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1Department of Chemistry, Tsinghua University, Beijing 10084, P. R. China. 2Departmentof Chemistry, College of Chemistry and Chemical Engineering, Xiamen University,Fujian 361005, P. R. China.*Corresponding author. Email: qmwang@tsinghua.edu.cn

Wan et al., Sci. Adv. 2017;3 : e1701823 6 October 2017

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Ligand effects in catalysis by atomically precisegold nanoclustersXian-Kai Wan,1,2 Jia-Qi Wang,1 Zi-Ang Nan,1,2 Quan-Ming Wang1,2*

Atomically precise gold nanoclusters are ideal model catalysts with well-defined compositions and tunablestructures. Determination of the ligand effect on catalysis requires the use of gold nanoclusters with protectingligands as the only variable. Two isostructural Au38 nanoclusters, [Au38(L)20(Ph3P)4]

2+ (L = alkynyl or thiolate), havebeen synthesized by a direct reduction method, and they have an unprecedented face-centered cubic (fcc)–typeAu34 kernel surrounded by 4 AuL2 staple motifs, 4 Ph3P, and 12 bridging L ligands. The Au34 kernel can be derivedfrom the fusion of two fcc-type Au20 via sharing a Au6 face. Catalytic performance was studied with these two na-noclusters supported on TiO2 (1/TiO2 and 2/TiO2) as catalysts. The alkynyl-protected Au38 are very active (>97%) inthe semihydrogenation of alkynes (including terminal and internal ones) to alkenes, whereas the thiolated Au38showed a very low conversion (<2%). This fact suggests that the protecting ligands play an important role in H2

activation. This work presents a clear demonstration that catalytic performance of gold nanoclusters can be modulatedby the controlled construction of ligand spheres.

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INTRODUCTIONAtomically precise gold nanoclusters are chosen as model catalystsbecause of their well-defined compositions and tunable structures,which build a solid basis for correlation between structures and proper-ties (1–7). Because these gold nanoclusters are protected by varioustypes of ligands, including thiolates, phosphines, alkynyls, etc. (8–35),study of the roles of ligands in structural control and their influenceon related properties is a very important issue (36–38).

Structural isomerism in thiolated Au nanoclusters has been re-ported, and their catalytic applications have also been investigated,showing that different surface structures may influence the catalytic ac-tivities (39, 40). It was reported that a bimetallic gold nanoclusterdisplayed distinct catalytic activities when it was tested with protectingligands or as a naked cluster (41). To understand the ligand effectinvolved in nanocluster catalysis, gold nanoclusters having identicalmetal kernels but with different kinds of organic ligands are ideal modelcatalysts. With protecting ligands being the only variable in comparingthe catalytic activities of two clusters, the ligand effect could be ad-dressed. To this end, we initiate the synthesis of these isostructural goldnanoclusters with either alkynyls or thiolates based on our previousfindings that alkynyls can be similar to thiolates in terms of binding pref-erence for gold (29, 30). Using the direct reductionmethod, we managedto synthesize two isostructural gold nanoclusters, such as [Au38(PhC≡C)20(Ph3P)4] (SO3CF3)2 (1) and [Au38(3-methylbenzenethiol)20(Ph3P)4](SbF6)2 {[Au38(m-MBT)20(Ph3P)4](SbF6)2} (2). Structural determinationrevealed that both 1 and 2 have an unprecedented Au34 kernel of face-centered cubic (fcc) type wrapped with L–Au–L monomeric staplemotifs [L = PhC≡C in 1 and SR in 2 (SR =m-MBT)]. To our surprise,cluster 1 is very active in the semihydrogenation of alkynes (includingterminal and internal ones) to alkenes, whereas cluster 2 has a very lowcatalytic activity under the same reaction conditions. These factsstrongly suggest the presence of ligand effects in gold nanoclustercatalysis.

RESULTS AND DISCUSSIONThe synthesis of Au38 nanoclusters was carried out by the direct reduc-tion of gold precursors with NaBH4 in CH2Cl2. For cluster 1, the goldprecursors are PhC≡CAu and Ph3PAuX (X = anion), and for cluster 2,the gold precursors are m-MBTAu and Ph3PAuX. The clusters werecharacterized by electrospray ionization–time-of-flight (ESI-TOF)massspectrometry (MS) in positive mode (Fig. 1). The spectra show cleansignals at a mass/charge ratio (m/z) of 5277.5 and 5498.3 for 1 and 2,respectively, which correspond to the molecular ions [Au38(PhC≡C)20(Ph3P)4]

2+ and [Au38(m-MBT)20(Ph3P)4]2+, respectively. The observed

isotopic pattern of the dication clusters agrees with the simulation(Fig. 1, inset). Thermogravimetric analysis (TGA) reveals that bothclusters are stable below 155°C (fig. S1). X-ray photoelectron spectros-copy (XPS) revealed that the twoAu38 nanoclusters have different Au 4fbinding energies (1, 84.4 eV for Au 4f7/2 and 88.1 eV for Au 4f5/2; 2,84.2 eV for Au 4f7/2 and 87.9 eV for Au 4f5/2), indicating different Aucharge states (fig. S2). Infrared (fig. S3) bands at 1033 and 1263 cm−1

of SO3 and 1150 and 1227 cm−1 of CF3 indicate the existence of SO3CF3

−

in 1 (42).The structure of 1was determined by single-crystal x-ray diffraction.

As shown in Fig. 2, the Au38 nanocluster has a D2 symmetry and con-sists of 38 Au atoms, 20 phenylethynyl ligands, and 4 Ph3P ligands. TheAu38 cluster has an fcc-type Au34 kernel surrounded by 4 PhC≡C–Au–C≡CPh staple motifs (Fig. 3A) and 12 bridging PhC≡C ligands (Fig.3B). The structure of this Au34 kernel can be derived from the fusion

Fig. 1. Mass spectra of the Au38 nanoclusters. Inset: Measured (black trace)and simulated (red trace) isotopic patterns. (A) [Au38(PhC≡C)20(Ph3P)4]

2+ in 1.(B) [Au38(m-MBT)20(Ph3P)4]

2+ in 2.

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of two fcc-type Au20 via sharing a Au6 {100} face (Fig. 3C, highlightedin green). As shown in Fig. 3D, the fcc-type rodlike Au20 is composedof two interpenetrating Au13 cuboctahedra (Fig. 4D), which was pre-

Wan et al., Sci. Adv. 2017;3 : e1701823 6 October 2017

viously observed in Au28(SR)20 (18). The Au34 kernel has well-definedcrystal faces exposed, that is, 8 trapezoid-shaped {111} facets on thefront and backside of the gold core (fig. S4A, pale blue shadow) and12 square facets {100} (fig. S4A, pale green shadow). Because of the fccsymmetry, the Au34 kernel exhibits layer-by-layer abc packing of{111} atomic planes (Fig. 3C). The structure of 2 is very similar to 1,with PhC≡C–Au–C≡CPh motifs replaced by RS–Au–SR staples andPhC≡C bridges replaced by SR ligands (Fig. 3E).

In the Au34 kernel of 1, the Au–Au distances from the four centeredAu atoms (fig. S4B, highlighted in pink) to the peripheral Au atomscan be classified into two groups: shorter distances ranging from2.7325(12) to 2.9138(9) Å and longer ones ranging from 2.9493(11)to 3.3017(12) Å with an average distance of 2.925 Å. It is comparableto the average Au–Au distance of 2.926 Å in 2. The Au–Au distancesof the peripheral Au atoms range from 2.6992(11) to 2.8340(2) Å and2.9085(12) to 3.4160(12) Å, giving an average distance of 2.940 Å. Acomparable average distance of 2.958 Å is found in 2. The averageAu–Au distances of Au34 kernel in 1 and 2 are 2.940 and 2.946 Å,respectively, which are slightly longer than the 2.88 Å bond length inbulk gold, indicating that the interaction between Au atoms in Au34is strong. The four Au atoms of staples are linked to the double Au20cores, with an average Au–Au distance of 3.147 Å for 1 and 3.217 Å for2. As shown in Fig. 3 (A and E), the SR groups link Au atoms via sbonds in 2, whereas the alkynyls display both s and p bonds in ligatingAu atoms of 1.

Ultraviolet-visible (UV-vis) spectra of 1 and 2 in CH2Cl2 are shownin Fig. 4. From 350 to 1000 nm, although their metal core structures arealmost identical and they are isoelectronic superatomic systems, the ab-sorption profiles are significantly different. This difference suggests thatthe electronic structure can be disturbed by the surrounding organicligands, for example, the alkynyl with a larger conjugated system. TheHOMO-LUMO (highest occupied molecular orbital–lowest un-occupied molecular orbital) gaps are also different, with 1.49 eV for1 and 1.37 eV for 2, which are comparable to the value of 1.5 eV ofthe body-centered cubic–type Au38S2(S-Adm)20, but are much largerthan the gap (0.9 eV) of the bi-icosahedral Au38(SCH2CH2Ph)24 (15, 43).

Clusters 1 and 2 are quite stable, as confirmed by monitoring theUV-vis spectra. No decomposition was observed after their solutionwas stored under ambient conditions for more than 1 month (fig. S5).The transmission electronmicroscopy (TEM) images of 1 and 2 indicatethat the particles are uniform in size of about 1.4 nm (Fig. 4, inset), and it

Fig. 2. Structure of the Au38 dications. (A) [Au38(PhC≡C)20(Ph3P)4]2+ in 1. (B) [Au38(m-MBT)20(Ph3P)4]

2+ in 2.

Fig. 3. Anatomy of the Au38 kernel structure. (A) Au34 kernel attached withfour PhC≡C–Au–C≡CPh staple motifs. (B) Au34 kernel with 12 bridging PhC≡C ligands.(C) Au34 (fcc type) kernel in Au38. (D) Model of interpenetrated bicuboctahedral Au20.(E) Au34 kernel attached with four RS–Au–SR staple motifs (SR = m-MBT). Orange/green/blue, Au atoms; yellow, S atoms; gray, C atoms.

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agrees with that determined by x-ray structural analysis. These data in-dicate that the clusters maintain their nanosized structures in the solu-tion and that their structural integrity in the solution is also supported by31P nuclear magnetic resonance (NMR) (fig. S6). Single peaks werefound at d = 48.87 parts per million (ppm) for 1 and 38.54 ppm for 2,which suggests that the four phosphorus atomsare in equivalent chemicalenvironments due to the D2 molecular symmetry.

Because 1 and 2 are isostructural, differing only in the protectingligands, they are ideal platforms for studying the ligand effects oncatalytic behavior. We chose the semihydrogenation of alkynes into

Wan et al., Sci. Adv. 2017;3 : e1701823 6 October 2017

alkenes as a probe reaction (44). The supported catalyst was made byimpregnation of oxide powders in a CH2Cl2 solution of the Au38nanoclusters, followed by drying and annealing at 130°C for 1 hourin a vacuum oven. Note that the free (unsupported) Au38 nanoclustersare stable below 155°C, as indicated by TGA (fig. S1).

To further confirm the thermal stability of Au38 nanoclusters, wetreated the unsupported Au38 nanoclusters (powder) at 130°C in a vac-uum tube for 1 hour and measured their absorption spectra. As shownin Fig. 5 (A and B), the Au38 nanoclusters have identical profiles beforeand after the thermal treatment, which indicates that they remain intactafter the thermal treatment. ESI-MS analysis (Fig. 5, C and D) alsoconfirms that no decomposition happened during the thermal treat-ment of Au38 nanoclusters because clean single peaks were observed atm/z values of 5277.5 and 5498.3, which correspond to molecular ionpeaks of [Au38(PhC≡C)20(Ph3P)4]

2+ and [Au38(m-MBT)20(Ph3P)4]2+,

respectively. Furthermore, the TEM images (fig. S7, A to C) show thatthe cluster size did not change significantly after being loaded on theTiO2 support and subjected to the thermal treatment.

The Au38/TiO2 catalysts were used in the semihydrogenation ofalkynes, and the reaction products were analyzed by 1HNMR spectros-copy (fig. S8). As shown in Fig. 6, neat TiO2 almost yielded no conver-sion, but it is important for H2 activation. We tested the support effectby replacing TiO2 with Al2O3 and found that 1/Al2O3 yielded a muchlower conversion (<10%). Under the same conditions (Table 1), catalyst1/TiO2 yielded a >97% conversion (entries 1 to 5) for the semihydro-genation of alkynes, including terminal and internal ones. The turnovernumber is up to 88,195 within 24 hours. These results are in sharp

Fig. 4. Optical absorption spectra of Au38 in CH2Cl2 and their TEM images.(A) TEM image of 1. (B) TEM image of 2. Black and red lines represent 1 and 2,respectively.

Fig. 5. UV-vis spectra and ESI-MS of Au38 in CH2Cl2. (A and C) UV-vis spectraand ESI-MS of 1. (B and D) UV-vis spectra and ESI-MS of 2. Black and red linesrepresent before and after thermal treatments, respectively.

Fig. 6. Activities of TiO2 and 1/TiO2 for the semihydrogenation of alkynes.(A) The substrate is ethynylbenzene [H2 (10 bar)]. (B) The substrate is diphenyla-cetylene [H2 (20 bar)].

Table 1. Semihydrogenation of terminal* and internal† alkynes onAu38/TiO2 catalyst.

‡ ‡

Entry Catalyst R1 R2 Conversion (%) Selectivity (%)

1

1 Ph H 100 100

2…

PhC2H4

……………

H……………

100……………………………

100………………………...

3…

Ph……………

Ph……………

100……………………………

93………………………...

4…

Ph……………

4-Br-Ph……………

97……………………………

94………………………...

5…

Ph……………

CH3

……………

97……………………………

94………………………...

6

2 Ph H 1.6

7…

PhC2H4

……………

H……………

1.4……………………………

………………………...

8…

Ph……………

Ph……………

1.7……………………………

………………………...

9…

Ph……………

4-Br-Ph……………

<1……………………………

………………………...

10…

Ph……………

CH3

……………

<1……………………………

………………………...

*Reaction conditions: 80 mg of Au38 [0.4 weight % (wt %)]/TiO2 catalyst,0.2 mmol of alkynes, 0.4 mmol of pyridine, 1.0 ml of EtOH/H2O (10:1, v/v),80°C, H2 (10 bar), 15 hours. †Reaction conditions: 80 mg of Au38 (0.4 wt%)/TiO2 catalyst, 0.2 mmol of alkynes, 0.4 of mmol pyridine, 1.0 ml of EtOH/H2O (10:1, v/v), 110°C, H2 (20 bar), 20 hours. ‡The conversion and stereo-selectivity for Z-alkenes were determined by 1H NMR.

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contrast to those reported by Li and Jin (45). In their work, the terminalalkynes can be catalyzed by “ligand-on” Au25 catalysts, but internal al-kynes cannot be catalyzed. With ligand-on catalysts, there is no roomfor the approaching of internal alkynes, so the alkynesmust be activatedby 1 in a different activation pathway; that is, it is not necessary for al-kynes to be adsorbed on the cluster catalysts. The key step will be theactivation of H2. The small H2 can be adsorbed on the nanocluster andthen dissociate with the promotion of the base (pyridine). The gener-ated hydrides are transferred to alkynes for reduction (46).

Whereas 1/TiO2 is very active, 2/TiO2 showed very low conversions(Table 1, entries 6 to 10). This fact indicates that the activation of H2 isinfluenced by the different electronic structures of the Au38 nanoclus-ters. Because the only structural difference between 1 and 2 is theprotecting ligands, the electronic structures are disturbed by the typesof the ligands. This hypothesis is supported by two facts: (i) The 31PNMR chemical shifts of 1 and 2 are different (fig. S6), and (ii) theirUV-vis spectra are significantly different (Fig. 4). The triple bondcharacter of the alkyne favors the electronic mixing into the clusterkernel, which gives amore suitable electronic structure for the activationofH2.Note thatOuyang and Jiang (47) found that it is difficult for directH2 activation on thiolate-protected gold nanoclusters.

Because 1 and 2 have different accompanying anions (SO3CF3versus SbF6), we also prepared [Au38(m-MBT)20(Ph3P)4](SO3CF3)2 (3)to make sure that the counterions do not affect the activities. Cluster 3was prepared by anion exchange of 2 with KSO3CF3. We observed verysimilar activities on 2/TiO2 and 3/TiO2, confirming that the anionshave no effect on the activity difference between 1/TiO2 and 2/TiO2.

To further reveal the roles of the ligands in H2 activation, we testedthe catalytic performance of the supported nanoclusters with ligandsbeing partially removed. We pretreated 1/TiO2 and 2/TiO2 at differenttemperatures to obtain catalysts with different ligand coverages. Wefound that 1/TiO2 pretreated at 175° or 220°C, that is, ligands par-tially removed, showed 100% conversion in the semihydrogenationof diphenylacetylene (table S1). More E isomers were obtained with1/TiO2 pretreated at a higher temperature (220°C) because the clustersurface is more accessible for the trans-type approach of the substrate.However, when all the ligandswere removed, the naked gold clusterwashardly active (1/TiO2 pretreated at 400°C). Note that no severe aggre-gation of Au38 was observed, as confirmed by TEM (fig. S9). This factindicates that alkynyl plays a key role in the activation of H2. As for 2/TiO2, the activity is very lowunder all tested conditions (table S1, entries6 to 9).

Thus, this work presents a clear example that the catalytic performanceof gold nanoclusters can be modulated by the controlled construction ofligand spheres. Although the detailed activation mechanism is uncertain atpresent, future theoretical studies should be able to map it out.

CONCLUSIONSTwonovel fcc-type gold nanoclusters, [Au38(L)20(Ph3P)4]

2+ (L = alkynylor thiolate), have been synthesized, and they are isostructural, with theonly difference being the L ligands. The unprecedented fcc-type Au34kernel in the Au38 nanoclusters consists of two face-sharing Au20 units.Because the only structural difference between 1 and 2 is the protectingligands, it offers ideal models to study the ligand effects in catalysis. Theelectronic structures are disturbed by the types of the ligands, which ac-count for the different catalytic performances in the semihydrogenationof alkynes into alkenes. This work presents the first example of atomic-level understanding of the ligand effect on the catalytic properties of gold

Wan et al., Sci. Adv. 2017;3 : e1701823 6 October 2017

nanoclusters. In addition to the previously studied support and sizeeffects, this work will draw more attention from synthetic, catalytic,and theoretical chemists in terms of understanding of the ligand effect.

MATERIALS AND METHODSSynthesisThe detailed information about the synthesis of Au38 nanoclustersis provided in the Supplementary Materials. Briefly, for 1, a freshlyprepared solution of NaBH4 (0.76 mg in 1.0 ml of ethanol) was addeddropwise to a CH2Cl2 suspension (4.0 ml) of 0.15 mmol of PhC≡CAuand 0.05 mmol of Ph3PAuX (X = anion) under vigorous stirring. Thecolor changed from pale brown to brown-red and finally to dark brown.After the reaction continued for 24 hours in air in the dark, the mixturewas dried using a rotavapor to yield a dark solid, which was washedwith ether (2 × 5 ml), dissolved in CH2Cl2 (1.6 ml), and then centrifugedfor 4 min at 10,000 rpm. The resulting solution was subjected to dif-fusion with ether to afford black crystals after 3 weeks. Cluster 2 wasprepared in a similar procedure, with m-MBTAu replacing PhC≡CAu.

Preparation of Au38/oxide and evaluation ofcatalytic performanceTypically, 2 mg of the Au38 nanoclusters was dissolved in 10 ml ofCH2Cl2, and 500 mg of oxide was added. After stirring for 16 hoursat room temperature, the Au38/oxide catalysts were collected by cen-trifugation and dried in vacuum. The catalysts were then annealedat 130°C for 1 hour in a vacuum tube. For the terminal alkynes, thereaction conditions were as follows: 80 mg of Au38 (0.4 wt %)/TiO2

catalyst, 0.2 mmol of alkynes, 0.4 mmol of pyridine, 1.0 ml of EtOH/H2O (10:1, v/v), 80°C, H2 (10 bar), and 15 hours. For the internal al-kynes, the reaction conditions were as follows: 80 mg of Au38 (0.4 wt%)/TiO2 catalyst, 0.2 mmol of alkynes, 0.4 of mmol pyridine, 1.0 ml ofEtOH/H2O (10:1, v/v), 110°C, H2 (20 bar), and 20 hours. The conver-sion and selectivity were determined by 1H NMR. The detailed methodand characterization are available in the Supplementary Materials.

X-ray crystallographyCrystal data for 1 (C234H168F6O10P4S2Au38) were as follows: a =25.6643(17) Å; b = 42.8361(16) Å; c = 44.304(3) Å; V = 48706(5) Å3;Fddd, Z = 8; T = 100 K; 74,545 reflections measured; 8763 unique (Rint =0.1055); final R1 = 0.0901; wR2 = 0.2915 for 7267 observed reflections[I > 2s(I)]. Intensity data of 1 were collected on an Agilent SuperNovaDual system (Cu Ka). CCDC 1553284.

Crystal data for 2 (C210H202O4F12P4S20Sb2Au38) were as follows:a = 25.3576(5) Å; b = 25.3576(5) Å; c = 83.603(5) Å; V = 53757(4) Å3;I41/acd, Z = 8; T = 100 K; 44,481 reflections measured; 9656 unique(Rint = 0.0882); final R1 = 0.0792; wR2 = 0.2256 for 7828 observedreflections [I > 2s(I)]. Intensity data of 2 were collected on an AgilentSuperNova Dual system (Cu Ka). CCDC 1553285.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/10/e1701823/DC1Physical measurementsSynthesisCharacterizationfig. S1. TGA of 1 and 2.fig. S2. XPS spectra of Au 4f in 1 and 2.fig. S3. Fourier transform infrared spectrum of 1.

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fig. S4. The Au34 kernel in Au38.

fig. S5. The stability check of 1 and 2 in CH2Cl2 solution.

fig. S6. 31P NMR spectra of 1 and 2 in CD2Cl2.

fig. S7. TEM images of Au38 catalyst at different periods.

fig. S8. 1H NMR spectra of the catalytic products.

fig. S9. TEM images of 1/TiO2 and 2/TiO2 catalysts treated at different temperatures.

fig. S10. The anion exchange of 2 with KSO3CF3 in CH2Cl2 solution.

fig. S11. The relationship between reaction time and conversion and selectivity in thesemihydrogenation of alkynes.

table S1. The catalytic performance of Au38 (0.4 wt %)/TiO2 catalyst pretreated at differenttemperatures in the semihydrogenation of diphenylacetylene.

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Acknowledgments: We thank Q.-X. Wang and J.-Y. Chen for the assistance with TEMmeasurements. Funding: This work was supported by the National Natural ScienceFoundation of China (21473139, 21521091, and 21631007) and the 973 Program (2014CB845603).Author contributions: X.-K.W. conceived and carried out the synthesis and crystallization

Wan et al., Sci. Adv. 2017;3 : e1701823 6 October 2017

of the clusters and performed the catalysis. J.-Q.W. assisted in the synthesis. Z.-A.N. assisted inthe structural refinements. Q.-M.W. designed the study, supervised the project, analyzed thedata, and wrote the manuscript. Competing interests: The authors declare that they haveno competing interests. Data and materials availability: All data needed to evaluate theconclusions in the paper are present in the paper and/or the Supplementary Materials.Additional data related to this paper may be requested from the authors.

Submitted 31 May 2017Accepted 15 September 2017Published 6 October 201710.1126/sciadv.1701823

Citation: X.-K. Wan, J.-Q. Wang, Z.-A. Nan, Q.-M. Wang, Ligand effects in catalysis by atomicallyprecise gold nanoclusters. Sci. Adv. 3, e1701823 (2017).

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Ligand effects in catalysis by atomically precise gold nanoclustersXian-Kai Wan, Jia-Qi Wang, Zi-Ang Nan and Quan-Ming Wang

DOI: 10.1126/sciadv.1701823 (10), e1701823.3Sci Adv 

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