Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Alkynyl Approach toward the Protection of Metal Nanoclusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Zhen Lei,† Xian-Kai Wan,†,‡ Shang-Fu Yuan,† Zong-Jie Guan,†,‡ and Quan-Ming Wang*,†,‡ †
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China
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‡
CONSPECTUS: The past decades have witnessed great advances in the synthesis, structure determination, and properties investigation of coinage metal nanoclusters. These monodisperse clusters have well-defined molecular structures, which is advantageous in correlating structures and properties. Metal nanoclusters are large molecules consisting of many components, so it is a big challenge to prepare them in a rational way. Strenuous efforts have been made to control their geometric and electronic structures, in order to optimize their various properties. A metal nanocluster normally contains a metal core and a peripheral ligand shell. The ligands do not only function as simple stabilizing agents. It has been revealed that these ligands are able to influence the formation processes of the nanoclusters, and they may also dictate the sizes, shapes, and properties of nanoclusters. There are mainly three types of ligands that are widely used as surface anchors on coinage metal nanoclusters: thiolates, phosphines, and halides. Recent ligand engineering has extended the scope to alkynyl ligands. As alkynyl ligands are versatile in interacting with metal atoms, interesting alkynyl−metal interfacial structures including linear, L-shaped, and V-shaped staple motifs can be generated, as well as a series of novel coinage metal nanoclusters that exhibit intriguing molecular geometries. The staple motifs do not simply resemble the surface structures of thiolate-protected nanoclusters, because the incorporation of alkynyl ligands may significantly alter diverse properties of nanoclusters. Compared with thiolate-protected gold nanoclusters, alkynyl-protected ones with identical metal cores exhibit distinctly different absorption profiles and show much improved catalytic activities for semihydrogenation of alkynes. In addition, the participation of alkynyl ligands could profoundly affect the luminescent properties of nanoclusters. These “ligand effects” are mainly attributed to the different nature of alkynyl ligands, as electronic perturbation through π-conjugated units may largely modulate the electronic structure of the whole cluster. In this Account, we describe the development of coinage metal nanoclusters protected with alkynyl ligands. We will first briefly bring up the emergence of alkynyl ligands as anchoring groups on the surfaces of nanoclusters. Then we present the direct reduction method for the synthesis of the following four categories of nanoclusters: (a) gold nanoclusters with mixed-ligand shells, (b) all alkynyl-protected gold nanoclusters, (c) heterobimetallic gold nanoclusters, and (d) silver nanoclusters. Their molecular structures are described, and their various alkynyl−metal interfacial structures are compared with thiolate−metal staples. Finally, ligand effects on the properties of the clusters, including optical absorption, luminescence, and catalysis, are discussed. The alkynyl ligands play an important role in terms of both structural and property aspects. We believe this Account will attract increasing attention to alkynyl ligands, which have shown promising potential in generating new structures and properties of coinage metal nanoclusters.
1. INTRODUCTION
the scope of protecting ligands by recruiting alkynyls as candidates in the synthesis of coinage metal nanoclusters.17−20 Alkynyl ligands are quite similar to the extensively used thiolates, being negatively charged and forming strong interactions with coinage metals. An important feature of an alkynyl ligand is that its CC bond may anchor on a metal surface via both σ and π bondings, which is different from the case of a thiolate.21 As a result, interesting staplelike motifs with different geometries were observed. In addition, alkynyl ligands process large conjugated systems (e.g., PhCC−) may participate in the electronic structures of the clusters, which
Monodisperse and structurally well-defined coinage metal nanoclusters have been sparking great research interests in the past decades. This is not only due to their intriguing structural diversity, novel luminescence, catalytic activities and potential bioapplications, but also because they provide a promising platform to correlate structures with properties.1−6 Organic ligands are usually used to cap on the cluster surface in order to prevent aggregation and to facilitate the isolation of target nanoclusters. These ligands not only influence the formation of nanoclusters but also determine their properties.7 Inspired by the fact that alkyne may be adsorbed on surface of bulk-gold and gold nanoparticles,8−16 recent ligand engineering has extended © XXXX American Chemical Society
Received: July 19, 2018
A
DOI: 10.1021/acs.accounts.8b00359 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research leads to the changes in the optical absorptions22 and luminescence properties.23−25 More recently, ligand effects in catalysis were also revealed.26 Up to date, more than two dozen alkynyl-protected coinage metal nanoclusters have been synthesized, isolated, and characterized by single crystal X-ray diffraction. The remarkable advances in the past 3 years are largely attributed to the development of various synthetic methodologies. In this Account, synthesis, structures, and properties of alkynylprotected coinage metal nanoclusters will be presented. We will focus on the alkynyl−metal interfacial structures and related optical, luminescent, and catalytic properties. We only cover those clusters containing free electrons, i.e. Au(0), Ag(0), or Cu(0) (so-called coinage metal nanoclusters).27
Au(111) surface and a tetrahedral Au20 model nanocluster. It was found that a flat-lying orientation of ligands is more preferred, rather than the upright model. In addition, a linear PhCC−Au−CCPh motif with π interactions between C C and surface gold atoms (Figure 1, motif E) is energetically favored.
3. MIXED LIGAND-PROTECTED GOLD NANOCLUSTERS In 2015, Wang et al. reported a direct reduction method for preparing alkynyl-protected gold nanoclusters by using PhC CAu and LAuSbF6 (L is a phosphine) as precursors. With this direct reduction protocol, Wang and co-workers managed to prepare a series of gold nanoclusters including [Au19(C CPh)9(Hdppa)3](SbF6)2 (Hdppa = N,N-bis(diphen ylphosphin o)amin e) (Au 1 9 ), 3 0 [Au 2 3 (C CPh)9(PPh3)6](SbF6)2 (Au23),31 [Au24(CCPh)14(PPh3)4](SbF6)2 (Au24),32 and [Au38(CCPh)20(PPh3)4](SO3CF3)2 (Au38)26 (Figure 2). The formation of Au23, Au24, and Au38 is controlled by varying the ratio between PhCCAu and Ph3PAu+ precursors. The optimized ratios of Ph3PAu+ to PhCCAu in the preparations are 2:1 for Au23, 3:1 for Au24, and 5:1 for Au38. These ratios correspond with the increasing trend of the relative amount of PhCC− components in Au23, Au24, and Au38. This finding also suggests that larger gold nanoclusters might be obtained with higher ratios of PhCCAu to Ph3PAu+. With the success in crystal structure determination of these clusters, surface binding structures of alkynyl ligands were revealed. Unprecedented V-shaped staple motif PhCC−Au− CC(Ph)−Au-CCPh (Figure 2e, motif F) was first observed in Au19, and later was also found in Au23. L-Shaped staple motifs PhCC−Au−CCPh (Figure 2f, motif G and Figure 2g, motif H) were observed in Au24 and Au38. Clusters Au19 and Au23 can be viewed as typical core−shell structures. They have icosahedral Au 13 and hexagonal prismatic Au 17 cores, respectively, which are wrapped by three V-shaped staple motifs to form configurations of C3 symmetry. Two and four L-shaped staple motifs are attached on opposite sides of the Au22 and Au34 kernels in Au24 and Au38, respectively, and many other PhC C− ligands adopt the simple bridging coordination mode (motif D). The V- and L-shaped motifs closely resemble the RS-(AuSR)n (n = 1 or 2) staples of thiolate-protected gold nanoclusters. However, two PhCC− ligands in the L-shaped motif are perpendicular to each other, which is quite different from the linear motif predicted by Jiang and Tang.21 This difference is due to the coordination nature of alkynyl and thiolate ligand: the triple bond of an alkynyl ligand may be orientated horizontally or perpendicularly, whereas a thiolate ligand normally interacts with metal atoms via two equivalent Au−S σ-bonds. The steric hindrance of ligands is plausible to be responsible for the formation of L-shaped staple motifs.
2. THE EMERGENCE OF ALKYNYL-PROTECTED METAL NANOCLUSTERS In 2011, Tsukuda and co-workers18−20 isolated a series of organogold nanoclusters protected by alkynyl ligands. In their synthetic protocol, polyvinylpyrrolidone (PVP) protected gold precursors were employed to react with PhCCH or PT−C CH (PT = 9-C14H10) under thermal conditions in a biphasic solvent system. The compositions of products are identified to be Au34(CCPh)16, Au43(CCPh)22, Au54(CCPh)26, Au30(CC-PT)13, and Au35(CC-PT)18. By tuning the synthetic parameters (solvent, temperature, etc.), certain species could be selectively isolated, e.g., Au54(CCPh)26. Although no crystal structures were available, FTIR, Raman, and EXAFS spectroscopic studies suggested that the interfacial structures of these gold nanoclusters involve μ2 or μ3 coordinated alkynyl ligands (Figure 1, motifs A and B). Similarly, the PhCC−
Figure 1. Coordination modes of PhCC− on the surface of gold nanoclusters.
group can terminally bind one gold atom via σ-bonding (Figure 1, motif C), as reported in gold nanoclusters [Au8(C CPh)2(dppp)4](NO3)2 (Au8, dppp = 1,3-bis(diphenylphosphino)propane) and [Au13(CCPh)2(dppe)5](PF6)3 (Au13, dppe = 1,2-bis(diphenylphosphino)ethane) by Konishi and co-workers.23−25 Jin and Li prepared a rodlike nanocluster [Au25(C CPh)5(PPh3)10X2]2+ (Au25, X = Cl or Br), and its composition was identified by mass spectroscopy.28 Five alkynyl ligands are presumed to be located on the waist site of cluster, and this binding mode is similar to the thiolate in rodlike [Au25(SCH2CH2Ph)5(PPh3)10Cl2]2+ (Figure 1, motif D).29 Jiang and Tang21 performed TDDFT calculations and predicted possible binding modes of PhCC− on flat
4. ALL ALKYNYL-PROTECTED GOLD NANOCLUSTERS Although the structures of alkynyl and phosphine coprotected gold nanoclusters have been revealed, the structure determination of homoleptic alkynyl-protected gold nanoclusters has not been achieved until Au36(CCPh)24 (Au36) and Au44(C CPh)2833 (Au44) were reported. Very recently, the total structure determination of Au144(CCC6H4-2-F)6022 (Au144) was also reported. Interestingly, all of these gold nanoclusters are B
DOI: 10.1021/acs.accounts.8b00359 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Structures of Au19 (a), Au23 (b), Au24 (c), and Au38 (d), and staple motifs in Au19, Au23 (e, motif F), Au24 (f, motif G), and Au38 (g, motif H), respectively. Hydrogen atoms are omitted for clarity. Alkynyl ligands of staple motifs are highlighted by larger atoms and thicker bonds. Color legend: C, gray; N, blue; P, purple; Au, orange.
Figure 3. Structures of Au36 (a) and Au44 (b), and staple motifs in Au36 and Au44 (c−e). Hydrogen atoms are omitted for clarity. Color legend: C, gray; Au, orange.
not in the series of Aum(CCR)n found by Tsukuda et al.,18−20 and they can be viewed as the counterparts of Au36(SR)24,34 Au44(SR)28,35 and Au144(SR)60.36 The emergence of these new species in the alkynyl family is largely owing to the development of new synthetic protocol (direct reduction), which is different from the etching method by Tsukuda et al. In the synthesis of Au36 and Au44, PhCCAu was used as the only precursor to
react with NaBH4, consequently nanoclusters with an all alkynyl protecting sphere were isolated. Structures of Au36 and Au44 are illustrated in Figure 3. One can see that their gold skeletons are almost identical to that of Au 36 (SR) 24 (SR = 4-tert-butylbenzenethiolate) 34 and Au44(SR)28 (SR = phenylethanethiolate), respectively.35 For example, Au36 comprises a Au28 core, which is surrounded by C
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Figure 4. (a) Structure of Au144. Hydrogen atoms are omitted for clarity. Color legend: C, gray; F, light green; Au, orange. (b-e) Anatomy of the kernel structure in Au144. Innermost shell, Au12 Mackay icosahedron (orange); second shell, Au42 Mackay icosahedron (green); third shell, Au60 anti-Mackay icosahedron (purple); and overall kernel structure with the outmost shell (Au30) highlighted in blue. (f) Staple motif on surface of Au144.
Figure 5. (a) Structure of cationic part of Au80Ag30. Hydrogen atoms are omitted for clarity. Color legend: C, gray; Cl, light green; Ag, green; Au, orange. (b−e) Anatomy of the kernel structure in Au80Ag30. Innermost shell, Au6 octahedron; second shell, Au35 (purple); third shell, Au18Ag30; and overall kernel structure with the outmost shell (Au21) highlighted in blue.
The linear staple motifs (motif K) on the surface of Au144 are slightly different from that in Au44. Only Au−C σ bonding presents in Au144 while both σ and π interactions appear in Au44. Au144 contains a spherical metal core, so it is hard for linear staple motifs to form CC/Au π interactions on a curvature surface.
four V-shaped staple motifs (motifs I and J). In Au44, four similar V-shaped staples (motif I) can also be found around the Au34 kernel. The Au34 kernel in Au44 has an identical geometry as that in Au38. Two additional gold atoms are attached on opposite sides of the Au34 kernel, which can be taken as two linear PhC C−Au−CCPh staples (Motif E) flat-lying on Au34. However, a notable difference between Au36, Au44, and their thiolate counterparts is that Au−Au distances in the former are shorter. For example, the average Au−Au distances in fcc-type Au28 kernels of Au36 and Au36(SR)24 are 2.974 and 2.985 Å, respectively. Besides, the average Au−Au distance in staple motifs of Au36 is also shorter than that of Au36(SR)24 (3.262 and 3.405 Å, respectively). Thus, the structures of alkynyl-protected clusters are more compact in comparison with their thiolate analogues. Note that the V-shaped staples in Au36 and Au44 are slightly different from those in Au19 and Au23. The right alkynyl binds Au atoms with solely σ bonding (motifs I and J), while the left PhCC− could have two orientations: horizontal in motif I and perpendicular in motif J. By co-reduction of preformed gold complex Au-dpa (Hdpa = 2,2′-dipyridylamine) and 2-F−C6H4CCH in the presence of triethylamine, all alkynyl-protected Au144(CCC6H4-2-F)60 (Au144) was obtained and structurally characterized.22 Au144 has a Russian doll-like spherical architecture, and the metal kernel is arranged in a four-shelled Au12@Au42@Au60@Au30 manner. Sixty alkynyl ligands and 30 gold atoms form 30 linear staple motifs on the surface (Figure 4).
5. HETEROBIMETALLIC GOLD NANOCLUSTERS AND THEIR ALKYNYL−METAL INTERFACES The introduction of a secondary metal into gold clusters can generate structural diversity and tailor the properties. Heterometallic gold nanoclusters can be obtained through the use of PhCCAu and PhCCAg precursors. Wang and co-workers reported two giant gold−silver nanoclusters with compositions of [Au 8 0 Ag 3 0 (CCPh) 4 2 Cl 9 ]Cl 3 7 (Au 8 0 Ag 3 0 ) and Au57Ag53(CCPh)40Br1238 (Au57Ag53). Although they are synthesized via similar approaches, and both are 58 electron systems with 110 metal atoms, they show totally different molecular geometries. It was found that silver precursors (AgCl and AgSbF6 for Au80Ag30, AgBr and PhCCAg for Au57Ag53) play a key role in determining the formation of different structures, which suggests counteranions can induce the formation of different types of nanoclusters. As illustrated in Figures 5, Au80Ag30 has D3 symmetry. Specifically, Au80Ag30 exhibits a distorted octahedral Au6 core, which is wrapped by a Au35 shell. A bimetallic transition shell around Au6@Au35 contains 18 gold atoms and 30 silver atoms. Both of the Au35 and Au18Ag30 shells are D3 symmetric. The D
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Figure 6. (a) Structure of Au57Ag53. Hydrogen atoms are omitted for clarity. Color legend: C, gray; Br, dark yellow; Au/Ag, light blue; Ag, green; Au, orange. (b−e) Anatomy of the kernel structure in Au57Ag53. Innermost shell, Au2M3 (M = 1/3 Au + 2/3 Ag; light blue for M, purple for Au); second shell, Au34; third shell, Ag51; and overall kernel structure with the outmost shell (Au20) highlighted in blue.
Figure 7. (a) Structure of Au34Ag28. Hydrogen atoms are omitted for clarity. Color legends: C, gray; Ag, green; Au, orange. (b−d) Anatomy of the kernel structure in Au34Ag28. Inner core, Au17 with a centered silver atom; second shell, Ag27; and overall kernel structure (Ag@Au17@Ag27@Au17) with the outmost shell highlighted in blue.
Figure 8. (a) Structure of cationic part of Au19Cu30. Hydrogen atoms are omitted for clarity. Color legends: C, gray; P, purple; Cl, light green; Cu, light blue; Au, orange. (b−d) Anatomy of the kernel structure in Au19Cu30. Innermost shell, Au13 icosahedron; second shell, Cu30 icosidodecahedron; and overall kernel structure Au13@Cu30@Au6.
ture, where M = 1/3 Au + 2/3 Ag, and none of the shells is
outmost layer consists of 21 linear PhCC−Au−CCPh staple motifs and 9 chlorides. In sharp contrast, the 110 metal atoms of Au57Ag53 are arranged in an irregular manner to form a structure of C1 symmetry (Figure 6). The metal atoms are distributed in a four-shell Au2M3@Au34@Ag51@Au20 architec-
symmetrically arranged. The PhCC−Au−CCPh staple motifs and halides of the protecting sphere are also irregularly arranged. E
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Accounts of Chemical Research The alkynyl ligands of Au80Ag30 and Au57Ag53 exhibit slightly different coordination modes. Multiple CC/metal π interactions can be found between each linear PhCC−Au−C CPh motif and the metal kernel in Au57Ag53 (similar to motif E). However, CC/metal π interactions in some of the surface staple motifs are absent in Au80Ag30 (similar to motif K in Au144). Both clusters have excellent stabilities. Au80Ag30 can be stable in solution under ambient conditions for about 3 weeks, while Au57Ag53 is robust upon 60 °C thermal treatment and is stable under oxidative (H2O2) and basic conditions (CH3ONa). Gold precursors are found having effects on the formation of the products. Au 34 Ag 28 (CCPh) 34 39 (Au 34 Ag 28 ) and [Au19Cu30(CCPh)22(Ph3P)6Cl2](NO3)340 (Au19Cu30) were prepared by co-reduction of PhCCAuPPh3 and silver or copper precursors. Note that the employment of PhCCAu in place of PhCCAuPPh3 could not generate these bimetallic species under similar conditions. Au34Ag2839 exhibits an ellipsoidal three-shelled metal core with a silver atom located in the center (Ag@Au17@Ag27@Au17, Figure 7). The inner Au17 shell is an interpenetrating biicosahedron without central atoms. Each triangular or square face of the inner Au17 shell is capped by one or two silver atoms of the Ag27 shell. The outer Au17 shell exhibits identical geometry to that of the inner Au17 shell of a larger size. Each of the gold atom in this shell caps one pentagon of the Ag27 shell. All 34 PhCC− form 17 linear staple motifs with the surface gold atoms. Au19Cu3040 is the first alkynyl-protected Au−Cu alloy nanocluster, and it also represents a rare example containing three kinds of protecting ligands (alkynyl, phosphine and halide). It has an icosahedral Au13 core, which is encapsulated by an icosidodecahedral Cu30 shell. Six Ph3PAu units are attached on the surface of Cu30, and the protecting sphere is further fulfilled by another 22 alkynyl ligands and 2 chloride atoms (Figure 8). Interestingly, the 49 metal cores can also be seen as part of the M55 Mackay icosahedron with 6 surface atoms being removed. Note that a two-shell (Au12@Au42) hollow Mackay icosahedron was also observed in recently solved Au144 structure.22 Different from that on gold surface, it was found that alkynyl ligands do not form staple motifs in Au19Cu30. Instead, all the 22 PhCC− function as μ3-bridges on M3 triangles (M = Au or Cu). Au19Cu30 can be stable in solution under ambient conditions for 1 week, but less stable than Au80Ag30, Au57Ag53, and Au34Ag28. By introducing a 2-pyridyl group with an extra ligating site into alkynyl ligands, μ3-coordinating 2-py-CC− were also observed on surface of [Au10Ag2(CC-2-py)3(dppy)6](BF4)541 (Au10Ag2, dppy = 2-pyridyldiphenylphosphine) (Figure 9). Au10Ag2 exhibits a Au10 kernel bicapped by two silver atoms. Three 2-py-CC− are located on the middle mirror plane.
Figure 9. Structure of cationic part of Au10Ag2. Hydrogen atoms are omitted for clarity. Color legend: C, gray; N, blue; P, purple; Ag, green; Au, orange.
Ag19 and Ag25 display unprecedented anti-cuboctahedral Ag13 cores, and three Ag2 and Ag4 subunits are attached to the cores of Ag19 and Ag25, respectively (Figure 10). Each Ag2 in Ag19 is bridged by one dppm, whereas each Ag4 in Ag25 is formed via the ligation of two alkynyls and two P donors, thus there are six more alkynyl ligands in Ag25. Similar to that in Au19Cu30, all alkynyl ligands in Ag19 and Ag25 adopt μ3 modes to cap on Ag3 triangles. No staplelike motifs are observed. In order to enhance the stability of alkynyl-protected silver nanoclusters, macrocyclic ligands such as thiacalixarenes have been incorporated. With the protection of alkynyl and calixarene ligands, a silver nanocluster [Ag35(CCtBu)16(H2L)2(L)](SbF6)3 (Ag35, H4L = p-tert-butylthiacalix[4]arene) was synthesized.43 Structural analysis revealed that Ag35 comprises an icosahedral Ag13 core, which is hemispherically surrounded by 22 silver atoms. All tBuCC− ligands are ligated to the peripheral silver atoms, while three thiacalixarenes connect the Ag13 core and the Ag22 shell (Figure 11a). The thiacalixarenes adopt μ4- and μ5-modes as shown in Figure 11b and c. Interestingly, some tBuCC− ligands in Ag35 adopt μ4-η1(Ag), η1(Ag), η1(Ag), η2(Ag) or μ4-η1(Ag), η1(Ag), η2(Ag), η2(Ag) modes, which are quite different from those in Ag19 and Ag25. Similar μ3- and μ4-alkynyl ligands were also found in Ag74(C CPh) 44 (NO 3 ) 2 , 44 [Ag 42 Pt(CCC 6 H 4 -4-Me) 28 (SbF 6 ) 4 ](SbF6)2,45 Cu20(CCPh)12(CH3CO2)6,46 and Cu13 nanoclusters.47,48
7. STABILITY AND ELECTRON COUNTING The above-mentioned nanoclusters are relatively stable and some of them are very stable. Au19 and Ag35 are composed of M13 (M = Au or Ag) cores, and both of them are 8-electron systems. Au80Ag30 and Au57Ag53 have large spherical metal kernels, and both of them are classical 58-electron superatoms. It is noteworthy that their free electron numbers are not always the shell-closing numbers. In some cases, the symmetry of the structure should be taken into account, for example, Au23 is an unusual 12 electron system, but DFT calculations suggest that the splitting of 1D orbitals under D3h symmetry of the Au17 kernel contributes to its stability significantly. The stabilities of Au19Cu30 (22-electron), Au34Ag28 (28-electron), and Au144 (84-electron) are also believed to be associated with their high symmetric structures. A rare case is Au24, which has 8 free electrons formally, but the rodlike structure suggests that it cannot be regarded as a magic number cluster.
6. SILVER NANOCLUSTERS AND BRIDGING ALKYNYLS The direct reduction approach is also applicable in the synthesis of alkynyl-protected silver nanoclusters. The reduction of PhCCAg with AgSbF6 in the presence of phosphine ligands led to the isolation of [Ag19(CCPh)14(dppm)3](SbF6)3 (Ag19, dppm = 1,1-bis(diphenylphosphino)methane), and [Ag25(CCC6H4-4-MeO)20(dpppe)3](SbF6)3 (Ag25, dpppe = 1,5-bis(diphenylphosphino)pentane) could be prepared similarly with 4-MeO−C6H4CCAg as the precursor.42 F
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Figure 10. Structures of (a) Ag19 and (b) Ag25. Hydrogen atoms are omitted for clarity. Color legend: C, gray; O, red; P, purple; Ag, green.
Figure 11. (a) Structure of cationic part of Ag35. Hydrogen atoms are omitted for clarity. (b, c) Coordination modes of thiacalixarenes. Color legend: H, black; C, gray; O, red; S, yellow; Ag, green.
Figure 12. Optical absorption spectra of (a) Au36 (red trace) and Au36(SR)24 (blue trace, ref 33); (b) Au144 (red trace) and Au144(SR)60 (blue trace, ref 22) in CH2Cl2.
8. LIGAND EFFECTS ON THE PROPERTIES OF LIGAND-PROTECTED METAL NANOCLUSTERS Alkynyl ligands, especially those containing large π-conjugated systems (e.g., PhCC−), may influence the electronic structures of metal nanoclusters through delocalization of electrons over the metal cores.30 As a result, the optical properties and catalytic performance of the nanoclusters can be significantly affected by the ligand sphere.
analogues. One can see that Au36 and Au36(SR)24 (SR = 4-tertbutylbenzenethiolate) have distinctly different absorption spectra, and significant absorption difference is also found between alkynyl-protected Au144 and thiolate-protected Au144(SR)60 (SR = phenylethanethiolate). Au144(SR)60 exhibits a rather unstructured spectrum, whereas sharp absorption peaks can be observed in Au144. These examples strongly suggest that the electronic structures of Aum(CCR)n and Aum(SR)n can be disturbed by protecting ligands of different nature. Interestingly, although the profiles are different, the molecular absorbance coefficients do not change much, i.e., the ε values at 600 nm are 4.0 × 104 for Au144 and 3.8 × 104 M−1 cm−1 for Au144(SR)60, respectively.
8.1. Optical Absorption
Au36, Au44,33 and Au14422 serve as ideal platforms to correlate absorptions with surface ligands, because their thiolate analogues have almost identical metal arrangements to them with the only difference being the protecting ligands. Figure 12 shows the UV−vis spectra of Au36, Au144, and their thiolate G
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Figure 13. (a) Emission spectrum of Au10Ag2 in CH2Cl2/MeCN = 9:1 (v:v, excitation at 300 nm). The dip at about 1157 nm is due to solvent/ligand absorption. (b) Projected densities of states (PDOS) of Au10Ag2.
8.2. Luminescence
Scheme 1. Catalytic Performance of Supported Gold Nanoclusters for the Semihydrogenation of Alkynesa
Luminescence of coinage metal nanoclusters is well-known for their strong emissions, large Stokes shifts, long lifetimes, and high resistance to photobleaching.7 It was found that some of the alkynyl-protected nanoclusters exhibit intriguing emission behaviors. For example, Au 8 2 3 and [Cu 1 2 Au(C CPh)4(S2CNnBu2)6](CuCl2)48 emit in the visible range, Au2432 is intensely emissive in the NIR region, whereas Au1324 and [Au7Ag8(CCtBu)12]Cl49 show broad emission bands extending from the visible to near-infrared range. An interesting example is Au10Ag2,41 which shows two emission bands in the visible and NIR regions, respectively (Figure 13a). By performing detailed experimental and theoretical investigations, the origin of the dual emission of Au10Ag2 is revealed.41 The visible component is associated with metal-toligand charge transfer (MLCT) state resulting from the capping silver atoms to pyridyl groups of phosphine ligands. The intense NIR emission is largely associated with excited state generated from the electronic transition from HOMO-1 to LUMO, in which the 2-pyridylethynyl actively participates (Figure 13b).
a
R = 3-Me-C6H4; R1 = Ph or PhC2H4; R2 = H, CH3, Ph or 4-Br-C6H4.
conditions. This fact suggests that the organic ligands are important for H2 activation. This work clearly demonstrates that the catalytic activity of gold nanoclusters can be controlled through rational construction of ligand spheres. More importantly, the direct comparison at atomic level revealed the dissimilarity between alkynyl and thiolate ligands, and further highlighted the ligand effect of coinage metal nanoclusters in catalysis. The catalytic conditions employed here with gold nanoclusters are relatively mild (hydrogen pressure: 10−20 bar, temperature: 80−110 °C), but not mild enough in comparison with Lindlar catalyst. The ligand effects observed on gold nanoclusters suggest that the activity and selectivity could be tuned through appropriate ligand engineering. It will be interesting to check the possibility of optimizing the catalytic conditions via rational construction of the cluster surface.
8.3. Catalysis
It was found that ligand-protected gold nanoclusters can catalyze diverse organic reactions including oxidation, hydrogenation, and cross-coupling reactions.3,5,6 Recent advances have been made in employing alkynyl-protected nanoclusters as catalysts, and the organic ligands are proved to be vital in dictating their catalytic performance. Atomically precise Au34Ag28 is catalytically active in the hydrolytic oxidation of organosilanes to silanols.39 The cluster supported on activated carbon gave 100% conversion, while the bare cluster generated from the removal of the capping ligands showed negligible activity. Rod-shaped Au25 was found to have high catalytic activity for the semihydrogenation of terminal alkynes.28 FT-IR spectroscopy suggested that the terminal alkynes are activated through deprotonation, which is attached to the waist sites of Au25. Ligand effects on catalysis based on gold nanoclusters are clearly demonstrated by the investigation on the catalytic performance of isostructural gold nanoclusters protected by different ligands. Wang et al.26 studied the activities of alkynylprotected Au38 and thiolate-protected [Au38(SR)20(Ph3P)4](SbF6)2 (SR = 3-methylbenzenethiolate) supported on TiO2 in the semihydrogenation of terminal and internal alkynes (Scheme 1). The alkynyl-protected Au38 is very active with conversion >97% while the thiolate-protected analog exhibited low or negligible catalytic activities (
