High-yield synthesis and crystal structure of a green Au30 cluster co-capped by thiolate and sulfide

Since the crystallographic structure determination of Au102( p-MBA)44 in 2007, thiolate-stabilized metal nanoclusters have attracted increasing research attention owing to their well-defined molecular structures. A series of atomically precise thiolated/selenolated Au nanoclusters have been synthesized and crystallographically characterized. In reported thiolated Au nanoclusters, staple Aux(SR)x+1 units are commonly observed in their surface protected layers. Numerous subsequent studies have demonstrated the influence of both the metal species in the core and capping ligands on the surface features of thiolated metal nanoclusters. No staple units were revealed on the surfaces of thiolated metal nanoclusters where metals other than Au were introduced. Ag44 and Au12Ag32 clusters reported to date do not include staple units. Non-staple units were proposed on the surface of Au41(S-Eind)12 where a bulky thiol is used. 17 A m3-like bonding of that ligand to Au was suggested. The potential for a diverse array of structure motifs beyond simple Aux(SR)x+1 staple units is envisaged to enrich the structures of thiolated metal nanoparticles and expand the range of surface properties beyond those of known thiolated-Au nanoclusters. Here we report the high-yield synthesis of Au30S(StBu)18 and its crystal structure. A trace amount of Na2S is used as the S 2 source to explore incorporation of S on the surface of thiolated nanoclusters. The crystal structure analysis reveals that each cluster comprises an Au22 core capped by a mixed layer of staple Au-thiolate units, bridging thiolates and a m3-S 2 . As part of the surface protecting layer, S serves as a m3-ligand coordinating to three Au atoms. The structure of Au30S(StBu)18 gives direct evidence that m3-S 2 readily acts as a surface protecting ligand to introduce a new surface structure giving potential for preparing more structurally diverse thiolated Au nanoclusters. To incorporate S onto the surface of thiolated Au nanoclusters, Na2S was introduced deliberately in the synthesis of Au nanoclusters. In a typical synthesis of Au30S(StBu)18 nanoclusters (ESI†), HAuCl4 and tert-butylthiol (t-C4H9SH) (1 : 3 molar ratio) were combined in tetrahydrofuran (THF). After stirring for 15 min at 55 1C, aqueous solutions of NaBH4 and Na2S were added simultaneously into the mixture of HAuCl4 and t-C4H9SH. The ratio of HAuCl4 : NaBH4 : Na2S was 50 : 500 : 1. The reaction mixture turned darkbrown immediately and was kept under stirring at 55 1C for another hour. After the aqueous layer was removed, toluene and excess t-C4H9SH were added to the reaction mixture whose temperature was then raised to 60 1C. The colour of the solution gradually changed from dark-brown to dark-green in the following 6 h stirring at 60 1C. Brown sheet-like single crystals of Au30S(StBu)18 were recrystallized by diffusing hexane into the cluster solution in CH2Cl2 at 4 1C over 15 days. The crystals were readily redissolved in toluene to give a green solution. The molecular structure of Au30S(StBu)18 was determined by X-ray single crystal analysis.§ Au30S(StBu)18 clusters are crystallized in the triclinic space group P% 1 (Fig. S1, ESI†). As illustrated in Fig. 1 and Fig. S2 (ESI†), Au30S(StBu)18 is formulated as a neutral cluster with a rod-like Au22 core capped by a mixed layer of thiolate ligands, gold– thiolate complex units and S . The mixed surface capping layer consists of two Au3(SR)4 staple units, two Au(SR)2 units, six bridging thiolate SR ligands and one S . Although the Au-thiolate staple units and bridging thiolates have been previously revealed as important surface capping motifs in the reported structures of thiolated-Au nanoclusters, the presence of surface m3-S 2 motifs has not been previously observed in thiolated Au nanoclusters. a State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen University, Xiamen 361005, China. E-mail: nfzheng@xmu.edu.cn; Web: http://chem.xmu.edu.cn/groupweb/nfzheng/ index.asp; Fax: +86 592 2183047; Tel: +86 592 2186821 b Bragg Institute, Australian Nuclear Science and Technology Organization, New Illawarra Road, Lucas Heights, New South Wales 2234, Australia † Electronic supplementary information (ESI) available: Experimental details and crystallographic data, and the MALDI mass spectra of pure Au30S(StBu)18 with the laser irradiation of increased power. CCDC 986161. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc01773k ‡ H.Y. and Y.W. contributed equally to this work. Received 9th March 2014, Accepted 24th September 2014

][10][11][12][13] In reported thiolated Au nanoclusters, staple Au x (SR) x+1 units are commonly observed in their surface protected layers.Numerous subsequent studies have demonstrated the influence of both the metal species in the core and capping ligands on the surface features of thiolated metal nanoclusters.No staple units were revealed on the surfaces of thiolated metal nanoclusters where metals other than Au were introduced.5][16] Non-staple units were proposed on the surface of Au 41 (S-Eind) 12 where a bulky thiol is used. 17A m 3 -like bonding of that ligand to Au was suggested.The potential for a diverse array of structure motifs beyond simple Au x (SR) x+1 staple units is envisaged to enrich the structures of thiolated metal nanoparticles and expand the range of surface properties beyond those of known thiolated-Au nanoclusters. 17,18ere we report the high-yield synthesis of Au 30 S(StBu) 18 and its crystal structure.A trace amount of Na 2 S is used as the S 2À source to explore incorporation of S 2À on the surface of thiolated nanoclusters.The crystal structure analysis reveals that each cluster comprises an Au 22 core capped by a mixed layer of staple Au-thiolate units, bridging thiolates and a m 3 -S 2À .As part of the surface protecting layer, S 2À serves as a m 3 -ligand coordinating to three Au atoms.The structure of Au 30 S(StBu) 18 gives direct evidence that m 3 -S 2À readily acts as a surface protecting ligand to introduce a new surface structure giving potential for preparing more structurally diverse thiolated Au nanoclusters.
To incorporate S 2À onto the surface of thiolated Au nanoclusters, Na 2 S was introduced deliberately in the synthesis of Au nanoclusters.In a typical synthesis of Au 30 S(StBu) 18 nanoclusters (ESI †), HAuCl 4 and tert-butylthiol (t-C 4 H 9 SH) (1 : 3 molar ratio) were combined in tetrahydrofuran (THF).After stirring for 15 min at 55 1C, aqueous solutions of NaBH 4 and Na 2 S were added simultaneously into the mixture of HAuCl 4 and t-C 4 H 9 SH.The ratio of HAuCl 4 : NaBH 4 : Na 2 S was 50 : 500 : 1.The reaction mixture turned darkbrown immediately and was kept under stirring at 55 1C for another hour.After the aqueous layer was removed, toluene and excess t-C 4 H 9 SH were added to the reaction mixture whose temperature was then raised to 60 1C.The colour of the solution gradually changed from dark-brown to dark-green in the following 6 h stirring at 60 1C. Brown sheet-like single crystals of Au 30 S(StBu) 18  were recrystallized by diffusing hexane into the cluster solution in CH 2 Cl 2 at 4 1C over 15 days.The crystals were readily redissolved in toluene to give a green solution.
The molecular structure of Au 30 S(StBu) 18 was determined by X-ray single crystal analysis.§ Au 30 S(StBu) 18 clusters are crystallized in the triclinic space group P% 1 (Fig. S1, ESI †).As illustrated in Fig. 1 and Fig. S2 (ESI †), Au 30 S(StBu) 18 is formulated as a neutral cluster with a rod-like Au 22 core capped by a mixed layer of thiolate ligands, goldthiolate complex units and S 2À .The mixed surface capping layer consists of two Au 3 (SR) 4 staple units, two Au(SR) 2 units, six bridging thiolate SR ligands and one S 2À .Although the Au-thiolate staple units 1,[8][9][10][11][12] and bridging thiolates 10 have been previously revealed as important surface capping motifs in the reported structures of thiolated-Au nanoclusters, the presence of surface m 3 -S 2À motifs has not been previously observed in thiolated Au nanoclusters.
As depicted in Fig. 2, the Au 22 core of Au 30 S(StBu) 18 can be better described structurally as a rod-like Au 20 unit face-capped by two Au atoms.Au 20 is a bicuboctahedral unit consisting of two distorted interpenetrating Au 13 cuboctahedrons, similar to the Au 20 core of Au 28 (TBBT) 20 . 10In the core of Au 30 S(StBu) 18 , however, the Au 20 unit is further face-capped by two Au atoms at either end, resulting in the formation of a rod-like Au 22 core.The average Au-Au bond length of the Au 20 bicuboctahedron is 2.89 Å which is comparable to the bond length in bulk gold (2.88 Å) and shorter than the bond length of the Au 20 kernel of Au 28 (2.92 Å).
The presence of two additional face-capping Au atoms (Au cap ) in the core of Au 30 S(StBu) 18 significantly modifies the surface binding structure when compared with that of Au 28 (TBBT) 20 . 10ile the Au 20 core in Au 28 (TBBT) 20 is protected by four Au 2 (SR) 3 staple units and eight bridging SR ligands, no comparable Au 2 (SR) 3 staple units are revealed on the surface of Au 30 S(StBu) 18  for which a diversity of surface motifs are identified.As shown in Fig. 1b and Fig. S3 (ESI †), each of the two Au 3 (SR) 4 staple units is capping an end of the Au 22 core with the terminal thiolates binding to Au atoms on the bicuboctahedral unit.The two Au(SR) 2 units bind at the sides of the rod-like Au 22 core with one thiolate coordinating to an Au atom on the bicuboctahedral unit and the other to a capping Au atom.Similar to the situation in [Au 23 (SC 6 H 11 ) 16 ] À , 12 each of the two face-capping Au atoms in the core forms a linker between the Au 3 (SR) 4 and Au(SR) 2 units.The Au 22 core is further bound by four bridging SR at its sides, two bridging SR ligands on both ends, and one m 3 -S 2À on one end.The average Au-S bond length/Au-S-Au bond angle are 2.311 Å/96.2811 and 2.330 Å/97.3371 in the Au 3 (SR) 4 and Au(SR) 2 units, respectively.Compared with those in staple units, the average Au-S-Au bond angle (92.221) at the six bridging SR ligands is smaller and their average Au-S bond length (2.338 Å) is slightly longer.
Only one m 3 -S 2À is present on the surface of Au 30 S(StBu) 18 , rendering the cluster asymmetric due to its location at one end of the Au 22 core.The m 3 -S 2À binds to two adjacent Au atoms on the bicuboctahedral Au 20 unit and to one Au cap atom as described above.Such a coordination mode differentiates the binding structures of the two face-capping Au atoms in the Au 22 core.While the Au cap at the end without m 3 -S 2À binding caps an Au 4 square, the other Au cap only caps three Au atoms of the bicuboctahedral unit.Although observed in small Au clusters, 19 the presence of m 3 -S 2À surface motifs has not been well reported in large thiolated Au nanoclusters.The unique m 3 -coordinated sulfide on an Au cluster presents an opportunity to create custom-modified thiolated Au nanoclusters with the potential for targeted functionality and applications.
As illustrated in Fig. 3a, the UV-Vis spectrum of Au 30 S(StBu) 18 in toluene displays one major absorption peak at 620 nm and two shoulder peaks around 375 nm and 475 nm (Fig. 3b).Such an optical absorption of Au 30 S(StBu) 18 is very similar to that of the all-thiolate-protected Au 30 (S-tBu) 18 cluster. 20Very recently, Dass and coworkers also obtained single crystals of Au 30 S(StBu) 18 during the crystallization of chromatographically purified Au 30 (StBu) 18 . 21ith the use of a trace amount of Na 2 S, we demonstrate a highyield, high-purity synthesis of Au 30 S(StBu) 18 in a one-pot method  without purification.The crude reaction product of this synthesis displayed an identical UV-Vis absorption to that of single crystals of Au 30 S(StBu) 18 dissolved in CH 2 Cl 2 (Fig. 3a).To confirm the high purity of Au 30 S(StBu) 18 in the reaction mixture, the mass spectra of both the crude product and the pure crystals were further analysed.A clean and strong peak with m/z of 7457 is clearly observed in both mass spectra (Fig. 3c and Fig. S4, ESI †).The peak is assigned to [Au 30 S(StBu) 17 ] + which corresponds to the loss of one StBu À group from the Au 30 S(StBu) 18 cluster.These results demonstrate the high purity of Au 30 S(StBu) 18 in the crude product.When the laser intensity used in the MALDI-MS measurements was increased, further loss of organic ligands from the cluster was observed (Fig. S5, ESI †).The high-yield production of Au 30 S(StBu) 18 on introducing Na 2 S could be rationalized by the presence of m 3 -S 2À which helps to relieve the steric pressure caused by the adjacent five StBu groups.It should be noted that Au 30 S(StBu) 18 is not luminescent while strongly absorbing in the UV-Vis region.
In conclusion, a new gold-nanocluster, Au 30 S(StBu) 18 , was successfully synthesized and structurally determined by single crystal analysis.The introduction of a trace amount of Na 2 S was found to be critical for achieving the high-yield synthesis of Au 30 S(StBu) 18 .While the core of Au 30 S(StBu) 18 is an Au 22 unit that can be described as a bicuboctahedral Au 20 unit capped by two Au atoms, the surface layer of the cluster consists of two Au 3 (SR) 4 staple units, two Au(SR) 2 units, six bridging SR ligands and one m 3 -S 2À .The presence of m 3 -S 2À on a thiolated Au nanocluster provides opportunities to create and manipulate surface structures of thiolated Au nanoparticles.
We thank the Ministry of Science and Technology of China (2011CB932403) and the National Natural Science Foundation of China (21131005, 21420102001, 21390390, 21227001) for financial support.
Notes and references § The diffraction patterns measured for the compound are dominated by the scattering from the Au 30 S 19 core with almost 3000 electrons whereas the less ordered tertiary butyl shell contains fewer than 600 electrons distributed in a considerably large volume.The structure analysis is complicated by the occurrence of twinning which appears to be an inherent property of crystals of this material.Review of the diffraction images confirmed this diagnosis and calculated precession layers demonstrated that additional smaller twin components may also be present.Residual electron density in the core region is ascribed to minor twin components which were not modeled.A large void in the structure, B22% of the cell volume, has a calculated electron content of B400 and is consistent with occupation by disordered solvent.A detailed description of the structure analysis is provided in the CIF.

Fig. 1
Fig. 1 The molecular structure of Au 30 S(StBu) 18 from X-ray diffraction analysis.All hydrogen (a, b) and carbon (b) atoms are omitted for clarity.Colour legend: orange/green/cyan/red spheres, Au; pink sphere, S 2À ; yellow sphere, S on thiolate; grey stick, C.

Fig. 2
Fig. 2 (a, b) The structure of Au 20 in the Au 22 core.(c) The overall structure of the Au 22 core.Colour legend: orange/yellow-green spheres, Au atoms of Au 20 bicuboctahedron; green sphere, the two capping Au atoms of Au 22 .