Homoleptic alkynyl-protected gold nanoclusters with unusual compositions and structures†‡

Zong-Jie Guan ^ab, Feng Hu ^a, Jiao-Jiao Li ^a, Zi-Rui Liu ^a and Quan-Ming Wang *^ab
aDepartment of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing, 100084, P. R. China. E-mail: qmwang@tsinghua.edu.cn; Web: http://www.wangqmlab.org.cn
^bDepartment of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China

First published on 9th June 2020

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Atomically precise metal nanoclusters have attracted great attention owing to their structural diversities and potential applications in catalysis, sensing, optics, and bioimaging.^1–6 In the past decade, great advances have been made in terms of the synthesis, structure determination and property studies of nanoclusters.^4,7–13 A large number of thiolate-protected gold nanoclusters have been extensively studied,^14–35 but only a handful of alkynyl-protected gold nanoclusters have been reported.^36–45 Alkynyl-protected gold nanoclusters have been identified by mass spectrometry, including Au₄₃(CCPh)₂₂, Au₄₆(CCPh)₂₄, Au₅₂(CCPh)₂₆, Au₅₄(CCPh)₂₆, Au₅₉(CCPh)₂₇, Au₇₁(CCPh)₃₂, Au₉₀(CCPh)₃₆, Au₉₄(CCPh)₃₈, Au₁₀₁(CCPh)₃₈ and Au₁₁₀(CCPh)₄₀.^46,47 So far, only six all-alkynyl-protected Au nanoclusters with defined structures have been reported,^40–45 namely Au₃₆,⁴⁰ Au₄₄,⁴⁰ Au₁₄₄,⁴¹ Au₂₅,⁴² Au₂₂^43,44 and Au₂₃.⁴⁵ It is worth noting that most of the alkynyl-gold nanoclusters were discovered as the counterparts of Au_n(SR)_m, for example, Au₃₆(CCPh)₂₄ and Au₃₆(SPh-^tBu)₂₄ have an identical metal core and metal-to-ligand ratio.^22,40 Recently we have revealed that the metal-to-ligand ratio in Au₂₃(CCBu^t)₁₅ is different from all known Au_n(SR)_m nanoclusters and has not been observed among all-alkynyl-protected gold clusters Au_x(CCR)_y.⁴⁵ This finding indicates that various gold-alkynyl nanoclusters with new compositions can be expected.

Herein, we report two novel alkynyl-protected gold nanoclusters, i.e., Au₄₂(CCC₆H₄-2-CF₃)₂₂ (1) and Au₅₀(CCC₆H₄-3-F)₂₆ (2), which are synthesized by direct reduction of [AuCCR]_n (R = C₆H₄-2-CF₃ or C₆H₄-3-F) precursors. Interestingly, cluster 2 is the first Au₅₀ nanocluster, and the metal-to-ligand ratios of 1 and 2 are different from those of known Au_n(SR)_m or Au_x(CCR)_y nanoclusters. In addition, the metal kernels of these two clusters are built up with nprecedented units.

The chemical compositions of 1 and 2 were determined by matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry in the positive-ion mode with trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as the matrix. The mass spectrum of 1 shows a dominant peak at about 11.9 kDa, corresponding to Au₄₂(CCC₆H₄-2-CF₃)₂₂ (Fig. 1a), and the signal at around 12.9 kDa, as shown in Fig. 1b, for 2 matches the formula weight of 12945 of Au₅₀(CCC₆H₄-3-F)₂₆.

Single-crystal structural analysis⁴⁸ revealed that 1 and 2 have novel metal kernels, which are in contrast to previously reported Au_n(SR)_m and Au_x(CCR)_y clusters. As shown in Fig. 2, cluster 1 has D₂ symmetry, which is composed of an Au₃₀ kernel and 12 peripheral gold atoms, and surrounding protection is provided by 22 2-CF₃-C₆H₄CC ligands. The Au₃₀ kernel can be divided into two Au₁₅ units, which join together (Fig. 3a and b). Each Au₁₅ unit comprises three layers (a–b–a) arranged in a hexagonal-close-packed (hcp) fashion (Fig. 3a), and the dihedral angle between the closest packing layers in two Au₁₅ units is about 50° (Fig. S1a‡). The whole Au₃₀ kernel is surrounded by two “RCC–Au–CC(R)–Au–CCR” dimeric staples (Fig. 3c) and eight “RCC–Au–CCR” staples (Fig. 3d). These eight linear “RCC–Au–CCR” motifs connect the two Au₁₅ units like bridges (Fig. S1b‡), and the two “V-shaped” staples provide terminal protection to the Au₃₀ kernel (Fig. 3c). In each Au₁₅ unit, the average Au–Au distance is 2.816 Å, which is significantly shorter than that between two Au₁₅ units (2.932 Å). The average Au–Au distance from the 12 peripheral Au atoms to the Au₃₀ kernel is 3.169 Å. There is only one known example of Au₄₂, the thiolated Au₄₂(TBBT)₂₆ (TBBT: 4-tert-butylbenzenethiol). Its kernel adopts a face-centered cubic (fcc) structure different from the hcp in 1, and its metal-to-ligand ratio is 42 to 26 vs. 42 to 22 in 1.⁴⁹

Interestingly, we obtained a new alkynyl-protected gold nanocluster 2 by an identical synthetic method with an alkynyl ligand having a less bulky R group. This cluster contains 50 gold atoms and 26 alkynyl ligands (Fig. 4), which is the first Au₅₀ nanocluster. As shown in Fig. 5a, cluster 2 has C₂ symmetry. There are two anti-pentagonal prismatic Au₁₁ basic units in 2, each Au₁₁ unit can be seen as derived from icosahedral Au₁₃ with the removal of two opposite gold atoms. Each pentagonal edge at the bottom of the Au₁₁ connects a gold atom to form a Au₁₆ unit (Fig. 5b), and such two Au₁₆ units join together to form a Au₃₂ core (Fig. 5c). There are four additional peripheral gold atoms surrounding Au₃₂ to form the Au₃₆ kernel (Fig. 5d, highlighted in light blue). The average Au_p–Au_core bond length between the peripheral gold atoms and the Au₃₂ core is 2.880 Å, indicating that these four gold atoms are strongly bound to the kernel.

Similar to 1, there are two types of staples in 2, i.e., dimeric staple and monomeric staple. The Au₃₆ kernel of 2 is surrounded by two “RCC–Au–CC(R)–Au–CCR” dimeric staples (Fig. 5e) and ten “RCC–Au–CCR” staples (Fig. 5f). Two dimeric staples serving as “bridges” tightly connect two Au₁₆ units. Ten monomeric staples can be divided into two sets, five staples are linked with the upper Au₁₆ unit, and the other five staples surround the lower Au₁₆ unit (Fig. S2‡). The linear RCC–Au–CCR motif is the main surface binding structure in the title cluster, which was also extensively found in many alkynyl-protected Au or Au–Ag nanoclusters, such as Au₁₄₄,⁴¹ Au₂₃,⁴⁵ Au₈₀Ag₃₀,⁵⁰ Au₅₇Ag₅₃⁵¹ and Au₃₄Ag₂₈.⁵²

The Au–Au bond lengths in the Au₁₁ unit range from 2.708 to 3.417 Å with an average length of 2.882 Å. The average Au–Au bond length between two Au₁₁ units is 2.875 Å, which is shorter than the average bond length between two Au₁₅ units in 1. The average Au–Au distance between Au atoms in the waist (Fig. 5c, green) and the Au₁₁ unit is 2.819 Å. It is noteworthy that the Au–Au distances in the whole Au₃₆ kernel give an average of 2.861 Å, which is significantly shorter than that (2.944 Å) of the Au₃₆ kernel in Au₄₄(CCPh)₂₈,⁴⁰ which indicates that the gold atom packing in 2 is more compact than the FCC packing of the Au₃₆ kernel in Au₄₄(CCPh)₂₈.

The optical absorption spectra of these two clusters were recorded and are shown in Fig. 6. Cluster 1 exhibits a distinct absorption band at 815 nm, and cluster 2 shows a shoulder peak at 745 nm. In contrast to the expected red shift in the absorption from 1 to 2, the blue shift indicates that the structural types are important for determination of the electronic structures. To understand the electronic structure and optical absorption properties of these two clusters, we performed TDDFT calculations using the quantum chemistry program Gaussian 09. The calculated significant transitions of 1 and 2 are in good agreement with the experimental spectra (Fig. S3‡). For cluster 1, the lower-energy absorption band at 815 nm arises primarily from the HOMO−4 → LUMO transition. Notably, the HOMO−4 orbital of 1 is located over the metal core and surface of gold-alkynyl staples, while the LUMO is mainly contributed by the Au₃₀ kernel (Fig. S4a‡). The band at 745 nm for 2 corresponds to a HOMO to LUMO+1 transition, and both HOMO and LUMO+1 orbitals are mainly contributed by the metal core, therefore, this band is mainly attributed to the core-confined transitions (Fig. S3b and S4b‡). Because Au₂₂(CCR)₁₈ is strongly luminescent,^43,44 we examined the emission spectra of 1 and 2. Unfortunately, these two clusters are not emissive.

The metal-to-ligand ratio of a nanocluster determines the composition and the number of free electrons, which influences its structure and properties. Fig. 7 shows a plot of the number of ligands against the number of gold atoms for the compositions of well-defined Au_x(CCR)_y and Au_n(SR)_m nanoclusters. In many cases, the x and y values in Au_x(CCR)_y is exactly the same as n and m in Au_n(SR)_m, respectively. However, it catches our attention that the open squares representing Au₂₃, Au₄₂ and Au₅₀ in the alkynyl family lie at a lower place in comparison with the thiolated ones (black triangles) in the plot, which indicates that these gold-alkynyl nanoclusters have larger metal-to-ligand ratios with respect to the thiolated ones. The multiple bridging ability of alkynyl accounts for this difference, i.e. less alkynyl ligands are needed for protecting the same number of gold atoms. As a result, the number of free electrons is higher, e.g. 20 in Au₄₂(CCC₆H₄-2-CF₃)₂₂ and 16 in Au₄₂(TBBT)₂₆. In addition, it was noted that the steric repulsion between ligands is responsible for higher metal-to-ligand ratios.^53,54

Moreover, this work also provides additional evidence to prove that the direct reduction method is a convenient and effective approach for synthesizing a variety of alkynyl-protected gold nanoclusters.

Conclusions

In summary, we have synthesized and structurally determined two homoleptic alkynyl-protected gold nanoclusters, Au₄₂(CCC₆H₄-2-CF₃)₂₂ and Au₅₀(CCC₆H₄-3-F)₂₆. Their compositions, i.e. the metal-to-ligand ratios, have not been observed in the Au_n(SR)_m series and Au_x(CCPh)_y series. They possess novel metal kernels, which have not been observed previously in ligand-protected gold nanoclusters. The success of the direct reduction in the synthesis of 1 and 2 suggests that a large number of various alkynyl-protected gold nanoclusters are yet to be discovered.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21631007, 91961201, and 21971136). Z. -J. G. acknowledges the support from the Shuimu Tsinghua Scholar Program. We thank the Tsinghua Xuetang Talents Program for providing instrumentation and computational resources.

Notes and references

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Footnotes

† In celebration of the 60th anniversary of the Fujian Institute of Research on the Structure of Matter.

‡ Electronic supplementary information (ESI) available. CCDC 1996850 and 1996851. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0nr02986f

This journal is © The Royal Society of Chemistry 2020