Chenwanli Qinab,
Qianqin Yuanab,
Peng Li*ab,
Shuxin Wang
ab,
Shuang Chen*abc and
Manzhou Zhu
*ab
aDepartment of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, P. R. China. E-mail: peng-li@ahu.edu.cn; chenshuang@ahu.edu.cn; zmz@ahu.edu.cn
bKey Laboratory of Structure and Functional Regulation of Hybrid Materials, Anhui University, Ministry of Education, Hefei, 230601, P. R. China
cInstitutes of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, P. R. China
First published on 20th March 2020
The C–H⋯π interaction and the C–H⋯Cl–C van der Waals interaction play a crucial role in the crystallization of nanoclusters. In this paper, we present an example of a crystal system transformation of Au11(PR3)7Cl3 from monoclinic (M) to trigonal (T) by surface modification. Atomically-resolved gold nanoclusters containing tris(4-chlorophenyl)phosphine and chloride ligands were synthesized and determined to be Au11(p-ClPPh3)7Cl3 (p-ClPPh3 = tris(4-chlorophenyl)phosphine) by X-ray crystallography. Crystal data demonstrated that the C–H⋯Cl–C interaction is dominant in a trigonal crystal system of Au11(p-ClPPh3)7Cl3 with a R space group. However, the C–H⋯π interaction is the major driving force to form a monoclinic crystal system of Au11(PPh3)7Cl3 (PPh3 = triphenylphosphine) with a P2(1)/n space group. Moreover, UV-vis absorption spectra and X-ray photoelectron spectra reveal that the electronic structure of the Au11(p-ClPPh3)7Cl3 nanocluster is greatly influenced by p-ClPPh3. This work provides critical implications for the crystallization of metal nanoclusters, as well as a better understanding of the non-covalent interaction on the nanocluster assembly and the crystal engineering by surface modification.
Ligands play a crucial role in the stabilization of nanoclusters.14–17 The non-covalent interactions between surface ligands not only alter properties of clusters but also have significant effects on nanocluster assembly and self-assembly.18–30 For instance, Wu and co-workers demonstrated that C–H⋯π interactions facilitated the chiral arrangement of ligands, and the assembled ligands presented rotational and parallel patterns on the surface of the nanoparticles.14 Pradeep and colleagues reported that C–H⋯π interactions are dominant in a cubic lattice and result in a higher luminescence efficiency.16 Crudden et al. showed that multiple C–H⋯π and π⋯π interactions rigidify the ligands and contribute to the high photoluminescent quantum yields in the Au13 nanocluster.31 Zheng et al. reported that π⋯π interactions play a crucial role in stabilizing silver nanoclusters.32 Xie et al. found that the assembly behaviour of clusters can be regulated by metal ions (i.e., Ag+/Cs+).33 Non-covalent interactions play an important role in nanocluster chemistry.34,35 To date, a series of Au11(PR3)Cl3 nanoclusters with adjustable ligands have been extensively studied comparing with other nanoclusters.15,36–39 Therefore, Au11(PR3)Cl3 clusters are chosen as models to investigate non-covalent interactions in the crystallization of nanoclusters by surface modification.
In the following, we synthesized Au11(p-ClPPh3)7Cl3 (Au11-Cl for short) and Au11(PPh3)7Cl3 (Au11-H for short) nanoclusters by using tris(4-chlorophenyl)phosphine and triphenylphosphine as the protecting ligand. The composition of the Au11-Cl was characterized by electron spin ionization (ESI-MS), and the structure of the Au11-Cl was determined by X-ray crystallography. We are engaged in illuminating the crystal system transformation ascribed to C–H⋯Cl–C interactions and C–H⋯π interactions. The different patterns of non-covalent interactions can alter the packing of nanoclusters into monoclinic or trigonal lattice. The distance between H and H atom (DHH for short) reveals the non-covalent interactions of the intra-molecular can also be changed through surface modification. Furthermore, UV-vis absorption spectra and X-ray photoelectron spectra reveal the difference in the electronic structure due to surface modification.
Red-brown crystals were collected, and the structure of Au11-Cl was determined by X-ray crystallography. Single crystal X-ray diffraction data of the Au11-Cl was collected on a Bruker Smart 1000 CCD. The area detector used graphite-monochromatized Mo Kα radiation (λ = 0.71069 Å). A piece of red-brown block-shaped crystal (0.25 × 0.24 × 0.23 mm) was mounted onto a MiTeGen capillary with Fluorolube, and all structures were solved by direct methods using SHELXS-97. Data collection was performed under room temperature (296 K).
The electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) measurement was recorded using UPLC H-Class XEVO G2 XS Qtof (Waters. Crop.). The Au11-Cl was recorded in the positive ion mode when dissolving in CH3OH, containing 50 mM CsOAc, and centrifugation for 5 min (12000 rpm). The sample was infused at 180 μL h−1. The source temperature was kept at 50 °C with the spray voltage keeping at 4 kV. The mass spectra were processed using Masslynx 4.1 software (Waters Corp.).
The Thermo ESCALAB 250 instrument was used to measure X-ray photoelectron spectroscopy (XPS), which configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, and the analysis chamber base pressure lower than 1 × 10−9 mbar. Data were collected with FAT = 20 eV.
The structure of Au11-Cl was determined by the single crystal X-ray crystallography (Fig. 2). The molecular structure of the Au11-Cl is similar to that of the Au11-H. Ten gold atoms (yellow) form an incomplete icosahedral structure with one gold atom in the centre. And the ligands of six p-ClPPh3 (pink) and three Cl (green) atoms cap the metal core along the C3 axis with one p-ClPPh3 ligand on the C3 axis. In comparison with the Au11-H, the Au11-Cl crystal possesses a larger cell volume (23297.7 Å3 versus 12
398 Å3) and less density (2.066 g cm−3 versus 2.243 g cm−3), which is possibly attributed to distinct non-covalent interactions in the unit cell. More comparisons of the cell details are given in Table S1.†
The packing of these two nanoclusters along (100), (010), and (001) planes is shown in Fig. 3, respectively. For the T system of Au11-Cl, the nanoclusters are organized layer by layer with an fcc-like ABCABC style viewing along a-axis, b-axis, and c-axis (Fig. 3a–c). Especially, viewing along the c-axis, the Au11-Cl gets organized into a hexagonal lattice, while the Au11-H gets decorated into a rectangular lattice. In the case of the Au11-H, the nanoclusters are packed layer by layer with an ABAB style viewing along the a-axis, b-axis, and c-axis (Fig. 3d–f). Although the Au11-Cl and the Au11-H are constructed with the same core structure, they display totally different lattices. The obvious packing difference of the Au11 nanoclusters is related to the difference of non-covalent interactions between nanoclusters.14
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Fig. 3 Packing of the Au11-H and the Au11-Cl view from the (a and d) a-axis, (b and e) b-axis and (c and f) c-axis. |
C–H⋯π interactions play a crucial role in the crystal packing of the Au11-H, which are the major driving force to form monoclinic. In the M system, each unit contains four Au11-H nanoclusters. Fig. 4 shows the C–H⋯π interactions (red/blue dashed line) of the Au11-H. C–H⋯π interactions exist between the C–H groups (blue) of the PPh3 ligands and adjacent benzenes (green) of the adjacent cluster, evidenced by their distance. The distance of C–H⋯π ranges from 2.80 to 2.89 Å, with an average of 2.835 Å (Fig. S3†). Strong C–H⋯π interactions form a chain to hold each Au11-H nanocluster and restrict the motion of the entire nanocluster.
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Fig. 4 The C–H⋯π interactions in the Au11-H. Yellow = Au; pink = P; green = C; blue = H; bright green = Cl; blue red dashed, C–H⋯π interactions. |
Interestingly, most H atoms form C–H⋯π interactions are located on the para position of the benzene rings in the Au11-H, which inspires us to speculate whether these interactions could be destroyed by introducing an atom with a larger atomic radius in the para position. As we expected, by replacing PPh3 with p-ClPPh3 ligands that contain Cl atoms on the para position of aromatic rings, C–H⋯π interactions are not found in the unit cell of the as-obtained Au11-Cl. In order to discuss the non-covalent interactions of Au11-Cl, four Au11-Cl clusters are marked by the green quadrilateral (Fig. S7†).
As shown in Fig. 5, the inter-cluster distances are 19.80 Å and 17.42 Å, respectively, and the corresponding obtuse is 125.42°. However, the lengths (16.41 Å and 16.59 Å) and obtuse (127.86°) of Au11-H are different from those of Au11-Cl. When zooming into the surfaces between nanoclusters, C–H⋯Cl–C interactions (bright green/blue dashed line) of the Au11-Cl lead to form T systems in the unit. In the T lattice of Au11-Cl, the average distance of the C–H⋯Cl–C interactions is 2.94 Å, with a range from 2.85 to 2.99 Å (Fig. S4†). This distance is consistent with the reported value.40 Overall, comparing the C–H⋯Cl–C interactions and the C–H⋯π interactions suggests that the insertion of chlorine atoms breaks C–H⋯π interactions and changes the crystal system from M to T.
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Fig. 5 The C–H⋯Cl–C interactions in the Au11-Cl. Yellow = Au; pink = P; green = C; blue = H; bright green = Cl; bright green blue dashed, C–H⋯Cl–C interaction. |
Another significant aspect is that the non-covalent interactions of the intra-molecular also changed because of the introduction of chlorine atoms in the surface ligands. The average distance of H and H (DHH for short) in the Au11-Cl is 2.72 Å (Fig. 6a and S5†). In contrast, the DHH in the Au11-H is 2.80 Å, varying from 2.53 to 2.99 Å (Fig. 6b and S6†), which is slightly larger than that of the Au11-Cl. Viewing along the C3 axis, the triangle constituted by nearest H atoms is slightly rotating compared with the one in Au11-Cl, due to the introduction of the Cl atoms. The total numbers and average distances of the non-covalent interactions and the DHH were shown in Table 1. All these results verify that non-covalent interactions of intra-cluster induce the rotation and arrangement of ligands, which further enable every single nanocluster to be compacted and ordered with reduced intra-cluster vibration, contributing to the formation of high-quality single crystals.
Interaction (distance) | Au11-H | Au11-Cl |
---|---|---|
CH⋯π | 6 (2.84 Å) | × |
C–H⋯Cl–C | × | 20 (2.94 Å) |
Inter-DHH | 46 (2.80 Å) | 83 (2.81 Å) |
Intra-DHH | 24 (2.79 Å) | 21 (2.72 Å) |
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1966633. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ra01288b |
This journal is © The Royal Society of Chemistry 2020 |