Decahedral Structures

2.3.2.1 Case of Au102(SR)44

Au102(SR)44 is one of the thiolate-protected decahedral gold nanocluster structures determined by X-ray crystallography [33]. This nanocluster possesses 58 nominal valence electrons based on the counting rule of the number of Au (i.e., 102) subtracting the number of ligands/charge (i.e., 44 and 0) [34]. The single crystal was, however, grown from a trace amount of sample, and so detailed studies on the Au102(SR)44 properties had to wait for the subsequent development of large-scale synthesis with the atomic level of precision [35].

The synthesis was performed in aqueous phase under the basic condition with p-mercaptobenzoic acid (pMBA) as the surface ligand. The crystal structure of Au102(SR)44 is made up of a Au79 core with a shell-by-shell structure starting from the innermost Au7 decahedron (Fig. 2.11A) [33]. The decahedral Au7 is covered by an additional layer of Au32 to form Au39 (Fig. 2.1 IB). The Au39 shows 10 (111) facets on its top and bottom in total and five {100} facets on the waist (Fig. 2.11C). The {100} facets are further each covered by two atoms to form Au49, and the Au49 geometry is known as Marks decahedron [13]. The unique decahedral structure exhibits five truncated corners compared to the regular decahedron, and the truncated decahedron shows reduced surface energy according to the classical theory. The 10 {111} facets are each covered by a triangle of three atoms to form Au79 (Fig. 2.11C). Then, the top and bottom halves of the Au79 core are each protected by five monomeric staples, each bridging two triangular facets (Fig. 2.11D-F). The five monomeric staples either on the top or on the bottom are placed in the symmetry of fivefold rotation. Surface protection on the waist of Au102 is made up of nine monomeric and two dimeric staples, which are arranged in the symmetry of twofold rotation along the lateral direction.

Crystal structure o

Figure 2.11 Crystal structure of Au102(SR)44. (A) Au7/ (B) Au39, and (C) Au79core structures of Au102 in shell-by-shell illustration. (D) Surface protection of the Au79 core by staple motifs. (E, F) Side and top views of surface staple motifs on Au102. Carbon and hydrogen atoms are omitted for clarity [36].

2.3.2.2 Case of Au103S2(SR)41

Au103S2(SR]41 was synthesized by LEIST from Au99(SPh)42 [36] using 2-naphthalenethiolate as exchanged protecting ligands [37]. According to the electron counting rule, Au103S2(SR]41 also possesses 58 valence electrons (103 -41-2x2], noting that one sulfido takes two electrons since its valence is two. The transformation from Au99 to Au103 is completed after 48 h at 80 °C in a mixed solvent of toluene and 4-tert-butyltoluene. Interestingly, the crystal structure of Au103S2(SR]41 is made up of exactly the same Marks decahedral Au79 core in Au102 [33, 37] (see Fig. 2.11В]. Both Au103 and Au102 have 58 valence electrons, and the observation of the same core structure in these two nanoclusters could provide atomic insights into the structural rules of the nanoclusters that share the same number of valence electrons. On the surface of Au103, the top and bottom halves of each Au79 core are protected by five monomeric staples, similar to AU102 (Fig- 2.12; see Fig. 2.11D-F for details). However, remarkable differences are observed on the waist of Au103 [37]. In contrast to nine monomeric and two dimeric staples on Au102, Au103 exhibits six monomeric, one dimeric, and two trimeric staples on its waist. The trimeric staple motifs contain one /r3-sulfido at the position indicated by the small arrow in top middle panel in Fig. 2.12. The overall core/ staple geometry shows the symmetry of twofold rotation along the lateral direction.

Comparison of surface structure between AuS(SR) and Au(SR)44 [37]

Figure 2.12 Comparison of surface structure between Au103S2(SR)41 and Au102(SR)44 [37].

The 2-naphthalene carbon tails on the Au103 exhibit surface patterns via C-H—tt and tt—tt interactions (Fig. 2.13) [37]. For example, a dimeric pattern is formed by two naphthalene groups via the edge-to-face C-H—tt interaction with an angle of 51° (Fig. 2.13 I,II). A cyclic tetramer pattern is further constructed from a pair of dimeric patterns with a distance of 3.02 or 2.76 A (Fig. 2.131,11). Such tetrameric units of naphthalene ligands resemble the "herringbone pattern” [38] that is often observed in crystal structures of polycyclic aromatic hydrocarbons. In contrast, the bottom of Au103 shows tt—tt stacking between four naphthalene ligands with an angle of 97° with 3.40 ± 0.10 A spacing (Fig. 2.13 III).

Crystal structure of Au

Figure 2.13 Crystal structure of Au103. Total structure of Au103 (upper left), Marks decahedral Au79 core (upper middle) and surface staple motifs (upper right). Intracluster ligand interactions on the surface (lower left). The anatomy of the tetrameric herringbone structure via С-Н---П interactions (I and II) and the parallel tetramer via staggered n---n interactions (III). Color labels: magenta = Au in the kernel, blue = Au in the staple motifs, yellow = S, white = H, and all the other colors are used for C in different positions [37].

2.3.2.3 Case of Au130(SR)50

The synthesis of Au130(SR)50 is performed by the size-focusing method [39]. The preparation starts with polydisperse nanoclusters protected by pora-methylbenzenethiolate QoMBT). Then the polydisperse precursor is treated with excess thiol at high temperature. In this reaction, the position of the substituent methyl group plays a critical role in controlling the size/structure of final nanoclusters [39]. For example, when meta- or ortho-methyl benzenethiol (mMBT or oMBT for short) is used for surface protection, the product is, respectively, Au104(mMBT)41 or Au40(oMBT)24 (Fig. 2.14) [39]. After the work-up procedure, the as-synthesized Au130(pMBT)50 is crystallized by vapor diffusion of acetonitrile into the nanocluster solution in toluene [40].

Tuning the magic size of thiolate-protected gold nanoclusters through the careful choice of the position of the substituent methyl group [39]

Figure 2.14 Tuning the magic size of thiolate-protected gold nanoclusters through the careful choice of the position of the substituent methyl group [39].

The crystal structure of Au130(SR)50 is made up of a Au10s core with a shell-by-shell structure starting from the innermost AuL center (shown in green in Fig. 2.15) [40]. The inner core is covered by a Au12 (in magenta) shell, forming Au13, which is called Ino decahedron (Fig. 2.15A) [41]. The decahedral Au13 is covered by a shell of Au42 (in gray) to form Au55 (Fig. 2.15B). The Au55 shows 10 (111) facets on its top and bottom in total and five {100} facets on the waist (Fig. 2.15C). The {100} facets are further covered by four atoms on each facet, and the 10 {111} facets are instead covered by a triangle of three atoms on each facet to form Au10s (Fig. 2.15C). The surface on Au10S is then protected by 25 monomeric staple motifs.

Crystal structure of Au(pMBT). Top and side views of the core structure made up of (A) Au, (B) Au, and (C) Aus. (D) Surface protection of the core by 25 monomeric staples [40]

Figure 2.15 Crystal structure of Au130(pMBT)50. Top and side views of the core structure made up of (A) Au13, (B) Au55, and (C) Au10s. (D) Surface protection of the core by 25 monomeric staples [40].

The monomeric staple motifs are placed with the symmetry of fivefold rotation (Fig. 2.16) [40]. In each row, five monomeric motifs form a pentagonal circle, making up five rows of stripe pattern (Fig. 2.16A). The diameter of the middle circle (shown in blue) is the largest, while the ones on the top and bottom possess the smallest diameter. The overall structure of 25 monomeric motifs has the dimension of 1.9 x 1.6 nm with a barrel-like shape, where each row of pentagonal circle resembles metal hoops to bind the wooden pieces for a barrel. The core and staple structure of Au130(SR)50 shows quasi-Ds symmetry with chirality due to the difference in orientation of staple motif circles (Fig. 2.16B,C).

Surface protection pattern of Au(pMBT). (A) Five pentagon ripples of 25 monomeric staple motifs. (B) Top and (C) side views showing fivefold rotation symmetry [40]

Figure 2.16 Surface protection pattern of Au130(pMBT)50. (A) Five pentagon ripples of 25 monomeric staple motifs. (B) Top and (C) side views showing fivefold rotation symmetry [40].

2.3.2.4 Case of Au246(SR)80

The synthesis of Au246(SR)80 is performed by the size-focusing methodology, followed by solvent fractionation for further purification [15]. During the first size-focusing process, the size distribution of the product is converged to Au246 with ~90% purity under thermal treatment with excess thiol. The following solvent fractionation successfully removes the minor product of smaller nanoclusters for molecular purity of Au246. Crystallization of Au246 is performed by vapor diffusion of acetonitrile into the nanocluster solution of toluene [15].

The crystal structure of Au246 is made up of a decahedral Au206 core with a shell-by-shell structure starting from an innermost Au7

decahedron, which is covered by a layer of Au32 to form Au39 (Fig. 2.17A) [15]. The Au39 is further covered by an additional layer of Au77 to form Au116 (Fig. 2.17В]. The Au116 shows 10 (111) facets on its top and bottom in total, as well as five {100} facets on the waist (Fig. 2.17С]. The five {100} facets are further covered by six atoms on each facet to form an additional layer on it, and the ten {111} facets are instead covered by a triangle with six atoms on each facet to form Au2o6 (F'g- 2.17С]. Then, the top and bottom halves of the Au206 core are protected by five surface protecting dimeric staples by bridging across the triangular facets (Fig. 2.17D). The six-atom triangles (i.e., {111} of Au206] are connected by 10 bridging thiolates in total for top and bottom. Additional five monomeric staple motifs are used for the connection between six-atom triangles on top/bottom {111} and six-atom squares on the middle {100}. On the waist of Au206, six- atom squares on {100} are connected by two monomeric staples at each junction.

Crystal structure of Au(pMBT). Top and side views of the core structure made up of (A) Au, (B) Au, and (C) Au6- (D) Surface protection of the core by staple motifs [15]

Figure 2.17 Crystal structure of Au246(pMBT)80. Top and side views of the core structure made up of (A) Au39, (B) Au116, and (C) Au206- (D) Surface protection of the core by staple motifs [15].

The surface of Au246 shows aesthetic patterns of carbon tails (Fig. 2.18} [15]. At the top and bottom sites of Au246, pMBT ligands form pentagonal circles, which is defined as а-rotation (Fig. 2.18A,B). The waist of Au246 possesses alternating parallel pairs of pMBT ligands, which is defined as р-parallel (Fig. 2.18A,C). The overall structure shows chirality induced by these arrangements of pMBT ligands on the surface.

Surface patterns of pMBT ligands on Au6-

Figure 2.18 Surface patterns of pMBT ligands on Au246- (A) Overall structures as well as pole and waist. (B) The а-rotation arrangement of pMBT at the top and bottom. (C) The р-parallel packing of pMBT at the waist [15].

Face-Centered Cubic Structures

2.3.3.1 Case of Au146(SR)57

Aui46(SR)57 is synthesized via kinetically controlled reduction in aqueous phase [42], similarly to the case of Au102(SR)44, where both nanoclusters are protected by pora-mercaptobenzoic acid. The crystal structure of Au146(SR)57 was solved by electron diffraction and synchrotron X-rays [42]. The core of Au146(SR)57 is made up of Au109 with the twinned fee structure. The innermost Aux (shown in green) and Au12 (in magenta) form a Au13 anti-cuboctahedron (Fig. 2.19A). The Au13 is then covered by a layer of Au42 (in gray) to form Au55 (Fig. 2.19B). The Au55 shows eight (111) facets and six {100} facets (Fig. 2.19C). The {100} facets are then covered by four atoms on each facet. For the two {100} planes in the front side (top, Fig. 2.19C), two pairs of Au2 are additionally placed on the four-atom layer. The {111} facets are instead covered by incomplete triangles with five atoms on each facet, except for the two {111} facets on the back {111} covered by one atom each at the center of the triangular

{111} facet. The incomplete additional shell makes up a Au119 core (Fig. 2.19C). The surface on Au119 is then protected by staple motifs as shown in Fig. 2.19D.

Crystal structure o

Figure 2.19 Crystal structure of Au146(pMBA)57. Front and back views of the core structure made up of (A) Au13, (B) Au55, and (C) Au119. (D) Surface protection of the core by staple motifs. Reprinted with permission from Ref. [42], Copyright 2017, American Chemical Society.

2.3.3.2 Case of Au279(SR)84

The synthesis of Au279(SR)84 is performed either by size focusing followed by chromatographic isolation [43] or by LEIST from the larger sized Au333(SR)79 nanocluster [16]. Crystallization is performed by vapor diffusion of acetonitrile into the nanocluster solution of toluene or vapor diffusion of pentane into CH2C12 solution of the nanocluster [16, 43]. The core of Au279 is made up of Au211 with the fee structure. The innermost Au4 (shown in dark blue) and Au12 (in green) form a Au13 cuboctahedron (Fig. 2.20A). The Au13 is covered by a layer of Au42 (in violet) to form Au55 (Fig. 2.20B). The Au55 is further covered by another layer of Au92 to form Au147 (Fig. 2.20C). The Au147 shows eight {111} facets and six {100} facets (Fig. 2.20D). Each {100} facet is covered by a square of nine atoms. On the other hand, each {111} facet is covered by a triangle of six atoms, with some distortion on some facets. The incomplete additional shell makes up the Au211 core (Fig. 2.20D). The surface on Au211 is then protected by staple motifs as shown in Fig. 2.20E.

Crystal structure of Au7(TBBT)4. The fee core structure made up of (A) Au, (B) Au, and (C) Au, and (D) Au. (E) Surface protection of the core by staple motifs [43]

Figure 2.20 Crystal structure of Au279(TBBT)84. The fee core structure made up of (A) Au13, (B) Au55, and (C) Au147, and (D) Au249. (E) Surface protection of the core by staple motifs [43].

 
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