Article pubs.acs.org/JACS
Thermally Robust Au99(SPh)42 Nanoclusters for Chemoselective Hydrogenation of Nitrobenzaldehyde Derivatives in Water Gao Li, Chenjie Zeng, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: We report the synthesis and catalytic application of thermally robust gold nanoclusters formulated as Au99(SPh)42. The formula was determined by electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry in conjunction with thermogravimetric analysis. The optical spectrum of Au99(SPh)42 nanoclusters shows absorption peaks at ∼920 nm (1.35 eV), 730 nm (1.70 eV), 600 nm (2.07 eV), 490 nm (2.53 eV), and 400 nm (3.1 eV) in contrast to conventional gold nanoparticles, which exhibit a plasmon resonance band at 520 nm (for spherical particles). The ceria-supported Au99(SPh)42 nanoclusters were utilized as a catalyst for chemoselective hydrogenation of nitrobenzaldehyde to nitrobenzyl alcohol in water using H2 gas as the hydrogen source. The selective hydrogenation of the aldehyde group catalyzed by nanoclusters is a surprise because conventional nanogold catalysts instead give rise to the product resulting from reduction of the nitro group. The Au99(SPh)42/CeO2 catalyst gives high catalytic activity for a range of nitrobenzaldehyde derivatives and also shows excellent recyclability due to its thermal robustness. We further tested the size-dependent catalytic performance of Au25(SPh)18 and Au36(SPh)24 nanoclusters, and on the basis of their crystal structures we propose a molecular adsorption site for nitrobenzaldehyde. The nanocluster material is expected to find wide application in catalytic reactions.
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INTRODUCTION Gold nanoclusters protected by thiolate ligands have emerged as a new class of nanomaterials in recent years.1−7 Such nanoclusters have attracted wide research interest because of their promising applications, such as catalysis, sensing, and photovoltaics.8−14 A number of size-discrete, atomically precise gold nanoclusters have been reported,15−23 such as Au25(SR)18, Au36(SR)24, Au38(SR)24, Au102(SR)44, Au144(SR)60, etc. With respect to catalysis by nanoclusters, a major goal is to achieve a fundamental understanding of the size dependence and correlation between the catalytic properties and the nanocluster structure. The Aun(SR)m nanoclusters (where n and m represent the numbers of gold atoms and thiolate ligands, respectively) exhibit high stability, and some of them have been crystallographically characterized.2,16,17,24−26 In recent research, gold has been reported to exhibit good catalytic performance in reactions such as selective oxidation, hydrogenation, and C−C coupling reactions.9,27−33 In the hydrogenation reactions, Zhu et al.31 found that free or oxidesupported Au25(SR)18 nanoclusters can be applied in the chemoselective hydrogenation of α,β-unsaturated aldehydes to the corresponding α,β-unsaturated alcohols using H2 gas as the reducing agent in toluene as the solvent. Scott and co-workers reported stable and recyclable Au25(SR)18 nanoclusters for the reduction of 4-nitrophenol to 4-aminophenol using NaBH4 as the hydrogen source and tetrahydrofuran (THF) as the solvent.33 On the other hand, conventional gold nanoparticles © 2014 American Chemical Society
have also been utilized in catalytic hydrogenation reactions, and there have been a few reports of the use of supported gold nanoparticle catalysts for reducing aldehydes to alcohols as well as reducing nitro groups to amine groups.34−40 In principle, nitrobenzaldehyde could be reduced to three major products: aminobenzaldehyde, nitrobenzyl alcohol, and aminobenzyl alcohol (Scheme 1). For example, Corma and Serna reported Scheme 1. General Pathways for Reduction of Nitrobenzaldehyde Catalyzed by Gold Nanocatalysts
the chemoselective hydrogenation of p-nitrobenzaldehyde to paminobenzaldehyde with 97% selectivity using oxide-supported gold nanoparticles (e.g., Au/TiO2 and Au/Fe2O3) as catalysts and H2 (10−25 bar) as the reducing agent under thermal conditions (T > 100 °C).35 Compared with the conventional Au/oxide catalysts, ultrasmall (97%; entries 4 and 5), similar to the case of Au99(SPh)42/CeO2. The catalytic performance of these three sizes of Aun(SPh)m nanoclusters is comparable on the cluster molar basis, and the turnover frequency (TOF) can reach up to 97−100 h−1 (i.e., moles of product per hour per mole of clusters) on the supported Aun(SPh)m/CeO2 catalysts versus 28−42 h−1 over the unsupported Aun(SPh)m nanoclusters. Mechanistic Insights. The Au36(SR)24 and Au25(SR)18 nanoclusters have been crystallographically characterized in previous reports.16,24,25 Each Au25(SR)18 nanocluster consists of an icosahedral Au13 kernel protected by six −S(R)−Au−S(R)− Au−S(R)− staplelike motifs (Scheme 2A), and the Au36(SR)24 Scheme 2. (A, B) Crystal Structures16,24,25 of (A) Au25(SCH2CH2Ph)18 and (B) Au36(SPhtBu)24; (C, D) Corresponding Proposed Nitrobenzaldehyde Adsorption Modes
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EXPERIMENTAL SECTION
Chemicals. Tetrachloroauric(III) acid (HAuCl4·3H2O) (99.99% metal basis, Aldrich), tetraoctylammonium bromide (TOAB) (98%, Fluka), sodium borohydride (99.99% metals basis, Aldrich), thiophenol (99%, Sigma-Aldrich), toluene (HPLC grade, 99.9%, Aldrich), methanol (absolute, 200 proof, Pharmco), methylene chloride (HPLC grade, 99.9%, Sigma-Aldrich), and acetone (HPLC grade, 99.9%, Sigma-Aldrich) were obtained from the indicated suppliers. All chemicals were used as received. Nanopure water (resistivity 18.2 MΩ cm) was purified with a Barnstead NANOpure deionized water system. All glassware was thoroughly cleaned with aqua regia (3:1 v/v HCl/HNO3), rinsed with copious Nanopure water, and then dried in an oven prior to use. Synthesis and Characterization of Au99(SR)42 (R = SPh, SPhCH3) Nanoclusters. HAuCl4·3H2O (0.21 mmol, dissolved in 10 mL of water) and TOAB (0.24 mmol, dissolved in 15 mL of dichloromethane) were combined in a three-neck round-bottom flask. The solution was vigorously stirred (∼600 rpm) to facilitate phase transfer of the Au(III) salt into the organic phase. After 15 min, phase transfer was completed; the aqueous layer was removed, and the organic layer was washed with water (2 × 10 mL). Thiol (2.0 equiv relative to gold) was added. The solution color slowly changed from deep red to light yellow. After 1 h, 5 mL of an aqueous solution of NaBH4 (10 equiv relative to gold) was rapidly added to the solution. After NaBH4 addition, the reaction was allowed to proceed overnight. The organic phase was then removed and washed with methanol. The black solid was extracted with 1 mL of toluene and transferred to a flask. Thiol (1 mL) was added, and the solution was heated to 80 °C and maintained at this temperature overnight. During the long-time etching process, the initial polydisperse Aun(SR)m nanoclusters were finally converted to monodisperse Au99(SR)42 clusters. The organic phase was thoroughly washed with methanol to remove excess thiol, after which the pure Au99(SR)42 nanoclusters were simply extracted with dichloromethane. Characterization of Gold Nanoclusters. TEM images of the nanoclusters were obtained on a Hitachi 7000 transmission electron microscope operated at 75 kV. The specimen was made by placing one drop of a CH2Cl2 solution of nanoclusters onto a carbon thin-filmcoated TEM grid. The optical spectra of the clusters (dissolved in CH2Cl2) were acquired on a Hewlett-Packard Agilent 8453 diode array spectrophotometer (190−1100 nm) and a Varian Cary 5000
structure comprises a truncated tetrahedral Au28 kernel protected by four −S(R)−Au−S(R)−Au−S(R)− staples and 12 simple bridging thiolates (Scheme 2B). In previous work on Au25(SR)18-catalyzed reduction of CO2, density functional theory (DFT) simulations of the CO2 adsorption identified that one carbonyl group of OCO adsorbs to the surface gold atoms in the −S(R)−Au−S(R)−Au−S(R)− staples.10 Following this model, herein we propose that the OC(H)− group of nitrobenzaldehyde adsorbs onto the exposed gold sites in the staple motifs of Au25(SR)18 (Scheme 2C) and similarly in the case of Au36(SR)24 (Scheme 2D). While the crystal structure of Au99(SR)42 has not been attained, staplelike surface motifs are expected to be present in Au99(SR)42, and the OC(H)− adsorption mode should be similar. Future work will provide further insight into the molecular activation pathways in the chemoselective hydrogenation reaction.
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CONCLUSION We have successfully synthesized thermally robust Au99(SPh)42 nanoclusters for catalytic application in the chemoselective hydrogenation of nitrobenzaldehyde and its derivatives. ESI-MS 3677
dx.doi.org/10.1021/ja500121v | J. Am. Chem. Soc. 2014, 136, 3673−3679
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(5) Maity, P.; Xie, S.; Yamauchi, M.; Tsukuda, T. Nanoscale 2012, 4, 4027−4037. (6) Zhu, X.; Jin, S.; Wang, S.; Meng, X.; Zhu, C.; Zhu, M.; Jin, R. Chem.Asian J. 2013, 8, 2739−2745. (7) Malola, S.; Lehtovaara, L.; Knoppe, S.; Hu, K.-J.; Palmer, R. E.; Bürgi, T.; Häkkinen, H. J. Am. Chem. Soc. 2012, 134, 19560−19563. (8) Tsukuda, T.; Tsunoyama, H.; Sakurai, H. Chem.Asian J. 2011, 6, 736−748. (9) Li, G.; Jin, R. Acc. Chem. Res. 2013, 46, 1749−1758. (10) Kauffman, D. R.; Alfonso, D.; Matranga, C.; Qian, H.; Jin, R. J. Am. Chem. Soc. 2012, 34, 10237−10243. (11) Kauffman, D. R.; Alfonso, D.; Matranga, C.; Gao, L.; Jin, R. J. Phys. Chem. Lett. 2013, 4, 195−202. (12) Kwak, K.; Kumar, S. S.; Lee, D. Nanoscale 2012, 4, 4240−4246. (13) Wu, Z.; Wang, M.; Yang, J.; Zheng, X.; Cai, W.; Meng, G.; Qian, H.; Wang, H.; Jin, R. Small 2012, 8, 2028−2035. (14) Sakai, N.; Tatsuma, T. Adv. Mater. 2010, 22, 3185−3188. (15) Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. J. Am. Chem. Soc. 2008, 130, 1138−1139. (16) Zeng, C.; Qian, H.; Li, T.; Li, G.; Rosi, N. L.; Yoon, B.; Barnett, R. N.; Whetten, R. L.; Landman, U.; Jin, R. Angew. Chem., Int. Ed. 2012, 51, 13114−13118. (17) Qian, H.; Eckenhoff, W. T.; Zhu, Y.; Pintauer, T.; Jin, R. J. Am. Chem. Soc. 2010, 132, 8280−8281. (18) Levi-Kalisman, Y.; Jadzinsky, P. D.; Kalisman, N.; Tsunoyama, H.; Tsukuda, T.; Bushnell, D. A.; Kornberg, R. D. J. Am. Chem. Soc. 2011, 133, 2976−2982. (19) Qian, H.; Jin, R. Nano Lett. 2009, 9, 4083−4087. (20) Nimmala, P. R.; Yoon, B.; Whetten, R. L.; Landman, U.; Dass, A. J. Phys. Chem. A 2013, 117, 504−517. (21) Negishi, Y.; Sakamoto, C.; Ohyama, T.; Tsukuda, T. J. Phys. Chem. Lett. 2012, 3, 1624−1628. (22) Tang, Z.; Robinson, D. A.; Bokossa, N.; Xu, B.; Wang, S.; Wang, G. J. Am. Chem. Soc. 2011, 133, 16037−16044. (23) Nimmala, P. R.; Dass, A. J. Am. Chem. Soc. 2011, 133, 9175− 9177. (24) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883−5885. (25) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754−3755. (26) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430−433. (27) Julius, M.; Roberts, S.; Fletcher, J. C. Q. Gold Bull. 2010, 43, 298−306. (28) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239−3265. (29) Li, G.; Liu, C.; Lei, Y.; Jin, R. Chem. Commun. 2012, 48, 12005− 12007. (30) Li, G.; Jiang, D.; Liu, C.; Yu, C.; Jin, R. J. Catal. 2013, 306, 177− 183. (31) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Angew. Chem., Int. Ed. 2010, 49, 1295−1298. (32) Yamamoto, H.; Yano, H.; Kouchi, H.; Obora, Y.; Arakawa, R.; Kawasaki, H. Nanoscale 2012, 4, 4148−4154. (33) Shivhare, A.; Ambrose, S. J.; Zhang, H.; Purves, R. W.; Scott, R. W. J. Chem. Commun. 2013, 49, 276−278. (34) Claus, P.; Brückner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem. Soc. 2000, 122, 11430−11439. (35) Corma, A.; Serna, P. Science 2006, 313, 332−334. (36) Corma, A.; Serna, P.; Garcia, H. J. Am. Chem. Soc. 2007, 129, 6358−6359. (37) Ren, D.; He, L.; Yu, L.; Ding, R.-S.; Liu, Y.-M.; Cao, Y.; He, H.; Fan, K. J. Am. Chem. Soc. 2012, 134, 17592−17598. (38) Yan, M.; Jin, T.; Chen, Q.; Ho, H. E.; Fujita, T.; Chen, L.; Bao, M.; Chen, M.; Asao, N.; Yamamoto, Y. Org. Lett. 2013, 15, 1484− 1487. (39) Barrio, L.; Liu, P.; Rodriguez, J. A.; Campos-Martin, J. M.; Fierro, J. L. G. J. Chem. Phys. 2006, 125, No. 164715. (40) Ide, M. S.; Hao, B.; Neurock, M.; Davis, R. J. ACS Catal. 2012, 2, 671−683.
UV−vis−NIR spectrophotometer (190−1600 nm) at room temperature. MALDI-MS was performed with a PerSeptive Biosystems Voyager DE super-STR time-of-flight (TOF) mass spectrometer. trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as the matrix in MALDI analysis.23 Typically, 1 mg of matrix and 0.1 mL of analyte stock solution were mixed in 100 μL of CH2Cl2, and 10 μL of this solution was applied to the steel plate and then air-dried prior to MALDI analysis. ESI-MS was performed using a Waters Q-TOF mass spectrometer equipped with a Z-spray source. The sample was dissolved in toluene (1 mg/mL) and then mixed 1:1 (v/v) with a dry methanol solution of CsOAc (50 mM). TGA was conducted on ∼2 mg samples under a N2 atmosphere (flow rate ∼50 mL/min) on a TG/DAT 6300 analyzer (Seiko Instruments, Inc.) at a heating rate of 10 °C/min. Typical Procedure for Chemselective Hydrogenation of Nitrobenzaldehyde. In a typical selective hydrogenation reaction, 4nitrobenzaldehyde (0.05 mmol), pyridine (0.1 mmol), and Au99(SPh)42 nanoclusters (∼1 mg) or Au99(SPh)42/oxide (100 mg, 1 wt % loading) were added to a reactor (Parr Instrument Company, series 4700, 22 mL capacity) under 20 bar H2. The reaction mixture was kept at 80 °C as indicated in Table 1. After the reaction, the mixture was extracted with ethyl acetate. The crude product was obtained after removal of the solvent. The conversion of 4nitrobenzaldehyde and the selectivity for the 4-nitrobenzyl alcohol product were determined by 1H NMR (300 MHz) spectroscopic analysis (Figure S6 in the Supporting Information). In the recycling tests, the Au99(SPh)42/CeO2 catalyst was collected after the reaction by centrifugation at 1000 rpm for 5 min and then reused in a fresh reaction. The reported conversion and selectivity values were averaged over three measurements, and the error is ∼0.5%.
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ASSOCIATED CONTENT
S Supporting Information *
UV−vis spectrum of the Au n (SPhCH 3 ) m nanoclusters, isothermal stability analysis at 150 °C and UV−vis spectra of the nanoclusters before/after 150 °C thermal treatment, TEM image of Au99(SPh)42 nanoclusters after the catalytic reaction, UV−vis and MALDI-MS spectra of Au 25 (SPh) 18 and Au36(SPh)24 nanoclusters before and after hydrogenation reaction, and 1H NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Grant DE-FG0212ER16354). We thank Joseph Suhan for assistance with TEM imaging of nanoclusters.
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REFERENCES
(1) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Acc. Chem. Res. 2012, 45, 1470−1479. (2) (a) Das, A.; Li, T.; Nobusada, K.; Zeng, C.; Rosi, N. L.; Jin, R. J. Am. Chem. Soc. 2013, 135, 18264−18267. (b) Zeng, C.; Li, T.; Das, A.; Rosi, N. L.; Jin, R. J. Am. Chem. Soc. 2013, 135, 10011−10013. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27−36. (4) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397−406. 3678
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(41) Jin, R.; Qian, H.; Wu, Z.; Zhu, Y.; Zhu, M.; Mohanty, A.; Garg, N. J. Phys. Chem. Lett. 2010, 1, 2903−2910. (42) Fields-Zinna, C. A.; Sampson, J. S.; Crowe, M. C.; Tracy, J. B.; Parker, J. F.; de Ney, A. M.; Muddiman, D. C.; Murray, R. W. J. Am. Chem. Soc. 2009, 131, 13844−13851. (43) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175−192. (44) Chng, L. L.; Erathodiyil, N.; Ying, J. Y. Acc. Chem. Res. 2013, 46, 1825−1837. (45) Stakheev, A. Y.; Kustov, L. M. Appl. Catal., A 1999, 188, 3−35.
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dx.doi.org/10.1021/ja500121v | J. Am. Chem. Soc. 2014, 136, 3673−3679
Thermally Robust Au99(SPh)42 Nanoclusters for Chemoselective Hydrogenation of Nitrobenzaldehyde Derivatives in Water Gao Li, Chenjie Zeng, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: We report the synthesis and catalytic application of thermally robust gold nanoclusters formulated as Au99(SPh)42. The formula was determined by electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry in conjunction with thermogravimetric analysis. The optical spectrum of Au99(SPh)42 nanoclusters shows absorption peaks at ∼920 nm (1.35 eV), 730 nm (1.70 eV), 600 nm (2.07 eV), 490 nm (2.53 eV), and 400 nm (3.1 eV) in contrast to conventional gold nanoparticles, which exhibit a plasmon resonance band at 520 nm (for spherical particles). The ceria-supported Au99(SPh)42 nanoclusters were utilized as a catalyst for chemoselective hydrogenation of nitrobenzaldehyde to nitrobenzyl alcohol in water using H2 gas as the hydrogen source. The selective hydrogenation of the aldehyde group catalyzed by nanoclusters is a surprise because conventional nanogold catalysts instead give rise to the product resulting from reduction of the nitro group. The Au99(SPh)42/CeO2 catalyst gives high catalytic activity for a range of nitrobenzaldehyde derivatives and also shows excellent recyclability due to its thermal robustness. We further tested the size-dependent catalytic performance of Au25(SPh)18 and Au36(SPh)24 nanoclusters, and on the basis of their crystal structures we propose a molecular adsorption site for nitrobenzaldehyde. The nanocluster material is expected to find wide application in catalytic reactions.
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INTRODUCTION Gold nanoclusters protected by thiolate ligands have emerged as a new class of nanomaterials in recent years.1−7 Such nanoclusters have attracted wide research interest because of their promising applications, such as catalysis, sensing, and photovoltaics.8−14 A number of size-discrete, atomically precise gold nanoclusters have been reported,15−23 such as Au25(SR)18, Au36(SR)24, Au38(SR)24, Au102(SR)44, Au144(SR)60, etc. With respect to catalysis by nanoclusters, a major goal is to achieve a fundamental understanding of the size dependence and correlation between the catalytic properties and the nanocluster structure. The Aun(SR)m nanoclusters (where n and m represent the numbers of gold atoms and thiolate ligands, respectively) exhibit high stability, and some of them have been crystallographically characterized.2,16,17,24−26 In recent research, gold has been reported to exhibit good catalytic performance in reactions such as selective oxidation, hydrogenation, and C−C coupling reactions.9,27−33 In the hydrogenation reactions, Zhu et al.31 found that free or oxidesupported Au25(SR)18 nanoclusters can be applied in the chemoselective hydrogenation of α,β-unsaturated aldehydes to the corresponding α,β-unsaturated alcohols using H2 gas as the reducing agent in toluene as the solvent. Scott and co-workers reported stable and recyclable Au25(SR)18 nanoclusters for the reduction of 4-nitrophenol to 4-aminophenol using NaBH4 as the hydrogen source and tetrahydrofuran (THF) as the solvent.33 On the other hand, conventional gold nanoparticles © 2014 American Chemical Society
have also been utilized in catalytic hydrogenation reactions, and there have been a few reports of the use of supported gold nanoparticle catalysts for reducing aldehydes to alcohols as well as reducing nitro groups to amine groups.34−40 In principle, nitrobenzaldehyde could be reduced to three major products: aminobenzaldehyde, nitrobenzyl alcohol, and aminobenzyl alcohol (Scheme 1). For example, Corma and Serna reported Scheme 1. General Pathways for Reduction of Nitrobenzaldehyde Catalyzed by Gold Nanocatalysts
the chemoselective hydrogenation of p-nitrobenzaldehyde to paminobenzaldehyde with 97% selectivity using oxide-supported gold nanoparticles (e.g., Au/TiO2 and Au/Fe2O3) as catalysts and H2 (10−25 bar) as the reducing agent under thermal conditions (T > 100 °C).35 Compared with the conventional Au/oxide catalysts, ultrasmall (97%; entries 4 and 5), similar to the case of Au99(SPh)42/CeO2. The catalytic performance of these three sizes of Aun(SPh)m nanoclusters is comparable on the cluster molar basis, and the turnover frequency (TOF) can reach up to 97−100 h−1 (i.e., moles of product per hour per mole of clusters) on the supported Aun(SPh)m/CeO2 catalysts versus 28−42 h−1 over the unsupported Aun(SPh)m nanoclusters. Mechanistic Insights. The Au36(SR)24 and Au25(SR)18 nanoclusters have been crystallographically characterized in previous reports.16,24,25 Each Au25(SR)18 nanocluster consists of an icosahedral Au13 kernel protected by six −S(R)−Au−S(R)− Au−S(R)− staplelike motifs (Scheme 2A), and the Au36(SR)24 Scheme 2. (A, B) Crystal Structures16,24,25 of (A) Au25(SCH2CH2Ph)18 and (B) Au36(SPhtBu)24; (C, D) Corresponding Proposed Nitrobenzaldehyde Adsorption Modes
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EXPERIMENTAL SECTION
Chemicals. Tetrachloroauric(III) acid (HAuCl4·3H2O) (99.99% metal basis, Aldrich), tetraoctylammonium bromide (TOAB) (98%, Fluka), sodium borohydride (99.99% metals basis, Aldrich), thiophenol (99%, Sigma-Aldrich), toluene (HPLC grade, 99.9%, Aldrich), methanol (absolute, 200 proof, Pharmco), methylene chloride (HPLC grade, 99.9%, Sigma-Aldrich), and acetone (HPLC grade, 99.9%, Sigma-Aldrich) were obtained from the indicated suppliers. All chemicals were used as received. Nanopure water (resistivity 18.2 MΩ cm) was purified with a Barnstead NANOpure deionized water system. All glassware was thoroughly cleaned with aqua regia (3:1 v/v HCl/HNO3), rinsed with copious Nanopure water, and then dried in an oven prior to use. Synthesis and Characterization of Au99(SR)42 (R = SPh, SPhCH3) Nanoclusters. HAuCl4·3H2O (0.21 mmol, dissolved in 10 mL of water) and TOAB (0.24 mmol, dissolved in 15 mL of dichloromethane) were combined in a three-neck round-bottom flask. The solution was vigorously stirred (∼600 rpm) to facilitate phase transfer of the Au(III) salt into the organic phase. After 15 min, phase transfer was completed; the aqueous layer was removed, and the organic layer was washed with water (2 × 10 mL). Thiol (2.0 equiv relative to gold) was added. The solution color slowly changed from deep red to light yellow. After 1 h, 5 mL of an aqueous solution of NaBH4 (10 equiv relative to gold) was rapidly added to the solution. After NaBH4 addition, the reaction was allowed to proceed overnight. The organic phase was then removed and washed with methanol. The black solid was extracted with 1 mL of toluene and transferred to a flask. Thiol (1 mL) was added, and the solution was heated to 80 °C and maintained at this temperature overnight. During the long-time etching process, the initial polydisperse Aun(SR)m nanoclusters were finally converted to monodisperse Au99(SR)42 clusters. The organic phase was thoroughly washed with methanol to remove excess thiol, after which the pure Au99(SR)42 nanoclusters were simply extracted with dichloromethane. Characterization of Gold Nanoclusters. TEM images of the nanoclusters were obtained on a Hitachi 7000 transmission electron microscope operated at 75 kV. The specimen was made by placing one drop of a CH2Cl2 solution of nanoclusters onto a carbon thin-filmcoated TEM grid. The optical spectra of the clusters (dissolved in CH2Cl2) were acquired on a Hewlett-Packard Agilent 8453 diode array spectrophotometer (190−1100 nm) and a Varian Cary 5000
structure comprises a truncated tetrahedral Au28 kernel protected by four −S(R)−Au−S(R)−Au−S(R)− staples and 12 simple bridging thiolates (Scheme 2B). In previous work on Au25(SR)18-catalyzed reduction of CO2, density functional theory (DFT) simulations of the CO2 adsorption identified that one carbonyl group of OCO adsorbs to the surface gold atoms in the −S(R)−Au−S(R)−Au−S(R)− staples.10 Following this model, herein we propose that the OC(H)− group of nitrobenzaldehyde adsorbs onto the exposed gold sites in the staple motifs of Au25(SR)18 (Scheme 2C) and similarly in the case of Au36(SR)24 (Scheme 2D). While the crystal structure of Au99(SR)42 has not been attained, staplelike surface motifs are expected to be present in Au99(SR)42, and the OC(H)− adsorption mode should be similar. Future work will provide further insight into the molecular activation pathways in the chemoselective hydrogenation reaction.
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CONCLUSION We have successfully synthesized thermally robust Au99(SPh)42 nanoclusters for catalytic application in the chemoselective hydrogenation of nitrobenzaldehyde and its derivatives. ESI-MS 3677
dx.doi.org/10.1021/ja500121v | J. Am. Chem. Soc. 2014, 136, 3673−3679
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UV−vis−NIR spectrophotometer (190−1600 nm) at room temperature. MALDI-MS was performed with a PerSeptive Biosystems Voyager DE super-STR time-of-flight (TOF) mass spectrometer. trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as the matrix in MALDI analysis.23 Typically, 1 mg of matrix and 0.1 mL of analyte stock solution were mixed in 100 μL of CH2Cl2, and 10 μL of this solution was applied to the steel plate and then air-dried prior to MALDI analysis. ESI-MS was performed using a Waters Q-TOF mass spectrometer equipped with a Z-spray source. The sample was dissolved in toluene (1 mg/mL) and then mixed 1:1 (v/v) with a dry methanol solution of CsOAc (50 mM). TGA was conducted on ∼2 mg samples under a N2 atmosphere (flow rate ∼50 mL/min) on a TG/DAT 6300 analyzer (Seiko Instruments, Inc.) at a heating rate of 10 °C/min. Typical Procedure for Chemselective Hydrogenation of Nitrobenzaldehyde. In a typical selective hydrogenation reaction, 4nitrobenzaldehyde (0.05 mmol), pyridine (0.1 mmol), and Au99(SPh)42 nanoclusters (∼1 mg) or Au99(SPh)42/oxide (100 mg, 1 wt % loading) were added to a reactor (Parr Instrument Company, series 4700, 22 mL capacity) under 20 bar H2. The reaction mixture was kept at 80 °C as indicated in Table 1. After the reaction, the mixture was extracted with ethyl acetate. The crude product was obtained after removal of the solvent. The conversion of 4nitrobenzaldehyde and the selectivity for the 4-nitrobenzyl alcohol product were determined by 1H NMR (300 MHz) spectroscopic analysis (Figure S6 in the Supporting Information). In the recycling tests, the Au99(SPh)42/CeO2 catalyst was collected after the reaction by centrifugation at 1000 rpm for 5 min and then reused in a fresh reaction. The reported conversion and selectivity values were averaged over three measurements, and the error is ∼0.5%.
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ASSOCIATED CONTENT
S Supporting Information *
UV−vis spectrum of the Au n (SPhCH 3 ) m nanoclusters, isothermal stability analysis at 150 °C and UV−vis spectra of the nanoclusters before/after 150 °C thermal treatment, TEM image of Au99(SPh)42 nanoclusters after the catalytic reaction, UV−vis and MALDI-MS spectra of Au 25 (SPh) 18 and Au36(SPh)24 nanoclusters before and after hydrogenation reaction, and 1H NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Grant DE-FG0212ER16354). We thank Joseph Suhan for assistance with TEM imaging of nanoclusters.
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REFERENCES
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