Research ArticleNANOMATERIALS

Thiacalix[4]arene: New protection for metal nanoclusters

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Science Advances  12 Aug 2016:
Vol. 2, no. 8, e1600323
DOI: 10.1126/sciadv.1600323

Abstract

Surface organic ligands are critical for the formation and properties of atomically precise metal nanoclusters. In contrast to the conventionally used protective ligands such as thiolates and phosphines, thiacalix[4]arene has been used in the synthesis of a silver nanocluster, [Ag35(H2L)2(L)(C≡CBut)16](SbF6)3, (H4L, p-tert-butylthiacalix[4]-arene). This is the first structurally determined calixarene-protected metal nanocluster. The chelating and macrocyclic effects make the thiacalix[4]arene a rigid shell that protects the silver core. Upon addition or removal of one silver atom, the Ag35 cluster can be transformed to Ag36 or Ag34 species, and the optical properties are changed accordingly. The successful use of thiacalixarene in the synthesis of well-defined silver nanoclusters suggests a bright future for metal nanoclusters protected by macrocyclic ligands.

Keywords
  • Calixarene
  • crystal structure
  • silver nanocluster
  • alkynyl ligand
  • coordination mode
  • ligand-protected metal nanocluster

INTRODUCTION

Ligand-protected metal nanoclusters have attracted great interest because of their importance in fundamental study and promising applications in biological sensing, catalysis, and nanoscale optoelectronics (15). With the protection of various organic ligands, a number of coinage metal nanoclusters have been isolated, and some of them have been structurally characterized by single-crystal x-ray diffraction (620). It is well known that surface organic ligands are critical in determining the stability, atom packing, and properties of metal clusters. Conventionally, thiolate and phosphine ligands have been used to protect metal clusters, leading to the formation of a variety of nanoclusters with distinctly different structures. It has been found that the coordination preference, bulkiness, and electronic nature of ligands can have significant effects on the structures and properties of the clusters. Jin et al. reported that even the small difference in the paragroup on the benzenethiolate could give gold nanoclusters with different sizes and structures (21). Accordingly, it is an effective strategy to obtain new functional metal nanoclusters by using ligands beyond the conventional ones. To this end, we are initiating a project to use macrocyclic ligands such as calixarenes for the stabilization of well-defined metal nanoclusters.

Calixarenes are good candidates for binding metal ions (22) because of their preorganized multidentate coordination sites. Calixarene-capped nanoparticles have been used for applications in sensing, molecular recognition, and tuning of optical properties (23, 24). In addition to the protection of nanoparticles, calixarenes are also promising surface capping ligands for new functional nanoclusters. First, a calixarene with macrocyclic cone-shaped conformation should adopt completely different surface binding motifs in contrast to the typical staple motifs of thiolate/alkynyl or monodentate terminal coordination of phosphine. Second, a bulky calixarene provides a rigid shell that protects a metal core, which may increase the stability of nanoclusters. Third, a calixarene can be modified at the “lower rim,” the “upper rim,” and/or the bridging atoms, which can bring in specific functionalities (25). Fourth, a calixarene is a supramolecular host; thus, host-guest interaction features can be integrated in a calixarene-protected nanocluster.

Recent attempts to use calixarenes in the synthesis of nanoclusters have been proved to be successful (2628). Katz et al. synthesized a group of phosphine-bound calixarene-modified Au11 nanostructures (26), and the structures are theoretically verified by Häkkinen et al. (27). The bulkiness and rigidness of calixarene ligands make the metal core partly exposed, which is accessible for small molecules. Ras et al. reported a series of Au25(Calix-4S)x(BuS)y nanoclusters with tunable number of calixarenes (28). However, the crystal growth of calixarene-protected metal nanoclusters is a big challenge, and there is no report on the crystal structure determination of calixarene-protected nanomaterials. The surface structural ordering is unknown, and the influence of calixarenes on a metal core has not been addressed.

On the basis of the consideration that the affinity of a calixarene to gold/silver atoms will be enhanced by incorporating thioether moieties in the ligand, we use a thiacalix[4]arene that bears sulfur bridges to stabilize silver nanoclusters (Scheme 1). Ancillary alkynyl ligands are also included for better surface arrangements. Herein, we report the synthesis, total structure determination, and optical properties of a novel thiacalixarene-protected metal nanocluster, [Ag35(H2L)2L)(C≡CBut)16](SbF6)3, (H4L, p-tert-butylthiacalix[4]-arene) (1). Compound 1 represents the first structurally determined calixarene-protected metal nanocluster.

Scheme 1 Structure of thiacalix[4]arene H4L.

RESULTS AND DISCUSSION

Cluster 1 was prepared from the reduction of AgC≡CBut and AgSbF6 by NaBH4 in the presence of H4L and triethylamine. Single crystals were obtained by vapor diffusion of n-pentane into a solution of the product in toluene and methylene chloride. The composition and purity of 1 were supported by energy dispersive x-ray spectroscopy (EDX) and powder x-ray diffraction (PXRD) techniques. The PXRD pattern matches the simulated one generated from its single-crystal data (fig. S1). An infrared band at 2012 cm−1 confirms the presence of the C≡C group in 1. (fig. S2). The EDX result shows the Ag/Sb/S atomic ratio to be 69.1:6.0:24.9, which is close to the calculated value of 70.0:6.0:24.0 (fig. S3). 1H nuclear magnetic resonance in CD3COCD3 shows the ratio between the number of hydrogen atoms of phenyl groups and that of methyl groups to be 1:10.6, consistent with the calculated value 1:10.5 for 1 (fig. S4). Inductively coupled plasma atomic emission spectroscopy also gives satisfactory analytical results. The Ag-to-Sb ratio is measured to be 11.53, which is close to the calculated value of 11.67.

Single-crystal structural analysis revealed that 1 comprises a tricationic cluster [Ag35(H2L)2(L)(C≡CBut)16]3+ and three SbF6 counteranions. The total number of valence electrons is calculated to be 8 (n = 35 – 8 − 16 − 3), corresponding to the stable superatom electronic configuration (1S)2(1P)6. The cationic cluster consists of a centered icosahedron Ag13 core that is capped by 22 peripheral silver atoms. Surrounding protection is provided by three thiacalixarene ligands and 16 alkynyl ligands (Fig. 1). Guest species are found inside the bowls of thiacalixarenes (Fig. 1, A and B). A toluene molecule is enclosed in one thiacalixarene bowl with its methyl group pointing down into the cavity forming C–H···π interactions. The other two thiacalixarene bowls are occupied by a dichloromethane molecule each. Three thiacalixarene ligands bind four, four, and five silver atoms, respectively (Fig. 2, A and C). The remaining 12 silver atoms are surrounded by alkynyl ligands, and these silver atoms can be divided into three groups. In each group, four peripheral silver atoms and two atoms from the central Ag13 form a distorted triangular prism (Fig. 2, B and D). The bulky thiacalixarene ligands are located at the top of the cluster core, whereas alkynyl ligands surround the bottom half of the cluster core (Fig. 2, E and F).

Fig. 1 Representative crystal structure of [Ag35(H2L)2(L)(C≡CBut)16]3+ cluster.

(A) Overall structure of cationic cluster 1. (B) Space-filling view of the metal core structure. (C) The arrangement of thiacalixarene ligands and the metal core. Pink/green, silver; red, oxygen; yellow, sulfur; gray/blue, carbon; bright green, chloride. Dash line represents hydrogen bonding. The hydrogen atoms were omitted for clarity.

Fig. 2 The arrangements of metal core and ligands.

(A to D) Top view: Position of 10 peripheral Ag atoms (green) held by thiacalixarene ligands onto the Ag13 core (pink) (A and C). Bottom view: Position of 12 peripheral Ag atoms (green, triangular prisms) capped by alkynyl ligands (B and D). (E and F) Top and side views of the position of surface ligands with respect to the Ag35 core. Purple, sphere, Ag35 core; blue, thiacalixarene ligand; gray, alkynyl ligand.

The icosahedral Ag13 kernel consists of 20 triangular faces with an encapsulated Ag atom. The Ag···Ag distances between the central Ag atom to the Ag12 shell have an average value of 2.778 Å, which is shorter than the average value in Ag12 shell (2.923 Å). The values are comparable with the Ag···Ag distances of Ag13 observed in [Ag21{S2P(OiPr)2}12]+ (2.779 and 2.922 Å) (15). The Ag···Ag distances of Ag12 shell are larger than the average Ag···Ag distances (2.829 Å) in the empty icosahedron Ag12 in [Ag44(SPhF2)30]4− (14). The average Ag···Ag distances from the 22 exterior Ag atoms to the Ag13 icosahedron are 3.02 Å, indicating a compact metal core structure in this nanocluster.

The prominent structural feature in 1 is that diversified surface binding geometries can be generated through the combination of thiacalixarene and alkynyl ligands. In contrast to conventional thiolate or phosphine ligands with one or two coordination donors, thiacalix[4]arene has eight. The introduction of bridging sulfur atoms in thiacalix[4]arene is very important because it offers four additional coordinating donors and thereby increases the chelating ability. As shown in Fig. 3, there are two types of coordination motifs of thiacalixarene with silver atoms. One thiacalixarene keeps its cone-shaped conformation and binds five silver atoms within the bowl (Fig. 3A). Four silver atoms are symmetrically distributed, and each is coordinated by two phenoxyl oxygen atoms and one bridging sulfur donor. There is one silver sitting in the center of the bowl, which is coordinated by four phenoxyl oxygen donors. The central silver atom has interactions with the four silver atoms, with the Ag···Ag distances being in the range of 2.941 to 3.232 Å. The other two thiacalix[4]arenes each hold four silver atoms without the central silver atom inside the bowl (Fig. 3B). Two silver atoms are coordinated by two oxygen atoms and one bridging sulfur atom, whereas the other two silver atoms are each coordinated by one oxygen atom and one sulfur atom. Two hydroxyl groups in the bowl are not deprotonated, and they interact with the other two phenoxyl oxygens to form intramolecular hydrogen bondings with O···O separations in the range of 2.442 to 2.543 Å. Because of the unsymmetrical distribution and coordination of silver atoms in the bowl, each thiacalixarene adopts a distorted cone conformation, indicated by the dihedral angles between the phenyl rings and the plane of the four oxygen atoms, which are 112.5° to 145.3° and 108.3° to 162.7°, respectively.

Fig. 3 The coordination motifs A and B of the thiacalix[4]arene with silver atoms.

The silver atoms in pink belong to the Ag13 core.

It is well known that a calixarene can be modified at the lower rim, the upper rim, or the bridging atoms. In the reported gold nanostructures, calixarenes were modified at the lower rim with phosphines or thiols replacing hydroxyl groups. The ligand provides S or P donors to bind cluster cores, serving a similar role as thiolate or phosphine ligands (26, 28). Here, we use thiacalixarene with the bridging CH2 groups replaced by S atoms, offering four additional coordinating donors. Because of the chelating and macrocyclic effects presented by the four O and four S donors of each thiacalixarene, a rigid protection shell is available for the silver cluster core.

In addition to thiacalixarenes, 16 alkynyl ligands are also presented in the protection sphere of the cluster. Their triple bond character is retained because the C–C bond lengths are in the range of 1.161 to 1.245 Å. Alkynyl ligands adopt five types of coordination modes in 1: three in μ41112, four in μ41122, six in μ31, two in μ3112, and one in μ3122 (fig. S5). These types of binding modes have been commonly found in high-nuclearity silver alkynyl clusters (29).

Thiolate and phosphine ligands have been extensively used as protective agents for gold/silver nanoclusters (3034), but we demonstrate that calixarene and alkynyl can also function as excellent surface-protecting ligands, as shown by the crystal structure. A calixarene may generate diversified and complicated interfacial structures in comparison with those protected by conventional ligands.

Cluster 1 was further characterized by electrospray ionization mass spectrometry (ESI-MS) in positive mode, using CH2Cl2 as a solvent (Fig. 4A). The most prominent peak at mass/charge ratio (m/z) = 2409.8 corresponds to [Ag35(H2L)2(L)(C≡CBut)16]3+. The observed isotopic pattern of the tricationic cluster fully agrees with the simulation. Only a small amount of fragment species are observed, namely, [Ag35(H2L)(HL)(L)(C≡CBut)15Cl]2+ and [Ag35(H2L)(HL)(L)(C≡CBut)16]2+, suggesting that Ag35 nanocluster is relatively stable under the conditions of the measurement. Furthermore, transmission electron microscopy (TEM) imaging shows that the particle size is about 2.0 nm, which is comparable to the size of cluster 1 determined by single-crystal x-ray structural analysis, suggesting that 1 maintains its nanosized structure in solution (Fig. 5).

Fig. 4 ESI-MS spectra.

(A) Experimental (blue) and simulated (red) spectra of [Ag35(H2L)2(L)(C≡CBut)16]3+ (1). (B) Formation of Ag36 cluster by addition of 2 equiv of NEt3 and 1 equiv of AgSbF6 to Ag35. (C) The deprotonated product of 1 by addition of 2 equiv of NEt3. (D) Formation of Ag34 cluster by addition of 2 equiv of HBF4 to Ag35.

Fig. 5 TEM image of 1.

In cluster 1, one thiacalixarene ligand holds five silver atoms within the bowl, whereas the other two thiacalixarene ligands each only bind four silver atoms. In the latter type of thiacalixarene, there is a vacant site in the middle of the bowl. Hydrogen bondings are found between the hydroxyl groups. It is interesting to see whether the central vacant position can bind an additional silver ion; thus, the silver cluster core can be modified in an ordered way. Experimentally, different amounts of silver salt were added in the solutions of 1 in the presence of a small amount of NEt3. The reactions were monitored by ESI-MS. The addition of 1 equiv of AgSbF6 salt in the solution of 1 immediately led to formation of a new silver nanocluster, as indicated by a prominent ESI-MS peak at m/z = 3667.36 that corresponds to [Ag36(H2L)(L)2(C≡CBut)16]2+ (Fig. 4B). The addition of one more equivalent of silver salt did not give any Ag37 species because the thiacalixarene bowl occupied by a toluene is not accessible for a silver ion. With the help of the base NEt3, the hydrogen bondings within the thiacalixarene bowl are destroyed, which facilitates the trapping of an additional silver ion. ESI-MS confirmed the formation of a deprotonated dicationic cluster [Ag35(H2L)(HL)(L)(C≡CBut)16]2+ when NEt3 was added (Fig. 4C). It is structurally expected that Ag36 is an analog of Ag35 with one silver atom filling in the middle of one of the thiacalixarene ligands.

Furthermore, the addition of 1 equiv of HBF4 in the solution of Ag35 led to the formation of a [Ag34(H2L)3(C≡CBut)16]4+ cluster because lower pH value favors the hydrogen bonding between the hydroxyl groups, which squeezes the the central silver ion out of the thiacalixarene bowl on Ag35 (Fig. 4D). The ready addition or removal of silver ion from Ag35 is controlled by hydrogen bonding, suggesting that thiacalixarene can not only provide a rigid shell for protection of the metal core but also adjust the silver atom numbers of the nanocluster structure (Scheme 2).

Scheme 2 Transformation among Ag34, Ag35 and Ag36 cluster species.

The alkynyl ligands have been omitted for clarity.

The transformation of the cluster species influences the optical properties significantly. As shown in Fig. 6, the UV-visible absorption spectrum of Ag35 cluster (red trace) exhibits two prominent absorption bands at 495 and 336 nm, along with one shoulder at 300 nm. The organic ligands account for the absorptions around 300 nm. The 495-nm absorption is related to the metal core. Upon removal of one silver atom, the 495-nm peak was shifted to 482 nm for Ag34, whereas bathochromic shift was observed (501 nm) for Ag36 as a result of the addition of one silver atom onto Ag35.

Fig. 6 The optical absorption spectra of Ag35 and analogous Ag34 and Ag36 species in CH2Cl2.

Abs., absorbance.

To better understand the electronic structures of these nanoclusters, time-dependent density functional theory calculations were performed to investigate their frontier orbitals (fig. S6) and optical absorption (fig. S7). The computed absorption spectra were compared with the experimental data (Fig. 7, and fig. S7). The main experimental absorption peaks around 500 nm are reproduced well in the simulated spectra. These absorption bands are attributed to the transitions from the highest occupied molecular orbital (HOMO) to LUMO+2 for Ag35 and from HOMO-5 to the lowest unoccupied molecular orbital (LUMO) for both Ag34 and Ag36, respectively. It is noted that the characters of HOMOs are different (fig. S6). For Ag35, the HOMO spreads over the π orbital of the L ligand (motif A shown in Fig. 3A). For Ag36, the HOMO is located over the two L ligands (motif A shown in Fig. 3A). The low-energy absorption bands around 500 nm in Ag35 and Ag36 originate from primarily ligand-to-metal transitions. For Ag34, because no motif A is presented, the HOMO appears at the metal core, which suggests the absorption originates from primarily metal-to-metal transitions. The subtle adjustment of the silver atom numbers in the nanoclusters changes the nature of electronic transitions and results in the observation of optical gaps in absorption spectra accordingly.

Fig. 7 The experimental absorption in comparison with the calculated spectrum of Ag35.

a.u., arbitrary unit.

CONCLUSION

We have demonstrated total structure determination of a metal nanocluster containing the protection of thiacalixarenes for the first time. A new structure of the silver cluster and novel coordination modes of the thiacalixarene have been revealed. In addition to being a protective ligand, the thiacalixarene provides a coordination pocket for tuning of the surface structure of the silver nanocluster; that is, Ag34 and Ag36 species can be generated from the Ag35 cluster. The competition between the hydrogen bonding of hydroxyl groups and the coordination of silver to hydroxyl groups results in the formation of various cluster species, showing the power of supramolecular chemistry. The successful use of thiacalixarene in the synthesis of well-defined silver nanoclusters suggests a bright future for metal nanoclusters protected by macrocyclic ligands. The elucidation of the crystal structure of 1 sheds light on the metal-calixarene interface, which should stimulate study of the modulating of structures and properties of a new family of functional metal nanoclusters.

MATERIALS AND METHODS

Synthesis

The detailed information about the synthesis of [Ag35(H2L)2(L)(C≡CBut)16](SbF6)3 (1) is provided in the Supplementary Materials. Briefly, to a methanol solution containing AgC≡CBut and AgSbF6, a freshly prepared solution of NaBH4 was added dropwise under vigorous stirring. Then, a CHCl3 solution of L was added to the mixture and followed by addition of trimethylamine. The reaction continued for 20 hours, and the mixture was evaporated to dryness to give a dark solid. This solid was washed with n-pentane and then dissolved in mixed solvents of toluene and CH2Cl2. After centrifugation, the supernatant solution was subjected to the diffusion of n-pentane to afford sheetlike dark crystals within 1 week in 11% yield (5.1 mg, based on Ag). EDX analysis calculated for 1: Ag, 70.0 atomic %; Sb, 6.0 atomic %; S, 24.0 atomic %; found: Ag, 69.08 atomic %; Sb, 5.98 atomic %; S, 24.94 atomic %.

X-ray crystallography

Crystal data for 1 were as follows: M = 7990.41, triclinic, P-1, a = 21.7517 Å, b = 22.6953 Å, c = 34.1752 Å, α = 91.304, β = 101.977, γ = 118.038, V = 14,424.7(5) Å3, Z = 2, T = 100 K, 109,699 reflections measured, 51,338 unique (Rint = 0.0406), final R1 = 0.0720, and wR2 = 0.2366 for 37283 observed reflections [I > 2σ(I)]. Intensity data of 1 were collected on an Agilent SuperNova Dual system (Mo Kα). CCDC 1473428.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/8/e1600323/DC1

Supplementary Materials and Methods

table S1. Crystal data and structure refinement for 1.

fig. S1. The experimental and simulated PXRD spectra of 1.

fig. S2. Infrared spectrum of 1.

fig. S3. EDX analysis of 1.

fig. S4. 1H nuclear magnetic resonance spectrum of 1 (CD3COCD3).

fig. S5. The coordination modes of alkynyl ligands in 1.

fig. S6. The main frontier orbitals of Ag34, Ag35, and Ag36 clusters.

fig. S7. The experimental absorption spectra in comparison with calculated spectra of Ag34 (left) and Ag36 (right).

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REFERENCES AND NOTES

Acknowledgments: Dedicated to T.C.W. Mak on the occasion of his 80th birthday. We thank Z.-M. Cao for assistance in TEM measurements. Funding: This work was supported by the 973 Program (2014CB845603) and the National Natural Science Foundation of China (21125102, 21390390, 21301145, 21473139, 20720150038, and 21521091). Author contributions: Z.-J.G. conceived and carried out the synthesis and crystallization of the cluster and performed the theoretical calculations. J.-L.Z. assisted in the synthesis. Z.-A.N. assisted in the structural refinements. X.-K.W. assisted in ESI-MS measurements. Y.-M.L. and Q.-M.W. designed the study, supervised the project, analyzed the data, and wrote the paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions 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.
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