Å) and Au (1.44 Å) atoms. In contrast, the Cu atoms in Au−Cu bimetallic clusters tend to be located inside the clusters due to the lower surface energy of Au compared to that of Cu(111) (1.83 J m−2)9 and the smaller atomic size of Cu (1.28 Å).10 Recently, a variety of ligand-protected bimetallic clusters with well-defined compositions have been synthesized.11−23 The mixing pattern of these clusters is probably the most difficult structural parameter to control and to identify, owing to the technical challenges associated with obtaining single crystals. Another difficulty stems from the complex ligand−metal interactions that occur in addition to the determinant factors listed above. Interestingly, several bimetallic clusters have monometallic counterparts composed of the same total quantities of metal atoms and ligands. Table 1 lists examples of such bimetallic clusters. Scheme 1 depicts the structures of several monometallic clusters as determined by X-ray crystallography. Thiolate (RS)-protected [Ag44(SR)30]4− has a Received: August 23, 2014 Revised: September 27, 2014

A

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atoms (M = Ag or Cu) in [Au25−xMx(SC2H4Ph)18]− were investigated by Ag and Cu K-edge extended X-ray absorption fine structure (EXAFS) with the help of DFT calculations. We focused on the structures obtained at a low mixing ratio (x ∼ 1) since this limited the number of possible structures of singleatom-doped [Au24M1(SR)18]− clusters to only three (MC, MS, and MO), as shown in Scheme 2. DFT calculations concerning Au24Ag1(SR)18 (refs 12, 31 and 32) and Au24Cu1(SR)18 (ref 18) predicted that the AgS and CuS structures should be the most energetically stable.

Table 1. Examples of Au-Based Bimetallic Clusters and Locations of the Dopant Atoms monometallic clusters [Au25(SR)18]−

doped bimetallic clusters Au24Pt1(SR)18 Au24Pd1(SR)18 [Au25−xAgx(SR)18]− (x ≤ 13)

Au38(SR)24

[Au25−xCux(SR)18]− (x ≤ 5) Au36Pd2(SR)24

[Ag44(SR)30]4−

[Au12Ag32(SR)30]4−

location of dopant (analysis method) center site of icosahedral core (EXAFS28) center site of icosahedral core (EXAFS, 197Au Mössbauer,29 DFT30,31) surface site of icosahedral core (single-crystal XRD,13 DFT12,31,32) surface site of icosahedral core (DFT18) center sites of bi-icosahedral core (DFT16) hollow icosahedral core (singlecrystal XRD20)

Scheme 2. Possible Structures for [Au24M1(SR)18]−a

Scheme 1. Schematic Structures of Monometallic Clusters as Determined by X-ray Crystallographya a

R groups are omitted for the sake of simplicity. Key: red = M; orange and yellow = Au; green = S.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Highly purified samples of [Au 25−xAgx(SC2 H4 Ph)18]− and [Au 25−xCu x(SC 2H 4Ph) 18 ]− were synthesized according to a slightly modified version of previously reported methods.11,18 2.1.1. [Au25−xAgx(SC2H4Ph)18]−. HAuCl4·4H2O (0.747 mmol) and (C8H17)4NBr (0.757 mmol) were dissolved in tetrahydrofuran (THF) (25 mL). After stirring for 15 min, PhC2H4SH (4.66 mmol) was added to the THF solution, and the solution was stirred. Following a further 15 min, a mixture of 30 mM aqueous solution (0.1 mL) of AgNO3 with THF (8.9 mL) was added to the resulting solution, and the solution was stirred for another 15 min. A chilled (0 °C) aqueous solution (5.8 mL) containing NaBH4 (8.73 mmol) was then rapidly added. After 6 h, the organic phase was evaporated to dryness, and the residue was washed with methanol to remove excess thiol and other byproducts. The [Au25−xAgx(SC2H4Ph)18]− clusters were extracted from the dried residue with acetonitrile and further purified by high-performance gel permeation chromatography. 2.1.2. [Au25−xCux (SC2H4Ph)18]−. HAuCl4·4H2O (0.552 mmol), CuCl2·2H2O (0.048 mmol), and (C8H17)4NBr (0.695 mmol) were dissolved in methanol (30 mL). After 5 min of stirring, PhC2H4SH (7.21 mmol) was added to the methanol solution, and the solution was stirred for another 15 min. A chilled (0 °C) aqueous solution (10 mL) containing NaBH4 (6.03 mmol) was then rapidly added, following which the solution was stirred for 2 h and the precipitate separated by centrifugation. The product was washed three times with methanol to remove excess thiol and other byproducts. The [Au25−xCux(SC2H4Ph)18]− clusters were extracted from the dried residue with acetonitrile and further purified by highperformance gel permeation chromatography. 2.2. Determination of Chemical Compositions. Matrixassisted laser desorption ionization (MALDI) mass spectra were obtained using a spiral time-of-flight mass spectrometer (JEOL Ltd. JMS-S3000) equipped with a semiconductor laser (349 nm).

a

R groups are omitted for the sake of simplicity. Key: dark green and light blue = Ag; orange and yellow = Au; green = S.

three-layered structure: a hollow icosahedral Ag12 core, a dodecahedral Ag20 shell, and a protective layer made of six Ag2(SR)5.20 It was determined based on single-crystal X-ray diffraction (XRD) data that the Au−Ag bimetallic cluster [Au 12 Ag 32 (SR) 30 ] 4− has the same structural motif as [Ag44(SR)30]4− except that the Ag12 core is replaced with a Au12 core.20 This result strongly suggests that the other Aubased bimetallic clusters in Table 1 are obtained by replacing some of the Au atoms of [Au25(SR)18]− and Au38(SR)24 with dopant atoms while maintaining the original structural frameworks (Scheme 1), in which icosahedral Au13 and biicosahedral Au23 cores, respectively, are protected by −(SR− Au)n−SR− oligomers.24−26 The quantities of Pt and Pd atoms that can be doped into [Au25(SR)18]− and Au38(SR)24 are limited to one and two, respectively (Table 1),16,19,27 suggesting that these dopant atoms are located at specific positions. In fact, density functional theory (DFT) calculations on Au36Pd2(SCH3)24 suggest that two Pd dopant atoms occupy the center positions of the bi-icosahedral core of Au38(SR)24.16 X-ray absorption fine structure (XAFS) and 197Au Mössbauer spectroscopy data have demonstrated that the Pt and Pd atoms exclusively occupy the center positions of the icosahedral cores of Au24Pt1(SR)18 (ref 28) and Au24Pd1(SR)18 (ref 29), respectively, which is in good agreement with the theoretical prediction.30,31 In contrast, more than one atom of the coinage metals (Ag and Cu) can be doped into [Au25(SR)18]−, and so there are variations in the possible quantities of the doped atoms.12,13,17,18 Very recently, it was demonstrated by X-ray crystallography that Ag atoms in [Au25−xAgx(SR)18]− (4 ≤ x ≤ 8) are exclusively located at the surface sites of the icosahedral core.13 In the present study, the locations of coinage metal B

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2.3. DFT Calculations. Three stable structures (MC, MS, and MO) have been reported for [Au24M1(SCH3)18]− (M = Ag or Cu) by the Kohn−Sham DFT approach employing the hybrid functional (PBE0).18 In the present study, for comparison purposes, the geometry of [Au25(SCH3)18]− was also optimized using the same level of calculations applied for [Au24M1(SCH3)18]−. In addition, the relative stabilities for three models of [Au24Ag1(SCH3)18]− were calculated using the reported method.18 2.4. XAFS Measurements and Analysis. Ag and Cu Kedge XAFS measurements were carried out at beamline BL01B1 at the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (proposal nos. 2014A1680, 2014A1458, 2013B1421, 2012B1986, and 2012B1527). A Si(311) two-crystal was used to monochromatize the incident X-ray beam. The energy resolutions at the Cu K-edge (∼8.98 keV) and Ag K-edge (∼25.5 keV) were 0.15 and 0.30 eV, respectively. A solid sample of each [Au25−xMx(SC2H4Ph)18]− (M = Ag or Cu) diluted with boron nitride powder was pressed into a pellet and mounted on a copper holder attached to the cryostat. Ag and Cu K-edge XAFS spectra were then recorded in the fluorescence mode using an ion chamber as the I0 detector and 19 solid state detectors as the I detector. The Xray energy was calibrated using Cu and Ag foils for Cu and Ag K-edge data, respectively. Data analysis was carried out using the REX2000 Ver. 2.5.9 program (Rigaku Co.). X-ray absorption near edge structure (XANES) spectra were obtained by subtracting the atomic absorption background from the μt spectra by cubic spline interpolation and were compared after normalization of the edge height. The EXAFS spectra were analyzed as follows. The χ spectra were extracted by subtracting the atomic absorption background by cubic spline interpolation and were normalized to the edge height. The k3-weighted χ spectra within the k range of 3.0−14.0 Å−1 underwent Fourier transform (FT) to generate the r spectra. In the curve fitting analysis, the phase shifts and backscattering amplitude functions of Ag−S, Ag−Au, Cu−S, and Cu−Au were extracted from the MS structure using the FEFF8 program33 by setting σ2 = 0.0036 (σ: Debye−Waller factor) and S02 = 1 (S02: passive electron reduction factor). The σ2 and S02 values did not significantly affect the phase shift and backscattering amplitude functions. The curve fitting analysis was performed for M−S and M−Au bonds over the r range of 1.5−3.2 Å. The XANES and EXAFS spectra of the bimetallic models in Scheme 2 were simulated using the FEFF934 and FEFF833 programs, respectively. During these simulations, we used optimized structures in which the bond lengths were reduced to 97.7% of their original values (see Section 3.2). The parameter values σ2 = 0.0081 and S02 = 1 were used to obtain the simulated EXAFS oscillation patterns and FT-EXAFS spectra of the three models. Finally, the locations of the Ag or Cu atoms were determined by comparing the EXAFS oscillations obtained experimentally with those obtained through the computational simulations.

Figure 1. Negative-ion MALDI mass spe ctra of (a) [Au25−xAgx(SC2H4Ph)18]− and (b) [Au25−xCux(SC2H4Ph)18]−.

[Au25−xCux(SC2H4Ph)18]− (x = 0−3). Although undoped [Au25(SC2H4Ph)18]− clusters were most abundant in both samples, these clusters do not contribute to the data of Ag and Cu K-edge XAFS data, and as a result, they are ignored in the following discussion. Thus, the main contributors to the EXAFS results were the single-atom doped clusters: [Au 24 Ag 1 (SC 2 H 4 Ph) 18 ] − which accounted for 84% of [Au25−xAgx(SC2H4Ph)18]− (x = 1−2) and [Au 24 Cu 1 (SC 2 H 4 Ph) 18 ] − which accounted for 68% of [Au25−xCux(SC2H4Ph)18]− (x = 1−3). The average quantities of Ag and Cu atoms doped were calculated to be 1.16 and 1.41, respectively. The two bimetallic clusters used in the subsequent a na l ys e s a re th e r e fo r e refer re d t o h e rea ft e r as [Au23.8Ag1.2(SC2H4Ph)18]− and [Au23.6Cu1.4(SC2H4Ph)18]−. 3.2. Model Structures for [Au24M1(SCH3)18]− Calculated by DFT. On the basis of the mass analysis, we considered only single-atom doped clusters [Au24M1(SCH3)18]−. Figure S135 (Supporting Information) shows the optimized structures of the MC, MS, and MO models (Scheme 2) previously reported for [Au24Ag1(SCH3)18]− and [Au24Cu1(SCH3)18]−.18 For comparison, Figure S1 (Supporting Information) also includes the structure of [Au25(SCH3)18]− optimized at the same level of DFT calculations; the atomic coordinates are given in Table S1 (Supporting Information).35 A comparison of the structures of [Au25(SCH3)18]− and [Au24M1(SCH3)18]− reveals several interesting features. The Ag−Au bond length in the AgC model (2.835 Å) is comparable to the corresponding Au−Au bond in [Au25(SCH3)18]− (2.841 Å), while the Ag−Au bonds in the AgS model (2.867 and 2.985 Å) are also similar in length to the corresponding Au−Au bonds in [Au25(SCH3)18]− (2.841 and 2.988 Å). These results are understandable considering the similar atomic radii of Au (1.44 Å) and Ag (1.45 Å). In contrast, the Ag−S bonds in the AgS and AgO models (2.519 and 2.451 Å) are both significantly longer than the Au−S bonds in [Au25(SCH3)18]− (2.461 and 2.365 Å). In the case of [Au24Cu1(SCH3)18]−, both the Cu−S and Cu−Au bonds are significantly shorter than the corresponding Au−S and Au−Au bonds in [Au25(SCH3)18]− due to the considerably smaller atomic radius of Cu (1.28 Å) compared to Au (1.44 Å). The Cu−Au bonds in the CuC model (2.784 Å) and the CuS model (2.633 and 2.833 Å) are shorter than the corresponding Au−Au bonds in [Au25(SCH3)18]− (2.841, 2.841, and 2.988 Å, respectively). The Cu−S bonds in the CuS and CuO models

3. RESULTS AND DISCUSSION 3.1. Compositions of Bimetallic Clusters. The compositions of [Au25−xAgx(SC2H4Ph)18]− and [Au25−xCux(SC2H4Ph)18]− samples were investigated by MALDI mass spectrometry. Figure 1 shows typical MALDI mass spectra, which exhibit a series of [Au25−xAgx(SC2H4Ph)18]− (x = 0−2) and C

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Table 2. Summary of XAFS Analyses of [Au23.8Ag1.2(SC2H4Ph)18]− and Structural Parameters of [Au24Ag1(SCH3)18]−a bimetallic clusters

relative energy (eV)

R2 factorb

−

[Au23.8Ag1.2(SC2H4Ph)18] (300 K)

[Au23.8Ag1.2(SC2H4Ph)18]− (8 K) [Au24Ag1(SCH3)18]−

AgC

0.57

9.5

AgS

0.00

0.49

AgO

0.14

1.23

Xc

CNd

r (Å)e

σ2f

R factor (%)

S Au S Au S Au S Au S Au

1.0(2) 3.4(9) 0.7(1) 6.5(1.3) 0.0 12 1.0 6.0 2.0 0.0

2.443(5) 2.837(5) 2.442(10) 2.872(13) − 2.835 [2.770] 2.519 [2.461] 2.965 [2.897] 2.451 [2.395] −

0.0062(30) 0.0123(36) 0.0010(31) 0.0135(29)

7.1 6.4

Numbers in parentheses are uncertainties; for example, 2.443(5) represents 2.443 ± 0.005. bR2 = (Σ(k3χsample(k) − k3χmodel(k))2)/Σ(k3χsample(k))2. Neighboring atom. dCoordination number. eBond lengths determined by simulations and the average bond lengths for the model structures. Numbers in brackets are bond lengths reduced to 97.7% of their original values. fDebye−Waller factor. a c

Table 3. Summary of XAFS Analyses of [Au23.6Cu1.4(SC2H4Ph)18]− and Structural Parameters of [Au24Cu1(SCH3)18]−a relative energyb (eV)

bimetallic clusters

R2 factorc

−

[Au23.6Cu1.4(SC2H4Ph)18] (300 K) [Au23.6Cu1.4(SC2H4Ph)18]− (8 K) [Au24Cu1(SCH3)18]− CuC

0.40

6.59

CuS

0.00

1.05

CuO

0.13

0.35

Xd

CNe

S

2.4(2)

S S Au S Au S Au

2.4(2) 0.0 12 1.0 6.0 2.0 0.0

σ2g

R factor (%)

2.244(8)

0.0081(21)

6.4

2.229(3) − 2.784 [2.720] 2.302 [2.249] 2.800 [2.735] 2.265 [2.213] −

0.0069(16)

11.4

r (Å)f

a Numbers in parentheses are uncertainties; for example, 2.244(58) represents 2.244 ± 0.008. bObtained from the DFT calculation reported in ref 18. cR2 = (Σ(k3χsample(k) − k3χmodel(k))2)/Σ(k3χsample(k))2. dNeighboring atom. eCoordination number. fBond lengths determined by simulations and the average bond lengths for the model structures. Numbers in brackets are bond lengths reduced to 97.7% of their original values. gDebye− Waller factor.

are 2.302 and 2.265 Å, respectively, whereas the corresponding Au−S bonds in [Au25(SCH3)18]− are 2.461 and 2.365 Å. The relative stabilities of the three models are summarized in Tables 2 and 3. In both clusters, the MS model is the most stable, and the stabilities decrease in the order of MS > MO > MC. This result is in sharp contrast to the results obtained for Au24Pd1(SR)18 and Au24Pt1(SR)18, in which case the MC model is the most stable structure.28−31 The preferential location of the Ag and Cu atoms on the core surface cannot be explained by their respective surface energies (1.55, 1.25, and 1.83 Jm−2 for Au, Ag, and Cu, respectively),1,9 but rather by their electronegativities (2.54, 1.93, and 1.90 for Au, Ag, and Cu, respectively).36 The more electropositive Ag and Cu atoms are preferentially replaced with Au atoms on the surface of the Au13 core and in the oligomers, whose formal charge states are +0.5 and +1.0, respectively. 3.3. Locations of Dopants in Bimetallic Clusters. 3.3.1. [Au23.8Ag1.2(SC2H4Ph)18]−. The EXAFS oscillations of [Au23.8Ag1.2(SC2H4Ph)18]− obtained at 300 K are shown as plot (a) in Figure 2(A). An oscillatory structure is clearly observed in the 3 Å−1 < k < 9 Å−1 range but is less evident at higher k values. A remarkable thermal effect has been observed in the EXAFS spectra of [Au25(SR)18]−,37 suggesting that the EXAFS oscillations are damped by thermal fluctuations of the Ag−S and/or the Ag−Au bonds softened due to ultrasmall size of the Au core.38 In order to test this hypothesis, we also acquired EXAFS data at 8 K. As shown in plot (b) in Figure 2(A), oscillations are clearly observed even in the high k region. FTEXAFS spectra acquired at 300 and 8 K are also presented as plots (a) and (b) in Figure 2(B), in which peaks assigned to the

Figure 2. (A) Ag K-edge EXAFS oscillations and (B) Ag K-edge FTEXAFS spectra of [Au23.8Ag1.2(SC2H4Ph)18]− acquired at (a) 300 and (b) 8 K and simulated spectra of (c) AgC, (d) AgS, and (e) AgO.

Ag−S and Ag−Au bonds are present at 1.8−2.5 Å and 2.5−3.0 Å, respectively. The associated bond lengths and CN values are listed in Table 2. The CN of the Ag−Au bond at 8 K was significantly larger than that at 300 K due to the appearance of EXAFS oscillations in the high k region (k > 9 Å−1) where the back scattering amplitude for Au is large. To determine the preferential location of the Ag dopant, the experimental data acquired for [Au23.8Ag1.2(SC2H4Ph)18]− at 8 K were compared with the simulation results for the AgC, AgS, and AgO models. During this fitting process, all bond lengths of the three models were reduced to 97.7% of the original values D

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since the average Au−Au bond length (2.946 Å) in [Au25(SCH3)18]− calculated by DFT (Figure S1 and Table S2, Supporting Information)35 overestimated the bond length determined for [Au25(SC2H4Ph)18]− by X-ray crystallography (2.878 Å).25 The R2 factor, a measure of the agreement between experiment and simulation, revealed that the most stable AgS model reproduced the experimental results to the greatest extent (Table 2). This AgS model quantitatively reproduced the CN and r values of the Ag−S and Ag−Au bonds of [Au23.8Ag1.2(SC2H4Ph)18]− (Table 2). However, there is a noticeable difference between the experimental data and simulated results in Figure 2, suggesting a non-negligible contribution from the other models. Indeed, the R2 values can be reduced slightly by mixing a small amount of the AgC and AgO models into the AgS model (Figure S2, Supporting Information).35 Another possible source of the discrepancy between the experimental and theoretical results in Figure 2 is that we ignored the small portion (16%) of [Au23Ag2(SC2H4Ph)18]− present in the sample (Figure 1). From these results, we conclude that the most probable site for the Ag dopant is the core surface of [Au23.8Ag1.2(SC2H4Ph)18]−, although the occupation of the oligomer site cannot be excluded completely. The AgS model is also supported by the Ag K-edge XANES data. Figure 3 shows the Ag K-edge XANES spectrum of

Figure 4. (A) Cu K-edge EXAFS oscillations and (B) Cu K-edge FTEXAFS spectra of [Au23.6Cu1.4(SC2H4Ph)18]− acquired at (a) 300 and (b) 8 K and simulated spectra of (c) CuC, (d) CuS, and (e) CuO.

the CuO model reproduces the closest match for the oscillation pattern of [Au23.6Cu1.4(SC2H4Ph)18]− at 8 K. The R2 values can be reduced only slightly by considering the contribution of the CuS model (Figure S3, Supporting Information).35 In the FTEXAFS spectra of [Au23.6Cu1.4(SC2H4Ph)18]− obtained at 300 and 8 K (Figure 4(B)), only peaks attributable to the Cu−S bond are observed at 1.5−2.3 Å. A similar FT-EXAFS spectrum was obtained using the CuO model. The results of the curve fitting analysis for [Au23.6Cu1.4(SC2H4Ph)18]− are summarized in Table 3. The CN and r values of the Cu−S bond in [Au23.6Cu1.4(SC2H4Ph)18]− at 300 and 8 K are close to those of the CuO model. Therefore, we conclude that the most probable location for the Cu dopant in [Au23.6Cu1.4(SC2H4Ph)18]− is the oligomer site. The electronic state of the Cu in [Au23.6Cu1.4(SC2H4Ph)18]− was also investigated by Cu K-edge XANES spectra, as shown in Figure 5. The XANES spectrum of

Figure 3. Ag K-edge XANES spectra of (a) Ag foil, (b) [Au23.8Ag1.2(SC2H4Ph)18]− at 8 K, and simulated XANES spectra of (c) AgC, (d) AgS, and (e) AgO.

[Au23.8Ag1.2(SC2H4Ph)18]− as well as simulated spectra for the Ag C , Ag S , and Ag O models. The XANES profile of [Au23.8Ag1.2(SC2H4Ph)18]− is seen to closely match that of the AgS model. We conclude from the above results that the first replacement of [Au25(SR)18]− with a Ag atom occurs at the surface site of the Au13 core. Our conclusion is consistent with the theoretical predictions for Au 24 Ag 1 (SCH 3 ) 18 and Au22Ag3(SCH3)18 (refs 12, 31, and 32) and the X-ray crystallographic results for Au18.3Ag6.7(SC2H4Ph)18.13 3.3.2. [Au23.6Cu1.4(SC2H4Ph)18]−. The Cu K-edge EXAFS oscillations of [Au23.6Cu1.4(SC2H4Ph)18]− obtained at 300 and 8 K are shown, respectively, as plots (a) and (b) in Figure 4(A). Both exhibit a sine-wave pattern, and there is no obvious effect of the temperature change. The EXAFS oscillations simulated for the CuC, CuS, and CuO models are also shown in Figure 4(A). During this fitting process, all the bond lengths in all three models were again reduced to 97.7% of their original values. A comparison of the R2 values (Table 3) indicates that

Figure 5. Cu K-edge XANES spectra of (a) Cu foil, (b) [Au23.6Cu1.4(SC2H4Ph)18]− acquired at 8 K, and simulated XANES spectra of (c) CuC, (d) CuS, and (e) CuO.

[Au23.6Cu1.4(SC2H4Ph)18]− acquired at 8 K is significantly different from that of Cu foil as well as simulated spectra for the CuC and CuS models. The peaked structures in the absorption edge data for [Au23.6Cu1.4(SC2H4Ph)18]− are also observed in the spectrum of the CuO model. These results clearly indicate that the first replacement of [Au25(SR)18]− with a Cu atom occurs at the oligomer site on the Au13 core. E

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The XAFS data for [Au23.6Cu1.4(SC2H4Ph)18]− were not explained by the most stable CuS model but rather by the second most stable CuO model (Table 3). We ascribe this discrepancy to the difference in the chemical stability of the model structures under the aerobic conditions employed in the synthesis. It is known that Cu(0) clusters stabilized by polymers, such as polyvinylpyrrolidone and dendrimers, are easily oxidized to the Cu+ and Cu2+ states when exposed to air.39 Therefore, the energetically most stable CuS structure formed immediately following the synthesis was likely decomposed by oxidation in air. In contrast, the CuO structure may survive under aerobic conditions because the Cu atom is already oxidized to the +1 charge state. Similarly, it has been reported that Cu atoms are not incorporated into the Au13 core but are bonded to the thiolates and pyrimidyl groups of phosphine ligands on the surface of the Au13 core in bimetallic Au13Cux clusters (x = 2, 4, 8).21 DFT calculations predict that the Cu atoms in the Au13Cux clusters will be positively charged, reflecting the more electropositive nature of Cu as compared to Au.

ASSOCIATED CONTENT

* Supporting Information S

Optimized structures of [Au25(SCH3)18]− and [Au24M1(SCH3)18]− (M = Ag, Cu), R2 values as functions of populations of the model structures, and atomic coordinates of [Au25(SCH3)18]−. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. SUMMARY The local structures of the dopant atoms Ag and Cu in [Au23.8Ag1.2(SC2H4Ph)18]− and [Au23.6Cu1.4(SC2H4Ph)18]− were probed by XAFS. It was revealed that the preferred doping location depends on the dopant elements. The Ag atom in [Au23.8Ag1.2(SC2H4Ph)18]− was located at the surface site of the icosahedral core, which is in good agreement with the predictions of DFT calculations. In contrast, the Cu atom in [Au 23.6 Cu 1.4 (SC 2 H 4 Ph) 18 ] − preferentially occupied the oligomer site, even though DFT calculations showed that the most stable site is the surface site, just as in the case of the Ag dopant. This discrepancy between the experimental results and theoretical calculations is explained in terms of the difference in the chemical durability of the Cu atom at various sites. The Cu atom at the oligomer site is stable against oxidation, whereas bimetallic clusters that have the Cu atom at the surface site are decomposed by aerobic oxidation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Funding Program for Next Generation World-leading Researchers (NEXT Program, GR-003), Elements Strategy Initiative for Catalysts & Batteries (ESICB), and a Grant-in-Aid for Scientific Research (Nos. 25102539, 25288009, 25288012, and 26248003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. F

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