Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold

Article Cite This: Acc. Chem. Res. 2019, 52, 23−33

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Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold Nanoclusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Yong Pei,* Pu Wang,† Zhongyun Ma,† and Lin Xiong

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College 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, Xiangtan, Hunan 411105, China CONSPECTUS: Understanding the structure and structure−property relationship of atomic and ligated clusters is one of the central research tasks in the field of cluster research. In chemistry, empirical rules such as the polyhedral skeleton electron pair theory (PSEPT) approach had been outlined to account for skeleton structures of many main-group atomic and ligandprotected transition metal clusters. Nonetheless, because of the diversity of cluster structures and compositions, no uniform structural and electronic rule is available for various cluster compounds. Exploring new cluster structures and their evolution is a hot topic in the field of cluster research for both experiment and theory. In this Account, we introduce our recent progress in the theoretical exploration of structures and evolution patterns of a class of atomically precise thiolate-protected gold nanoclusters using density functional theory computations. Unlike the conventional ligand-protected transition metal compounds, the thiolate-protected gold clusters demonstrate novel metal core/ligand shell interfacial structures in which the Aum(SR)n clusters can be divided into an ordered Au(0) core and a group of oligomeric SR[Au(SR)]x (x = 0, 1, 2, 3, ...) protection motifs. Guided by this “inherent structure rule”, we have devised theoretical methods to rapidly explore cluster structures that do not necessarily require laborious global potential energy surface searches. The structural predictions of Au38(SR)24, Au24(SR)20, and Au44(SR)28 nanoclusters were completely or partially verified by the later X-ray crystallography studies. On the basis of the analysis of cluster structures determined by X-ray crystallography and theoretical prediction, a structural evolution diagram for the face-centered-cubic (fcc)type Aum(SR)n clusters with m up to 92 has been preliminarily established. The structural evolution diagram indicates some basic structural and electronic evolution patterns of thiolate-protected gold nanoclusters. The fcc Aum(SR)n clusters show a genetic structural evolution pattern in which each step of cluster size increase results in the formation of another Au4 tetrahedron or Au3 triangle unit in the Au core, and every increase of a structural unit in the Au core leads to an increase of two electrons in the whole cluster. The unique one- or two-dimensional cluster size evolution, the isomerism of the Au−S framework, and the formation of a double-helical and cyclic tetrahedron network in the fcc Aum(SR)n clusters all can be addressed from this evolution pattern. The summarized cluster structural evolution diagrams enable us to further explore more stable cluster structures and understand their structure−electronic structure−property relationships. and Au25(SR)18− (SR = SCH2CH2Ph).4−6 Unexpectedly, in the Au102 and Au25− NCs, a brand new form of metal−ligand interaction was found. In each NC, some gold atoms were organized into a symmetrical nucleus of Au(0) atoms. The rest of the gold atoms were united with the mercaptan salts to form a new type of oligomeric gold−thiolate protecting units (e.g., −RS−Au−SR− and −RS−Au−SR−Au−SR−, also called staple motifs) capping the Au(0) core. The breakthroughs in total structure determination of Au102 and Au25− have opened up broad prospects for the study of the structure and structure−property relations of thiolate-capped Au NCs. Over the past 20 years, the scope of gold nanoclusters

1. INTRODUCTION The understanding of physical and chemical disciplines of structure and bonding patterns of atomic and ligated clusters is a major endeavor of cluster research. Over the past 20 years, the synthesis and characterization of atomically precise thiolate ligand (SR)-protected gold nanoclusters (NCs), denoted as Aum(SR)n or RS-AuNCs, have attracted great research interest because of their molecular-like physicochemical properties, intrinsic chirality, luminescence, etc.1−3 However, for a long time the structure determination of these atomically precise Aum(SR)n nanoclusters lagged seriously, which slowed down their practical applications because of a lack of understanding of their structure−property relationships. In 2007 and 2008, two major breakthroughs were made on the total structure determinations of Au102(SR)44 (SR = SC7O2H5) © 2018 American Chemical Society

Received: July 31, 2018 Published: December 14, 2018 23

DOI: 10.1021/acs.accounts.8b00385 Acc. Chem. Res. 2019, 52, 23−33

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Accounts of Chemical Research Scheme 1. 2n Valence Electron Array of Aum(SR)n Nanoclusters

Figure 1. (a) Proposed structural model of Au38(SR)24. The R groups are not displayed. (b) Comparison of theoretically simulated and experimental UV−vis absorption spectra. Reprinted from ref 7. Copyright 2008 American Chemical Society.

protected by thiol compounds has been greatly expanded, and they constitute one of the most widely studied classes of ligandprotected metal clusters. The substantial progress in precise structural determinations indicated that the Aum(SR)n NCs, regardless of their shape and composition, adopt a common structural pattern, i.e., they are composed of a symmetrical Au core and a certain number of oligomeric SR(AuSR)x (x = 0, 1, 2, 3, ...) protection motifs. In addition to the ubiquitous structural pattern, an interesting 2n valence electron (2ne) array is found, i.e., 2e, 4e, 6e, 8e, 10e, 12e, 14e, 16e, 18e, 20e, 34e, 58e, etc. (Scheme 1). The unique 2ne array and the novel cluster structures encouraged us to explore the fundamental structural and electronic rules of Aum(SR)n NCs. In this Account, we summarize our research progress on the structural and electronic rules of Aum(SR)n nanoclusters. Three aspects are discussed, including (1) proposing and validating structural rules of Aum(SR)n nanoclusters from known crystal structures, (2) how to use these rules to understand and predict cluster structures based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and (3) the structural and electronic rules of a class of face-centered-cubic

(fcc)-type Aum(SR)n clusters (with m up to 92). We hope that these discussions will enable researchers not only to understand the fundamental structural and electronic rules of Aum(SR)n NCs but also to explore more magic-sized clusters and new metal core−ligand structure patterns.

2. “INHERENT STRUCTURE RULE” OF Aum(SR)n AND STRUCTURAL STUDY OF Au38(SR)24 The “inherent structural rules” of Aum(SR)n clusters were inspired by structural analysis of Au25− and Au102. Both clusters can be divided into an ordered Au core (13-atom icosahedron and 79-atom decahedron, respectively) and certain numbers of protection motifs such as −RS−Au−SR− and −RS−Au−SR− Au−SR−. The structures of the two NCs can be therefore written as Au13[Au2(SR)3]6 and Au79[Au(SR)2]19[Au2(SR)3]2, respectively. On basis of the structural analyses, we tentatively suggested a structural partition formula for Aum(SR)n clusters, written as Aua+a′[Au(SR)2]b[Au2(SR)3]c[Au3(SR)4]d..., where a, a′, b, c, d, ... are integers.7 This proposed formula stated that any atomically precise Au−thiolate clusters are composed of a symmetrical Au(0) nucleus (Aua+a′) and certain number of level24


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Accounts of Chemical Research

Figure 2. (left) Theoretical structural models of Au20(SR)16 and Au24(SR)20 (R = CH3). (right) Structures of Au20(TBBT)16, Au24(SeC6H5)20, and Au24(SCH2Ph-t-Bu)20. The R groups are not displayed. Reprinted from ref 21. Copyright 2014 American Chemical Society.

Figure 3. (a) (left) Au26 core and assembly patterns of staple motifs of Au44(SR)28. (right) Chiral structure of Au44(SR)28. (b) Comparison of simulated UV−vis absorption curves of dianionic and neutral Au44(SR)28 (R = CH3) with earlier experimental results (red curve). (c) Electron densities of Auq cores in Au44, Au36, and Au28 NCs from adaptive natural density partitioning analysis.31 Polyhedra denote the Au4 tetrahedron units. Color code: red = S, others = Au. The R groups are not shown. Reprinted from ref 29. Copyright 2013 American Chemical Society.

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may be reversed. This was also the first theoretical investigation of ligand steric effects on cluster structures.

x oligomeric motifs (SR[Au(SR)]x with x = 1, 2, 3, ...), where a′ is the number of gold atoms in the core that are linked to the S terminal of staple motifs. We note that such a structural scheme is similar to the concept of “divide and protect” proposed by Häkkinen et al.8 The hypothetical structural rule was first applied to the structure of Au38(SR)24, which was synthesized in 1997, but the cluster structure was unknown for a long time.9 By application of the proposed structural formula, a low-energy isomer with a structural division of [Au]5+18[Au(SR)2]3[Au2(SR)3]6 was obtained, as shown in Figure 1a. The UV−vis absorption curve of the structural model (R = CH3) computed using TDDFT showed good agreement with the experimental curve (Figure 1b). Moreover, compared with the earlier theoretical structure model, this new structural model also showed very low electronic energy.10−14 Our proposed structural model was then slightly modified by Lopez-Acevedo et al.,15 who considered rotation of three [Au2(SR)3] motifs on one side of the icosahedral Au13 unit, which led to a D3-symmetric ligand shell. In 2010, Jin and co-workers reported the crystal structure of Au38(SC2H4Ph)24.16 The predicted structure is in good agreement with the experimental crystal structure.

4. Au44(SR)28: A CHIRAL fcc GOLD CLUSTER AND DOUBLE-HELICAL TETRAHEDRON CHAINS IN THE METAL CORE In 2012−2013, three fcc Aum(SR)n clusters were reported for the first time by means of the ligand strategy. In Au36(SR)24 (R = Ph-t-Bu)25 and Au28(SR)20 (R = Ph-t-Bu)26 as well as Au23(SR)16− (R = C6H12),27 the unprecedented fcc gold kernels were seen for the first time. For comparison, the previously known ligand -stabilized Au NCs were generally made of non-fcc complete or incomplete icosahedral or decahedral cores. The fcc cores found in Au23−, Au28, and Au36 NCs provided new insight into the structures and evolution pathways of Aum(SR)n nanoclusters. By learning the structural features of these fcc clusters, we carried out a structural exploration of Au44(SPh)282−. The Au44(SPh)282− is an 18e cluster that was first reported by Price and Whetten.28 Inspired by the crystal structures of Au23−, Au28, and Au36, an fcc-type Au26 core was built for Au44(SPh)282− by considering a structural partition formula of Au26[Au(SR)2]2[Au2(SR)3]8,29 as shown in Figure 3a. Several intriguing features are found in this fcc structural model. First, a D2-symmetric Au−S network is formed, which indicates that Au44 is a chiral cluster. Second, the proposed Au44 cluster structure model shows a one-dimensional growth of double-helix tetrahedron chains in the gold cores started from Au28(SR)20 and Au36(SR)24 NCs. In addition to the structural properties, the simulated UV−vis spectra suggest the Au44 cluster is neutral rather than anionic as previously found (Figure 3b). The stability and elongated cluster structure of the neutral Au44 cluster can be attributed to its 16e 1S2|1P6|1D8 shell (Figure 3c). We note that almost at the same time, Zeng et al.30 reported the synthesis of neutral Au44(SR)28 and made a similar structural prediction.

3. EXTENDED STAPLE MOTIFS AND LIGAND STERIC EFFECTS The correct structural prediction of the Au38 nanocluster partially rationalized the structure partition formula. Nonetheless, as more and more crystal structures were determined, some unexpected motifs were found. During the study of the structures of Au20(SR)1617 and Au24(SR)20,18 the possible existence of extended ligand motifs, e.g., Au3(SR)4, Au4(SR)5, and Au5(SR)6, were considered because of their low Au/SR ratios (1.25 and 1.20, respectively),19,20 which are between those of the metal-core-free homoleptic [Au(SR)]x polymers (1.00) and the clusters with closely packed cores (e.g., Au25(SR)18−). The lowest-lying isomers of Au20(SR)16 and Au24(SR)20 (R = CH3) obtained by DFT energy calculations are shown in Figure 2. Both of them were predicted to contain a bitetrahedral Au8 core. However, the later X-ray crystallography study of Au20(TBBT)16 (TBBT = SPh-t-Bu) NC indicated a largely different structure. In Au20(TBBT)16, an unprecedented Au8(SR)8 ring motif was found.21 The inconsistency between the theoretical prediction and the experimental structure of the Au20 NC results from neglecting the ring motifs. Therefore, in order to study the structures of the small-sized gold nanoclusters, the [Au(SR)]n ring motifs should be added into the structural partition formula, especially for clusters whose Au:SR number ratio is close to 1.0. Now, it has been known that the steric effect of the SR ligand significantly affects the Au−S framework structure of gold nanoclusters. For Au24 NCs, two structural forms were found. From Figure 2, Au24(SCH2Ph-t-Bu)2022 has a Au8 core protected by four identical Au4(SR)5 motifs, but Au24(SePh)2023 is composed of two sets of Au5(SeR)6 and Au3(SeR)4 motifs. It is of interest that our predicted Au24 cluster structure agreed very well with that of Au24(SePh)20 but was different from that of the Au24(SCH2Ph-t-Bu)20. By means of dispersion-corrected DFT (DFT-D) calculations, Jiang and co-workers24 concluded that the relative stabilities of the structural forms of Au24(SCH2Ph-tBu)20 and Au24(SePh)20 are affected by the steric effects of the substituent group R. Depending on the type SR protecting groups, the relative stabilities of the two kinds of structural form

5. STRUCTURAL EVOLUTION OF fcc-TYPE CLUSTERS 5.1. One-Dimensional Evolution of Au28, Au36, Au44, Au52, and Au76 Clusters

The total structure determination of Au44(TBBT)28 was achieved in 2016.32 The theoretical Au−S skeleton model is completely consistent with the experimental crystal structure.29 Moreover, Au44(TBBT)28 filled the missing link in the evolution of the Au28(SPh-tBu)20, Au36(TBBT)24, and Au52(TBBT)3233 clusters: they form a neat magic series Au20+8N(SR)16+4N with N = 1−4.32 Theoretically, this magic series can be further expanded, i.e., Au60(TBBT)36 (N = 5), Au68(TBBT)40 (N = 6), Au76(TBBT)44 (N = 7), and Au84(TBBT)48 (N = 8), etc.34 Shortly after crystal structure determination of Au52(TBBT)32 (N = 4), 32 Takano et al. reported the synthesis of Au76(SC9H11O2)44.35 The Au76 cluster exhibited a strong adsorption band at 1340 nm, which was distinctly different from the optical spectra of other thiolate-protected gold nanoclusters. Takano et al. proposed that the strong nearinfrared (NIR) absorption of Au76(SR)44 originated from the unique anisotropic Au core, because the linear Au clusters and nanorods showed strong adsorption peaks in the NIR region due to the photoexcitation of an electron confined in a onedimensional box.36 We proposed an alternative structural model by considering that Au76(SR)44 belonged to the Au20+8N(TBBT)16+4N series with N = 7, as shown in Figure 4.37 The energy computations 26


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Figure 4. (left) Comparison of the UV−vis optical curve simulated by TD-DFT and earlier experimental results. (right) Electron density diagram of the HOMO and LUMO of Au76(SH)44. Reprinted from ref 37. Copyright 2016 American Chemical Society.

formation a tetrahedron ring in Au40(SR)24. Starting from the structure evolution of Au22, Au34, and Au40, a Au28(SR)20 (N = 2) cluster is predicted,41 which has the same bitetrahedral Au core as two known Au 28 clusters (Au 28 (S-c-C 6 H 11 ) 24 and Au28(TBBT)24) but different ligand shell configurations. The DFT calculations with inclusion of dispersion energy corrections (DFT-D) confirmed that the new Au28 isomeric structure is a low-energy isomer relative to the two crystal structures.

indicated that the new structure model is a lower-energy isomer structure in comparison with the structure models constructed on the basis of a Au49 core.35 The intense near-IR absorption band of Au76(SR)44 centered at 1340 nm can be explained from its anisotropic Au core as well as the synergistic effects of the ligand shell. In addition to studying the structure and properties of the Au76 cluster, extension studies on the geometric structures, thermodynamic stabilities, and electronic structures of Au20+8N(SR)16+4N cluster with N up to 15 were carried out.34 A greatly enhanced absorption in the NIR region extending to the biologically important NIR window was seen in these linear clusters with N > 7.

5.3. Cubic Nanocrystal-like Au68 Nanocluster and Two-Dimensional Structure Evolution of fcc-Aum(SR)n

X-ray crystallography is the most commonly used method to determine the structures of nanoclusters. However, obtaining single crystals of RS-AuNCs for X-ray crystallography still remains challenging. In 2014, Azubel et al.42 explored the atomic structure of a 68 Au atom NC by single-particle transmission electron microscopy (SP-TEM) and DFT calculations. The advantage of the SP-TEM-centered approach is that the structural determination does not necessarily require the preparation of a single crystal. Nevertheless, to date SP-TEM can only determine the positions of heavy (herein, Au) atoms, and the TEM electron beam can alter the positions of several surface Au atoms during the measurement. In order to find the optimal structure model of the Au68 cluster, Gao and co-workers43 took the atomic positions of the Au atoms from the SP-TEM measurement and re-examined the pattern of the ligand shell on the basis of the inherent structure rule of the “divide and protect” scheme. DFT computations predicted four distinct low-energy isomers whose structures are all lower in energy by 3 to 4 eV than that reported previously by Azubel et al. The most stable isomer is composed of a quasi-fcc spherical-like Au core with four Au(SR)2 and eight Au2(SR)3 motifs. On the basis of these experimental and theoretical research advances, we proposed a new structural form for the Au68 NC. The NC has a molecular formula of Au68(SR)36.44 Unlike the quasi-fcc cores reported previously, the new Au68 species has a perfect 5 × 5 × 5 cubic crystalline fcc gold kernel, in contrast to similar-sized clusters such as Au68(SR)34 and Au67(SR)352−.45,46 A lower-energy fcc structure, Au68(SR)32, was also obtained by removing four SR groups of Au68(SR)36.44 The resulting structure is more stable than the quasi-fcc spherical ones by at least 0.47 eV. Notably, the crystalline-like gold atom arrange-

5.2. Structural Prediction of Au22(SR)18 and the Tetrahedron-Ring Growth Pattern

As discussed above, the notable structural feature of 4e Au20(TBBT)16 and Au24(SCH2Ph-t-Bu)20 is that the protecting motifs are longer than those of the Au25−, Au38, and Au102 NCs. In particular, the unique [Au8(SR)8] ring motif is formed in Au20(TBBT)16. In 2015, a Au22(SR)18 nanocluster was synthesized.38 Like Au20(SR)16 and Au24(SR)20, it also has four free valence electrons (4e) and can be regarded as an intermediate nanocluster between Au20(SR)16 and Au24(SR)20. We theoretically predicted an optimal structure of Au22(SR)18 by employing the structural partition formula and DFT calculations.39 The structure model of Au22(SR)18 contained a bitetrahedral Au7 core. In the ligand shell, a [Au6(SR)6] ring and three Au3(SR)4 staple motifs are formed to protect the Au7 core. In the proposed structure model, a Au13 cuboctahedron is formed in the inner part of Au22(SR)18 (Figure 5a) that consists of the gold atoms from the Au7 kernel and the [Au6(SR)6] complex. It is of interest that the predicted Au−S framework of Au22(SR)18 shows close connections with those of Au34(S-cC6H11)2240 and Au40(o-MBT)2433 (o-MBT = mercaptobenzothiazole). In view of the Au−S skeleton structures and numbers of Au and SR groups, Au22(SR)18, Au34(SR)22, and Au40(SR)24 constituted a new class of homogeneous clusters, denoted as Au16+6N(SR)16+2N with N = 1, 3, and 4, respectively, as shown in Figure 5b. Unlike the one-dimensional continuous evolution of the double-helix tetrahedron chains in Au28, Au36, Au44, and Au52, the metal cores of Au22, Au34, and Au40 show novel tetrahedron ring growth. In each step of cluster size increase, a Au(SR)2 unit in the [Au6(SR)6] complex is consumed and a bitetrahedral Au7 unit forms, which eventually leads to the 27


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Figure 5. (a) Structure model of Au22(SR)18 and comparison of the theoretical and experimental UV−vis absorption curves of Au22(SCH3)18. The R groups are not shown. (b) Au core structural evolution of Au22(SR)18, Au34(S-c-C6H11)22, and Au40(o-MBT)24. The missing cluster, Au28(SR)20, is predicted. Reprinted from (a) ref 39 and (b) ref 48. Copyright 2015 and 2018, respectively, American Chemical Society.

following: (1) the Au cores of fcc-type Aum(SR)n nanoclusters show a Au3 triangle or Au4 tetrahedron growth pattern; (2) with every increase of a new triangle or tetrahedron unit in the Au(0) core, the nanocluster increases by two valence electrons (2e); (3) a class of gold nanocrystals including Au20(SR)16, Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32, Au68(SR)36, and Au76(SR)44 are seen, which exhibit one- or two-dimensional layer-by-layer crystal facet growth (Figure 6); (4) the nucleation growth of gold nanoclusters originates from the smallest 2e clusters; (5) isomerism of the Au−S framework is seen in several clusters, such as Au21(SR)15 (two isomers with SR = S-Adm or St-Bu) and Au28(SR)20 (two experimentally determined isomers with R = C6H11 or Ph-t-Bu). The 2e increase has been verified by recent mechanistic studies of size growth reactions from Au25− to Au44 nanoparticles by Xie and co-workers,49 who found that the size growth of gold nanoparticles included monotonous LaMer growth and volcanic

ments observed in the SP-TEM measurement were wellreproduced by the new theoretical model. Furthermore, we found that the experimentally determined or theoretically predicted structures of Au44(TBBT)28, Au92(TBBT)44,47 and Au68(SR)36 showed two-dimensional layer-by-layer nanocrystal growth (Figure 6). From Au44(SR)28 to Au68(SR)36 and then to Au92(SR)44, six Au3 triangle units and two Au4 tetrahedron units increased in the metal cores in each cluster size increase step. 5.4. Structural Evolution Diagram of fcc-Type Aum(SR)n Nanoclusters

By collecting the cluster structures reported experimentally and theoretically, we preliminarily constructed the structure evolution diagram of fcc-type Aum(SR)n nanoclusters with m up to 92, as displayed in Figure 7.48 It was found that the theoretically predicted and experimentally reported cluster structures were compatible very well and demonstrated genetic structural and electronic evolution patterns, including the 28


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Figure 6. View of the one- and two-dimensional crystal facet growth patterns and metal core evolutions in the Au28(SR)20, Au36(SR)24, and Au44(SR)28 series and the Au52(SR)32, Au68(SR)36, and Au92(SR)44 series. Color code: red = S, green = Au. The R groups are not shown. Reprinted from ref 44. Copyright 2017 American Chemical Society.

Figure 7. Structural evolution diagram of fcc Aum(SR)n clusters with n up to 92. Color code: red = S, green = Au. The R groups are not shown. Reprinted from ref 48. Copyright 2018 American Chemical Society.

Whetten and co-workers.53 All of these structural models suggest a tetrahedral core in the 2e NCs.

aggregation growth, which is driven by the continuous 2e boosting of the valence electron number of gold nanoparticles.48 The stable 2e NCs including Au11(SR)9 and Au15(SR)13 were detected.49,50 We recently carried out an elaborative structural search of Aum(SR)n clusters with m and n ranging from 5 to 12.51 The theoretical analysis showed that the initial nucleation and growth of the core and protection units can be divided into three evolution paths with the increasing numbers of gold atoms and SR groups, namely, nuclei growth, nuclei dissolution, and staplemotif growth. The global-minimum structure of the 2e NCs such as Au11(SR)9 was predicted. It is similar to the theoretical model of the 2e Au15(SR)13 cluster predicted by Jiang et al.52 and

5.5. The “Gold Atom Insertion, SR Group Elimination” Mechanism

From Figure 7, the growth of Au3 units is a very popular Au core evolution pattern in several cluster size increase paths. However, the mechanism of such a core size increase has rarely been studied. In order to address the formation of Au3 triangle units, we suggested a “gold atom insertion, SR group elimination” mechanism.54 The mechanism involves two steps: first an exterior Au(0) atom is inserted between a pair of neighboring SR 29


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Figure 8. Illustration of size increase steps from (a) Au28(SR)20 to Au30(SR)18 and (b) Au20(SR)16 to Au21(SR)15 according to the “gold atom insertion, thiolate group elimination” mechanism. Color code: red = S, others = Au. The R groups are not shown.

Figure 9. Electron density diagrams of the valence MOs of the Au cores in 2e, 4e, 6e, and 8e clusters. The electron density was calculated from the extended Hückel molecular orbital method using the Au 6s and 5d orbitals only. Reprinted from ref 48. Copyright 2018 American Chemical Society.

groups in the ligand shell, and then the resulting μ4-SR group leaves to form a new Au3 triangle unit. As shown in Figure 8, the structural evolution from 8e Au28(SR)20 (R = C6H11) to 12e Au30(SR)1855 (R = t-Bu) as well as the recently reported Au20(SR)16 isomeric nanocrystal structure56 all can be explained from this mechanism. Recently, from the ligand exchange reaction of the Ag3@Au15(SR)13 cluster,57 we succeeded in the synthesis and determination of the cluster structure of a Ag@ Au16(SR)13 alloy cluster. The analogue of Ag@Au16(SR)13, namely, Au17(SR)13, has a Au7 core made of a gold tetrahedron unit and a gold triangle unit, which is considered to be an intermediate cluster between 2e species and 6e Au21(S-t-Bu)15.

arise from the magic 2n electrons but are not completely discussed. Unlike the SAN and GUM models, the superatom complex model (SAC)61 explains the electronic stabilization effects on the basis of the jellium model. Since the fcc-type clusters are made of 2e building blocks such as tetrahedral Au42+ and triangular Au3+, their delocalized valence molecular orbitals (MOs) should be able to be expressed as linear combinations of the separated cluster MOs of the 2e building blocks, as shown in eq 1:

6. STRUCTURE−VALENCE ELECTRON COUNTING−ELECTRONIC STRUCTURE RELATIONSHIPS From the structural evolution diagram, a simple structure− valence electron counting−electronic structure relationship can be derived. An important feature of the evolution of fcc clusters is that each increase of a new Au3 triangle or Au4 tetrahedron unit in the core results in a 2e boost of the clusters. This valence electron counting has been rationalized by the superatom network model (SAN)58 and grand unified model (GUM).59,60 However, although these localized electronic structure models explain the valence electron counting, the electronic stabilities

where the ci and cj are the linear combination coefficients and ϕ(Tri) and ϕ(Tetra) are the MOs of the triangular and tetrahedral 2e units, respectively. The construction of delocalized MOs from the MOs of fragment 2e units provides a more efficient way to understand the localized and delocalized electronic properties of ligated gold NCs, particularly for the fcc-type clusters, which commonly have nonspherical shapes. For instance, Au23(SR)16−, Au28(SR)20, and Au25(SR)18− are three well-known 8e NCs. Among them, the Au23− and Au28 nanoclusters are fcc-structured, while Au25− is nearly spherical. Although the three nanoclusters have the same number of valence electrons (8e), their valence electronic structures are different. On the basis of eq 1, Au25− and Au28 have

N1

φ=

i=1

30

N2

∑ ciϕ(Tri) + ∑ cjϕ(Tetra) j=1

(1)


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Accounts of Chemical Research a perfect 1S2|1P6 superatom electronic shell formed by the interactions of four Au42+ tetrahedron units, while Au23− shows a quite different valence electron density that can be roughly assigned as 1S2|1P4|1D2, caused by its zigzag arrangement of the 2e building blocks (Figure 9). In our recent studies,48 we have examined this relationship for all of the already reported fcc clusters as well as some non-fcc clusters. The electron density analysis results confirmed the fragment MO linear combination scheme described in eq 1.

Zhongyun Ma received his Ph.D. from the Institute of Chemistry of the Chinese Academy of Sciences under the supervision of Prof. Zhigang Shuai, and he joined the chemistry faculty of XTU in 2017. His research interests are computational studies of ligand-protected gold clusters and new energy materials. Lin Xiong is a Ph.D. candidate at XTU under the supervision of Prof. Yong Pei. His current research interest is on the structural evolution and properties of ligand-protected metal nanoclusters.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21773201, 21422305, and 21503182) and the Project of Innovation Team of the Ministry of Education (IRT_17R90).

CONCLUDING REMARKS In this Account, we have summarized our recent progress in the theoretical exploration of the structural and electronic evolution of thiolate-protected gold clusters in the sub-2 nm region. Some inherent structural rules such as the “divide and protect” scheme and the Au3 triangle and Au4 tetrahedron associated gold core structure growth (TTG) patterns have been discussed. Following these genetic structural rules, we have carried out extensive theoretical studies in search of cluster structures of a variety of Aum(SR)n nanoclusters. Together with experimental X-ray crystallography, we have outlined a structural evolution diagram for the smaller-sized fcc Aum(SR)n clusters. This structural evolution map has allowed researchers to peer into the fundamental evolution rules of Aum(SR)n nanoclusters and their structure−valence electron counting−electronic structure relationships. Furthermore, we also note that additional research efforts are still needed on the structural determination of transition sizes, such as the NCs located in the region between 0e homoleptic Aum(SR)m complexes and core-stacked ones (m > n) and the small-to-medium sized NCs (52 < m < 92), as shown in Scheme 1. The understanding of these NC structures will deepen a thorough understanding of the structural evolution of these nanoscale gold−thiolate nanomaterials. In addition to cluster structure explorations, efforts are also necessary to discuss in depth the ligand effects, catalytic properties, alloy properties, and surface plasmon resonances, which will spur the practical applications of these precious atomically precise NCs.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Pei: 0000-0003-0585-2045 Zhongyun Ma: 0000-0002-8727-8608 Author Contributions †

P.W. and Z.M. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Yong Pei obtained his Ph.D. in 2006 at Nanjing University. He did postdoctoral research in Prof. Xiao Cheng Zeng’s group at the University of NebraskaLincoln during 2006−2010. He is now a Professor at Xiangtan University (XTU), and his research interests focus on theoretical studies of the structure and optical, catalytic, and magnetic properties of noble metal nanoparticles. Pu Wang recived her Ph.D. from XTU under the supervision of Prof. Yong Pei. Her current research interests are focused on the structrue, properties, and catalytic applications of gold nanoclusters. 31


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Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold

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