Cluster with a Face-Centered Cubic Core

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C: Physical Processes in Nanomaterials and Nanostructures 25

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An Au (SR) Cluster with a Face-Centered Cubic Core Tsubasa Omoda, Shinjiro Takano, Seiji Yamazoe, Kiichirou Koyasu, Yuichi Negishi, and Tatsuya Tsukuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03841 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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The Journal of Physical Chemistry

An Au25(SR)18 Cluster with a Face-Centered Cubic Core Tsubasa Omoda,1 Shinjiro Takano,1 Seiji Yamazoe,1,2,3 Kiichirou Koyasu,1,2 Yuichi Negishi,4,5 and Tatsuya Tsukuda1,2* 1

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. 3 Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Tokyo 1020076, Japan 4 Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan. 5 Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan. 2

Supporting Information Placeholder ABSTRACT: A representative thiolate (RS)-protected gold cluster, Au25(SR)18, shows a fingerprint-like characteristic spectral profile regardless of the R-groups, reflecting the common motif of the structural backbone made of Au and S: an icosahedral Au13 core fully protected by six staple units of Au2(SR)3. On the other hand, we reported in 2006 that an Au25(SPG)18 cluster (PGSH = N(2-mercaptopropionyl)glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au25(SR)18, suggesting the formation of a non-icosahedral Au core. Here, we investigated the structure of Au25(SPG)18 by UV-Vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au25(SPG)18 has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au15(SPG)4[Au2(SPG)3]2[Au3(SPG)4]2 in which an Au15(SPG)4 core with an FCC motif is protected by two types of staples with different lengths, Au2(SPG)3 and Au3(SPG)4. The formation of an FCC-based Au core is attributed to bulkiness around the α-carbon of the PGS ligand.

INTRODUCTION

whereas Au30(SAdm)18 (AdmSH = 1-adamanthanethiol) has a hexagonal-close-packed (HCP) core.16–18 Proper design of the RS ligands will lead to on-demand control of the size and structure of Aun(SR)m. Au25(SR)18 has long been regarded as a prototypical system since its discovery.20 In 2007, Nobusada theoretically proposed the first model in which a planar Au7 core is sandwiched by two Au3(SR)3 rings and surrounded by an Au12(SR)12 ring.21 Later, a new structure has been theoretically predicted by Grönbeck22 and experimentally resolved by Murray and Jin independently.23,24 Later crystallographic studies have shown that Au25(SR)18 has an Ih Au13 core fully protected by six staple units of Au2(SR)3 regardless of the R-group.23–29 UV-Vis spectra of non-crystallized Au25(SR)18, such as Au25(SG)18 (GSH = glutathione), exhibit very similar and characteristic profiles, suggesting the common structure motif of Au25(SR)18.20,30,31 An exception can be found in Au25(SPG)18 (PGSH = N-(2-mercaptopropionyl)glycine, Scheme 1), which shows a significantly different UV-Vis spectrum from that of the conventional Au25(SR)18.32 In this study, we synthesized Au25(SPG)18 with a higher purity than in the previous study32 and examined the structures by UV-Vis spectroscopy, extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations. An FCC-based Au core was proposed for Au25(SPG)18.

Gold clusters exhibit novel catalytic,1,2 magnetic,3,4 and photophysical5,6 properties significantly different from those of bulk gold because of their quantized electronic structures and non-closest-packed geometrical structures. State-of-the-art atomically precise synthesis of ligand-protected Au clusters revealed that their properties depend strongly on the number of gold atoms (size).7,8 Their electronic structures, stability and optical properties are understood through the superatomic concept.9 A unique feature of the superatoms as compared to conventional atoms is the presence of internal structures. X-ray absorption fine structure (XAFS) study of thiolate-protected Au cluster Aun(SR)m revealed that the Au core is protected by Au(SR) staple motifs through Au–S bonds much stiffer than Au–Au bonds.10 It is expected that the deformation of the Au core is induced by the force originating from interactions between the RS ligands, such as steric repulsion, π–π stacking, and hydrogen bonding, and is transmitted through stiff Au–SR bonds. For example, the Au cores Aun(SR)m are transformed from icosahedral (Ih)-based structures to face-centered cubic (FCC)-based structures by exchanging the ligands from 2phenylethanethiol to 4-tert-butylbenzenethiol.11 In some cases, such structural conversion by ligand exchange proceeds reversibly.12,13 It has been reported that Au28(SR)20,14,15 Au30(SR)18,16–18 and Au144(SR)60 (Ref. 19) exhibit polymorphism depending on the R-group structures while retaining the chemical compositions. For example, Scheme 1. PGSH structure. Au30(StBu)18 (tBuSH = tert-butylmercaptan) has an FCC core ACS Paragon Plus Environment

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(JMS-T100LP AccuTOF; Jeol). The sample of fraction 4 was prepared by reprecipitation using 2% acetic acid and EtOH/DCM = 1:1 (v/v) to remove the counter cation of Tris+. A 50% water/MeOH (v/v) dispersion of Aun(SPG)m in fractions 1–6 thus purified was mixed with 0.1 vol% of Et3N at a concentration of 0.5 mg/mL. The mixed dispersion was electrosprayed at 100–150ºC in negative-ion mode. The mass spectra were calibrated against that of (CsI)nI− measured under the same conditions. X-ray absorption spectra (XAS) of Aun(SPG)m in fraction 4, [Au25(SC2Ph)18]− and [Au23(Sc-C6)16]− at the Au L3-edge were measured using BL01B1 beamline at the SPring-8 facility of the Japan Synchrotron Radiation Research Institute. The incident X-ray beam was monochromatized by an Si(111) double-crystal monochromator. Solid samples were ground in an agate mortar with boron nitride powder and pressed into square pellets (5×10 mm). For hygroscopic Au25(SPG)18, it was first dissolved in 50% water/1.5 M Tris-HCl buffer and dried up by a lyophilizer with boron nitride before grinding. The sample pellets were wrapped in aluminum foil and mounted on a copper holder attached to a cryostat.10 Data analyses were performed using the program REX2000 (Rigaku Co.) Calculation. Density functional theory (DFT) calculations were performed on [Au25(SCH3)18] − using the B3LYP functional. Basis sets used in this study were LanL2DZ for Au atoms and 6-31G(d) for C, H, and S atoms. Frequency calculations were conducted to confirm that each optimized structure was located at the potential minima. All calculations were conducted using the Gaussian 09 package.36

EXPERIMENTAL DETAILS Chemicals. All chemicals were commercially available and used without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O), sodium borohydride (NaBH4), ethanol (EtOH), methanol (MeOH), dichloromethane (DCM), 2-phenylethanethiol (PhC2SH), 1-cyclohexanethiol (c-C6SH), acetic acid, triethylamine (Et3N), and boron nitride were obtained from Wako Pure Chemical Industries. 0.5 M tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer (pH 6.8), 1.5 M Tris-HCl buffer (pH 8.8), and Trisglycine (Tris = 0.25 M, glycine = 1.92 M) buffer were obtained from Bio-Rad Laboratories. PGSH was obtained from Tokyo Chemical Industry. Glycerol was obtained from ICN Biomedicals. Water was Milli-Q grade (ρ > 18 MΩ·cm). Synthesis of Au25(SPG)18. Crude Au:SPG clusters were prepared by the method reported32 with slight modifications: the biggest difference is the usage of EtOH in this study instead of MeOH. PGSH (1.5 mmol) was added to the EtOH solution of HAuCl4·4H2O (5 mM, 50 mL). The mixed solution was stirred for 2.5 h at room temperature until it became colorless, and then was cooled to ~0°C in an ice bath. To this solution, an ice-cooled aqueous solution of NaBH4 (0.2 M, 12.5 mL) was added dropwise in ~10 min. The sticky black precipitate that formed was collected, dissolved in a Tris-HCl buffer solution (1.5 M, 2 mL), and reprecipitated using EtOH. After washing with EtOH three times, the black precipitate was dried in vacuo and used further for synthesis of Aun(SPG)m. Crude Au:SPG clusters were separated by polyacrylamide gel electrophoresis (PAGE)20,32,33 using a single (NA-1120, Nihon Eido) or double (BE-S12, Bio Craft) slab gel units with a size of 3t × 160 × 160 mm. The concentration of acrylamide in the preparative gel and stacking gel was 30 and 3 wt%, respectively (acrylamide:bis-acrylamide = 93:7). The crude Au:SPG clusters were dissolved in a 2.5% (v/v) aqueous solution of glycerol. The sample dispersion (40 mg/mL, 300 µL) was applied for a single gel. The eluting buffer consisted of 25 mM Tris and 192 mM glycine. The Au:SPG clusters were eluted for 10.5 h at a constant voltage mode (150 V) in an incubator kept at 5°C to suppress decomposition of the clusters. After the elution, six fractions 1–6 (Figure 1(a)) were cut out, crushed using a spatula, and placed in 1.5 M Tris-HCl buffer (2 mL) at 2ºC for 2 h to extract the clusters from the gel. Then, the gel lumps were filtered (0.2 µm pore size) followed by centrifugal ultrafiltration (Vivaspin 20; Vivascience; MWCO = 30 kDa) to remove remaining gel lumps. The filtrate was concentrated by repeated centrifugal ultrafiltration (Vivaspin 20; Vivascience; MWCO = 5 kDa) and dried by a lyophilizer (FDU-2000; EYERA). The dried sample was dissolved in 2% (v/v) acetic acid (200 µL) and reprecipitated in EtOH by centrifugation (10,000 rpm). The obtained precipitates were washed with EtOH repeatedly and dried in vacuo. A typical yield was ~1 mg (~1 % based on Au). Synthesis of reference clusters. [Au25(SC2Ph)18] − and [Au23(Sc-C6)16]− were synthesized by reported methods.34,35 Characterization. UV-Vis optical spectra of Aun(SPG)m in fractions 1–6, [Au25(SC2Ph)18] − and [Au23(Sc-C6)16] − were measured using spectrophotometers (V-670, V-770; Jasco). The raw spectral data obtained as a function of the wavelength was converted to the energy-dependent data according to the procedure reported in Ref. 20. Electrospray ionization (ESI) mass spectra of Aun(SPG)m in fractions 1–6 were measured with using a mass spectrometer

RESULTS AND DISCUSSION Figure 1(a) shows a typical result of the PAGE separation of crude Au:SPG clusters. They were clearly separated into fractions, six of which were collected and named as 1–6 in this work. Figure 1(b) shows the negative-ion mode ESI mass spectrum of fraction 4. A major series of mass peaks can be assigned to multiply-charged anions [Au25(SPG)18–x(SPG–)x]y, where x and y represent the number of the deprotonated SPG ligand (SPG–) and the net charge of the Au core, respectively. Small progressions observed in the higher mass region are assigned to the complexes with Tris+ cations contained in the buffer solution. The total charge of Au25(SPG)18 was determined by the combination of [x, y]. The inset of Figure 1(b) compares the enlarged view of the most intense mass peak observed at m/z ~ 1568 and those simulated for [x, y] = [4, –1], [5, 0] and [6, +1]. A comparison indicates that the net charge of

Figure 1. (a) Typical PAGE result for crude Au:SPG. The notation (n, m) represents the composition of Aun(SPG)m contained in each fraction. (b) Negative-ion mode ESI mass spectrum of fraction 4. Inset shows an expanded mass peak at m/z ~ 1568 and mass peaks simulated for [Au25(SPG)18–x(SPG–)x]y with [x, y] = [4, −1] (black), [5, 0] (red), and [6, +1] (blue).

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The Journal of Physical Chemistry Au25(SPG)18 is −1: the formal number of valence electrons in the Au core was eight as in the case of the conventional [Au25(SR)18]− cluster.9 This electron count suggests that the valence electrons are confined in a nearly spherical Au core so that three 1P superatomic orbitals are nearly degenerated. The compositions of Aun(SPG)m contained in the other fractions were determined by ESI-MS (Figures S1). The results of PAGE and mass analysis obtained in this study are different from those reported previously.32 The most probable reason for this discrepancy is the purity of the PGSH used rather than the difference in the solvent: mass analysis suggested that the PGSH used in the previous study contained a greater amount of unidentified impurities32 than that used here. The following focuses on the characterization of [Au25(SPG)18]− collected as fraction 4. Figure 2(a) shows the UV-Vis spectrum of [Au25(SPG)18]−, showing a distinct peak at ~593 nm (2.1 eV). The spectrum looks different from that reported previously, probably due to contamination of the PGSH sample (Figure S2).32 Notably, the spectral profile of [Au25(SPG)18]− is significantly different from that of [Au25(SC2Ph)18]− (Figure 2(b)). Because the spectral profile is sensitive to the atomic structure of the Au core, this result indicates that [Au25(SPG)18]− does not have an Ih Au13 core. We noticed that the spectral profile of [Au25(SPG)18]− is similar to that of [Au23(Sc-C6)16]− (Figure 2(c)) having an FCC Au15 core, among other thiolate-protected Au clusters having 8 electrons.14,15,35,37 This result suggests that [Au25(SPG)18]− has an FCC-based core similar to that of [Au23(Sc-C6)16]−. To confirm the hypothesis, the atomic structure of the Au core was examined by Au L3-edge EXAFS. Figures 2(d)–(f) show the Au L3-edge EXAFS oscillations of [Au25(SPG)18]−, [Au25(SC2Ph)18]−, and [Au23(Sc-C6)16]− measured at 10 K, respectively. Measurement at low temperature is essential to gain structural information of such small clusters by suppressing thermal fluctuation.10 It is known that EXAFS oscillation in the high k region (k ≥ 10 Å−1) sensitively reflects the geometrical structure of the Au core.10 The oscillation profile of [Au25(SPG)18]− is significantly different from that of [Au25(SC2Ph)18]−, but similar to that of [Au23(Sc-C6)16]−. Au L3-edge FT-EXAFS spectra are shown in Figure S3 and the results of curve fitting analysis are summarized in Table S1. Both results support the conjecture that the structure of [Au25(SPG)18]− more closely resembles [Au23(Sc-C6)16]− than

[Au25(SC2Ph)18]−. From these results, it is concluded that [Au25(SPG)18]− has an FCC-based Au core similar to that of [Au23(Sc-C6)16]−. Next, we consider possible model structures of [Au25(SPG)18]− based on the crystallographically resolved structure of [Au23(Sc-C6)16]− (Figure S4): an FCC Au15 core capped by four Sc-C6 thiolates (Figure 3(a)) is ligated by two oligomers of Au1(Sc-C6)2 and two oligomers of Au3(Sc-C6)4.35 The simplest model structures of [Au25(SPG)18]− were constructed by protecting the Au15(SR)4 core (Figure 3(a)) with Aun(SR)n+1 oligomers of different lengths. This modeling is reasonable given that Au28(SPhtBu)20 (tBuPhSH = 4-tertbutylbenzenethiol) and Au28(Sc-C6)20 having a common Au core motif show a similar UV-Vis spectrum profile regardless of the different staple configurations.15 In the models, four staple motifs must cap all the Au surface atoms of the Au15(SR)4 core. There are five possible combinations of the staple motifs as listed in Table S2. However, two combinations that allow symmetrical protection of the Au15(SR)4 core with inverse symmetry are more reasonable. Figures 3(b) and 3(c) show the schematic construction of two models, A and B. Both are constructed by elongating the Au1(SR)2 and Au3(SR)4 staples in [Au23(Sc-C6)16]− to Au2(SR)3 and Au4(SR)5, respectively. However, the distance between the binding sites for Au1(SR)2 in [Au23(Sc-C6)16]− (~3.5 Å) is too short for the Au2(SR)3 staples, which requires ~5 Å according to the previous single crystal X-ray data.7 Therefore, two elongated oligomers of Au2(SR)3 in model A occupy the different sites on the Au15(SR)4 core from those for the Au1(SR)2 staples in [Au23(SR)16]−. On the other hand, in model B, the Au4(SR)5 staples are bonded to the same anchor sites of Au3(SR)4 in [Au23(SR)16]− since Au4(SR)5 is flexible enough to be bonded on the same anchor sites of Au3(SR)4. [Au25(SR)18]− (R = CH3) with models A and B were successfully optimized and the

Figure 2. (Left) UV-Vis spectra of (a) [Au25(SPG)18]−, (b) [Au25(SC2Ph)18]−, and (c) [Au23(Sc-C6)16]−. (Right) Au-L3 edge EXAFS oscillations of (d) [Au25(SPG)18]−, (e) [Au25(SC2Ph)18]−, and (f) [Au23(Sc-C6)16]− measured at 10 K. (a), (b), (d), and (f) are offset for clarity.

Figure 3. Structures of (a) the Au15(SR)4 core in [Au23(SR)16]−, (b) model A, and (c) model B. Two Au(SR) units added to staple units in [Au23(SR)16]− are shown by colored ovals. Au = orange, red, blue. S = green.



optimized structures are presented in Figures 4(a) and (b), respectively. The stability of models A and B is compared with the structure models of [Au25(SR)18]− predicted so far using DFT calculations. First, structural optimization of [Au25(SR)18]− was conducted at the same calculation level as for the models conventionally observed (model Ih) and those proposed by Nobusada (model N) and Grönbeck (model G)21,22 by assuming R = CH3. Figures 4(c)–(e) show the optimized structures of models Ih, N and G, respectively. The stability of models A and B is comparable to that of model Ih, whereas models N and G are far less stable as compared with the other three models. The stability comparison led us to conclude that models A and B in this study are plausible candidates for the structures of [Au25(SPG)18]−. Since model B is constructed by elongating two Au3(SR)4 units of [Au23(SR)16]− to Au4(SR)5, we can expect the concurrent formation of [Au24(SR)17]−, in which one of the Au4(SR)5 staples of model B are shortened to Au3(SR)4. However, [Au24(SPG)17]− was not observed by mass spectrometry. Considering this result, a structure having different binding sites for the staples from [Au23(SR)16]− like model A is preferable as a model for [Au25(SPG)18]−.

Au23(SSA)16 (SASH = mercaptosuccinic acid) having a secondary α-carbon was similar to that of [Au23(Sc-C6)16]−.32 These results led us conclude that the bulkiness at the α-carbon of thiolates played an essential role in forming the FCC Au core although the formation of an Ih Au core in [Au25(SR)18]− was reported when 3-mercaptobenzoic acid with bulky α-carbon is used as ligands.31

CONCLUSIONS In summary, we synthesized [Au25(SPG)18]− (PGSH = N-(2mercaptopropionyl)glycine) with an improved purity as compared with the previous study.32 The optical spectrum and EXAFS data of [Au25(SPG)18]− were very similar to that of [Au23(Sc-C6)16]−, which has been crystallographically proven to have an Au15(Sc-C6)4 core with a face-centered cubic (FCC) motif. The spectral similarity indicated that [Au25(SPG)18]− has an FCC Au core rather than an Ih Au13 core found in the conventional [Au25(SR)18]−. Based on DFT calculations, we proposed a model structure for [Au25(SPG)18]− formulated as Au15(SPG)4[Au2(SPG)3]2[Au3(SPG)4]2 in which an Au15(SPG)4 core with an FCC motif is protected by two types of staples Au2(SPG)3 and Au3(SPG)4. The formation of [Au25(SR)18]− with an FCC-based Au core is attributed to the structures of the ligands, especially the bulkiness around the α-carbon of the thiolates.

ASSOCIATED CONTENT Supporting Information. Synthetic scheme and characterization results of [Au23(SPG)16]−, details of ESI-MS analysis of fractions 1–6; comparison between present study and results in Ref. 32, EXAFS analyses of [Au25(SPG)18]−, [Au25(SC2Ph)18]−, and [Au23(Sc-C6)16]−, crystal structures of [Au23(Sc-C6)16]−, and lists of candidates for model structures of [Au25(SPG)18]−. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Present Address S. Y.: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan.

Notes

Figure 4. Optimized structures of [Au25(SCH3)18]− with structure models of (a) A, (b) B, (c) Ih, (d) N, and (e) G. Relative total energy is shown. CH3 group is shown by a wireframe. Au = orange. S = green.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is dedicated to Prof. Katsuyuki Nobusada (Institute for Molecular Science) who passed away this January at the age of 49. We thank Prof. Henrik Grönbeck (Chalmers University of Technology) for providing us with the coordinates of the isomers reported in Ref. 22. This research was financially supported by the Elements Strategy Initiative for Catalysts & Batteries (ESICB) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and a Grant-in-Aid for Scientific Research (A) (Grant No. 17H01182) from the Japan Society for the Promotion of Science (JSPS). The synchrotron radiation experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2016A1436, 2016B0910, 2017A0910, and 2017B0910).

What is the driving force for the formation of [Au25(SPG)18]− with an FCC Au core? Recently, Jin proposed that the crystalline phase of the Au core of Aun(SR)m can be controlled by the structure of the thiolate ligands.7,38 Previous examples including [Au23(Sc-C6)16]− suggest that an FCC Au core is favored when the α-carbon of the thiolate is bulky. This empirical rule can explain the formation of an FCC Au core in [Au25(SPG)18]− since the α-carbon of PGS is secondary. In support of this explanation, the [Au23(SPG)16]− clusters we synthesized in this study (see Supporting Information for details of synthesis, Figure S5) and reported in Ref. 32 exhibit similar UV-Vis spectra to that of [Au23(Sc-C6)16]− (Figure S6). A previous study also showed that the UV-Vis spectrum of


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The Journal of Physical Chemistry Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

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Cluster with a Face-Centered Cubic Core

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