LETTER pubs.acs.org/Langmuir
Isolation and Structural Characterization of an OctaneselenolateProtected Au25 Cluster Yuichi Negishi,* Wataru Kurashige, and Ukyo Kamimura Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
bS Supporting Information ABSTRACT: We report the isolation and structural characterization of an octaneselenolate-protected Au25 cluster ([Au25(SeC8H17)18]). Isolated [Au25(SeC8H17)18] was characterized by various analytical techniques. The results strongly suggest that [Au25(SeC8H17)18] possesses a similar geometric structure to the well-studied thiolate (RS)-protected Au25 cluster ([Au25(SR)18]) and that the charge transfer between the metal atoms and ligands in [Au25(SeC8H17)18] is lower than that in [Au25(SR)18]. To the best of our knowledge, this is the first report of the isolation of a selenolate-protected gold cluster. [Au25(SeC8H17)18] is an ideal compound for determining how changing the ligand from thiolate to selenolate affects the fundamental properties of a cluster.
’ INTRODUCTION Small gold clusters protected by ligands are of great interest in both fundamental and applied research because they exhibit sizespecific optical and physical properties. Of such clusters, thiolateprotected gold clusters (Au:SR) have been the most extensively studied. Recent advances in isolation and characterization techniques have enabled clusters such as [Au25(SR)18]/0, [Au38(SR)24]0, [Au68(SR)34]0, [Au102(SR)44]0, and [Au144(SR)60]0 to be synthesized and isolated in high purity120 and their structures and properties to be characterized in detail. For example, the structure of [Au25(SR)18] has been found to consist of an icosahedral Au13 core protected by six [SAuSAuS] oligomers.4,13,21 In addition, [Au25(SR)18] has been found to exhibit properties that are not observed in bulk gold, including photoluminescence3,7,2224 and paramagnetism.25 Recently, several studies have investigated gold clusters protected by selenolates (Au:SeR).2629 Compared with S, which belongs to the same periodic group, the atomic radius and electronegativity of Se are closer to those of Au. Consequently, the bonds between Au and selenolate (SeR) are more covalent than those between Au and thiolate (SR),29 resulting in Au:SeR clusters having different stabilities and properties from Au:SR clusters. However, there have been conflicting reports regarding how changing the ligand from thiolate to selenolate affects the fundamental properties of a cluster. For example, Yee et al. reported that changing the ligand from thiolate to selenolate increases the cluster stability,29 but Brust et al. reported the opposite.26 To elucidate the effects of changing the ligand, it is necessary to isolate Au:SeR clusters with well-defined chemical compositions and compare their properties with those of corresponding Au:SR clusters. However, to the best of our knowledge, there have been no reports of the successful isolation of Au:SeR clusters. We report herein the isolation of a Au25 cluster protected by an octaneselenolate ([Au25(SeC8H17)18]). r 2011 American Chemical Society
[Au25(SeC8H17)18] possesses the same number of metal atoms and ligand molecules as [Au25(SR)18] and is an ideal compound for determining how changing the ligand molecules affects the fundamental properties of clusters. We also determine the geometric structure and the nature of the bonding of isolated [Au25(SeC8H17)18].
’ EXPERIMENTAL SECTION Synthesis and Isolation. Dioctyl diselenide ((C8H17Se)2) was synthesized in our laboratory by the method described in ref 30. All other chemicals were commercially obtained and used without further purification. A mixture of Au:SeC8H17 clusters (Figures S1 and S2) was prepared by the well-established two-phase procedure pioneered by Brust and co-workers31 with a slight modification. First, 20 mL of a toluene solution of (C8H17)4NBr (30 mM, 0.6 mmol) was added to 1.4 mL of an aqueous solution of HAuCl4 (0.14 M, 0.2 mmol). After vigorous stirring at room temperature for 30 min, 2 mL of an aqueous solution of NaBH4 (1 M, 2 mmol) was rapidly added to the solution. After 1 to 2 s, 3 mL of a toluene solution containing (C8H17Se)2 (1.5 mmol) was rapidly added to this solution. The solution’s color immediately changed from orange to dark red. The solution was stirred for 1 h. The organic phase was subsequently washed thoroughly with water and dried in an evaporator. The product was centrifuged at 3500 rpm with acetonitrile to remove byproducts such as (C8H17Se)2 and (C8H17)4NBr. The title compound was extracted from the dried product with a yield of 67% using acetone. Characterization. The ultravioletvisible absorption spectrum of the cluster was recorded in toluene at ambient temperature using a double-beam spectrometer (Jasco V-630). The raw spectral data, I(w), which are functions of wavelength, were converted into energy-dependent Received: August 23, 2011 Revised: September 19, 2011 Published: September 20, 2011 12289
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Langmuir data, I(E), using the following relation such that the integrated spectral areas were conserved: IðEÞ ¼
IðwÞ µ IðwÞw2 ∂E=∂w
Electrospray ionization (ESI) mass spectrometry was performed using a Fourier transform mass spectrometer (Bruker, Solarix). A 1 mg/mL toluene/acetonitrile (1:1 v/v) solution of [Au25(SeC8H17)18] was electrosprayed at a flow rate of 800 μL/min. Matrix-assisted laser desorption ionization (MALDI) mass spectra and laser desorption ionization (LDI) mass spectra were collected using a linear time-of-flight (TOF) mass spectrometer (Applied Biosystems, Voyager Linear RD VDA 500) with a nitrogen laser (337 nm). MALDI mass spectrometry was used to obtain mass spectra of clusters nondestructively to determine their chemical compositions precisely. trans2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as the MALDI matrix. To minimize the dissociation of the clusters caused by laser irradiation, we used a cluster-to-matrix ratio of 1:1000. The lowest laser fluence was used that enabled us to
Figure 1. Optical absorption spectrum of the product. The optical absorption spectrum of [Au25(SC8H17)18] is shown for comparison.
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detect the ions. LDI mass spectrometry was used to obtain the dissociation pattern of the clusters generated by laser irradiation. A cluster on a stainless steel substrate was dissociated using a slightly higher laser fluence than that used for MALDI mass spectrometry. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (Bruker TGA2000SA) at a heating rate of 10 °C/min under N2 flow in the temperature range of 25500 °C. [Au25(SeC8H17)18]((C8H17)4N+) (4.3 mg) was used in this measurement. Transmission electron microscopy (TEM) images were recorded using an electron microscope (Hitachi H-7650) operated at 100 kV; the TEM images obtained typically had magnifications of 100 000. X-ray diffraction (XRD) measurements were performed on a Rigaku Rint2500 using Cu Kα radiation (λ = 1.54 Å). X-ray photoelectron spectra were collected using an electron spectrometer (JEOL JPS-9010MC) equipped with a chamber at a base pressure of ∼2 108 Torr. X-rays from the Mg Kα line at 1253.6 eV were used for excitation.
’ RESULTS AND DISCUSSION Figure 1 shows an optical absorption spectrum of the extracted fraction. The product exhibits significant absorption in the energy range of 1.22.0 eV. This spectral feature resembles that of [Au25(SC8H17)18] (Figure 1) and that of [Au25(SR)18].1,3,69,13,14 The product has an absorption coefficient of 1 104 M1 cm1 at 1.85 eV (670 nm), which is the same as that measured for [Au25(SC8H17)18] (1 104 M1 cm1). Figure 2a shows a negative ion ESI mass spectrum of the product. It contains only a peak attributed to [Au25(SeC8H17)18]. The isotopic distribution of this peak is in good agreement with that calculated for [Au25(SeC8H17)18] (Figure 2b). We synthesized the cluster [Au25(SeC12H25)18] (Figure S3) by the same method using dodecaneselenolate (C12H25Se) as the ligand and obtained its ESI mass spectrum (Figure S4). The mass difference of the two clusters reveals that they have 18 ligands. In addition, only peaks from Au25(SeC8H17)18 were observed in the MALDI
Figure 2. (a) Negative ion ESI mass spectrum of the product and (b) a comparison of the experimental data with the calculated isotope pattern of [Au25(SeC8H17)18]. (c) Negative-ion MALDI mass spectrum and (d) TGA of the product. The asterisk in c indicates the fragment ions of [Au25(SeC8H17)18] (Figure S5). 12290
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Langmuir
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Figure 3. (a) Negative ion LDI mass spectrum and (b) X-ray diffractogram of [Au25(SeC8H17)18]. The X-ray diffractogram of [Au25(SC8H17)18] is shown for comparion in b.
Figure 4. (a) Au 4f spectrum and (b) Se 3d spectrum of [Au25(SeC8H17)18]. In a, the Au 4f spectrum of [Au25(SC8H17)18] is shown for comparison.
mass spectrum (Figures 2c and S5). The metal-to-organic ratio (55.1:44.9) obtained by TGA was also in good agreement with that (55.6:44.4) for [Au25(SeC8H17)18]((C8H17)4N+)32 (Figure 2d). Only particles with diameters of approximately 1 nm (∼30 atoms) were observed in TEM images of the product (Figure S6). All of these results indicate that the extracted clusters have a chemical composition of [Au25(SeC8H17)18] and that [Au25(SeC8H17)18] was isolated in high purity by the present experimental method. The isolation of [Au25(SeC8H17)18] indicates that Au25(SeC8H17)18 is stable with a charge state of 1. Au25(SR)18 has also been found to be most stable with a charge state of 1.9,21,33 The high stability of [Au25(SR)18] is explained by the filling of the electronic shell (the superatom model33). The results of our study strongly imply that a similar electron-counting rule as that for Au:SR clusters33 is applicable to Au:SeR clusters. Isolated [Au25(SeC8H17)18] is expected to have a similar geometric structure to [Au25(SR)18] because [Au25(SeC8H17)18] has same number of metal atoms and ligands as [Au25(SR)18]. That is, [Au25(SeC8H17)18] is expected to possess a structure in which the thiolates of [Au25(SR)18] are replaced by selenolates (Figure S7). Actually, a comparison of the absorption spectra of [Au25(SeC8H17)18] and [Au25(SC8H17)18] (Figure 1) reveals that the absorption peak (1.22.0 eV) attributed to the Au13 core13,34,35 has relatively similar shapes in both spectra. However, the absorption peak (2.53.0 eV) attributed to the transition from the oligomer-derived orbital to the lowest unoccupied molecular orbital (LUMO)13,34,35 is quite different in shape. This demonstrates that both clusters possess similar metal cores and that only the ligands in the oligomers differ. To gain a deeper understanding of the geometric structure, we obtained the LDI mass spectrum and XRD pattern of [Au25(SeC8H17)18]. Figure 3a shows the negative ion LDI mass spectrum of [Au25(SeC8H17)18]. The peak assigned to [Au25Se12] is the most abundant peak in the mass spectrum. Wu et al.
reported that the laser irradiation of [Au25(SR)18] clusters results in the process [Au25(SR)18] f [Au25S12] + 6S + 18R and that the most abundant peak in the resulting LDI mass spectrum was from [Au25S12], which is formed by the desorption of six S atoms together with CS dissociation.14 In that study, the six lost S atoms were assigned to the S atom in the center of the [SAuSAuS] oligomers. Six S atoms are considered to be lost by laser irradiation because [Au25(SR)18] contains six [SAuSAuS] oligomers. Figure 3a shows that the [Au25Se12] clusters formed by the loss of six Se atoms and the dissociation of CSe have the highest ion intensity in the LDI mass spectrum.36 This result strongly suggests that isolated [Au25(SeC8H17)18] also contains six similar oligomers ([SeAuSeAuSe]). In other words, it strongly indicates that isolated [Au25(SeC8H17)18] has a framework similar to that of [Au25(SR)18]. The powder XRD pattern of [Au25(SeC8H17)18] supports this interpretation. Figure 3b shows XRD patterns for [Au25(SeC8H17)18] and [Au25(SC8H17)18]. Both patterns have similar peaks at the same diffraction angles (Figure 3b), which indicates that [Au25(SeC8H17)18] possesses a similar geometric structure to [Au25(SR)18]. It has been pointed out that the bonds between the metal and the ligands in Au:SeR clusters are more covalent than those in Au:SR clusters.29 In fact, the X-ray photoelectron spectra of [Au25(SeC8H17)18] demonstrate that charge transfer from the metal to the ligands is lower in [Au25(SeC8H17)18]. Figure 4a shows the Au 4f spectra of [Au25(SeC8H17)18] and [Au25(SC8H17)18]. Because partial charge transfer occurs from gold to SR in [Au25(SC8H17)18], the Au 4f peak of [Au25(SC8H17)18] (84.5 eV) is observed on the oxidation side (i.e., the higher-energy side) relative to that of Au(0) (84.0 eV, red line). Although the Au 4f peak of [Au25(SeC8H17)18] (84.2 eV) was also observed on the oxidation side, the position of this peak was near the position of the Au(0) peak. Figure 4b shows the Se 3d 12291
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Langmuir peak of [Au25(SeC8H17)18]. The Se 3d peak (54.9 eV) was also observed near the Se 3d peak (55.3 eV) of alkyl diselenide (RSe)2. These results indicate that very little charge transfer occurs from the metal to the ligands in [Au25(SeC8H17)18]. That is, these results indicate that the AuSeR bond is much more covalent than the AuSR bond. In summary, we have succeeded in isolating [Au25(SeC8H17)18], which has the same number of metal atoms and ligands as [Au25(SR)18], in high purity. The experimental results for the isolated clusters reveal that [Au25(SeC8H17)18] has a similar geometric structure to [Au25(SR)18] and that less charge transfer occurs between the metal atoms and the ligands in [Au25(SeC8H17)18] than in [Au25(SR)18]. Further investigation of [Au25(SeC8H17)18] is required to determine how changing the ligands from thiolate to selenolate affects the stability and properties of clusters and the interaction between Au and SeR.
’ ASSOCIATED CONTENT
bS
Supporting Information. Characterization of as-prepared Au:SeC8H17 clusters and [Au25(SeC8H17)18]. This material is available free of charge via the Internet at http://pubs.acs. org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Dr. T. Kimura for technical advice regarding the synthesis of (C8H17Se)2. The ESI-MS analysis was supported by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. This work was financially supported by a grant-in-aid for scientific research (grant no. 20038045) and the Nanotechnology Network from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. ’ REFERENCES (1) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630. (2) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (3) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193. (4) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (5) Fields-Zinna, C. A.; Sardar, R.; Beasley, C. A.; Murray, R. W. J. Am. Chem. Soc. 2009, 131, 16266. (6) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Am. Chem. Soc. 2004, 126, 6518. (7) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (8) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464. (9) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322. (10) Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, 8608. (11) Negishi, Y.; Kurashige, W.; Niihori, Y.; Iwasa, T.; Nobusada, K. Phys. Chem. Chem. Phys. 2010, 12, 6219. (12) Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. J. Am. Chem. Soc. 2008, 130, 1138.
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