J. Phys. Chem. C 2009, 113, 16983–16987
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What Protects the Core When the Thiolated Au Cluster is Extremely Small? De-en Jiang,*,† Wei Chen,‡ Robert L. Whetten,§ and Zhongfang Chen‡ Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Department of Chemistry, Institute of Functional Nanomaterials, UniVersity of Puerto Rico, San Juan, Puerto Rico 00931, and School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: July 19, 2009; ReVised Manuscript ReceiVed: August 13, 2009
The title question is motivated by the fact that extremely small thiolated-gold clusters such as Au20(SR)16 have been isolated, but their undetermined structures cannot be fully rationalized by the present knowledge derived from single-crystal structures of larger clusters. One needs to go beyond the linear monomer (RSAuSR) and V-shaped dimer (RSAuSRAuSR) motifs that were found to protect larger clusters. We hypothesize that the U-shaped trimer motif (RSAuSRAuSRAuSR) is required to protect the core of some extremely small thiolated-gold clusters, which have about 20 or fewer Au atoms. We test this hypothesis by proposing structural models for Au10(SR)8 based on two trimer motifs protecting a tetrahedral Au4 core and for Au20(SR)16 based on four trimer motifs protecting an Au8 core. 1. Introduction The past two years saw great advances in our understanding of the geometrical construction and electronic structure of thiolated gold clusters, catalyzed by the remarkable total structure determinations of Au102(SR)44 and Au25(SR)18-.1-3 The abundances of certain stoichiometry (so-called “magic numbers”) in thiolated gold clusters can be explained by the electronshell model; for example, Au102(SR)44 and Au25(SR)18- correspond to shell-closing electron counts of 58 and 8, respectively.4,5 On the geometric construction, these two structures demonstrate that RSAuSR and RS(AuSR)2 motifs, not isolated thiolate groups, are employed to protect their high-symmetry Au cores. This knowledge is a confirmation of the “divide-and-protect” concept that the Au atoms in a thiolated gold cluster are partitioned into the pure Au core and the RS(AuSR)x (x ) 1, 2, 3,.. .) outer layer.4,6 The protection of the core by the ligand layer has two contributions: (1) the bonding between terminal thiolates and the Au core and (2) the Au-Au interaction between the Au core and Au in RS(AuSR)x. Thermodynamically, this new interaction scheme wins over the adsorption of isolated thiolates, as demonstrated by density functional theory (DFT) studies on both Au clusters7 and surfaces.8-10 On the basis of the structure of Au102(SR)44, Jiang et al.7 hypothesized that RSAuSR (the monomer) motifs dominate the gold-thiolate interface for thiolated gold clusters. This turns out to be true only for larger [comparable to Au102(SR)44 or bigger] thiolated gold clusters11 or on a flat gold surface,8-10 as Au25(SR)18- has been found to have exclusively six RS(AuSR)2 (the dimer) motifs.2,3 Further, Tsukuda and co-workers12 proposed that the protective layer consists of only the monomer and dimer motifs with more dimers than monomers for smaller [Au25(SR)18- comparable or smaller] clusters. Tsukuda’s principles were successfully employed to predict the core structure and the composition of the protective layer for Au38(SR)24,12 which was also independently confirmed by Zeng and co* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (865)574-5199. Fax: (865) 576-5235. † Oak Ridge National Laboratory. ‡ University of Puerto Rico. § Georgia Institute of Technology.
TABLE 1: Combinations of Monomer (RSAuSR) and Dimer [RS(AuSR)2] Motifs for Composing the Au20(SR)16 Cluster index 1 2 3
monomer
dimer
anchora
coreb
2 5 8
4 2 0
12 14 16
10 11 12
a The total number of terminal thiolates. atoms in the core.
b
The number of Au
workers13 who proposed similar ideas and predicted a more stable model for Au38(SR)24 than previous ones.6,7,14,15 Tsukuda’s principles have also been successfully applied to predict the smallest thiolated gold superatom complexes, which have a shell-closing electron count of two: the best candidate is predicted to be Au12(SR)9+, an Au6 octahedron protected by three dimer motifs.16 Despite the successes of applying Tsukuda’s structural principles, the assumption that the protective layer consists of only the monomer and dimer motifs may not hold for all cases. The argument for the need of the dimer motif is that smaller clusters have larger curvatures, and therefore the dimer motif is needed for its V shape.12,15 However, this V shape is not capable of wrapping around the two neighboring faces of extremely small cores, such as the tetrahedral Au4 cluster.16 Moreover, some small gold-rich thiolated gold clusters with high S/Au ratios (close to 1) such as Au20(SR)16,17 Au18(SR)14,18 Au15(SR)13,18 and Au13(SR)11,19 cannot satisfy all requirements from Tsukuda’s structural principles. For example, when the principles are applied to Au20(SR)16 (Table 1), the number of terminal thiolates (anchors) is at least greater by 2 than the number of the core atoms. This means at least two pairs of terminal thiolates sharing a common core atom in Au20(SR)16, a highly unfavorable scenario due to steric repulsion in such a small cluster. Therefore, one needs to consider longer RS(AuSR)x (x > 2) motifs for these high S/Au ratio small clusters. The logical step is to include the trimer [RS(AuSR)3] motif (Figure 1) in the “divide-and-protect” scheme. In this paper, we propose to use this trimer motif (1) to protect the tetrahedral
10.1021/jp906823d CCC: $40.75 2009 American Chemical Society Published on Web 09/04/2009
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Figure 1. The linear monomer, V-shaped dimer, and U-shaped trimer motifs of gold-thiolate complexes. The dashed line indicates the terminal thiolate that binds to a gold atom of the cluster core.
Au4 core and (2) to predict structures for Au20(SR)16, based on DFT computations. 2. Methods We used Turbomole V5.10 to perform parallel resolutionof-identity density functional theory (RI-DFT) calculations.20 The nonempirical Tao-Perdew-Staroverov-Scuseria (TPSS)21 form of meta-generalized gradient approximation (meta-GGA) was used for electron exchange and correlation, which has been shown22 to describe the aurophilic interactions in gold clusters and gold complexes better than the local density approximation, GGA, and hybrid functionals. The def2-TZVP orbital and auxiliary basis sets23 were used for all atoms for structural optimization. Effective core potentials, which have 19 valence electrons and include scalar relativistic corrections, were used for Au.24 Force convergence criterion was set at 1.0 × 10-3 a.u. Powder X-ray diffraction (XRD) was simulated following the previous example.5 We used CH3S- for RS- for convenience in our studies of Au10(SR)8 and Au20(SR)16. 3. Results and Discussion Using the Trimer Motif to Protect the Tetrahedral Au4 Core. The sulfur atom in each thiolate group can strongly bind to two Au atoms. Therefore, the two terminal thiolate groups in each RS(AuSR)x oligomer motif are the “hands” that grip the Au core through Au-S bonds, while the Au atoms in the RS(AuSR)x motif can interact with the core Au atoms through aurophilic interactions. The typical Au-S bond length of 2.35 Å and S-Au-S bond angle of 180° in the RS(AuSR)x motif dictate that the monomer motif is preferred over the longer motifs on the large (small-curvature) thiolated gold clusters1,11 such as Au102(SR)44 and on the close-packed Au(111) surface9 where the next-nearest-neighbor distance matches well the S-S distance of the monomer. On the surface of an icosahedron2,3,5 or octahedron,16 the curvature as defined by the dihedral angle between two edge-sharing Au triangles (Table 2 and Figure 2) dictates that the dimer motif is preferred. However, when the dimer was used to wrap around two faces of the tetrahedron, dramatic structural relaxation due to the large curvature led to the dimer motif capping the edge.16 Table 2 indicates that the trimer motif may be a better protective motif for the tetrahedron. The construction is shown in Figure 3. We found that the structural relaxation is only minor, and the initial structure is well maintained. The resultant
Jiang et al. Au10(SR)8 cluster has a highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of 1.21 eV and the Au4 core undergoes a slightly distortion to an approximate D2d symmetry (within 0.05 Å tolerance). The Au atoms in the two trimer motifs interact well with the core (Figure 3), as evidenced by the short Au-Au bond lengths (0.1 eV than the other two. However, 1 and 2’s HOMO-LUMO gaps are very different, and the measured
Figure 5. Four optimized candidate structures for Au20(SR)16. Au8 core is highlighted in gray. Only Au (green) and S (blue) in the trimer motif are shown; R- is not shown.
TABLE 5: Relative Energy (∆E) and HOMO-LUMO Gap (HL Gap) for the Four Candidate Structures of Au20(SR)16 Shown in Figure 5 candidate ∆E (eV) HL Gap (eV)
1 0 1.30
2 0.05 2.07
3 0.15 1.84
4 0.21 1.89
optical absorption gap (2.1 eV) indicates that candidate 2 is a better choice. In their paper, Jin and co-workers17 implied that the stable Au20 cage26 could serve as a core for Au20(SR)16. We found that an initial structure of the Au20 cage covered with 16 isolated thiolates underwent dramatic structural change, and the final structure is significantly higher in energy (>6 eV) than models in Figure 5, indicating again that the “divide and protect” concept of RS(AuSR)x complexes protecting a high-symmetry core should be the right principle to predict models for structureunknown thiolated gold clusters. Besides the optical absorption, the other information that can be checked against the experiment is powder XRD. We computed the XRD patterns for both 1 and 2, and found that 1 and 2 display a strong peak at 4.0 and 4.2 nm-1, respectively, both rather off the measured peak (3.85 nm-1)27 with 1 in better
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Figure 6. Topology of connections for candidates 1 and 2 shows how the four trimer motifs are anchored to the Au8 core of Au20(SR)16. Each dashed or dotted line represents a trimer motif.
agreement. This rather smaller peak position at 3.85 nm-1 compared with measured patterns for larger clusters (usually at 4.2 nm-1)28,29 indicates that the plane spacing is rather large (2.6 Å) in the core of Au20(SR)16 and that the core is most likely not closely packed. The other issue with 1 and 2 is that the trimer motifs are rather floppy, and the Au-Au interaction between Au in the trimer motif and Au in the core is not fully utilized. These concerns indicate that better models should exist. The structural design we proposed here for Au20(SR)16 (that is, an Au8 core with four trimer motifs) is still sound and should be pursued further, but one may explore more core shapes beyond those in Figure 4. Hopefully, global minimum search can deliver more stable models, as the system size is not that prohibitive for a quantum-mechanical-based search. The Au20(SR)16 cluster has a valence electron count of 4, corresponding to filling 1S and a 1P orbital and leading to nonspherical core geometry. Since only one 1P orbital is occupied, this should lead to a prolate shape, which agrees with the fcc core in our candidates 1 and 2. Analysis of previous 4e ligated Au clusters also indicates a loose-packed, elongated core.30,31 Because Au10(SR)8 and Au20(SR)16 have the same S/Au ratio, we can compare their stabilities directly (Table 3, reaction 3). We found that Au10(SR)8 is unstable against dimerization into Au20(SR)16 by 0.60 eV based on candidate 1, indicating that Au20(SR)16 is thermodynamically more stable. By taking into account reactions 1 and 2, one can conclude that Au8(SR)6 and Au6(SR)4 are also unstable against formation of Au20(SR)16 with Au4(SR)4. The trimer motif together with the monomer and dimer motifs has been previously found by Garzon and co-workers14 in their structural relaxation of a model for Au38(SR)24, although the “divide-and-protect” concept was not put forward in that work. The total structure determinations1-3 of Au102(SR)44 and Au25(SR)18- helped establish the idea of RS(AuSR)x motifs protecting a high-symmetry core. In this work, we have further extended the “divide-and-protect” concept to the trimer motif. Of course, the final judge would be the total structure determination of Au20(SR)16, while the other example, Au10(SR)8, has not been experimentally identified and is a potential target for synthesis. Implications to Other High S/Au Ratio Clusters. The trimer motif can be used to design structures for other thiolated gold clusters. One example is Au18(SR)14, isolated by Tsukuda and co-workers,18 which can be partitioned as an Au8 core protected by two dimer and two trimer motifs. Since this cluster is also assumed to have an Au8 core, its structure may be closely related
Jiang et al. to that of Au20(SR)16. There are some other high S/Au ratio clusters such as Au15(SR)1318 and Au13(SR)1119 which can not be partitioned into equal numbers of terminal thiolates and core Au atoms with up to the trimer motif, based on the “divideand-protect” concept. Therefore, even longer motifs [RS(AuSR)x; x > 3] and Au-SR cyclomers may be needed. For some other small clusters with relatively low S/Au ratios, the trimer motif may not be needed. For example, Au20(SR)15, isolated by Choi and co-workers,19 is only one thiolate less than Au20(SR)16, but can be partitioned as an Au10 core protected by five dimer motifs, and the Au10 core could be a pentagon or pentagonal antiprism whose edges are capped by the terminal thiolates of the dimer motifs. How about Isolated Thiolates? In heteroleptic gold clusters, isolated thiolates have been observed in experimentally determined structures, such as Au11(S-4-NC5H4)3(PPh3)7.32 Although the preference of RS(AuSR)x complexes for the protective layer has been clearly demonstrated for large homoleptic thiolated gold clusters (comparable to Au38 or larger),7,15 whether or not isolated thiolates can protect the high-curvature core of small homoleptic gold clusters is an open question. For a simple comparison, we consider two isomers of the Au4(SR)4 cluster: (1) an experimentally known ring structure25 with an approximately square shape (each side is a linear RS-Au-SR motif) and (2) an Au4 tetrahedron with each vertex terminated by an RS- group (see Figure S1 in the Supporting Information). We found that the ring structure is more stable in energy by 3.1 eV, which indicates higher stability of RS(AuSR)x complexes than terminal, isolated RS- groups and is consistent with what we found for larger thiolated gold clusters.7,15 Since the stability of thiolated gold clusters strongly depends on the formal valence-electron count,4 detailed examination of various cluster compositions and charge states are needed to conclusively address this issue. Further work is warranted. 4. Summary and Conclusions On the ground of large curvature for the Au4 tetrahedron and high S/Au ratios for some small thiolated gold clusters (with ∼20 Au atoms or less), we argued for the need of the trimer motif [RS(AuSR)3] to protect the high-symmetry Au core. We proposed using two trimer motifs to protect the tetrahedral Au4 core, which results in the Au10(SR)8 cluster, another candidate for the smallest thiolated gold superatom. We have shown that this cluster is more stable than the other two Au tetrahedroncored clusters proposed previously, which are protected by the monomer and dimer motifs. We also predicted structures for Au20(SR)16 based on the trimer motif: an Au8 core protected by four [RS(AuSR)3]. Several promising candidates were found, with the most stable one featuring a prolate fcc core but a low HOMO-LUMO gap. More stable isomers should exist, and the structural design proposed here for Au20(SR)16 points toward a direction to look for such models. Added Note. After we had finished this work, we learned of a recently submitted paper by Y. Pei, Y. Gao, N. Shao, and X. C. Zeng, who proposed a similar structure design for Au20(SR)16 and predicted several models that show good agreement with experiments regarding optical absorption. Acknowledgment. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC, and by NSF Grant CHE-0716718, the Institute for Functional Nanomaterials (NSF Grant 0701525), and the US Environmental Protection Agency (EPA Grant No. RD-83385601). D.J. thanks
What Protects the Core of Small Thiolated Au Clusters? R. Jin for helpful discussion and providing the measured powder XRD pattern for their Au20(SR)16 sample. Supporting Information Available: Optimized structures for two isomers of Au4(SCH3)4 and coordinates for structures in Figure 5 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/. References and Notes (1) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430. (2) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (3) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883. (4) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Go¨nbeck, H.; Ha¨kkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157. (5) Akola, J.; Walter, M.; Whetten, R. L.; Ha¨kkinen, H.; Gro¨nbeck, H. J. Am. Chem. Soc. 2008, 130, 3756. (6) Ha¨kkinen, H.; Walter, M.; Gro¨nbeck, H. J. Phys. Chem. B 2006, 110, 9927. (7) Jiang, D. E.; Tiago, M. L.; Luo, W. D.; Dai, S. J. Am. Chem. Soc. 2008, 130, 2777. (8) Maksymovych, P.; Sorescu, D. C.; Yates, J. T. Phys. ReV. Lett. 2006, 97, 146103. (9) Gro¨nbeck, H.; Ha¨kkinen, H.; Whetten, R. L. J. Phys. Chem. C 2008, 112, 15940. (10) Jiang, D. E.; Dai, S. J. Phys. Chem. C 2009, 113, 3763. (11) Lopez-Acevedo, O.; Akola, J.; Whetten, R. L.; Gro¨nbeck, H.; Ha¨kkinen, H. J. Phys. Chem. C 2009, 113, 5035. (12) Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, 8608. (13) Pei, Y.; Gao, Y.; Zeng, X. C. J. Am. Chem. Soc. 2008, 130, 7830.
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