1Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA.2Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.3Department of Theoretical and Computational Molecular Science, Institute forMolecular Science, Myodaiji, Okazaki, 444-8585, Japan. 4Elements Strategy Initiative forCatalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan.*These authors contributed equally to this work.†Corresponding author. E-mail: [email protected]
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Magic-sized clusters, as the intermediate state between molecules and nanoparticles, exhibit critical transitions ofstructures and material properties. We report two unique structures of gold clusters solved by x-ray crystallography,including Au40 and Au52 protected by thiolates. The Au40 and Au52 clusters exhibit a high level of complexity, with thegold atoms in the cluster first segregated into four-atom tetrahedral units—which then coil up into a Kekulé-like ringin the Au40 cluster and a DNA-like double helix in Au52. The solved structures imply a new “supermolecule” origin forrevealing the stability of certain magic-sized gold clusters. The formation of supermolecular structures originates inthe surface ligand bonding–induced stress and its propagation through the face-centered cubic (FCC) lattice. More-over, the two structures reveal anisotropic growth of the FCC lattice in the cluster regime,which provides implicationsfor the important roles of ligands at the atomic level. The rich structural information encoded in the Au40 and Au52clusters provides atomic-scale insight into some important issues in cluster, nanoscale, and surface sciences.
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INTRODUCTION
Metal clusters containing tens to hundreds ofmetal atoms constitute animportant regime that bridges molecular materials (typically <1 nm)and nanoparticle materials (typically >3 nm). Because of their interme-diate state, metal clusters often exhibit distinct properties in catalysis,optics, electronics, andmagnetism, and thus hold great potential as func-tional materials (1–4). On the other hand, clusters might share somefeatures with molecules and nanoparticles: they can be viewed as theminiature of nanoparticles, or the “maxiature” of molecules. Importantmolecular and nanoscale information may be encoded inside thecluster, and many fundamental issues (for example, the origin of magicsizes and the structural evolution pattern) and real-world applicationsrequire the knowledge of atomic structures of clusters.
The thiolate-protected gold clusters referred to as Aun(SR)m haveemerged as a new frontier in cluster research, not only because of theirhigh stability, atomic precision, and wide size tunability (5) but also forthe rich gold thiolate chemistry—which is broadly used in surface func-tionalization of nanoparticles and two-dimensional (2D) films (6). It isthese merits of Aun(SR)m clusters that bestow on them the potential toshed light on mysterious issues in the cluster, nanoscale, and surfacesciences. The recent research efforts in structural characterization of ul-trasmall Aun(SR)m clusters with n≤ 38 and n≥ 102 have revealed somecommon features: (i) the inner cores of clusters are constructed fromvarious polyhedra and their derivatives; (ii) the surfaces of clusters areprotected by the Au-SR oligomeric staple or ring motifs (5, 7); and (iii)their stability is often interpreted by the shell-closing “superatom”model with stable electronic structure [for example, Au25(SR)18
− asan 8e superatom and Au102(SR)44 as a 58e superatom] (8, 9), which issimilar to other ligand-protected metal clusters and gas-phase clusters(10–15). However, certain Aun(SR)m clusters do not conform to thesuperatom category, such as Au38(SR)24 (16). The polyhedron-based
kernels, Au-SR surface oligomeric motifs, and superatom model havebeen serving as the basis for analysis and prediction of Aun(SR)m clusterstructures (7, 17–19).
However, despite the advances in Aun(SR)m cluster research, severalbasic and critical issues remain to be addressed. First, the origin of thestability of magic sizes in solution-phase clusters is still unclear. Thisissue lies in the center of cluster research. Although the well-knownsuperatom model can explain the stability of a few magic sizes (8–14),it cannot accommodate the newly discovered magic sizes. Second,the crystal structural information of medium-sized gold clusters (be-tween Au38 and Au102) is still missing (8, 16) except for some theoreticalwork (18–21), and most of the solved structures are concentrated on thesmaller end (5). This knowledge gap precludes the understanding of thegrowth pattern of clusters. Third, the thiolate bonding and patterningstructures on the gold crystalline facets remain largely unknown. Wehave recently revealed the formation of aesthetic “helical-stripe” pat-terns of -S-Au-S-motifs on the curved surface ofAu133(SR)52 nanoparticles(22); however, it remains to be seen whether the same protecting modeexists on gold crystalline surfaces.
In an effort to gain insight into the above issues, we herein reporttwo unique structures of Aun(SR)m clusters, that is, Au40(SR)24 andAu52(SR)32. The rich structural information encoded in these twoclusters provides atomic-scale insight into somemajor issues, includingthe origin of magic sizes in clusters, the shape control of nanocrystals,and the self-assembled monolayers of thiolates on gold crystallinefacets. Significantly, unlike the previously reported superatom orclose-shell clusters (8–14), Au40 and Au52 exhibit a high level ofcomplexity, with the gold atoms in the cluster first segregated intofour-atom tetrahedral units, which then coil up into a Kekulé-like ringin the Au40 cluster and a DNA-like double helix in Au52. Thesestructures are better viewed as “supermolecules” rather than super-atoms. Also, the Au40 and Au52 structures reveal the construction ofclusters in a manner similar to the anisotropic growth of nanocrystals,reflecting the early stage of “shape control” in the cluster regime.More-over, the anisotropic growth patterns of Au40 and Au52 lead to the ex-posure of the extended Au(111) and Au(100) facets on clusters, whichprovide valuable crystallographic information on thiolate bonding andpatterning on gold crystalline facets.
The Au40 and Au52 clusters were synthesized by a two-step “size-focusing” method (1). The key aspects involve careful control of reac-tion kinetics and a proper selection of protecting thiolates (SR); seeMaterials and Methods for details. The Au40 was synthesized with2-methylbenzenethiolate (o-MBT), formulated as Au40(o-MBT)24(23), whereas theAu52was synthesizedwith 4-tert-butylbenzenethiolate(TBBT), formulated as Au52(TBBT)32. Both Au40 and Au52 are highlystable because they were thermodynamically selected through harshsize-focusing processes (1, 23). Their structures were determined bysingle-crystal x-ray crystallography (tables S1 to S4). Both clusters arechiral, and the unit cells of Au40 andAu52 single crystals comprise a pairof enantiomers for each cluster (Figs. 1, A and B, and 2, A and B).
Supermolecular view of Au40(SR)24 and Au52(SR)32 clustersThe structure of Au40(SR)24 has long been a puzzle since the first isola-tion of this magic size from a mixture with the Au38(SR)24 cluster (24).Density functional theory (DFT) predicted bi-icosahedron–based corestructures for Au40(SR)24 (18, 19), which are similar to the structure ofAu38(SR)24. Here, we found that the overall structure of Au40(o-MBT)24is a unique oblate shape (that is, a hexagonal prism; Fig. 1, A and B),which is drastically different from the previous theoretical prediction.
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An anatomy of the structure reveals a 25–gold atom kernel resemblinga snowflake (Fig. 1C) and nine surface-protecting staples (Fig. 1, D andE). Significantly, we found that the gold atoms in the kernel aresegregated into eight tetrahedral Au4 units, evidenced by the Au-Aubond length differences (vide infra). Two of the tetrahedral units formthe central bi-tetrahedral antiprism (Fig. 1C, green), and the remain-ing six tetrahedra form a Kekulé-like external ring with alternativelyfacing-up-and-down arrangement of the tetrahedra (Fig. 1C, blue). TheKekulé ring is protected by six Au(SR)2 monomer staples (Fig. 1D),whereas the central bi-tetrahedron is protected by three Au3(SR)4 tri-mer staples (Fig. 1E). The kernel adopts achiral D3d symmetry, but theoverall Au40S24 framework has chiral D3 symmetry (Fig. 1F), which isdue to the rotative arrangement of the surface staplemotifs. The discov-ery of the Kekulé-like Au40 structure indicates molecular complexity inthis cluster, as opposed to a simple superatom-dimer as predicted on thebasis of the Au38(SR)24 structure (18, 19).
The segregation of tetrahedral Au4 units and the formation of anelegant superstructure are also observed in another magic-sized cluster,Au52(TBBT)32 (Fig. 2). This indicates certain generality of the super-molecular complexity in clusters. The Au52 cluster has a 32–gold atomkernel, which is segregated into 10 tetrahedral units (Fig. 2C). The tetra-hedra are assembled into a double helical superstructure resembling the
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Fig. 1. Total structure of the Au40(o-MBT)24 cluster. (A) Unit cell com-
prising two enantiomers. (B) Mirror symmetry of the enantiomers. (C)Snowflake-like Au25 kernel with tetrahedral units coiled up into a Kekulé-likesuperstructure. (D) Six monomeric staples protecting the Kekulé ring. (E)Three trimeric staples protecting the central Au7 bi-tetrahedron. (F) OverallAu40S24 framework. Blue/green, Au atoms in the kernel; orange, Au atoms inthe staples; yellow, sulfur; gray, carbon; pink, hydrogen.
Fig. 2. Total structure of the Au52(TBBT)32 cluster. (A) Unit cell compris-ing twoenantiomers. (B)Mirror symmetry of the enantiomers. (C) Twohelical
pentatetrahedral strands forming the double helical kernel. (D) Four mono-meric staples protecting the waist of the kernel. (E) Four dimeric staplesprotecting the top and another four protecting the bottom of the kernel.(F) Overall Au52S32 framework. Blue/green, Au atoms in the kernel; orange,Au atoms in the staples; yellow, sulfur; gray, carbon; pink, hydrogen.
double-strandedDNA.Within each helix, five tetrahedra are connectedby vertex sharing (Fig. 2C). This double helix is protected by fourAu(SR)2 monomer staples at the waist (Fig. 2D) and eight Au2(SR)3dimer staples at the top and bottom (Fig. 2E). Both the helical kerneland the overall Au52S32 framework (Fig. 2F) adopt the same D2
symmetry, hence a chiral cluster. The left- and right-handed helicalkernels are shown in fig. S1.
The segregation of tetrahedral Au4 units in both Au40 and Au52clusters is manifested in the Au-Au bond length differences. A plot ofAu-Au bond lengths according to the different Au positions shows four“steps” (Fig. 3). In the Au40(o-MBT)24, the Au-Au bonds within theeight tetrahedral units are the shortest (average, 2.78 and 2.76Å, respec-tively; Fig. 3A, group I), whereas the Au-Au bonds connecting the cen-tral bi-tetrahedron and external tetrahedra are longer, with an averageof 2.90 Å (Fig. 3A, group II). TheAu-Au bonds between the Kekulé-likekernel and the surface staple motifs are among the longest, with anaverage length of 3.01 Å for trimeric staples and 3.16 Å for the mono-meric staples (Fig. 3A, groups III and IV). The case of Au52(TBBT)32 issimilar, with Au-Au bonds within each tetrahedron-coiled helix beingshorter than those between the two helices (Fig. 3B). It is this in-homogeneous distribution of Au-Au bond lengths that distinguishesthe Au4 units in the clusters.
The electronic structures of bothAu40(o-MBT)24 andAu52(TBBT)32clusters were analyzed by DFT to provide some insights into the magicstability of both clusters. The entire ligands (that is, without simplifyingas–SCH3)were included in the calculations considering their importantroles in stabilizing the two structures. For the Au40(o-MBT)24 cluster, weindeed found that the distribution of the highest-occupied molecular or-bital (HOMO) follows a six-lobe hexagonal pattern (fig. S2, green circles),that is, a “supermolecular” picture. Each lobe is located in the cor-responding Au4 tetrahedron. The phase of the HOMO alternates (fig.S2, red and blue lobes) in line with the facing up/down of Au4 units intheKekulé ring. Furthermore, themolecular orbitals of theAu4units con-nect to the p orbitals of the ligands in the sixmonomer staples, exhibitingan external “blade”-like configuration (fig. S2). For the Au52(TBBT)32cluster, the HOMO exhibits some features of the segregated Au4 unitsand a helical pattern (fig. S3), but these are less prominent than thefeatures in planar Au40(o-MBT)24. Further analysis of the atomiccharges of Au40(o-MBT)24 indicates that the Au-S bond has an ionicbond character (table S5) and the core Au atoms stabilize the ionic bond.Similar features are also found in Au52(TBBT)32. Overall, the symmetricAu4 patterns plus interactions with the surface ligands stabilize themulti-tetrahedron superstructures.
From the valence electron count perspective, previous work showedthe Au4 cluster complexes [for example, Au4(PR3)4
2+ and Au4I2(PR3)4]having two electrons (25). Thiolate is monovalent and consumes oneAu 6s valence electron in bonding, so the remaining free 6s electronsin Au40(SR)24 are 40 − 24 = 16e, in agreement with the eight tetrahedralunits inAu40(SR)24. TheAu52(SR)32 cluster has 10 tetrahedra, hence 20e(that is, 52 − 32 = 20). The Au4 tetrahedron can be taken as a 2e super-atom, but the Au4-based Kekulé-like and helical patterns call for asupermolecular picture, rather than the simple superatomic model (9).The elegant patterns observed in Au40 and Au52 are not existent insmaller clusters such as Au28 and Au36 (26–28), although the Au4unit was identified in these small clusters. Also, the previous theore-tical work proposed an Au4 network picture for thiolate-protectedAu18, Au20, and Au24 clusters (29), but no regular pattern was involved.Notably, the tetrahedral configuration is also favored in gas-phase gold
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clusters, for example, the Au20 tetrahedral cluster (15). This super-molecular picture from the Au40 and Au52 clusters provides a morecomplex origin of stable magic sizes in gold clusters than the superatompicture, and it reveals the assembly of “atoms” into “molecules” in thecluster regime. The supermolecular picture is expected to accommodatemore magic sizes in the future.
Anisotropic growth of the clusters and origin ofsupermolecular structuresAn intriguing question is what drives the formation of the super-molecular patterns of tetrahedra in the Au40 and Au52 clusters. Toanswer this question, we first need to have the overall pictures of thetwo structures without considering the differences in the Au-Au bondlengths. Specifically, when viewed as an entity, the 40 gold atoms in theAu40(o-MBT)24 can all be fit into a face-centered cubic (FCC) latticewith some distortions (Fig. 4, A to D). Three layers of gold atoms arestacked along the [111] direction in an a-b-cmanner, forming a hexag-onal prism. For the Au52(TBBT)32, 48 of the 52 gold atoms can also be
Fig. 3. Au-Au bond length distributions in the clusters. (A) Au40(o-MBT) . (B) Au (TBBT) . Green/blue/magenta/orange, gold; yellow,
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fit into the FCC lattice, with the atoms assembled in the [100] direction,forming a tetragonal rod enclosed by {100} facets (Fig. 4, E to H). Cor-respondingly, the surface-protectingAux(SR)x + 1 staples are decomposedinto simple bridging thiolates (fig. S4). This alternative view of the Au40and Au52 clusters reveals the anisotropic layer-by-layer constructionmode of magic-sized gold clusters, similar to the anisotropic growthof 2D nanoprisms and 1D nanorods in shape-controlled nanocrystals(30, 31). It also provides atomic-scale insight into the effect of selectivesurface passivation in tailoring the particle shape (32).
The segregation of Au4 tetrahedral units in the FCC lattice is due tothe surface-protecting thiolates or, more specifically, the directional co-valent bonds between thiolates and gold atoms. The bonding of the sul-fur atom in the thiolate adopts the tetragonal configuration, with twoorbitals bonded to two gold atoms, one bonded to the carbon tail, andone for the lone-pair electrons (Fig. 5, A and B). The three atoms in theAu-S-Au bridge form a triangle, with an Au-S bond length of ~2.33 Åand an average Au-S-Au angle of 87.4 ± 3.6° in Au40 and 92.6 ± 7.3° inAu52. Because of the geometric restriction of the Au-S-Au triangle, thetwo Au atoms underneath the sulfur bridge are pushed apart, with anaverageAu-Aubond length of 3.21Å inAu40 and 3.35Å inAu52, longerthan the bulk Au-Au bond length of 2.88 Å (Fig. 5, A and B).
This thiolate bonding effect can be easily viewed on the 2D surface ofthe clusters, as reflected in the inhomogeneous distribution of Au-Aubond lengths (Fig. 5, C and D). As discussed above, the overall aniso-tropic growth of the Au40 and Au52 clusters leads to the exposure of
Fig. 4. Anisotropic growth of the gold FCC lattice into a hexagonal prism in Au40(o-MBT)24 and a tetragonal rod in Au52(TBBT)32. (A to C) Model of a43–gold atom hexagonal prism composed of three layers (green, orange, and blue) stacked along the [111] direction in an a-b-cmanner (the three arrows
indicate the three missing gold atoms in the real Au40 cluster). (D) Au40(o-MBT)24 as a hexagonal prism. (E to G) Model of a 48–gold atom tetragonal rodcomposedof six layers stacked along the [100] direction. (H) Au52(TBBT)32 as a tetragonal rod. The four gold atomsnot included in the FCC lattice are indicatedby arrows.
Fig. 5. Thiolate bonding and patterning on the crystalline facets
of Au40 and Au52. (A) Tetragonal configuration of sulfur atom of o-MBTin the Au40 cluster. (B) Tetragonal configuration of sulfur atom of TBBTin Au52. (C) Twelve–gold atom {111} facets on the Au40 cluster. (D) Twelve–gold atom {100} facets on the Au52 cluster. Orange/magenta, Au; yellow,S; gray, C.
the extended gold crystalline facets, that is, the 12-atom {111} facetsin theAu40 hexagonal prism (Fig. 5C) and the 12-atom {100} facets in theAu52 tetragonal rod (Fig. 5D). Every two gold atoms on the facet arebridged by one thiolate ligand (Fig. 5, C and D). With the thiolatesbridging onto the gold crystalline facets, the Au-Au distance underneaththe bridging S is expanded tomeet the coordination requirements of theS bridge. On the other hand, the Au-Au pairs adjacent to the bridgingthiolates are squeezed together by the forces from different directions,resulting in shorter Au-Au distances (Fig. 5, C and D, bottom). Such ademonstration of inhomogeneous distribution of Au-Au distances in2D surface can be applied in the 3D nanoclusters because the thiolatebonding force can further penetrate into a few layers of gold atoms (fig.S5). Together, the surface thiolate bonding causes a stress, and this stresspropagates into the anisotropic-shaped FCC cluster to induce segregationof Au4 tetrahedra and their further coiling up into hierarchical patterns.
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CONCLUSION
Here, we have presented two novel structures of thiolate-protectedgold clusters. The implications of the Au40 and Au52 structures are mani-fold. First, the two clusters illustrate the supermolecular complexity,and such a view can explain more magic-sizes of clusters than the earlysuperatom model. The supermolecular picture is reminiscent of thefact that an unlimited number of stable molecules can be assembledfrom a limited number of atoms. Second, they reveal an anisotropicgrowth of FCC lattice at the atomic level, as reflected in the 2D hex-agonal prism of the Au40 and the 1D tetragonal rod of the Au52 struc-ture. The two structures imply the important roles of ligands in theanisotropic growth at the atomic level. Third, the new structures adda new dimension in constructing highly stable clusters in an anisotropicfashion other than the isotropic shell-by-shell growth (22) or polyhedron-fusion mode (24, 33, 34). The new supermolecular picture is expected toadvance further understanding of the cluster structure and stability, andthe Au40 and Au52 cluster materials will provide more insights into shapeand surface-dependent properties and applications.
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MATERIALS AND METHODS
ChemicalsThe following chemicals were used: tetrachloroauric(III) acid(HAuCl4·3H2O, 99.99%metals basis, Sigma-Aldrich), tetraoctylammo-nium bromide (TOAB, 98%, Fluka), o-MBT (97%, TCI), TBBT (97%,Alfa Aesar), sodium borohydride (NaBH4, 99.9%, Sigma-Aldrich);methanol [high-performance liquid chromatography (HPLC) grade,99.9%, Sigma-Aldrich], pentane (HPLC grade, 99.9%, Sigma-Aldrich),dichloromethane (HPLC grade, 99.9%, Sigma-Aldrich), toluene (HPLCgrade, 99.9%, Sigma-Aldrich), and tetrahydrofuran (HPLC grade, 99.9%,Sigma-Aldrich). All chemicals were used as received.
SynthesisThe Au40(o-MBT)24 cluster was synthesized by a two-step size-focusing method (23). In the first step, 0.25 mmol of HAuCl4 was re-duced by 1.27 mmol of o-MBT to form Au(I)-o-MBT polymers in atoluene solution containing 0.29 mmol of TOAB. The Au(I)-oMBTpolymers were further reduced to size-mixed Aux(o-MBT)y clustersby 2.5 mmol of NaBH4 (dissolved in 5 ml of water). In the secondstep, the polydispersed Aux(o-MBT)y clusters were reacted with excess
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of o-MBT thiol at 90°C for 48 hours. The Au52(TBBT)32 cluster wassynthesized by a similar process. In the first step, 0.125 mmol ofHAuCl4 was reacted with 0.625 mmol of TBBT in 10 ml of tetrahy-drofuran, followed by reduction to size-mixed Aux(TBBT)y clusters by1.25 mmol of NaBH4 (in 5 ml of water). In the second step, the Aux(TBBT)ymixture was reacted with excess TBBT thiol at 80°C for 24 hours.Both clusters were separated from the reaction mixture by precipita-tion with methanol and crystallized in the pentane/CH2Cl2 solvents.
X-ray crystallographyData of both Au40(o-MBT)24 and Au52(TBBT)32 were collected on aBruker X8 Prospector Ultra equipped with an Apex II charge-coupleddevice detector and an ImSmicrofocus CuKa x-ray source (l = 1.54178 Å)under cold N2 flow at 150 K.
ForAu40(o-MBT)24, a piece of brown crystal with dimensions of 0.16 ×0.10 × 0.01 mm was mounted onto a MiTeGen MicroMeshes with fluor-olube.A triclinic unit cellwith dimensionsa=18.9983(5)Å, b=19.0751(5)Å, c = 35.6255(9) Å, a = 81.5670(17)°, b = 81.4980(16)°, and g = 60.9600(15)° was derived from the least-squares refinement of 9930 reflections inthe range of 2.660 < q < 55.122. Centrosymmetric space group P-1 wasdetermined on the basis of intensity statistics and the lack of systematicabsences. The data were collected to 0.94 Å. After integration of the databy the Bruker SAINTprogram, empirical absorption correctionwas ap-plied using the program SADABS. Themaximumandminimum trans-mittance (Tmax and Tmin) values were 0.6319 and 0.0455, respectively.The structure was solved with a direct method using the BrukerSHELXTL program. All the Au and S atoms were located, and all theC atoms were generated through subsequent difference Fourier synthe-ses. Idealized atom positions were calculated for all hydrogen atoms[with d-(Cmethyl-H) = 0.979 Å and d-(Cphenyl-H) = 0.95 Å]. All theAu, S, and C atoms were refined anisotropically, and all the H atomswere refined isotropically.
For Au52(TBBT)32, a piece of black crystal with dimensions of 0.20 ×0.08 × 0.02 mm was used for data collection. A triclinic unit cell withdimensions a = 24.3807(9) Å, b = 24.8559(10) Å, c = 39.973(2) Å, a =97.027(4)°, b = 99.189(4)°, and g = 117.673(2)° was derived from theleast-squares refinement of 9938 reflections in the range of 2.365 <q < 51.159. Centrosymmetric space group P-1 was determined on thebasis of intensity statistics and the lack of systematic absences. The datawere collected to 0.99Å. TheTmax andTmin valueswere 0.5367 and 0.0531,respectively. The structure was solved with a direct method using Bru-ker SHELXTL. All the Au and S atoms were located; all phenyl C andmost t-butyl C atoms were generated through subsequent differenceFourier syntheses. However, some of the t-Bu C atoms were difficultto locate because of disordering of t-Bu groups as well as interferenceof surrounding solvent electron density, so a rigid TBBT fragment wasused in these cases. Idealized atom positions were calculated for all hy-drogen atoms [with d-(Cmethyl-H) = 0.979Å and d-(Cphenyl-H) = 0.95Å].All the Au, S, and phenyl C atoms were refined anisotropically. The t-BuC and all H atoms were refined isotropically.
DFT simulationsTo gain insight into the electronic structures of the newly observedAu40(o-MBT)24 and Au52(TBBT)32 nanoclusters, we performed DFT calcu-lations with the RI (resolution of the identity) approximation. In thecalculations, the structures of the clusters are exactly the same ones ob-served by the single crystal x-ray crystallography, taking full account ofeach ligand, o-MBT, or TBBT. The TURBOMOLE version 6.6 package
of ab initio quantum chemistry programs was used in all the calculations.The double-z valence quality plus polarization basis in the TURBOMOLEbasis set library was adopted in the calculations along with a 60-electron,relativistic, effective core potential for the gold atom.
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SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500425/DC1Materials and MethodsFig. S1. The left- and right-handed isomers of the chiral double helical Au32 kernel in Au52(TBBT)32.Fig. S2. DFT-simulated HOMO distribution of Au40(o-MBT)24.Fig. S3. DFT-simulated HOMO distribution of Au52(TBBT)32.Fig. S4. Arrangement of thiolates on the flat surface of Au40(o-MBT)24 and Au52(TBBT)32 clusters.Fig. S5. Penetration of surface bridging forces into the kernel, leading to the segregation oftetrahedral units.Table S1. Crystal data and structure refinement for Au40(o-MBT)24.Table S2. Crystal data and structure refinement for Au52(TBBT)32.Table S3. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 ×103)for Au40(o-MBT)24.Table S4. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103)for Au52(TBBT)32.Table S5. Calculated atomic charges.
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Funding: R.J. received financial support from the Air Force Office of Scientific Research(AFOSR) under AFOSR Award no. FA9550-15-1-9999 (FA9550-15-1-0154) and the Camille Drey-fus Teacher-Scholar Awards Program. K.N. received financial support from Elements StrategyInitiative for Catalysts and Batteries and a Grant-in-Aid for Scientific Research (no. 25288012)from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Authorcontributions: C.Z., Y.C., and R.J. were responsible for synthesis and crystallization and forthe design of the project. C.L. and N.L.R. conducted the x-ray crystallographic analysis. K.N.carried out DFT calculations. All authors contributed to the writing of the manuscript.Competing interests: The authors declare that they have no competing interests. Data andmaterials availability: All data is available in the manuscript and the Supplementary Materials.
Submitted 4 April 2015Accepted 6 August 2015Published 9 October 201510.1126/sciadv.1500425
Citation: C. Zeng, Y. Chen, C. Liu, K. Nobusada, N. L. Rosi, R. Jin, Gold tetrahedra coil up: Kekulé-like and double helical superstructures. Sci. Adv. 1, e1500425 (2015).
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