On the Structure and Electronic Structure Evolution of Thiolate

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C: Physical Processes in Nanomaterials and Nanostructures

On the Structure and Electronic Structure Evolution of Thiolate-Protected Gold Nanoclusters Containing Quasi Face-Centered-Cubic (FCC) Kernels Lin Xiong, Sha Yang, Xiangxiang Sun, Jinsong Chai, Bo Rao, Lanhua Yi, Manzhou Zhu, and Yong Pei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02010 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

On the Structure and Electronic Structure Evolution of ThiolateProtected Gold Nanoclusters Containing Quasi Face-Centered-Cubic (FCC) Kernels Lin Xiong,†a Sha Yang,†b Xiangxiang Sun,a Jinsong Chai,b Bo Rao,b Lanhua Yi,a Manzhou Zhu*b and Yong Pei*a a

Department of Chemistry, Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,Xiangtan University, Hunan Province 411105, China

b

Department 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, China. RECEIVED DATE (automatically inserted by publisher) Corresponding Authors: [email protected] (Y. P.); [email protected] (M. Z.)

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ABSTRACT A structure evolution map of fcc-structured thiolate-ligand protected gold nanoclusters is outlined on basis of total structure determination of a new 6e Au21(SR)15 (R = tert-butyl, t-Bu) cluster. The structural evolution map described some basic structural evolution patterns such as a triangle-Au3 and tetrahedron-Au4 associated gold-core evolution pattern (denoted as the TTG mechanism) and the periodic or symmetric growth of gold cores and ligand shells. According to the structural evolution map, a topological structure-electronic structure relationship is also proposed. The delocalized valence electronic properties of any fcc-structured gold clusters may be expressed as the linear combinations of the molecular orbitals (MOs) of the fragment 2e units (Au3+ and Au42+). The structural disciplines and topological structure-electronic structure relationships reported in this work laid a basis for understanding the structural evolution and electronic structure of fcc-structured thiolate-protected gold nanoclusters. Particularly, the established structural evolution map provides a tool to explore new magic sized clusters and cluster structures. In this work, a new fcc-structured 4e Au17(SR)13 and a new isomer structure of the 8e Au28(SR)20 cluster have been predicted. The medium sized fcc-structured gold clusters locating in the size range of 52 to 92 gold atoms and even larger sized gold clusters can be also explored from the structural regularities described in the map.


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1. INTRODUCTION The coinage metals stabilized by phosphine or organothiolato-ligands are well known to form the multiple-twinned structure such as icosahedrons and the fcc structure was thought unstable until the cluster size reached a certain threshold.1-4 However, it was recognized that the type of ligands played a crucial role in determining the overall structure, size and composition of such nanoclusters.5 It was evidently shown that the aliphatic versus aromatic or bulky versus slim ligand may significant shape the structure of ligand-protected gold clusters.6,7 Upon changing the protection ligand from the aliphatic to aromatic, it is possible to tune the gold cluster structure from non-fcc icosahedra to fcc cubotahedra or even hcp and bcc configurations.5,8-11 Using the ligand strategy, it is not only to control the physical, chemical, and biological properties of ligand protected gold nanoclusters, but also provide an opportunity to capture some key intermediate cluster structures which bridges the cluster evolution pathways and therefore summarize some genetic structural evolution rules and the structure-electronic structure relationships of certain ligand protected clusters. In recent years, a number of magic sized thiolate-protected gold nanoclusters or nanoparticles (refer to Aum(SR)n cluster or RS-AuNPs) have been synthesized.5,12 Pioneered by the breakthroughs of successful crystallizations of two RS-AuNPs, Au25(SCH2CH2Ph)18-

13,14

and Au102(p-MBA)44,15

tremendous efforts have been devoted to find new magic size of Aum(SR)n clusters and explore their atomic structures. Until 2012, all determined atomic structures of Aum(SR)n and the phosphine/halide protected gold nanoclusters are generally composed of multiple-twinned gold cores and their cluster structure and valence electronic structure can be satisfactorily explained from a classic jellium model or a superatom complex model.16 However, by extensively exploring the type of SR ligands, in recent years many unprecedented magic sizes and cluster structures have been found. An important progress is the finding of a family of face-centered-cubic (fcc) or quasi-fcc structured Aum(SR)n clusters. Ever since the first report of fcc gold kernel in the Au36(TBBT)24 (TBBT: 4-tert-butylbenzenethiol),14 to date there are over 15 quasi-fcc-


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structured Aum(SR)n clusters including 48e Au92(TBBT)44,17 36e Au68(3-MBA)32 (3-MBA: 3mercaptobenzoic acid),19 20e Au52(TBBT)32,20 16e Au40(o-MBT)24 (o-MBT: mercaptobenzothiazole),20 Au44(TBBT)2821 and Au42(S-c-C6H11)26,22 12e Au34(S-c-C6H11)22,22 Au30(S-tBu)18,23 Au30S(S-tBu)18,24 8e Au28(S-c-C6H11)2025 and Au28(TBBT)20,26 Au24(S-Adm)16,27 Au23(S-c-C6H11)16‒,28 6e Au21(SAdm)15,29 and 4e Au24(SCH2Ph-tBu)20,30 Au20(TBBT)1631-33 were crystallized. The single XRD characterizations indicated that the gold atoms in these clusters adopted quasi-fcc configuration, which is contrast to the well-known multiple-twinned structures such as icosahedron and truncated decahedrons. Although there have been several reports of fcc-structured gold clusters, the structure evolution pattern and the topological structure-electronic structure of these clusters were rarely explored.21,34-37 Firstly, it was always thought that the fcc gold structure is unstable at ultrasmall size. The observation of fcc gold frameworks even at with very smaller cluster sizes such as Au23(SC6H11)16‒ and Au21(S-Adm)15 suggested that the nucleation growth of thiolate-stabilized gold nanoclusters do not necessarily proceed from the successive growth of a spherical shell as observed in several multiple-twinned gold clusters such as Au25(SCH2CH2Ph)18-,13-14 Au102(p-MBA)44,15 Au130(p-MBT)50,38 Au133(TBBT)5239,40 and Au246(p-MBT)80,41 etc.. Secondly, several electronic structure models including the superatom complex (SAC) model,16 superatom network (SAN) model,42 super-valence bond (SVB)43-45 model and grand unified model (GUM)43 have been developed. Nonetheless, many of these electronic structure models have limitations in explanation of the electronic structure properties of some fcc-structured gold nanoclusters. For example, the gold cores in fcc-structured 8e Au23(SR)16- and 20e Au52(SR)32 clusters are expected to adopt spherical symmetry according to the jellium model. Nonetheless, the much irregular or elongated gold core structures in two clusters did not obey the jellium model prediction. The GUM model elucidated the relationship between cluster valence electron number and component unit of the gold cores, while the model did not address the delocalized electronic properties. Furthermore, all these developed models only give explanations on known cluster structures, it is still a grand challenge to predict the unknown cluster configurations. The fundamental disciplines of structural evolution of


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fcc-structured ligand protected gold cluster as well as the development of new electronic structure models remained to be addressed. In this work, we reported the total structure determination of a new 6e gold nanocluster named Au21(S-tBu)15. On basis of the structure knowledge of Au21(S-tBu)15 and recently reported fcc-gold cluster crystal structures, a structure evolution map is summarized for the smaller sized fcc-structured Aum(SR)n clusters (m ≤ 52). The fcc-structured gold clusters have constituted six structural evolution pathways, in which, the gold cores are evolved from successive and even periodic growth of triangleAu3 and tetrahedron-Au4 units (denoted as the tetrahedron and triangle growth mechanism, TTG mechanism), and the ligand shell also show strong structural regularities. Following the established structure evolution map, many unique gold cores and magic sizes such as the Au34(SR)22 and the currently reported Au21(SR)15 etc. can be understood as the intermediate clusters at certain nucleation stages of fcc-structured gold clusters. Two missing links in the cluster structure evolution map including a new 4e Au17(SR)13 and a new isomer structure of the 8e Au28(SR)20 cluster are also predicted. On basis of the TTG evolution pattern of gold cores, a topological structure and electronic structure relationship is further suggested. The delocalized electronic structure of any fcc-structured thiolate-protected gold nanoclusters may be expressed as the linear combinations of the molecular orbitals of the fragment 2e units. The whole paper is organized as follows: In the first section, the crystal structure of the Au21(StBu)15 cluster is described. In the second section, a structure evolution map of smaller sized fccstructured Aum(SR)n clusters is tentatively established. In the third section, a new model is developed to address the topological structure-electronic structure relationships of the fcc-structured Aum(SR)n clusters. 2. EXPERIMENTAL AND THEORETICAL METHODS AND DETAILS 2.1 Experimental Methods and details


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The specific methods of experiment and characterization are displayed in supporting information (SI). Briefly, HAuCl4•3H2O (195 mg, 0.5 mmol) were dissolved in 20 mL water under vigorous stirring at 80℃. Then 500 mg glutathione (GSH, 1.62 mmol) was added into the above solution. With prolong the time, the solution turned from yellow turbid liquid to a colorless transparent solution. After 20 min, a solution of borane tert-butylamine complex (dissolved in 10 mL toluene) was added. The color of solution immediately turned black. After 0.5 h, some black precipitate (a mixture of [(Au)n(GSH)m] nanoclusters) was produced. 0.7 mL tert-Butyl mercaptan was added into the solution under vigorous stirring. After about 24 h, the Au nanocluster was transferred to the organic phase. After that, the aqueous phase was removed. The mixture in the organic phase was rotavaporated, and then washed several times with CH3OH to remove the redundant ligands and by-products. The Au21(S-tBu)15 nanoclusters were crystallized in CH2Cl2/CH3OH at room temperature. The structure of Au21(S-tBu)15 was determined by X-ray crystallography.

2.2 Theoretical Method and Computational Details The density functional theory (DFT) calculations were employed to optimize the geometric structure of clusters using Perdew-Burke-Ernzerhof (PBE) functional and the d-polarization included basis set (DND) is used for C, H, S elements. The DFT Semi-core Pseudopototential (DSPP) approximation with some degree of relativistic corrections into the core is used for the Au element implemented in the Dmol3 package.47,48 The TS method49 is performed for the DFT-D correction. The convergence criterion of the geometrical optimization was set to be 1.0×10-5 Hartree for energy change, 4.0×10-3 Hartree/Å for the gradient, and 5.0×10-3 Å for the displacement, respectively. The extended Hückel molecular orbital (EHMO) calculations carried out using the Gaussian 09 package50 and the VESTA package51 is used to draw the electronic density diagrams of MOs. In plotting the electronic density diagrams of the MOs, the extended Hückel molecular orbital calculations are carried out using the Au 6s and 5d orbitals only. As a semi-empirical method, the EHMO calculations can provide clearer picture of Au 6s atomic orbital interactions than that from the ab initio molecular orbital calculations.


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3. RESULTS AND DISCUSSIONS 3.1 Structure and UV-vis Absorption Spectra of the Au21(S-tBu)15 Cluster The new precise atomic structure of Au21 cluster, i.e., Au21(S-tBu)15, has been determined from the single crystal X-ray crystallography and the Au21S15 framework is shown in Figure 1a. The single crystal X-ray crystallography revealed the Au21(S-tBu)15 crystallized in a monoclinic P2/c space group (Table S1). Two pairs of enantiomers were found in the unit cell. We note that the Au21(S-tBu)15 has the same numbers of Au atom and SR groups to the recently reported Au21(S-Adm)1529 cluster. However, the Au21S15 framework of two clusters differs greatly. The structural analysis of Au21(S-tBu)15 cluster indicates a 10-atoms Au-core, two “V-shaped” ‒SR(AuSR)2‒ motifs, one trimeric ‒SR(AuSR)3‒ motif, and one longer ‒SR(AuSR)4‒ motif (Figure 1a). The average bond length and the bond angle of two Au21 nanoclusters are compared in Table S2. It is found that the average Au(kernel)-Au(kernel) and Au(kernel)-Au(staple) bond lengths in Au21(S-tBu)15 (2.94 and 3.11 Å) are longer than those in Au21(S-Adm)15 (2.75 and 3.05 Å) by 6.5% and 1.9%, respectively. This shows that the kernel structure of Au21(S-tBu)15 is looser than that of Au21(S-Adm)15. The Au(staple)-S bond lengths in two clusters are similar, while the Au(kernel)-S and S-C bond lengths of Au21(S-tBu)15 (2.32 and 1.84 Å) are shorter than those in Au21(S-Adm)15 (2.35 and 1.97 Å). In terms of average bond angles, except for Au(staple)-S-Au(staple)- and -S-Au(staple)-S- bond angles in two clusters are close to each other, the average -Au(kernel)-S-Au(staple)- bond angle differs by 11.1%. This may be owing to the different binding patterns of the ligands in the two clusters. Although the structure of the two clusters is markedly different, the gold atoms in the cluster show the same quasi-fcc configuration.


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Figure 1. (a) Atomic structure of Au21(S-tBu)15 cluster. A: the crystal structure of Au21(S-tBu)15 cluster; B: Au-S skeleton structure of Au21 cluster. (b) C and D: a comparison of atomic structures of two Au21 clusters; E and F: the complete and incomplete cuboctahedron cores in Au21(S-tBu)15 and Au21(S-Adm)15 clusters, respectively. Color label: red balls, S; Green, yellow and pink balls, Au.

From Figure 1b, both Au21 clusters have fcc gold atom packing, while the Au21(S-tBu)15 cluster is composed of a complete Au-cuboctahedron and the Au21(S-Adm)15 cluster contains an incomplete Aucuboctahedron. To the best of our knowledge, the Au21(S-tBu)15 is the smallest thiolate-gold cluster comprising a complete cuboctahedron unit. A comparison of the relative stabilities of two Au21 clusters ACS Paragon Plus Environment

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is made by means of density functional theory (DFT) calculations. It is found that with the S-tBu protection, the currently discovered cluster structure is more stable than the recently reported one by 0.23 eV. When the protection ligand is switched to S-Adm, the recently reported Au21 cluster structure is more stable by 0.34 eV. Such energy differences indicated that the van der Waals interactions of ligands played a key role in stabilizing different cluster configurations. 5, 8-9 3.2 The Structural Evolution Map of Smaller Sized Fcc-Aum(SR)n Clusters (m ≤ 52) The determination of atomic structure of Au21(S-tBu)15 has provided important clues to disclose some genetic structural evolution patterns of the smaller sized thiolate-protected gold nanoclusters. To date, although the atomic structures of a number of fcc-structured thiolate-protected gold nanoclusters have been resolved.5, 6 The underlying geometric patterns and the structure evolution tendency of these clusters are less explored. By analyzing the gold-gold bond lengths, we find that the Au10-core in the Au21(S-tBu)15 can be divided into a triangle Au3 unit plus a bi-tetrahedral Au7 unit (Figure 1c). We note that the similar geometric patterns of gold cores have been seen in the Au20(SPh-tBu)1631,32 and Au23(SC6H11)16‒.28 In the two clusters, their gold cores contain either a tetrahedron Au4 unit or a tetrahedron unit plus two triangle Au3 units. From comparing three gold core structures, the unique Au10-core in the Au21(S-tBu)15 cluster can be viewed as a key intermediate that bridges the gold core evolution from Au20(SPh-tBu)16 to Au23(SC6H11)16‒, as shown in Figure 2 (route B). Inspired by this finding, we further explored the gold core configurations of other reported fccstructured gold clusters. As shown in Figure 2, it is found that the gold cores in fcc-structured clusters including 4e Au24(SCH2Ph-tBu)20, 8e Au28(S-c-C6H11)20 and Au28(TBBT)20, 12e Au30(S-tBu)18, 12e Au36(TBBT)24, 16e Au44(TBBT)28 and 20e Au52(TBBT)32 etc. displayed a genetic geometric evolution pattern that the Au-cores are evolved via continuous growth of triangle-Au3 or tetrahedron-Au4 unit. We noticed the analysis of the structure of Au28 was reported in the previous work.52 Häkkinen et al. divided


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the 14-atom gold core into two Au7 units and a further decomposition of the Au7 units into joined tetrahedra is mentioned, but the property of tetrahedron network was not discussed. Four kinds of gold core evolution patterns are found: 1) Gold core evolved via symmetric increase of triangle-Au3 or tetrahedron-Au4 unit at one side or both sides of a tetrahedron Au4 unit (paths A and C) or a bi-tetrahedral Au7 unit (paths B and E). 2) Periodic growth of gold cores along one-dimensional as shown in routes D. The corresponded gold clusters are Au28(TBBT)20, Au36(TBBT)24, Au44(TBBT)28, and Au52(TBBT)32. 3) Growth of unique tetrahedron-ring (route F). As shown in Figure 2, the gold cores in Au22(SR)18,53 Au34(S-c-C6H11)2222 and Au40(o-MBT)2420 clusters demonstrated a unique tetrahedron-ring growth surrounding a central bi-tetrahedral Au7-unit. 4) A collectively symmetric growth of triangle-Au3 and tetrahedron Au4 units, as seen in gold cores of Au44(SR)28 and Au92(SR)44 clusters. (Figure 2) In Figure 2, we show that the triangle-Au3 and tetrahedron-Au4 associated gold core evolutions is not only found in the fcc structured clusters. For the recently reported non-fcc gold clusters including the pairs of Au44(2,4-DMBT)2654 and Au38(SCH2CH2Ph)2455, Au25(SCH2CH2Ph)18- 13,14 and Au38(PET)2456, as well as Au40(o-MBT)24-20 and Au49(2,4-DMBT)27- 57, their gold core structures demonstrated a similar evolution pattern. In all kinds of evolution pathways, every increase of a tetrahedron or triangle gold units in the core, the cluster increases two free valence electrons (2e).


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Figure 2. The explanation of structural evolution of Au-cores in fcc-structured clusters and non fcc-structured Aum(SR)n clusters. Olive, turquoise and magenta balls denoted the Au atoms in the golden kernel of polyhedron representation. The [m, n] represents certain Aum(SR)n cluster. The Au-core structures surrounded by the dash frames are predicted based on the discovered structural evolution route.


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3.3 A Structural Evolution Map for the Smaller Sized Aum(SR)n Clusters By displaying both gold core and ligand structures, a structural evolution map is outlined for the smaller sized fcc-structured gold clusters. As illustrated in Figure 3, the smaller sized fcc-structured gold clusters demonstrate structural regularities not only in the gold cores, the motif layer also shows certain evolution trends.

Figure 3. The structural evolution map of smaller sized fcc-structured gold clusters. Clusters in the red dash frames are predicted from the structural regularities.

The established cluster structural evolution map shown in Figure 3 is meaningful for understanding the nucleation growth process of fcc-structured gold nanoclusters. Following the structural regularities demonstrated in the structural evolution map, all fcc-structured gold clusters seemed grow from a smallest 2e intermediate species containing a tetrahedron gold core such as


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Au15(SR)1358. The clusters with different sizes can be considered as the “intermediates” at the different nucleation growth stages in different kinds of structural evolution pathways. Taking the evolution path F as the example, the unique gold atom assembly pattern in the recently reported Au34(SR)2222 may be considered an intermediate at certain nucleation stage of the Au40(SR)24 cluster, as the Au20(SR)18, Au34(SR)22 and Au40(SR)24 clusters demonstrated a periodic growth of ligand shell and gold cores, e.g. Au22(SR)18 + 6Au + 2SR → Au28(SR)20 + 6Au + 2SR → Au34(SR)22 + 6Au + 2SR → Au40(SR)24 (Figure 4). Similar periodic growth of cluster structures is also seen in other fcc clusters such as Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32, as well as Au28(SR)20, Au29(SR)19, and Au30(SR)18, as shown in Figure S3.

Figure 4. Gold atom framework evolution in the fcc gold clusters involved in the cluster evolution routes F displayed in Figure 2. The SR groups in the fcc clusters are not displayed for clarity. The core and ligand shell gold atoms are distinguished by different colors. (pink balls, ligand shell gold atoms; indigo balls, core gold atoms; turquoise balls, core gold atoms that increases or decreases in a sequence of clusters.)

3.3 Application of Structural Evolution Map to Predict New Magic Sizes and Cluster Structures The structural evolution map displayed in Figure 3 may serve as a protocol to find out new magic sized clusters and new cluster structures. At present, the structural evolution map indicates two obvious missing linkages. The first one is a blank between the 6e Au21(S-Adm)15 and 2e clusters (evolution route


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A), which should be a 4e cluster comprising a Au7-core (Figure 2). The second missing linkage is in the evolution route F (Figure 3) between Au22(SR)18 and Au34(SR)22 clusters. 4e Au17(SR)13 Cluster A 4e Au17(SR)13 cluster is derived from the Au/S frameworks of both Au21(S-Adm)15 and Au21(S-tBu)15 clusters by simply reducing one triangle Au3 unit in the core and then properly patch the ligand shell. As shown in Figure 5a, the Au21(S-Adm)15 can be viewed as a combination of two mirror structures, each of which is an Au17(SR)13. It is also possible to derive the Au17(SR)13 cluster from the Au21(S-tBu)15 by reducing a triangle Au3 unit and an [Au2(SR)3] motif and then adding an Au-SR fragment between SR group and kernel gold atom (Figure 5a). We note that this is the first report of 4e cluster comprising a novel Au7 gold core that is comprised of a tetrahedron and a triangle Au3 unit.

(a)

(b)

Figure 5. (a) Derive the gold-sulfur framework of a new 4e Au17(SR)13 cluster from two Au21 clusters.; (b) The structure of the Au17(SR)13 cluster.

From a ligand exchange reaction of Au17Ag1(S-C6H11)14 cluster, recently an Ag/Au alloy cluster with a molecular formula of Au16Ag1(S-Adm)13 (17 metal atoms and 13 SR groups) was synthesized,59 which has the same number of metal atom and SR ligand to the presently predicted 4e Au17(SR)13 cluster. The currently predicted Au17(SR)13 cluster has exactly the same metal atom/sulphur framework


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to that of Au16Ag1(S-Adm)13 cluster59. These results validated the proposed structural regularities of fccstructured thiolate-gold clusters as described in the structural evolution map. 8e Au28(SR)20 Cluster From the structural regularity of Au22(SR)18, Au34(SR)22 and Au40(SR)24 clusters (displayed in Figure 3), a new isomer structure of Au28(SR)20 is also derived. It has been known that the Au28(SR)20 has two isomer Au28S20 frameworks.25,26 As shown in Figure 6, in comparison to two known structure forms of Au28(SR)20 cluster (R = Ph-tBu and R = C6H11), this new isomer structure has the same 14-atom gold core but much different ligand shell configurations. The energy comparison indicates that three Au28 isomer structures have very similar electronic energies, suggesting this new isomer structure form is viable in future synthesis.

Figure 6. The atomic structures and relative energies of three of Au28(SR)20 clusters. When calculating the relative energies, the R group is replaced by methyl group. A, the structure of the new isomer of Au28(SR)20 cluster; B, the structure of Au28(S-c-C6H11)20 cluster; C, the structure of Au28(TBBT)20 cluster.

Moreover, we propose that the structural regularities described in the structure evolution maps (Figures 2-4) enable us to find out more undiscovered magic cluster sizes and cluster structures which are not limited in the smaller size region (m ≤ 52). To date, the finding of new magic sizes and determination of atomic structure of Aum(SR)n clusters locating in a small-to-medium size region (52≤ m ≤ 92) is still a grand challenge. Here, with the aid of the proposed structure evolution map, it possible to construct medium sized cluster structures manually based on the TTG mechanism and the disciplines


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of the ligand shell structure evolution (Scheme 1). The exploration of stable fcc-structured cluster species in this size region will make the structural evolution map more complete and enrich our knowledge on the structural evolution of fcc-structured gold nanoclusters. Scheme 1. Illustration of the basic idea to find out the possible structural models of fcc-structured gold nanoclusters according to the TTG mechanism of gold cores. The ligand shells are constructed according to the ‘divide-and-protect’ concept.60-62

3.5 Topological Structure and Electronic Structure Relationship of fcc-Structured Gold Clusters Finally, on basis of the TTG evolution pattern of gold cores, a genetic topological structure and electronic structure relationship is further suggested for the fcc-structured gold clusters. As the fccstructured gold nanoclusters can be viewed as the combination of 2e units, according to the Mulliken methodologies,63,64 their valence orbitals may be described in terms of linear combinations of molecular orbitals (MOs) of 2e units. In Figure 7 and Figure S5-S6, the electronic density diagrams of MOs in the Au-6s bands of various charged [AuN-core]q+ species are plotted based on the extended Hückel molecular orbital (EHMO) calculations using the Au 6s and 5d orbitals only. It is seen clearly that (in Figure 7 and Figure S6): i) the delocalized MOs in the Au-6s1 band all can be viewed as the linear combinations of molecular orbitals of seperated 2e units; ii) the delocalized MOs demonstrate obvious nodel planes,


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which can be assigned as certain superatomic 1S, 1P, 1D states, etc.. iii) the relationship between the delocalized electronic states and the MOs of local geometric units such as Au42+ and Au3+ therefore can be expressed as eq. (1). ∑ ∑

eq. (1)

Following the eq. (1), the delocalized MOs formed by the Au-6s1 atomic orbitals are the linear combinations of the MOs of local geometric units such as Au42+ and Au3+. Taking the clusters involved in the evolution route A as the example (Figure 7), every increase of a 2e unit in the cluster corresponds to addition of a new 1s2 orbital (analogous to a He atom). The linear combinations of two, three, and four 1s2 orbitals in the 4e, 6e and 8e clusters lead to formation of 1S2|1P2, 1S2|1P4 and 1S2|1P4|1D2 superatom like electronic configurations, respectively. We note that for the much irregularly shaped 8e Au23(SR)16‒ cluster, due to the anti-symmetric distribution of two triangle Au3 units in the core, the linear combination of four 2e units results in a 1S2|1P4|1D2 superatom like orbital configuration, rather than the close-shell 1S2|1P6. For comparison, due to the spherically arrangment of four 2e units, the 8e Au28(SR)20 shows a perfect 1S2|1P6 configuration.

Figure 7. The electronic density diagrams of occupied EHMOs of Au-cores in 2e, 4e, 6e and 8e clusters.


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For other fcc-structured clusters with the magic 2Ne valence electrons, as shown in Figure S6, their delocalized valence electronic states all can be addressed from the linear combinations of molecular orbitals of 2e units as well. For the 4e and 6e clusters, because of the 2e units only take linear or triangular arrangment, these clusters demonstrate the same 1S2|1P2 or 1S2|1P4 superatomic orbital configuration, respectively. While for the clusters possessing valence electron number above 8, caused by the diverse packing styles of 2e units, their MO diagrams become much more complicated and some of them do not show well-defined superatomic orbital shapes.

CONCLUSIONS The atomic structure of a new 6e Au21(S-tBu)15 cluster is determined from the single crystal XRD characterizations. On basis of the total structure determination of the Au21(S-tBu)15 cluster, we outlined a structural evolution map that describes the structural evolution patterns of all known fcc-structured thiolate-protected gold clusters. Some basic structural evolution patterns underlying these fcc gold clusters are summarized, including a triangle-Au3 and tetrahedron-Au4 associated gold-core structure evolution pattern (TTG mechanism) and the symmetric and periodic growth of gold core and ligand layers. According to the TTG mechanism, a topological structure-electronic structure relationship is also proposed. It is found that the delocalized superatom-like electronic states in any fcc-structured gold clusters can be described by the linear combination of molecular orbitals of the 2e Au42+ and Au3+ units within the gold cores. The established cluster structure evolution map is a useful tool to explore new magic sized clusters and cluster structures. In the present work, a new 4e Au17(SR)13 and a new isomer structure of Au28(SR)20 are predicted. According to the structural evolution map, we believe there are a number of fcc-structured ligand protected gold clusters will be discovered in the future.


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Supporting Information The details of crystal structure of the Au21(S-tBu)15 and the electronic density of various Aum(SR)n clusters are given. These materials are available free of charge via the internet at http://pubs.acs.org.

Corresponding Author [email protected] (Y. P.); [email protected] (M. Z.) Notes The authors declare no competing financial interests. Author Contributions L. X. and S. Y. contributed equally. ACKNOWLEDGMENT Y. P. acknowledge financial support by National Natural Science Foundation of China (21773201 and 21422305) and the project of innovation team of the ministry of education (IRT_17R90). M. Z. is supported by NSFC (21372006, U1532141, 21631001), the Ministry of Education, the Education Department of Anhui Province, and 211 Project of Anhui University. References (1) Zhang, H.; Teo, B. K. Stereochemical and electronic evidence of icosahedricity and polyicosahedricity. Inorg. Chimica Acta 1997, 265, 213-224. (2) Vigderman, L.; Khanal, B. P.; Zubarev, E. R. Functional Gold Nanorods: Synthesis, Self-Assembly, and Sensing Applications. Adv. Mater. 2012, 24, 4811−4841. (3) Loh, N. D.; Sen, S.; Bosman, M.; Tan, S. F.; Zhong, J. A.; Nijhuis, C. A.; Král, P.; Matsudaira, P.; Mirsaidov, U. Multistep nucleation of nanocrystals in aqueous solution. Nat. Chem. 2017, 9, 77– 82. (4) Häkkinen, H. The gold–sulfur interface at the nanoscale. Nat. Chem. 2012, 4, 443–455. (5) Tlahuice-Flores A., Whetten R. L., Jose-Yacaman. M. Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters. J. Phys. Chem. C 2013, 117, 20867-


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On the Structure and Electronic Structure Evolution of Thiolate

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